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Quaternary International xxx (2013) 1e21

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Ostracod shell chemistry as proxy for paleoenvironmental change

Nicole Börner a,*, Bart De Baere b, Qichao Yang c, Klaus Peter Jochumc, Peter Frenzel d,Meinrat O. Andreae c, Antje Schwalb a

a Institut für Geosysteme und Bioindikation, Technische Universität Braunschweig, Langer Kamp 19c, 38106 Braunschweig, GermanybDepartment of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, CanadacBiogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germanyd Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Jena, Germany

a r t i c l e i n f o

Article history:Available online xxx

* Corresponding author.E-mail address: nicole.boerner@tu-bs.de (N. Börne

1040-6182/$ e see front matter � 2013 Elsevier Ltd ahttp://dx.doi.org/10.1016/j.quaint.2013.09.041

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a b s t r a c t

The application of ostracod shell chemistry as a paleoenvironmental tool has grown within the pastdecades. Most studies have investigated Mg and Sr in ostracod shells as proxies for temperature andsalinity, and the use of a wide range of trace elements as prospective paleoenvironmental indicators hasyet to be developed for lacustrine systems. Only a few preliminary studies have used trace metals inpaleolimnological studies such as Cd, Ba and Zn as paleonutrient indicators, or Mn, Fe and U as redox andoxygenation indicators. This paper reviews the state of the art of geochemical analyses in microfossilssuch as ostracods, foraminifera, and corals, and provides insights in new trace element proxies with thegoal to promote the use of trace elements in ostracod shells as paleoenvironmental proxies.

In paleoceanography, foraminifera and corals are most prominently used to reconstruct past climateconditions. Well-established proxies such as d18O, Mg/Ca and Sr/Ca provide information about changes insea surface temperatures. In addition, a great number of new proxies have been developed recently, suchas radiogenic isotopes and redox sensitive trace elements. In paleolimnology, ostracod shell chemistry iswidely used to assess paleohydrological changes. Reconstruction of temperature and salinity changes inlake environments is often achieved by oxygen isotopes as well as Mg/Ca and Sr/Ca ratios, but dependingon the hydrological and geological settings of the lake system, local calibrations are needed to assesswhich proxy is suited to reflect which processes.

New proxies need to be tested by novel techniques that recently have become available. Compared toconventional instrumentation used in ostracod shell chemistry, methods such as Laser Ablation ICP-MSand NanoSIMS allow single shell analysis and provide high-resolution data. The potential of ostracods inpaleolimnology is not yet fully assessed, but can be developed by learning from paleoceanographicstudies.

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1. Introduction

Biological remains are particularly well suited for the recon-struction of paleoenvironments and paleohydrology because theyare abundant and very diverse, comprising hundreds of taxaincluding diatoms, chrysophytes, charophytes, ostracods, corals,mollusks and foraminifera (Gasse et al., 1987). In lacustrine sedi-ments, the most abundant calcareous organism remains are shellsfrom ostracods (Holmes, 2003). These provide a discrete source ofbiogenic carbonate, an extremely valuable Quaternary paleoenvir-onmental indicator.

r).

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et al., Ostracod shell chemist13.09.041

In recent decades ostracods, have been increasingly used asmicrofossil indicators in lake systems, while the majority ofgeochemical approaches in paleoclimate research using biogeniccarbonates focus on marine foraminifera. In order to interpretcoastal, brackish, or freshwater environments, ostracods are moresuitable, as they tolerate a wide salinity range. Ostracod assem-blages have initially been used as a salinity proxy (De Deckker andGeddes, 1980; De Deckker, 1981, 1982; Forester, 1983, 1986; Smith,1993; Curry, 1997), as they allow establishing conductivity transferfunctions (Mezquita et al., 2005; Mischke et al., 2007).

In the marine environment, Mg/Ca in planktonic foraminiferaand Sr/Ca in corals are used as recorders of sea surface tempera-tures (SST), Mg/Ca in benthic foraminifera and ostracods as proxiesof bottom water temperature, and the coupling of d18O and Mg/Cacan be used to adjust for the temperature-dependency of d18O by

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isolating the temperature signal from the d18Owater record, whichthen can be used to reconstruct local changes in evaporation andprecipitation (Rosenthal and Linsley, 2006). Primary productionand decay of organic matter can be assessed using carbon isotopes(Kahn, 1979; Gasse et al., 1987; Basak et al., 2009). In coastal en-vironments the freshwater influx can be deduced using O-, C- or Sr-isotopes (Janz and Vennemann, 2005). In lacustrine environments,d18O, d13C, Mg/Ca and Sr/Ca ratios in ostracod shells are useful toolsfor reconstructing paleotemperatures, paleosalinity, paleo-productivity as well as methanogenesis (Schwalb, 2003; Mischkeet al., 2008b; Decrouy et al., 2012; Schwalb et al., 2013).Following recent developments in foraminifera studies, many othertrace metals become important in studies of ostracod shell chem-istry, as trace metals play an essential role in many geochemicalcycles (Anbar and Knoll, 2002; Algeo et al., 2012).

The ability to track chemical changes in the oceans, continentalwaters and the atmosphere through time depends on a thoroughunderstanding of the processes that influence the trace elementuptake by sediment and biogenic carbonate, and on the develop-ment of suitable trace element proxies for paleoceanographic andpaleolimnological research (Canfield, 1998; Saito and Sigman,2003; Algeo et al., 2012). The use of redox sensitive trace ele-ments in paleoceanographic studies has been extensively investi-gated (Ricketts et al., 2001; Tribovillard et al., 2006; Boiteau et al.,2012). Because of their conservative behavior and long residencetimes in oxic seawater, molybdenum and uranium (800 ka for Moand 450 ka for U, respectively) were found to be most useful inmarine environments (Algeo et al., 2012). The MneFe redox cyclingwithin the water column represents an important additional proxy(Algeo et al., 2012). Patterns in redox-sensitive trace elementenrichment can thus be used in paleoceanographic studies toevaluate deepwater restriction, deepwater residence time andchanges in deepwater chemical composition (Algeo and Lyons,2006; Algeo and Tribovillard, 2009; Algeo et al., 2012). The inves-tigation of the controls of co-variation, such as the co-variation of Uand Mo, is a relatively new area of research (Tribovillard et al.,2012).

New proxies need to be tested by novel techniques that recentlyhave become available. Compared to conventional instrumentationused in ostracod shell chemistry, methods such as Laser AblationICP-MS and NanoSIMS allow single shell analysis and provide high-resolution data. The potential of ostracods in paleolimnology is notyet fully explored, but can be developed by learning from paleo-ceanographic studies.

This study will give an overview of stable isotope and traceelement proxies in paleoceanographic and paleolimnologicalresearch. A wide range of proxies have found application in fora-miniferal, coral or molluscan studies. Prominent proxies such asstable oxygen and carbon isotopes as well as Mg/Ca and Sr/Ca ratioshave found wide application in ostracod shell chemistry. On theother hand, the use of radiogenic isotopes or trace elements such asbarium, cadmium, or boron has not yet found their way intoostracod research. But these parameters may become available forostracod research, if they show similar behavior in marine andlacustrine environments. Thus, the development of the use of traceelements in ostracod shells may significantly broaden the range ofexisting paleoenvironmental proxies.

2. History

During the last three decades trace element and stable isotopegeochemistry of ostracods has become a prominent tool in paleo-environmental reconstruction. First attempts to develop quantita-tive tools using oxygen isotopes in biogenic carbonates had alreadybeen undertaken in the middle of the twentieth century by Urey

Please cite this article in press as: Börner, N., et al., Ostracod shell chemist(2013), http://dx.doi.org/10.1016/j.quaint.2013.09.041

(1947), who suggested that oxygen isotopes reflect changes inboth temperature and ice volume. His students Epstein andMayeda(1953) and Emiliani (1955, 1966) developed the first paleo-temperature reconstructions using foraminifera from deep-seacores to calculate changes in ocean temperature. Shackleton et al.(1973) described the fractionation of oxygen isotopes in forami-nifera calcite and their relation to ice volume and temperature.Comparisons of stable isotope based reconstructions with thosefrom other proxies confirmed the outstanding paleoenvironmentalinformation derived from geochemical data (Fairbanks et al., 1980;Gasse et al., 1987; Lewis and Anderson,1992; Fritz et al., 1994). Untilthe 1980s, species assemblages and stable carbon and oxygen iso-topes were the major paleoclimatic tools, recently complementedby new proxies in foraminifera. Besides stable oxygen and carbonisotopes, Mg/Ca ratios (e.g., Nürnberg et al., 1996) and Sr/Ca ratiosas well as calcium isotopes (d44Ca) are used to reconstruct paleo-temperatures (Kisakürek et al., 2011). Proxies have been developedto reconstruct past salinity changes in marine and marginal marine(Carpenter et al., 1991; Corrège and De Deckker, 1997; Gillikin et al.,2006) as well as lacustrine environments (Chivas et al., 1985; DeDeckker et al., 1988a; Engstrom and Nelson, 1991). Other authorshave attempted to reconstruct past ocean circulation using Cd/Caratios and radiogenic isotopes (Vance et al., 2004; Colin et al., 2010;Makou et al., 2010; Van de Flierdt et al., 2010). Especially for themarine environment, additional proxies have been developed, suchas the reconstruction of the carbon cycle including proxies forocean productivity (231Pa/230Th, U concentration) (Tribovillardet al., 2006; Boiteau et al., 2012), nutrient utilization (Cd/Ca, d15N,d30Si) (Boyle, 1981; Perga, 2010; Versteegh et al., 2011), alkalinity(Ba/Ca) (Boyle, 1981; Chivas et al., 1983), pH (d11B), carbonate ionconcentration (Eggins et al., 2003; Elmore et al., 2012) and atmo-spheric CO2 (d11B, d13C) (Mii et al., 2001). Stable radiogenic isotopes(87Sr, 187Os, 143Nd) can be used to assess fluxes from continent toocean (Vance and Burton, 1999; Henderson, 2002; Kober et al.,2007).

3. Ostracods as paleoenvironmental proxies

Ostracods are bivalved micro-crustaceans and can be found inmost aquatic environments, e.g., in continental, estuarine, marineand hypersaline waters (Holmes, 1992). They are generally 0.2e1.0 mm long and weigh between 20 and 200 mg (Rosenthal andLinsley, 2006). Ostracods have up to 8 molting stages until theybecome adults, each time secreting an exoskeleton (carapace) oflow-magnesium calcite using Ca2þ and HCO3

� ions from ambientwater. Prior to molting, large amounts of calcium and phosphateare stored in the outer epidermis to have calcium quickly availablefor calcifying the new carapace (Keyser and Walter, 2004). Theprocuticle, the calcified part, is between 20 and 45 mm thick(Sylvester-Bradley and Benson, 1971) and covered by a continuouscuticular integument, consisting of a thin chitinous and organicmembrane (Bennett et al., 2011). Keyser and Walter (2004) sum-marized that the carapace consists of 80e90% calcite, 2e15%organic material (chitin plus protein) and various minor and traceelements (Delorme, 1970).

Shell calcification takes place very quickly, typically within a fewhours to several days, in geochemical equilibriumwith the ambientwater. After molting, the components used to form the shell aretaken directly from the host waters without storage prior tomolting (Turpen and Angell, 1971). Thus, the chemistry of ostracodshells reflects the chemical conditions of the ambient water (e.g.,temperature, salinity, dissolved ion composition, hydrology) at thetime of molting and new valve growth (Chivas et al., 1985; Holmes,1992; Smith, 1993; Smith and Horne, 2002; Mischke et al., 2007;Mischke and Holmes, 2008) and can be used as proxy for

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environmental variables (Chivas et al., 1986a; Dean and Schwalb,2002; Ito et al., 2003; Alvarez Zarikian et al., 2005; Anadón et al.,2006; Mischke et al., 2010b; Marco-Barba et al., 2013).

4. Ostracod shell chemistry

For the use of ostracod shell chemistry as paleoenvironmentalproxy and for appropriate data analysis it is crucial to know theecology and phenology of the target species (Pérez et al., 2013).Phenological variation often occurs with respect to the hatchingand molting of juveniles, females and males (Xia et al., 1997a), andinsight in timing and location of calcification with respect to thepreferred microhabitat is invaluable (Van der Meeren et al., 2011).The major driving factors of variations in ostracod valve chemistryare spatiotemporal variability in the physicochemical environmentof the host water, life history, habitat and biocalcification (Van derMeeren et al., 2011). Ostracod shells have been used as sourcematerial for geochemical analysis of stable isotope and traceelement composition in paleolimnological reconstruction of lakehydrochemistry and climate as they provide insight into past waterbalance and solute evolution of lakes (Lister, 1988; Chivas et al.,1993; Haskell et al., 1996; Bridgwater et al., 1999; Yu and Ito,1999; Gouramanis et al., 2010). A review about proxies and theirinterpretation is summarized in Table 1. However, the challengeremains to adequately clean samples prior to analysis, becausesurface coatings, added after formation of the carbonate, oftencontain significant quantities of the trace metal of interest (Holmes,1996; Ito, 2002; Keatings et al., 2006). In addition, studies of traceelement and stable isotope composition of ostracod shells frompre-Pleistocene sediments are relatively sparse. Extending ostracodshell chemistry to longer time scales is complicated, as diagenesisbecomes a greater problem and water chemistry is not well known(Mucci andMorse,1983; De Deckker et al., 1999; Dwyer et al., 2002;Bennett et al., 2011).

Table 1Summary of paleoenvironmental proxies and their interpretation in ostracod research.

Proxy & indicated environmental parameter Species

d18OTemperature Candona candida, Cytherissa lacustr

Herpetocypris brevicaudata/chevreuEucypris mareotica, Fabaeformiscandanielopoli, Limnocythere inopinataSeveral species

Temperature & d18Owater (d18Owater

reflects d18Oprecipitation and thus airtemperature or source of inflow)

Candona candida, Cytherissa lacustr

Candona subtriangulata

Candona sp., Cytherissa lacustrisCandona negIecta, Leucocythere mirLimnocythere sanctipatriciiCandona candida, C. levanderi, C. mCytherissa lacustris, Limnocythere saPseudocandona marchicaCandona neglectaCandona neglecta, Ilyocypris gibba/bPrionocypris zenkeriLeucocythere mirabilis, Limnocythersanctipatricii,

d18Owater (reflecting shifts in watersource, such as input of melt-,groundwater or river water)

Candona rawsoni, C. subtriangulata

Cytheridella ilosvayiCyprideis torosa

Temperature, P/E balance & water source Eucypris inflata, Limnocythere inopiLimnocythere sappaensisLimnocythere ceriotuberosaCandona neglecta

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4.1. Paleotemperature and paleosalinity

Information about global ice volume, salinity and temperatureare provided by oxygen isotopes in biogenic carbonates (Chivaset al., 1986a, 1986b; Talbot, 1990). Isotopically depleted water ispreferentially stored in polar ice sheets, whereas the ocean ischaracterized bymore positive d18O values (Shackleton et al., 1973).Hence, more positive oxygen isotope values in fossil specimensfrom the marine realm are interpreted as indicative of glacialconditions; more negative d18O as interglacial conditions. Tem-perature affects the fractionation of oxygen isotopes during for-mation of carbonate. An increase in temperature by 1 �C results in acorresponding decrease in d18Ovalve of 0.2& (Craig, 1965; Chivaset al., 1986a; Bennett et al., 2011). Oxygen isotope values inostracod valves are also affected by salinity changes. Marco-Barbaet al. (2012) reported a significant relationship between ostracodand water d18O for Cyprideis torosa, except at high Total DissolvedSolids (TDS >20 g L�1) where d18Ovalve values were lower thanexpected. Evaporation and hence higher salinity results in higherd18Owater values and thus higher d18Ovalve (Chivas et al., 1986a;Wrozyna et al., 2010; Pérez et al., 2013). A strong correlation be-tween TDS and Mg/Ca or Sr/Ca ratios has been shown in numerousstudies. Van der Meeren et al. (2011) suggested that Mg/Ca mighthave the highest potential as salinity proxy. Both Mg/Cavalve and Sr/Cavalve correlate strongly with the respective Mg/Cawater and Sr/Cawater (Hu et al., 2008). Other studies described a relationshipbetween Sr/Ca ratios and salinity, as high Sr/Ca ratios are indicativefor lacustrine environments and a rapid decrease in Sr/Ca coincideswith the transition from lacustrine to marine environments(Torgersen et al., 1988; De Deckker et al., 1988a). The uptake of Srand Mg is primarily a function of their concentration in ambientwater and water salinity; in the case of Mg it is also related towatertemperature (Carpenter and Lohmann, 1992; Hu et al., 2008; Itoand Forester, 2009). Teeter and Quick (1990) reported a negative

Reference Location of statement

is,xi

Hammarlund, 1999 Oxygen isotope records,Conclusion

dona Mischke et al., 2008b Section 5.

Decrouy et al., 2012 Section 4.is Lister, 1988 Oxygen Isotopes,

ConclusionLewis and Anderson, 1992 Climatic interpretation and

discussionVon Grafenstein et al., 1992 e

abilis, Schwalb et al., 1994 e

archica,nctipatricii

Von Grafenstein et al., 1994, 1996 Water temperature effects;Section 3.3

Von Grafenstein et al., 2000 Sections 4.1 & 4.2Anadón et al., 2006 Concluding remarks

radyi, Holmes et al., 2010 Section 4.5

ina Schwalb et al., 2013 Section 5.1, Conclusion

Last et al., 1994 Stable isotopes inostracodes

Alvarez Zarikian et al., 2005 Section 7.2, ConclusionMarco-Barba et al., 2012 Section 4.5

nata Lister et al., 1991 ConclusionSchwalb et al., 1999 Sections 4. & 5.Cohen et al., 2000 Stable IsotopesRicketts et al., 2001 Section 4.2.1

(continued on next page)

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Table 1 (continued )

Proxy & indicated environmental parameter Species Reference Location of statement

Several species Schwalb, 2003 Summary and conclusionsSeveral species Janz and Vennemann, 2005 Section 5.2, Fig. 7,

ConclusionEucypris lutzae, Ilyocypris binocularis, Strandesiarisgoviensis

Tütken et al., 2006 Section 5.2

Fabaeformiscandona breuili, F. spelaea,Pseudocandona compressa

Anadón et al., 2008 Geochemistry: discussion

Cyprideis torosa Anadón and Gabàs, 2009 DiscussionEucypris inflate Zhu et al., 2009 Section 4.2Eucypris mareotica, Fabaeformiscandonarawsoni, Leucocythere sp.

Mischke et al., 2010b Section 4.3

Several species Wrozyna et al., 2010 Section 5.Limnocythere inopinata Van der Meeren et al., 2011 Section 5.2Cytheridella ilosvayi, Limnocythere opesta,Pseudocandona sp.

Escobar et al., 2012 Sections 4.1 & 7.

Several species Pérez et al., 2013 Section 5.2, ConclusionP/E balance & Salinity Australocypris, Mytilocypris Chivas et al., 1993 Oxygen Isotopes,

ConclusionsCyprideis torosa Marco-Barba et al., 2013 Discussion

P/E balance & water source Several species Gouramanis et al., 2010 Section 6.2Eucypris mareotica Mischke et al., 2010a Discussion of lake evolution

d13CProductivity Cyprideis torosa Gasse et al., 1987 Biogenic carbonates

Candona negIecta, Leucocythere mirabilis,Limnocythere sanctipatricii

Schwalb et al., 1994

Candona candida, Cytherissa lacustris,Herpetocypris brevicaudata/chevreuxi

Hammarlund, 1999 Carbon isotope record,Conclusion

Limnocythere ceriotuberosa Cohen et al., 2000 Stable IsotopesEucypris mareotica, Fabaeformiscandonadanielopoli, Limnocythere inopinata

Mischke et al., 2008b Section 5.

Cyprideis torosa Anadón and Gabàs, 2009 DiscussionSeveral species Wrozyna et al., 2010 Section 5.Several species Decrouy et al., 2012 Section 4.

Productivity & CO2 exchange Candona neglecta Ricketts et al., 2001 Section 4.2.2Ilyocypris binocularis Tütken et al., 2006 Section 5.8Eucypris mareotica, Fabaeformiscandonarawsoni, Leucocythere sp.

Mischke et al., 2010b Section 4.3

Eucypris mareotica Mischke et al., 2010a Discussion of lake evolutionProductivity & carbon source (possible carbonsources are organic matter decay, CO2

exchange, methanogenesis)

Candona candida, Cytherissa lacustris Lister, 1988 Carbon isotopes,Conclusion

Limnocythere sappaensis Schwalb et al., 1999 Sections 4. & 5.Several species Schwalb, 2003 Summary and conclusionsCytheridella ilosvayi Alvarez Zarikian et al., 2005 ConclusionSeveral species Janz and Vennemann, 2005 Section 5.3, ConclusionCandona neglecta Anadón et al., 2006 Concluding remarksFabaeformiscandona breuili, F. spelaea,Pseudocandona compressa

Anadón et al., 2008 Geochemistry: discussion

Several species Gouramanis et al., 2010 Section 6.2Candona neglecta, Ilyocypris gibba/bradyi,Prionocypris zenkeri

Holmes et al., 2010 Section 4.4

Cytheridella ilosvayi, Limnocythere opesta,Limnocythere sp., Pseudocandona sp.

Escobar et al., 2012 Sections 4.2 & 7.

Cyprideis torosa Marco-Barba et al., 2012 Section 4.4Cyprideis torosa Marco-Barba et al., 2013 DiscussionSeveral species Pérez et al., 2013 Section 5.2, ConclusionLimnocytherina sanctipatricii, Leucocytheremirabilis

Schwalb et al., 2013 Sections 5.2 & 5.3

Mg/CaTemperature Mytilocypris henricae Chivas et al., 1983 Conclusions

Ilyocypris bradyi Holmes et al., 1992 Section 5.2Cypretta brevisaepta Holmes et al., 1995 Section 5.1, ConclusionsBythocypris, Krithe Corrège and De Deckker, 1997 Section 3.3Cyprideis australiensis De Deckker et al., 1999 Section 3., DiscussionLimnocythere ceriotuberosa Cohen et al., 2000 Mg/Ca and Sr/Ca resultsSeveral species Janz and Vennemann, 2005 Section 5.4, Fig. 7Australocypris, Diacypris, Mytilocypris, Reticypris Gouramanis and De Deckker, 2010 DiscussionKrithe Elmore et al., 2012 Sections 3.2 & 3.3

Temperature & Salinity (reflectingP/E balance, lake level, groundwaterinflow)

Several species Chivas et al., 1986b ConclusionCyprideis torosa Gasse et al., 1987 Biogenic carbonatesIlyocypris microspinata, Lineocypris jiangsuensis Hu et al., 2008 Section 4.3, ConclusionsDiacypris spp., Reticypris spp. De Deckker et al., 2011 Section 8.

Temperature & Mg/Cawater (indicativefor lake level changes)

Cyprideis De Deckker et al., 1988a Fig. 3, Discussion; ResultsCyprideis De Deckker and Williams, 1993Several species Decrouy et al., 2012 Section 4.

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Table 1 (continued )

Proxy & indicated environmental parameter Species Reference Location of statement

Cyprideis torosa Marco-Barba et al., 2012 Section 4.2, ConclusionsSalinity Cyprideis americana Teeter and Quick, 1990 Observation and

Discussion, ConclusionsCandona rawsoni Engstrom and Nelson, 1991 Salinity calibrationCandona rawsoni Fritz et al., 1994 ResultsCandona rawsoni Haskell et al., 1996 Section 4.2Candona rawsoni Yu and Ito, 1999 Geochemical analysisLimnocythere inopinata Van der Meeren et al., 2011 Section 5.1

Mg/Cawater (but no correlation to salinity) Australocypris, Mytilocypris Chivas et al., 1986a ConclusionsCyprideis australiensis De Deckker et al., 1999 Section 3., DiscussionCandona neglecta Ricketts et al., 2001 Section 4.2.3Cyprideis ruggierii, Ilyocypris cf. gibba,Loxoconcha minima

Anadón et al., 2002 Section 6.1

Cypridopsis vidua Ito and Forester, 2009 Summary and ConclusionsMg/Cawater & Salinity (reflecting changes inwater source and P/E balance)

Candona patzcuaro, Heterocypris punctata Bridgwater et al., 1999 Sections 4.3.1 & 5.Eucypris mareotica, Fabaeformiscandonadanielopoli, Limnocythere inopinata

Mischke et al., 2008b Section 5.

Cyprideis torosa Anadón and Gabàs, 2009 DiscussionSr/CaSalinity (signal can be biased by aragoniteprecipitation)

Australocypris robusta, Mytilocypris praenunciaand several other species

Chivas et al., 1985; 1986a Conclusion

Cyprideis De Deckker et al., 1988a,b Fig. 3, DiscussionCyprideis, Ilyocypris McCulloch et al., 1989 Comparison of 87Sr/86Sr and

Sr/Ca ratiosCandona rawsoni Engstrom and Nelson, 1991 Salinity calibrationIlyocypris bradyi Holmes et al., 1992 Section 5.2Australocypris, Mytilocypris Chivas et al., 1993 Paleosalinity, ConclusionsLimnocythere ceriotuberosa Cohen et al., 2000 Mg/Ca and Sr/Ca resultsCandona neglecta Ricketts et al., 2001 Section 4.2.3Ilyocypris microspinata, Lineocypris jiangsuensis Hu et al., 2008 Section 4.3, Conclusions

Sr/Cawater (but no correlation to salinity) Cyprideis australiensis De Deckker et al., 1999 Section 3., DiscussionCyprideis ruggierii, Ilyocypris cf. gibba,Loxoconcha minima

Anadón et al., 2002 Section 6.1

Cyprideis torosa Anadón and Gabàs, 2009 DiscussionCypridopsis vidua Ito and Forester, 2009 Summary and ConclusionsAustralocypris, Diacypris, Mytilocypris, Reticypris Gouramanis and De Deckker, 2010 DiscussionSeveral species Gouramanis et al., 2010 Section 6.2Cyprideis torosa Marco-Barba et al., 2012 Section 4.3, Conclusions

Sr/Cawater & Salinity (information aboutP/E balance and water source)

Cyprideis torosa Gasse et al., 1987 Biogenic carbonatesCyprideis De Deckker et al., 1988a,b Fig. 3, Discussion; ResultsCypretta brevisaepta Holmes et al., 1995 Section 5.1, ConclusionsCandona patzcuaro, Heterocypris punctata Bridgwater et al., 1999 Section 4.3.1 & 5.Fabaeformiscandona breuili, F. spelaea,Pseudocandona compressa

Anadón et al., 2008 Geochemistry: discussion

Eucypris mareotica, Fabaeformiscandonadanielopoli, Limnocythere inopinata

Mischke et al., 2008b Section 5.

Candona neglecta, Ilyocypris gibba/bradyi,Prionocypris zenkeri

Holmes et al., 2010 Section 4.3

Diacypris spp., Reticypris spp. De Deckker et al., 2011 Section 8.Cyprideis torosa Marco-Barba et al., 2013 Discussion

Temperature & Sr/Cawater (lake level changes) Cyprideis De Deckker and Williams, 1993 e

Several species Decrouy et al., 2012 Section 4.Carbonate mineralogy (aragonite vs. calcite) Candona rawsoni Haskell et al., 1996 Section 4.2

Ba/CaTemperature Mytilocypris henricae Chivas et al., 1983 Conclusions

U/CaOxygenation (vertical mixing, organic matterdecay)

Candona neglecta Ricketts et al., 2001 Section 4.2.4

87Sr/86SrWater source (continental weathering, watermass mixing, transition from marine tolacustrine and v.v., freshwater inflow)

Cyprideis, Ilyocypris McCulloch et al., 1989 Paleoenvironmentalimplications

Several species Janz and Vennemann, 2005 Section 5.1, ConclusionCyprideis torosa Vasiliev et al., 2006 Sections 5. & 6.Cypretta brevisaepta Holmes et al., 2007 Section 3.5Several species Kober et al., 2007 Section 4.3Candona neglecta, Ilyocypris gibba/bradyi,Prionocypris zenkeri

Holmes et al., 2010 Section 4.3

143Nd/144NdWater mass tracer Several species Janz and Vennemann, 2005 Section 5.1, Conclusion

Li/CaTemperature Eucypris inflata Zhu et al., 2009 Section 4.3

Fe, MnOxygenation Cyprideis torosa Gasse et al., 1987 Biogenic carbonates

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correlation between the Mg concentration in shells of living Cyp-rideis americana and salinity, with little or no temperature depen-dence of Mg. No temperature effect was also observed by Marco-Barba et al. (2012) as well as no influence of Mg/Cawater on theMg/Ca ratio in ostracods. In addition, the authors found that inwaters with Mg/Ca ratios �6 all Cyprideis torosa shells showed thesameMg/Ca content over the whole salinity range, indicating a lackof correlation between salinity and ostracod Mg/Ca ratios in low-magnesium waters. The temperature dependence can easily bemasked by small changes in the Mg/Ca ratio of the ambient water(De Deckker et al., 1999). Factors influencing the Mg/Ca and Sr/Caratios in ostracod shells are vital offsets, as well as the influence ofother soluble ions that may change the complexing of the divalentcations in the host waters and hence partitioning of Sr, Mg, and Cainto the shells (see Section 5.).

Van der Meeren et al. (2011) assessed how valve chemistry isaffected by regional patterns and seasonal trends in solute evolu-tion, the physicochemical stability of the host water, and the timeand place of biocalcification. The authors found, that the correlationof Sr/Cavalve to Sr/Cawater was more significant than that of Mg/Cavalve to Mg/Cawater, and suggested a lesser influence by calcifica-tion rate and temperature on the Sr incorporation. A local reductionin dissolved Sr can be caused by aragonite precipitation (Haskellet al., 1996; Hu et al., 2008; Mischke et al., 2008b), and reducedMg uptake by ostracods can occur in waters with high Mg-content(De Deckker et al., 1999; Gouramanis and De Deckker, 2010), bothmasking the temperature-dependent signal. An overview of in-terpretations of geochemical parameters is given in the publica-tions by Miller et al. (1991), Griffiths and Holmes (2000), andZachos et al. (2001), based on non-marine ostracods from closed-basin lakes where hydrochemical changes occur as a function ofprecipitation and evaporation as well as groundwater input.

According to Zhu et al. (2009) another possible tracer for pasttemperatures is provided by Li/Ca ratios in ostracod shells. Theyfound a negative correlation between the Li/Ca ratio and temper-ature in Eucypris inflata from Lake Qinghai, Tibet. A temperaturedependency of Li/Ca ratios was also described by Hall and Chan(2004b) and Lear et al. (2010) for several foraminifera species.Hall and Chan (2004b) showed that the conservative behavior oflithium in the ocean (1.5 Ma residence time) results in constant Li/Ca ratios of water on centennial to millennial scales, indicating thatthe Li/Ca ratios of carbonates are not mainly controlled by solubi-lity. Tomascak et al. (2003) calculated a residence time of 28 ka forlithium in Mono Lake, California, but Zhu et al. (2009) suggestedthat lithium in larger lakes, like Lake Qinghai, has longer residencetime than in Mono Lake. Thus, in large lakes, the Li/Ca ratios ofcarbonates are primarily controlled by temperature and not biasedby the Li/Ca ratios of the ambient water.

4.2. Carbon cycle

The d13C content of carbonate depends on the isotopic compo-sition of the Total Dissolved Inorganic Carbon (DIC) from which itprecipitates, influenced by the rate of local production of CO2 andthe rate of isotopic exchange with the atmospheric reservoir(Hammarlund, 1999; Alvarez Zarikian et al., 2005; Tütken et al.,2006; Mischke et al., 2010b). A temperature dependency of thecarbon isotope composition of the carbonate precipitated inostracod shells could be excluded in many studies (Durazzi, 1977;Marco-Barba et al., 2012). Additional factors that impact d13C arebiological productivity, pH, organic matter decay, and bacterialprocesses (De Deckker and Forester, 1988; Schwalb et al., 1999; Liuet al., 2008; Escobar et al., 2012). Freshwater or groundwater inflowhas been documented to shift the d13C signal toward lighter values(Janz and Vennemann, 2005; Gouramanis et al., 2010; Holmes et al.,

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2010). Methanogenesis and methane oxidation also influence thed13C signal in ostracods (Schwalb, 2003; Escobar et al., 2012;Schwalb et al., 2013). Methane oxidation close to the sediment-water-interface results in 13C-depleted DIC that is used for incor-poration into ostracod shells.

4.3. New chemical proxies in ostracod shell chemistry

Very few studies exist that address the potential suitability ofproxies other than d18O, d13C, Mg/Ca and Sr/Ca. In an early study,Chivas et al. (1983) tested the suitability of Ba/Ca ratios in ostracods.Their experiments revealed a positive correlation between ostracodBa/Ca ratios and temperature and no relationship to the Ba/Caconcentrations of the ambient water. Palacios-Fest et al. (2003)tried to track anthropogenic influence by measuring Co, Ni, Cu,Zn, Cd, Pb, and REEs in ostracod shells, which may reflect levels ofurban and industrial pollution. The authors found that most of thestudied heavy metals as well as REEs show increased values inostracod valves from polluted waters and are thus suitable to trackanthropogenic influence. Contradictory results were obtained forCd and Pb as their concentration in polluted and unpolluted con-ditions did not show any differences.

Additionally, manganese and iron (Gasse et al., 1987; Holmes,1997) as well as uranium (Ricketts et al., 2001) to calcium ratioswere tested as proxies for redox and oxygenation cycles. The au-thors reported increased element concentrations in reducing en-vironments and related changes in Fe/Ca, Mn/Ca, and U/Ca to shiftsin bottom water oxygenation. Ricketts et al. (2001) suggested thatvertical water mass mixing and organic matter decay were possiblecontrolling factors of changes in oxygenation.

In the marine and marginal marine environment, ostracod valvechemistry was used to assess the extent to which a marine basin isisolated from the open ocean using Nd isotopes as the isotopiccomposition of this element, 2010 reflects different sources ofcarbonate, depending on surface/bedrock composition (Janz andVennemann, 2005). The 87Sr/86Sr ratios trace transitions frommarine to non-marine environments and vice versa as well asfreshwater inflow (McCulloch and Deckker, 1989; McCulloch et al.,1989; Vasiliev et al., 2006). Strontium isotopes have been applied tolimnological studies as they give information about the mixing ofwaters from different water sources, e.g., river inflow, groundwaterinflow, or marine influence (Holmes et al., 2007; Kober et al., 2007;Holmes et al., 2010).

5. Limitations of ostracod shell chemistry as environmentalproxy

Various theoretical and practical limitations to the use of stableisotopes and trace elements in ostracod shells as paleoenvir-onmental proxies have been described (Engstrom and Nelson,1991; Holmes et al., 1995; Ito and Forester, 2009), including non-equilibrium fractionation of stable isotopes and trace elements inostracod shells, diagenesis and alteration, and the effect of differentpre-treatment methods on the shell composition.

5.1. Non-equilibrium fractionation and vital effects

Vital effects are very important for calculating past environ-mental conditions, e.g., temperature. Vital effects describe species-or genus-specific offsets from the expected equilibrium. Disequi-librium fractionation in biogenic carbonates has been reported inmany papers. Most studies, however, focus on disequilibrium inplanktonic foraminifera (Moberly, 1968; Fairbanks et al., 1980;Duplessy et al., 1981; Erez and Honjo, 1981; Kahn and Williams,1981; Kozdon et al., 2009; Kisakürek et al., 2011) (see Section 7).

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Ostracod shells calcify with a constant species-specific offsetfrom isotopic equilibrium with the host water, as shown by resultsfrom oxygen isotope studies by Xia et al. (1997b), Von Grafensteinet al. (1999), Li and Liu (2010), Kalm and Sohar (2010) and Decrouyet al. (2011). Candona rawsoni shows a clear and consistent tem-perature dependence of oxygen isotope fractionation during itsbiological calcification (Xia et al., 1997b). At 25 �C, the optimummolting and calcification temperature, C. rawsoni specimensreached maturity faster than at 15 �C and showed an offset of þ2&in d18O values. In inorganic carbonate a temperature difference of10 �C results also in a difference of 2&, suggesting that d18O inostracod shells is well defined by d18O and the temperature of thehost water. An oxygen isotope fractionation similar for all species ofCandoninae was reported by Decrouy et al. (2011) with an offset ind18O of more than þ3& relative to equilibrium values for inorganiccalcite. In Cytheroidea the oxygen isotope fractionation is lessdiscriminative against 18O, resulting in an enrichment in d18O of1.7e2.3&. Von Grafenstein et al. (1999) reported offsets from iso-topic equilibrium to be greater than 2& for Candona species, 1.5&for Cytherissa lacustris, and around 0.8& for Limnocythere inopinataand Darwinula stevensoni. Li and Liu (2010) calculated a fraction-ation factor of 1.0311 at 13.5 �C in Eucypris mareotica, similar to thatof inorganic calcite (1.0308 at 13.5 �C). They concluded that in highpH and high salinity waters, such as in Lake Qinghai, the vital off-sets in E. mareotica are small enough to be negligible. They alsofound that fractionation factors move farther away from the equi-librium fractionation as temperature increases.

Factors affecting oxygen isotope fractionation are the tempera-ture dependent rate of calcification as well as incomplete calcifi-cation caused by environmental stress (Xia et al., 1997b). Erez andLuz (1983) suggested that non-equilibrium fractionation in plank-tonic foraminifera occurs only in the earlier stages of shell forma-tion when metabolic activity is more intense (Moberly, 1968) andthe more rapid calcification is characterized by less discriminationagainst 16O, resulting in lower d18O for not completely calcifiedshells. Similar observations were published by De Villiers et al.(1995) for coralline aragonite, describing stronger discriminationagainst 16O and thus more incorporation of 18O at slow growthrates. Opposite results were found by Xia et al. (1997b) with respectto ostracods, showing less discrimination against 16O at slowercalcification. In addition, Keatings et al. (2002b) reported that ox-ygen isotope fractionation into ostracod shell calcite is not only afunction of temperature but is also dependent on the pH of theambient water.

Vital effects affect not only d18O values, but also a wide range ofother isotopes and trace elements. The d13C values of differentgenerawere reported to be generally in or near isotopic equilibriumwith the host waters (Keatings et al., 2002b; Decrouy et al., 2011).Disequilibrium incorporation into ostracod calcite was observed forSr/Ca and Mg/Ca (Xia et al., 1997b; Wansard et al., 1998; VonGrafenstein et al., 1999). In high-Mg waters a large physiologicalenergy is required to exclude Mg and Sr during shell calcification(Reddy andWang,1980). Hence, an upper limit to the amount of Mgand Sr that ostracods will incorporate into their shells exist as ahigher Mg/Ca ratio in the solution leads to lower partition coeffi-cient of Mg (Morse and Bender, 1990). De Deckker et al. (1999) andGouramanis and De Deckker (2010) reported a reduced uptake ofmagnesium in high-Mg waters, and an additional influence of highwater temperatures on theMg uptake is given byMarco-Barba et al.(2012).

Vital effects and Sr and Mg partition coefficients vary betweenspecies (Von Grafenstein et al., 1999) and evenwithin individuals ofsame species (Wansard et al., 1998). Morishita et al. (2007) studiedthe distributions of Mg and Sr in the marine ostracod Neonesideaoligodentata and found a banded structure of chemical

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distributions. The three different bandings with variations in Mgand Sr were described by the authors as a high-Mg and high-Srouter band, a heterogenous low-Mg and low-Sr middle band, anda high-Mg and low-Sr inner band. They suggested that differencesin calcification rates account for the different Mg and Sr distribu-tions. Systematic differences in element distributions betweenright and left valve, female and male were not observed (Morishitaet al., 2007; Marco-Barba et al., 2012).

Another problem that should be accounted for is the seasonalityof shell formation. Little is known about seasonality for mostostracod species, for example whether shells form during arestricted season or throughout the year, and if seasonality con-tributes to the variability in the biogeochemistry of fossil assem-blages. If molting and shell calcification of A-1 instars (pre-adults)to adults occurs at different times of the year, the resulting intra-annual noise may overwhelm long-term paleoclimatic signals.Freshwater populations of Limnocythere inopinata disappear duringwinter and hatch in early spring, whereas saline water populationsoverwinter as adults or late juvenile instars (Yin et al., 2001; Vander Meeren et al., 2011). Another example is given in a study byXia et al. (1997a) investigating the effects of seasonality on thestable isotope variability in C. rawsoni. This species hatches inspring, but adults may calcify in late summer or the followingspring depending onwater temperature. Seasonal changes inwatertemperature and variations in water chemistry result in high vari-ability in d18O, d13C, Mg/Ca and Sr/Ca. If C. rawsonimolts over awiderange of temperatures, the mean isotope and trace element sig-natures of the population as a whole may give an integrated sea-sonal signal for both temperature and water chemistry.

5.2. Pre-treatment

For trace element analysis the removal of contaminantscomprising (1) adhering aluminosilicates, (2) carbonate minerals,(3) chemical precipitates, and (4) organic material (Keatings et al.,2006) is essential. Stable isotope studies need to remove organicmaterial (Keatings et al., 2006), as such contaminants can mask theoriginal biogenic signal. Most studies recommend that the valvesonly be cleaned manually with a fine brush, needle and deionizedwater whenever possible, as chemical pre-treatment methods havethe potential to alter the stable isotope and tracemetal compositionof the ostracod valves (Holmes, 1996). Only a few studies exist thathave investigated the effects of different pre-treatmentmethods onthe ostracod shell composition (Jin et al., 2006; Keatings et al.,2006; Li et al., 2007; Mischke et al., 2008a). More extensive workhas been done on the effect of pre-treatment on isotopic compo-sition of other biogenic and inorganic carbonates (Sarkar et al.,1990; Boiseau and Juillet-Leclerc, 1997; Grottoli et al., 2005;Serrano et al., 2008). Other carbonates (such as foraminifera) differstructurally and in some casesmineralogically from ostracod calciteand those studies do not necessarily provide valid models for howostracod calcite will behave under similar treatments (Keatingset al., 2006). A comprehensive overview of different cleaningtechniques and their effects is given by Keatings et al. (2006). In thisstudy, differences in valve composition following pre-treatmentwere measured by comparing treated and untreated valves fromthe same carapaces, as the composition of both valves of a singlecarapace is statistically identical. The removal of organic materialfor stable isotope studies is usually accomplished by soaking inhydrogen peroxide (Curtis and Hodell, 1993; Von Grafenstein et al.,1996) or sodium hypochlorite (Durazzi, 1977; Grossman et al., 1986;Scotchman, 1989), vacuum roasting or plasma ashing (Boomer,1993; Andrews et al., 1995). In order to remove adhering alumi-nosilicate and ferromanganeseminerals from the shell surface, as isessential for trace element analysis, reducing or complexing

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solutions are used. With respect to the use of hydrogen peroxide,Ito (2002) suggested that it insufficiently removes organiccontamination and that the resulting organic acids can partiallydissolve the carbonate (Keatings et al., 2006). Boiseau and Juillet-Leclerc (1997) found no exchange between hydrogen peroxideand coral aragonite when they exposed it to 16O enriched hydrogenperoxide. In addition, Keatings et al. (2006) stated that hydrogenperoxide cleaning is a suitable method for oxygen isotope analysis.On the other hand, Xia et al. (1997b) found that cleaning for 15 minin hot (80 �C) 5% H2O2 resulted in small (0.1e0.3&) nonsystematicshifts in oxygen isotope composition. Keatings et al. (2006)concluded that pre-treating should only be done if absolutelynecessary and with full awareness of any resulting signal bias. Theysuggested that no pre-treatment should be carried out unless avisual inspection revealed substantial organic or mineral contam-ination of valves. Furthermore, Keatings et al. (2006) suggested toemploy both hydrogen peroxide and plasma ashing for oxygenisotopes analyses, and only plasma ashing when measuring carbonisotopes. Sodium hypochlorite pretreatment is recommended forthe analysis of Mg/Ca and Sr/Ca. The most effective method for theremoval of the chitin sheath from living or recently dead ostracodvalves may be freeze drying of the valves, which loosens chitin foreasy removal, leaving only the mineralized portion of the carapacebehind, but this approach needs further testing. Vacuum roasting,hydrogen peroxide and sodium hypochlorite caused a reduction inthe mean d13C values and increased deviations in d18O values by0.8&. Sodium hypochlorite has less impact to Mg/Ca and Sr/Caratios than hydrogen peroxide, and hydroxylamine hydrochloritecaused no significant changes in Mg/Ca and Sr/Ca ratios. Bothtechniques led to a reduction in the shell weight, suggesting thatMg, Sr and Ca are removed in equal proportions (Keatings et al.,2006). In conclusion, no single pre-treatment method was foundto be free from any effects on the ostracod valves.

Even when using the most suitable pre-treatment method,sample drying can also significantly alter the isotopic signature ofthe ostracod shells. Mischke et al. (2008a) tested the effects ofdrying the sieve residues either from tap water, deionized water, orethanol rinsing. The stable isotope values of shells dried fromwater(deionized and tap water, respectively) were lower for both oxygenand carbon as a result of calcite crystals precipitated on the shellsurfaces during the drying process. In contrast, samples dried fromethanol showed a smooth and clean surface. The observed isotopicdepletion of water-dried shells in comparison to ethanol-driedshells is attributed to the precipitation of inorganic calcite on theostracod shell surfaces. Deionized water is very mildly acidic andmay cause carbonate dissolution and later precipitation duringdrying, while tap water is a source of additional carbonate. Theseeffects were also reported in foraminifera by Sperling et al. (2002).And as the precipitated crystals are not easily assessable by visualinspection of shells prior to analysis, water drying should begenerally avoided (Mischke et al., 2008a).

5.3. Diagenesis

A number of studies have suggested that post-mortem diagen-esis may significantly affect the chemical composition of non-marine ostracod valves (Mucci and Morse, 1983; Chivas et al.,1986b), but the question, under which circumstances diagenesiscan alter the shell chemistry, has received little attention in theliterature (Keatings et al., 2002a). Diagenetically altered specimensfeature pitted, dissolved or re-crystallized surfaces. A visualassessment of the state of preservation is possible using light mi-croscopy (Dwyer et al., 2002; Keatings et al., 2002a). The degree ofsurface recrystallization can be identified using scanning electronmicroscopy imagery (Mischke et al., 2008a). Both of these

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visualizing techniques are generally used for Quaternary material(Bennett et al., 2011).

Chivas et al. (1986b) stated that partial dissolution of valves inmildly acidic waters has no effect on the Sr/Ca ratios, but leaching ofMg occurred in under-calcified Cyprideis valves collected from sa-line lakes in Australia. These observations were confirmed by lab-oratory tests with Krithe and Loxoconcha valves, which showed 12%reduced Mg/Ca ratios but original Sr/Ca ratios after 40% of the valvematerial was removed by dissolution in deionized water (Dwyeret al., 2002). Similarly, De Deckker et al. (1999) found that Sr/Caratios of partially dissolved valves are very similar to unalteredshells, but Mg/Ca ratios are significantly lower in altered material.These results suggest that Sr is homogeneously distributed,whereas Mg is not. All authors suggested to use only pristine valvesin geochemical studies, but this is not always possible (Corrège,1993). There tends to be a relationship between the physicalpreservation of valves in the sediment and the presence or absenceof the enveloping chitinous cuticle. Removal or damage of thiscuticle tends to result in poorly preserved valves. Similar findingswere also reported for foraminifera by Lorens et al. (1977). Incontrast, an investigation into the effects of preservation on Mg/Ca,Sr/Ca, d18O and d13C values in late Quaternary non-marine ostracodvalves by Keatings et al. (2002a) yielded no evidence that thepreservation state affected valve geochemistry. Diagenetic over-printing on low-Mg calcite brachiopod shells is well documentedand shows that diagenesis tends to lead to lower carbon and oxy-gen isotope values (Banner and Kaufman, 1994; Mii et al., 2001).Diagenetic alteration by later meteoric water also tends to lead tolower carbon and oxygen isotope compositions compared toalteration by seawater (Immenhauser et al., 2002). Anotherimportant factor is burial depth, as increasing temperatures withdepth lead to lower d18O, which was shown in studies of benthicforaminifera bulk carbonate (Schrag et al., 1995).

Bennett et al. (2011) published a detailed study about thediagenesis of fossil ostracods, assessing the ultrastructure of thecalcite crystals of the shells for different diagenetic stages. Theauthors described six diagenetic stages: replacement of originalcalcite by (1) neomorphic calcite, (2) framboidal and euhedral py-rite, (3) ferroan calcite, (4) ferroan dolomite, (5) siderite growth, (6)sphalerite and barite mineralization. The values of d13C and d18Otend to becomemore negative with increasing diagenesis, althoughthe effect is often not measureable for carbon isotopes. Neverthe-less, the ostracod neomorphic calcite (stage 1) preserves a seawatersignal, even though there may have been some alteration soon aftershallow burial. As the recrystallization process is currently poorlyunderstood, it has been suggested that the original crystals may nothave completely dissolved but instead just grown in size (Bennettet al., 2011). For diagenetically altered planktonic foraminifera asimilar ultrastructure to that of neomorphic calcite in ostracods wasdescribed and interpreted to have grown by dissolution andrecrystallization of the test at shallow burial depth of less than300 m (Pearson et al., 2001).

6. State of the art in foraminiferal geochemistry

6.1. Paleothermometry and paleosalinity

For paleoenvironmental reconstruction, especially in themarineenvironment, both faunal and geochemical studies have concen-trated on foraminifera (Table 2). Sea surface temperature re-constructions using oxygen isotopes were originally developedusing foraminifera-based paleoproxies (Emiliani, 1955, 1966;Zachos et al., 2001; Sagawa et al., 2013). In addition to the effectof temperature, the d18O values in foraminifera tests are alsoinfluenced by the seawater d18O composition. The value of

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d18Oseawater reflects the hydrological balance between evaporationand precipitation (E/P) and surface salinity (Corrège and DeDeckker, 1997; Guildersen and Pak, 2005). On longer time scales,d18Oseawater is also a function of the extent of continental ice sheets(Shackleton et al., 1973; Miller et al., 1991; Mulitza et al., 2004).

Table 2Summary of established paleoenvironmental proxies in foraminiferal, coral and mollusc

Proxy & indicated environmental parameter Foraminifera

d18OTemperature Emiliani, 1955, 1966; Shackleton et al.,

1973; Sagawa et al., 2013

Temperature & salinity Corrège and De Deckker, 1997;Guildersen and Pak, 2005

Temperature & ice volume Zachos et al., 2001; Mulitza et al., 2004;Lear et al., 2010

Ice volume Miller et al., 1991Temperature & d18Owater (meltwater andfreshwater input)Vital effects (growth rate)Salinity & P/E balance

d13CProductivity & nutrient distribution Lea, 1995; Rosenthal et al., 1997b;

Bickert and Mackensen, 2004;Mackensen and Licari, 2004

Nutrient distribution and CO2 exchange Makou et al., 2010Productivity & carbon source

Mg/CaTemperature Rosenthal et al., 1997a; Gagan et al.,

2004; Sadekov et al., 2008; Hathorneet al., 2009; Sadekov et al., 2009

Temperature & salinity Nürnberg et al., 1996Temperature & saturation state Lear et al., 2010Vital effects (growth rate)

Sr/CaTemperature

Temperature and Salinity Kisakürek et al., 2011Saturation state Rosenthal et al., 1997aVital effects (growth rate)

Sr/Cawater

SalinityBa/CaAlkalinity & nutrient distribution Lea and Spero, 1994; Lea, 1995Productivity

Productivity & freshwater inputTemperatureSalinity

U/CaTemperature Russell et al., 1996; Yu et al., 2008

Temperature & freshwater inputRedox changes & Oxygenation Boiteau et al., 2012U/Cawater Russell et al., 1994

d11B, B/CapH Sanyal et al., 1996; Lemarchand et al.,

2000; Pearson and Palmer, 2000; Sanyalet al., 2000

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Foraminiferal Mg/Ca was proposed to be a proxy for tempera-ture, asMg/Ca inmarine carbonates varies with latitude, suggestinga temperature-dependence (Rosenthal et al., 1997b; Lea, 2003; Learet al., 2010). The application of Mg/Ca on longer time scales islimited by a lack of knowledge about past seawater composition.

an geochemical research (selected papers).

Coral Mollusk

Emiliani et al., 1978; Hart andCohen, 1996

Von Grafenstein et al., 1992; Brandet al., 1993; Von Grafenstein et al.,1994; Von Grafenstein et al., 2000;Wurster and Patterson, 2001; Duttonet al., 2002; Gillikin et al., 2005; Schöneet al., 2005; Gillikin et al., 2006; Elliotet al., 2009; Batenburg et al., 2011Carroll et al., 2009

Smith et al., 1997; Linsley et al.,2004

Anadón et al., 2008

Tripati et al., 2001; Apolinarska, 2013Gasse et al., 1987

Emiliani et al., 1978 Gasse et al., 1987; Elorza and Garcìa-Garmilla, 1996; Wurster and Patterson,2001; Dutton et al., 2002; Schöne et al.,2005; Gillikin et al., 2006; Carroll et al.,2009; Apolinarska, 2013

Anadón et al., 2008; Taft et al., 2013

Hart and Cohen, 1996; Sinclair et al.,1998; Fallon et al., 1999; Montagnaet al., 2007

Lazareth et al., 2003; Anadón et al.,2008; Carroll et al., 2009; Elliot et al.,2009; Batenburg et al., 2011Dodd, 1965; Dodd and Crisp, 1982

Dutton et al., 2002; Carré et al., 2006;Schöne et al., 2011; Schöne et al., 2013

Hart and Cohen, 1996; Sinclair et al.,1998; Fallon et al., 1999; Marshalland McCulloch, 2002; Gagan et al.,2004; Linsley et al., 2004; Cohenet al., 2006; Montagna et al., 2007

Dodd, 1965; Lazareth et al., 2003;Carroll et al., 2009

Dutton et al., 2002; Gillikin et al., 2005;Carré et al., 2006; Schöne et al., 2011;Schöne et al., 2013Brand et al., 1993; Anadón et al., 2002;Anadón et al., 2008Dodd and Crisp, 1982

Fallon et al., 1999 Lazareth et al., 2003; Elliot et al., 2009;Batenburg et al., 2011Carroll et al., 2009

Montagna et al., 2007Gillikin et al., 2006, 2008

Sinclair et al., 1998; Fallon et al.,1999Montagna et al., 2007

Hemming et al., 1998

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Table 2 (continued )

Proxy & indicated environmental parameter Foraminifera Coral Mollusk

pH & Temperature Yu et al., 2007; Hathorne et al., 2009Temperature Hart and Cohen, 1996; Sinclair et al.,

1998; Fallon et al., 1999; Montagnaet al., 2007

87Sr/86SrSr-source & continental weathering Hess et al., 1986; Hodell et al., 1989;

Martin and Macdougall, 1991; Milleret al., 1991; Dia et al., 1992

Dia et al., 1992 Fan et al., 2010

Sr-source & water masses mixing(freshwater input, ocean circulation)

Shijie, 1996 Van de Flierdt et al., 2006; Colinet al., 2010; Copard et al., 2010; Vande Flierdt et al., 2010; López Correaet al., 2012

Whittaker and Kyser, 1993; Tripatiet al., 2001; Anadón et al., 2002

Temperature & continental weathering Rüggeberg et al., 2008143Nd/144NdOcean circulation & continentalweathering

Palmer and Elderfield, 1985; Vance andBurton, 1999; Burton and Vance, 2000;Pomiès et al., 2002; Vance et al., 2004;Klevenz et al., 2008; Roberts et al., 2012

Sholkovitz and Shen, 1995 Whittaker and Kyser, 1993

Li/Ca, Mg/Li (corals)Temperature Marriott et al., 2004 Case et al., 2010Temperature & saturation state Hall and Chan, 2004a,b; Lear et al., 2010Seawater saturation Lear and Rosenthal, 2006

F/CaTemperature Hart and Cohen, 1996

Cd/CaProductivity and water mass mixing Van Geen et al., 1992; Lea, 1995;

Rosenthal et al., 1997b; Burton andVance, 2000; Makou et al., 2010

Nutrient distribution and exchange withCO2

Rosenthal et al., 1997a

Mn/CaProductivity Wyndham et al., 2004 Carroll et al., 2009Productivity & circulation Klinkhammer et al., 2009Nutrient distribution, freshwater input Lazareth et al., 2003Oxygenation Brand et al., 1993

d44CaTemperature De La Rocha and DePaolo, 2000; Nägler

et al., 2000; Kisakürek et al., 2011d15NProductivity, anthropogenic influence Versteegh et al., 2011

REEsAlkalinity Roberts et al., 2012Water mass tracer (riverine input,volcanic activity, aeolian influx)

Sholkovitz and Shen, 1995 Whittaker and Kyser, 1993

Productivity Wyndham et al., 2004V/CaRedox changes Hastings et al., 1996

P/CaNutrient distribution LaVigne et al., 2008

238Pu/239PuAnthropogenic influence Imai and Sakanoue, 1973Al, Fe, Cu, Cr, Hg, V, ZnAnthropogenic influence Bastidas and García, 1999

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However, because Mg as well as Ca have long residence times(13 Ma for Mg, 1 Ma for Ca) (Rosenthal and Linsley, 2006), thecomposition of seawater cannot have changed significantly duringthe Pleistocene and thus provides a good basis for sea surfacetemperature reconstructions at least at this time scale (Gagan et al.,2004; Sadekov et al., 2008; Sadekov et al., 2009). Nevertheless,inter-species variability in Mg/Ca ratios is correlated with forami-nifera calcification depth, as shallow mixed-layer dwellers likeGlobigerinoides ruber and Globigerinoides sacculifer have high Mg/Ca, while deep dwellers (e.g., Globigerinoides tumida, Globiger-inoides dutertrei) are relatively low in Mg/Ca (Rosenthal and Boyle,1993). Large compositional variability within individual chambersand within the test as a whole also occurs and depends on thevertical migration during adult life; shallow-dwelling species havea more homogenous composition than deep-dwelling species

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(Rosenthal and Linsley, 2006; Allison and Austin, 2008; Hathorneet al., 2009). Laser ablation ICP-MS microanalysis of the forami-nifera Globigerinoides ruber revealed large variations of Mg/Cacomposition within and between individual tests and suggested asource of significant uncertainty in Mg/Ca thermometry, but meanmolar values show a strong exponential correlation with meanannual sea surface temperatures (Sadekov et al., 2008; Hathorneet al., 2009; Sadekov et al., 2009). Mg/Ca ratios decrease withincreasing calcification depth and a large interspecific variabilityamong benthic species suggests that using a single species cali-bration may yield the best precision. The effect of salinity on theMg/Ca composition of foraminiferal calcite is currently poorlyconstrained (Sadekov et al., 2008), but Lear et al. (2010) stated thatMg/Ca may be a salinity-independent paleothermometer. Bycombining Mg/Ca signatures with d18O values it is possible to

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separate bottom water temperature and ice volume effects in thed18O record.

An additional, but yet largely untested tool for paleotemper-atures is provided by foraminiferal Ca isotopes, as d44Ca is morerobust to diagenesis than Mg/Ca. De La Rocha and DePaolo (2000)found that periods of lower global temperature result in lowd44Ca concentrations of foraminifera tests. A correspondence be-tween d44Ca and glacial/interglacial SST fluctuations was also re-ported by Nägler et al. (2000). In contrary, Kisakürek et al. (2011)found this relationship in benthic but not in planktonic speciesand also just for temperatures below 24 �C. Nevertheless, the po-tential of calcium isotopes as recorders of sea surface temperatureswas reported by all authors.

A relatively new proxy is the benthic foraminifera Li/Ca ratio,which reflects changes in both seawater saturation state and tem-perature (Burton and Vance, 2000; Marriott et al., 2004; Hall andChan, 2004b; Lear and Rosenthal, 2006). For this proxy, coupledMg/Ca and Li/Ca may be used to reconstruct both temperature andseawater saturation state (Lear et al., 2010). A decrease in the Li/Caratio reflects a lower calcification rate caused by decreasedseawater carbonate ion concentration (Hall and Chan, 2004b; Learand Rosenthal, 2006). The correlation between Li/Ca and temper-ature reveals a remarkable linear fit for temperatures above 5 �C(Lear et al., 2010). Furthermore, because it is possible that the Mg/Ca values might have been affected by non-temperature-relatedeffects, since modern foraminifera living in poorly saturated wa-ters have lower Mg/Ca than expected from global temperaturecalibrations (Lear et al., 2010), combining the Mg/Ca with the Li/Casignature may reveal significant information.

Temperature dependence was also reported for U/Ca ratios inforaminifera tests (Russell et al., 1996; Yu et al., 2008). Planktonicforaminifera studied by Yu et al. (2008) showed strong correlationbetween U/Ca and temperature, allowing the reconstruction of seasurface temperatures by combined U/Ca and Mg/Ca analyses.Russell et al. (1994, 1996) stated that variations in U/Ca ratios inforaminifera tests are a function of temperature effects as well asthe seawater uranium content, and Boiteau et al. (2012) interpretedthe latter relationship as sedimentary redox changes caused byorganic carbon flux and bottom water oxygen distribution.

For paleosalinity reconstructions, independent geochemicalproxies for salinity still have to be developed. To assess paleo-salinity, it is possible to use an independent temperature proxy,such as Mg/Ca, to correct the d18Ocarbonate signal for temperature, sothat residual d18O variations reflect changing salinity (Corrège andDe Deckker, 1997; Guildersen and Pak, 2005). In addition, forami-nifera assemblages can provide information about salinity changes(Lear et al., 2010).

6.2. Paleoproductivity

The reconstruction of paleoproductivity is based on d13C or Cd/Ca ratios. The carbon isotope signatures of foraminifera are afunction of the nutrient cycling and the exchange of CO2 with theatmosphere (Rosenthal et al., 1997a; Makou et al., 2010). The d13Csignal in foraminifera tests reflects the ocean surface water pro-ductivity, but is further influenced by water mass distributions andthe respiration of organic matter (Bickert and Mackensen, 2004;Mackensen and Licari, 2004). A proposed relationship of benthicforaminiferal d13C values to the bottomwater nutrient content wasrejected by Lea (1995).

To extract the atmospheric signature on the carbon isotopiccomposition of seawater, coupled cadmium and d13C data can beused (Boyle, 1988; Rosenthal et al., 1997b). Cd/Ca ratios of shallow-dwelling foraminifera species reflect the vertical distribution ofnutrients and allows the calculation of changes in total dissolved

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inorganic carbon (Lea, 1995; Makou et al., 2010). Burton and Vance(2000) considered that cadmium behaves like the nutrient PO4 andthus indicates fluctuations in the nutrient distribution. Van Geenet al. (1992) used Cd/Ca ratios to reconstruct changes in upwell-ing intensity in a coastal environment. Rosenthal et al. (1997a) re-ported the correlation between the Cd/Ca ratio of the foraminiferatest and the Cd concentration of the ambient water, which reflectssurface water nutrient concentration and hence productivity ordeepwater circulation, respectively.

6.3. New paleoenvironmental proxies in foraminifera research

Recently, a wide range of radiogenic isotopes and trace elementsin foraminifera were investigated for their suitability as paleo-environmental proxies. Rosenthal et al. (1997b) tested F/Ca ratios,but could not find a depth, temperature, or salinity dependence. F/Ca ratios of benthic foraminifera are generally constant throughoutdifferent calcification depths and thus differing temperatures, butthey show significant differences between species, governed pri-marily by biological processes (Rosenthal et al., 1997b). Anotherfocus was on the reconstruction of ocean circulation based on in-formation about water mass distribution from proxies that mimicnutrients (d13C, Cd/Ca) or rates of flow (radiogenic isotopes) (Vanceand Burton, 1999).

For the differentiation of water masses that have indistin-guishable nutrient signals, radiogenic isotopes, e.g., Nd, Pb, or Hf,can be used since they exhibit spatial variability due to spatialvariability in their continental sources (Vance and Burton,1999). Ndisotopes allow identifying different sources of neodymium and thusvariations in ocean circulation and continental input (Palmer, 1985;Vance and Burton, 1999; Burton and Vance, 2000). Klevenz et al.(2008) used Nd isotopes to identify varying contributions ofnorthern- vs. southern-sourced deepwater at the studied locationand thus changes in deepwater hydrography. Paleo-oceanic circu-lation was reconstructed by Pomiès et al. (2002), and Vance et al.(2004) used Nd isotopic signatures in planktonic foraminifera toassess continental weathering, material flux from the continent tothe ocean, and ocean circulation.

Information about ocean circulation and flow rates are given bytwo insoluble products of uranium decay: 231Pa (Protactinium) and230Th (Thorium) (Yu et al., 1996; Walter et al., 1997; Hendersonet al., 1999). Uranium is present in constant proportion to salinityin seawater, thorium is insoluble and removed quickly to the seafloor, whereas protactinium is more soluble and can be advected bycirculation. The Pa/Th ratio hence reflects Pa transport pathways,therefore water mass distribution and flow rates (Walter et al.,1997). Sedimentary fluxes of biogenic barite (BaSO4) as well asTh, Pa, and Be have been used to assess past productivity (Kumaret al., 1995). Biological productivity also prefers the uptake oflight isotopes of trace metals such as Zn, Fe, and Mo (Beard et al.,1999; Maréchal et al., 2000). The d15N values reflect the degree ofnitrate utilization in water; the higher d15N is, the more completelynitrate has been used (Henderson, 2002).

Alkalinity and pH can be reconstructed using foraminiferal Ba/Ca and d11B, respectively. Lea (1995) reported a remarkably linear fitbetween the Ba/Ca ratio of foraminifera tests and alkalinity. Inaddition, periods of higher Ba/Ca seem to be associated with glacialintervals, thus the authors suggested variations in the nutrientcomposition as additional controlling factor. Hall and Chan (2004a)used Ba/Ca ratios as a proxy for water mass distribution andnutrient input, and Lea and Spero (1994) reconstructed nutrientand alkalinity distributions using Ba/Ca ratios. Hönisch et al. (2011)reported that Ba/Ca in foraminifera tests is related to the Ba/Caconcentration of the host waters and that environmental parame-ters including pH, temperature, salinity, and symbiont

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photosynthesis have no effect on the Ba incorporation into plank-tonic foraminifera. The use of boron isotopes or B/Ca ratios toreconstruct changes in pHwas shown by numerous authors (Sanyalet al., 1996; Lemarchand et al., 2000; Pearson and Palmer, 2000;Sanyal et al., 2000). Boron occurs as two species in seawaterwhose relative concentration is pH dependent: B(OH)3 and the20& isotopically lighter B(OH)4- . However, only B(OH)4- is incorpo-rated into marine carbonate, thus the d11B in carbonates changeswith B speciation and therefore with pH (Lea, 1995; Pearson andPalmer, 2000). A small biological effect on the boron incorpora-tion was stated by Sanyal et al. (2000), but its influence on thecorrelation between the d11B composition of the foraminifera testand seawater pH is negligible. On longer time scales, however,changes in the marine boron isotope budget may mask the pHdependency of the d11B signal (Lemarchand et al., 2000). Hathorneet al. (2009) associated variations in the B/Ca concentration withthe vertical migration of the foraminifera and correspondingchanges in pH and temperature. A temperature effect on the boronincorporation was also found by Yu et al. (2007).

Roberts et al. (2012) investigated the suitability of rare earthelement (REE) ions for paleoenvironmental studies. All studied REEconcentrations were positively correlated, e.g., neodymium,cerium, and lanthanum. Neodymium isotopes are suitable forreconstructing source and flow direction of water masses (Vanceand Burton, 1999; Pomiès et al., 2002; Klevenz et al., 2008). TheREE3þ ions precipitate between the inner layers of foraminiferalcalcite and are mostly associated with coatings rather than incor-porated into the calcite structure. The adsorption and complexationof REEs may be related to the oxidation of organic matter betweeninner calcite layers of foraminifera, causing carbonate dissolutionand providing carbonate ions, which increase REE3þ complexationand adsorption (Tang and Johannesson, 2005; Johnstone et al.,2010). The REEs are also strongly coupled to Fe and Mn cyclingbecause of their strong affinity for ferromanganese oxides(Sholkovitz et al., 1992). High REE concentrations are consistentwith increased alkalinity and MnCO3 precipitation (Roberts et al.,2012), while Tang and Johannesson (2005) observed increasedREE adsorption with higher pH, as more carbonate ions wereavailable for complexation. The mobility of Mn and Fe (also U andCe) ions is controlled by the oxygen concentration in the porewaters (Froelich et al., 1979; Boiteau et al., 2012), which allowsreconstructing changes in the porewater environment. High Fe andMn concentrations measured in planktonic foraminifera coincidedwith periods of high U enrichment, consistent with oxide dissolu-tion under reducing conditions and re-mobilization of redox-sensitive ions (Roberts et al., 2012). Under sub-oxic pore waterconditions, authigenic MnCO3 precipitates between the innercalcite layers of planktonic foraminifera, preventing re-mobilization of the REE ions under more reducing conditions(Roberts et al., 2012).

7. Limitations in foraminiferal geochemistry

In foraminifera, similar limitations are present as in ostracodshell chemistry. Significant intratest and intertest variations, forexample in Mg/Ca ratios, in various planktonic foraminifera specieshave been reported (Brown and Elderfield, 1996; Barker et al.,2005). But to what extent this compositional heterogeneity in-fluences the reproducibility and the achievable precision and ac-curacy of seawater thermometry has not been rigorously assessed.In an interlaboratory comparison study of Mg/Ca and Sr/Ca ratios inthree carbonate reference materials, Greaves et al. (2008) foundmany uncertainties and showed that interlaboratory variability isdominated by inconsistencies among instrument calibrations. Theyshowed that repeatability of Mg/Ca determinations increased with

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decreasing Mg/Ca (0.78% at Mg/Ca ¼ 5.56 mmol/mol (CMSI 1767),0.82% at Mg/Ca ¼ 3.76 mmol/mol (ECRM 752-1) and 1.15% at Mg/Ca ¼ 0.79 mmol/mol (BAMRS3), respectively) and that interlabor-atory reproducibilities were noticeably worse than intralaboratoryprecision. Nevertheless, the uncertainty decreases when increasingnumbers of foraminifera tests are analyzed per sample. If at least 20tests are analyzed, the uncertainty reduces to less than �1 �C(Barker et al., 2003; Anand and Elderfield, 2005). Mg/Ca variabilityis also influenced by a range of site-specific biological and envi-ronmental factors: the amplitude of seasonal and inter-annualtemperature changes, differences in sedimentation rate anddepth of bioturbation, also redox conditions in the water column aswell as in the pore waters. The intratest Mg/Ca variability resultsfrom the presence of calcite layers featuring varying Mg/Cacomposition, and high- and low-Mg/Ca layers within differentchambers of the same foraminifera test (Sadekov et al., 2008).Differences occur also between morphotypes. For example, G. ruberpyramidalis shows consistently lower Mg/Ca composition thanG. ruber ruber (Sadekov et al., 2008) because of differing seasonalgrowth preferences or habitat depth. Other limitations arise bydiagenetic alteration as well as pre-treatment methods (Boyle,1981; Boyle, 1983). A decrease in carbonate saturation level withdepth leads to alteration of Mg/Ca in foraminifera by post-depositional dissolution on the sea floor (Dekens et al., 2002). Adetailed study on diagenesis in foraminifera and their effect onstable isotope and trace element composition is given by Sextonet al. (2006). Diagenesis causes a reduction in Mg, possibly byremoval of a Mg-rich zone that is present within the tests of manyplanktonic species (Rosenthal and Boyle, 1993). It was also foundthat benthic foraminifera are generally more resistant to dissolu-tion than planktonic ones (Izuka, 1988; Lear et al., 2000).

8. Geochemical proxies in other micro- and macrofossils

In the marine environment a variety of organisms exist thathave potential as paleoclimate archives, e.g., corals and mollusks(see Table 2). Corals are widely used to assess past sea surfacetemperature (SST), especially as they allow resolving intra-annualto weekly variations (Wurster and Patterson, 2001; Montagnaet al., 2007). Isotope and trace element studies in mollusk shellcarbonates have been used to reconstruct paleoenvironments(Rosenthal and Katz, 1989; Anadón et al., 2002; Schöne et al., 2011),not only in the marine realm but also in continental waters. Inaddition, biogenic silica from diatoms or sponges has been used forpaleothermometry (Leng and Marshall, 2004; Ellwood et al., 2006;Baines et al., 2011). Jochum et al. (2012a) assessed paleotemper-atures by the Mg/Ca and d18O composition in spicules from thedeep-sea sponge Monorhaphis chuni. Besides their use for paleo-thermometry, silicon isotopes and germanium composition werealso measured in sponges as well as in diatoms to reconstructinorganic germanium and silicon concentrations in seawater(Ellwood et al., 2006; Hendry and Robinson, 2012).

8.1. Trace elements in corals as a paleoenvironmental proxy

Cold water corals have been used as geochemical archives indeep sea research (Hart and Cohen, 1996; Smith et al., 1997) sincethey inhabit a wide variety of depths and geographic distributions,including ocean basins and high-latitude regions (Case et al., 2010).Their aragonite skeletons record changing deep-water compositionand past circulation patterns. Oxygen stable isotope values can beused to reconstruct mean annual temperatures (Emiliani et al.,1978; Hart and Cohen, 1996; Linsley et al., 2004), while intra-annual variability is overprinted by biological fractionation (Smithet al., 1997). Coralline Sr/Ca is a useful proxy for changes in coral

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growth environment offering intra-annual resolution, hence bothseasonal and inter-annual variability can be assessed (Marshall andMcCulloch, 2002; Gagan et al., 2004; Linsley et al., 2004). The Mg/Ca, Sr/Ca, U/Ca, B/Ca and F/Ca ratios show seasonal cycles consistentwith sea surface temperatures (Hart and Cohen,1996; Sinclair et al.,1998; Linsley et al., 2000; Montagna et al., 2007), but Mg and B alsoshow intra-annual fluctuations that could not be explained bytemperature alone (Fallon et al., 1999). The linear correlation for U/Ca and Sr/Ca to SST seems to be limited to temperatures above18 �C, as in times of extreme cold, an increased elemental incor-porationwas observed (Fallon et al., 1999). Marshall and McCulloch(2002) reported that thermal stress at extremely warm or coldtemperatures can result in lower SST estimates, because the bio-logical control on Sr/Ca fractionation breaks down. Hart and Cohen(1996) limited the use as paleothermometer to the surface watergenus Porites, because deep-sea corals seem more affected by vitaleffects. Cohen et al. (2006) also reported non-equilibrium frac-tionation of strontium caused by vital effects. In contrast to thetemperature dependence that dominates the Mg/Ca and Sr/Ca ra-tios, Hemming et al. (1998) attributed changes in the B/Ca signatureof Porites to changes in seawater pH or productivity and upwelling.

Investigations by Montagna et al. (2006) and LaVigne et al.(2008) revealed a linear correlation between seawater dissolvedinorganic phosphate (DIP) concentrations and P/Ca ratios in corals,allowing the reconstruction of past seawater nutrient concentra-tions. Paleoproductivity can also be assessed using Ba/Ca ratios incorals and can thus give information about seasonal upwelling, e.g.,that induced by winds (Lea et al., 1989; Fallon et al., 1999), as highBa concentrations occur in deep, cold and nutrient-rich waters. Incontrast, Montagna et al. (2007) found a relationship between Ba/Ca ratios and sea surface temperatures, but the Ba/Ca ratios mayalso be sensitive to river runoff.

Other possible trace element proxies, e.g., Cd, Pb, Mn, Zn, and V,were studied by Shen and Boyle (1988), but only Pb and Cd showedclear temporal variability associated with industrialization, and inthe case of Cd with natural perturbation in ocean circulation. Lewiset al. (2007) studied land-use changes in a river catchment usingBa, Y, and Mn in Porites. Ba and Y are attributed to soil erosioncaused by livestock. Yttrium concentrations rise with agriculturalintensification as they show a positive correlation with cattlenumbers, and thus indicate increasing sediment flux due to land-use changes and prolonged droughts. The authors stated that Y ismore reliable than Ba/Ca ratios, as Ba/Ca seems to be influenced byadditional factors. Mn is also clearly related to changes in catch-ment land-use and related erosion (Lewis et al., 2007). Otherstudies trying to trace anthropogenic impact used a wide range ofheavy metals (Al, Fe, Cu, Cr, Hg, V and Zn) as well as Pu isotopes(Imai and Sakanoue, 1973; Bastidas and García, 1999).

REEs have been studied in coral aragonite as they could provideinformation about river water discharge, rainfall and weathering(Scherer and Seitz, 1980; Sholkovitz and Shen, 1995; Wyndhamet al., 2004). Concentration of REEs are low in surface waters andincrease with depth, thus they provide information about the up-welling of sub-surface seawater. Additionally, suboxic and anoxicseawater and pore waters are characterized by high REE concen-trations and Ce anomalies, allowing the reconstruction of pastredox conditions. And in coastal corals, REEs also provide infor-mation about river water discharge and weathering, as water fromterrigenous sources are quite different in REE/Ca from the localseawater (Sholkovitz and Shen, 1995), and about biologic activityand productivity (Wyndham et al., 2004).

However, a number of potential complicating factors can limitthe use of coralline aragonite. Corals precipitate their aragoniticskeleton out of equilibriumwith seawater, resulting in a decrease ofSr, U, d18O and d13C, and an enrichment in Mg relative to abiotic

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aragonite (Bar-Matthews et al., 1993; De Villiers et al., 1995; Hendyet al., 2007; Meibom et al., 2007). Biological processes that frac-tionate trace elements in corals originate from two different typesof aragonite. Sinclair et al. (2006) reported Mg-rich opaque centersof calcification and U-rich large translucent crystals, with the Mg-rich material strongly depleted in d18O and d13C. In addition thecenters of calcification show enriched trace element concentrations(Meibom et al., 2006). Varying proportions of these crystal typesaccount for most of the variability in d18O and d13C. The disequi-librium offset of, for example, skeletal Sr/Ca from seawater Sr/Cacan vary between corals of the same species, thus only relativechanges in SST over time can be reconstructed accurately. Addi-tionally, intra-axis variability occurs, since significantly higher Sr/Ca ratios are associated with slower skeletal extension rates (Hartand Cohen, 1996; Adkins et al., 2003; Cohen et al., 2006; Meibomet al., 2007). It has been shown that sampling along the coralmaximum growth axis is best suited, because off axis sections havehigher Sr/Ca ratios, apparently due to smaller polyp size and lowerskeletal density (Cohen et al., 2001; López Correa et al., 2010).Cohen et al. (2006) suggested that a combination of temperature-dependent partitioning and changes in the saturation state of thecalcifying fluid can account for varying Sr/Ca and Mg/Ca ratios incorals.

Additional limitations are related to skeletal dissolution andsecondary aragonite overgrowth affecting trace element ratios.Secondary aragonite infillings and overgrowth also influence d18Oand d13C signatures, both resulting in cooler temperature re-constructions (Hendy et al., 2007). Contrary to the prevailingopinion that diagenesis occurs increasingly with increasing age,Hendy et al. (2007) reported that postdepositional artifacts canoccur in very recent coral skeletons, e.g., a decade old, whereascentury-old skeletons have been found to be nearly intact.

8.2. Mollusk geochemistry

Mollusk shells, especially bivalves and gastropods, are mainlyused to assess paleotemperatures and paleosalinities in marine orbrackish water environments. The advantage on using mollusksinstead of foraminifera is the preservation of seasonal and evendaily variability (Dutton et al., 2002; Schöne et al., 2005). Stableisotope and trace element profiles along the external growth lineprovide high-resolution records and the possibility to assign cal-endar years to geochemical proxies, if the growth rate can becalculated. The d18O values have been shown to provide informa-tion about sea surface temperatures (Gillikin et al., 2005; Schöneet al., 2005; Carroll et al., 2009; Batenburg et al., 2011) as theshell is precipitated in isotopic equilibrium with ambient water(Elliot et al., 2009). In lake systems the d18O signature can be used toreconstruct temperatures, but it also reflects the oxygen isotopiccomposition of the lake water (Brand et al., 1993; Von Grafensteinet al., 1994; Wurster and Patterson, 2001). In this way, isotopechanges in precipitation resulting from variations in air tempera-ture can be traced (Von Grafenstein et al., 1992; Von Grafensteinet al., 2000). A relationship between d18O in the mollusk shelland salinity could not be found by Dutton et al. (2002). In contrast,Carroll et al. (2009) reported the possible influence of seasonalchanges in salinity, occurring after the spring melt. In coastal en-vironments d18O signals can be biased by lower d18O values ofmeteoric water, suggesting warmer temperatures at lower salin-ities. But there are also limitations to the use of d18O as differencesin growth rate can bias the temperature signal (Tripati et al., 2001;Elliot et al., 2009; Apolinarska, 2013).

Carbon isotope signatures in bivalves and gastropods are areliable proxy for paleoproductivity. Fluctuations in the molluscand13C value are interpreted as changes in the carbon isotope

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composition of the dissolved inorganic carbon (DIC) and reflectwater mixing and primary productivity (Gasse et al., 1987; Elorzaand Garcìa-Garmilla, 1996; Wurster and Patterson, 2001; Gillikinet al., 2006; Carroll et al., 2009). Dutton et al. (2002) stated thatmaxima in d13C coincide with visible growth lines and can beattributed to periods of high productivity. In addition, the d13Ccurves agree verywell with intra-annual fluctuations in chlorophylla, and thus are correlated to primary productivity (Schöne et al.,2005).

Only few studies reported a temperature dependence of Mg/Ca(Lazareth et al., 2003; Batenburg et al., 2011) and Sr/Ca ratios (Dodd,1965). According to Gillikin et al. (2006), many trace elements thatare commonly used in foraminiferal or coral research, such as Sr,Mg and U, cannot be used as environmental proxies in bivalves(Gillikin et al., 2006). For Mg/Ca paleothermometry, contradictoryresults led to the suggestion thatMg/Ca incorporation in the bivalveshell is mainly controlled by biological processes (Carré et al., 2006;Elliot et al., 2009). The salinity dependence of Sr/Ca and Mg/Caratios was studied by Dodd and Crisp (1982) with the result thatthese ratios can be used as salinity proxy, but only if the salinitydoes not exceed 8&. Although Sr/Ca ratios are useful paleo-temperature proxies in corals and sponges, they seem to be mainlycontrolled by growth rate in aragonitic bivalves (Dutton et al., 2002;Carré et al., 2006). Gillikin et al. (2005) tested whether the strongcorrelation between growth rate and temperature allows indirecttemperature predictions from bivalve Sr/Ca ratios. Using twodifferent species they found a significant correlation betweengrowth rate and Sr/Ca ratio for Saxidomus giganteus but no corre-lation for Mercenaria mercenaria. They suggested that variability inSr/Ca ratios is mainly caused by biological processes and not underthermodynamic control. Later Carroll et al. (2009) published thatthe primary driver of bivalve growth is mainly food supply, ratherthan temperature.

Carré et al. (2006) also showed that environmental parametershave no significant influence on the incorporation of Mn, Mg, Baand Sr into the bivalve shell, and that crystal growth rate accountsfor most if the trace metal variability (e.g., 74% variance in Sr).Temperature and salinity had only a minor influence on traceelement incorporation, and there is also no correlation between theelement/calcium ratios of the bivalve shell and seawater (Carréet al., 2006). Elliot et al. (2009) found no correlation between Mgand Sr incorporation and growth rate in Tridacna gigas bivalves andsuggested varyingmetabolic rates as the cause for varying shell Mg/Ca. And also Shirai et al. (2008) stated that biological processesaccount for micro-scale elemental distributions, as the elementalcomposition was significantly associated with sublayer types inBathymodiolus platifrons shells. Schöne et al. (2013) also found largedifferences between Sr/Ca and Mg/Ca ratios from simultaneouslydeposited regions in shells of Arctica islandica and related them todifferent crystal fabrics, or to the processes that control their for-mation. Only at the annual growth lines were Sr and Mg depositedin equilibrium with the ambient environment.

In contrast, strontium isotopes seem to be a reliable proxy totrace different water masses. Sr isotopes reflect the Sr isotope ratioof the ambient water and allow to separate different strontiumsources. Water mass exchange between open ocean and marginalmarine environments (Whittaker and Kyser, 1993), freshwaterinput, and water exchange between separated ocean basins (Tripatiet al., 2001) could be reconstructed. In continental environments,saline influxes into lake systems (Anadón et al., 2002) as well asdiffering hydrochemistry during two lake phases, caused by dif-ferences in the relative contributions of water sources (Fan et al.,2010), were assessed.

Promising results were reported also for bivalve Ba/Ca ratios, asthis proxy is highly reproducible between specimens. Ba/Ca shell

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profiles show a flat background signal interrupted by sharp Ba/Capeaks (Gillikin et al., 2006; Schöne et al., 2013). Given a directrelation between the background and the Ba/Cawater, the Ba/Ca ra-tios of bivalve shells can be used as proxies for changes in salinitydue to the inverse relation between Ba/Cawater and salinity (Gillikinet al., 2006). A temperature dependence of the Barium incorpora-tion into the bivalve shell was not observed. Elliot et al. (2009)found that the Ba/Ca peaks correlate very well with chlorophyllpeaks in time and amplitude, indicating the association of thesepeaks with seasonal increases of local primary productivity. Incontradiction, Gillikin et al. (2008) reported that Ba/Ca peaks startaround 40 days after the crash of a bloom and thus the bloomscould not be the cause. Lazareth et al. (2003) suggested the possi-bility to use Ba and Mn as paleoproductivity proxies, since Ba andMn concentrations were directly related to nutrient input duringplankton blooms and increased run-off of freshwater. Carré et al.(2006) found no such correlation, as periodicity was lacking intheir Ba and Mn profiles, and thus rejected the relation to periodicplankton blooms. Caroll et al. (2009) reported that up to 60% of theannual variability in Ba/Ca ratios could be explained by riverdischarge, but only at sites close to rivers, as this effect becamenegligible at farther distances. The possible application of Mn/Caratios as indicator for primary production was also suggested byresults of Carroll et al. (2009).

Complicating factors that may limit the use of trace elementalratios and stable isotopes in bivalves are similar to those that affectall micro- and macrofossils (Elorza and Garcìa-Garmilla, 1996;Wurster and Patterson, 2001). Finding an appropriate pretreat-ment prior to analysis is essential also for bivalves. Krause-Nehringet al. (2011) tested the effects of different chemical pretreatmentmethods with respect to changes in elemental composition, theefficiency to remove organic matter, and the possibility to causealteration, and found that no pretreatment is without any side ef-fects. They suggested avoiding chemical treatment prior to analysiswhenever possible.

9. Techniques in paleoceanography and paleolimnology

The most routine analysis in paleoceanography as well aspaleolimnology is batch dissolution. Techniques for geochemicalanalysis of trace elements in foraminifera, mollusks or corals thatare becoming more widely used are Laser Ablation InductivelyCoupled Plasma Mass Spectrometry (LA-ICP-MS) (Sinclair et al.,1998; Eggins et al., 2003; Mertz-Kraus et al., 2009; Sadekov et al.,2009; Hetzinger et al., 2011; Jochum et al., 2012b), electron probemicroanalysis (EPMA) (Cohen et al., 2001; Morishita et al., 2007;Pena et al., 2008; Hathorne et al., 2009), and Nano Secondary IonMass Spectrometry (NanoSIMS) (Cohen et al., 2006; Kozdon et al.,2009; Fehrenbacher and Martin, 2010; Jochum et al., 2012a;Hoppe et al., 2013). These methods allow rapid analysis of solid-state samples as well as simultaneous measurement of a widerange of trace elements with high sensitivity (Montagna et al.,2007; Hetzinger et al., 2011). In foraminifera research, laser abla-tion ICP-MS was used to remove diagenetically modified surfacecalcites by pre-ablation prior to each analysis (Eggins et al., 2003;Pena et al., 2005). NanoSIMS analysis in corals revealed variationsin Mg associated with microstructure zoning (Meibom et al., 2007).A few studies have applied these methods to the analysis ofostracod shells (Morishita et al., 2007). Jochum et al. (2012b) car-ried out laser ablation ICP-MS on Leucocytherella sinensis valves andsummarized the detection limits of a large number of trace ele-ments. With respect to the proxies in foraminifera, elements suchas Nd and other REEs are detectable and can thus also be tested asproxies in lacustrine environments. Laser ablation analysis andNanoSIMS can also be used to obtain trace element distribution

ry as proxy for paleoenvironmental change, Quaternary International

Fig. 1. U/Th mapping on a 600 mm long ostracod shell (Leucocytherella sinensis) fromTibet determined by laser ablation ICP-MS at the Max Planck Institute for Chemistry,Mainz, using a spot size of 12 mm.

N. Börner et al. / Quaternary International xxx (2013) 1e21 15

patterns in ostracod valves, e.g., U/Th mapping using LA-ICP-MS(Fig. 1). For example, Sohn and Kornicker (1969) suggested thatcalcium is evenly distributed through the ostracod shell, whilephosphorous is concentrated near the shell margins. But the dis-tribution of most elements is still unclear. Detailed mapping ofostracod shells would thus also give new insights in possible bio-logical controls on the element incorporation in the shells.

10. Conclusions

Shell chemistry in ostracods and other micro- and macrofossilssuch as foraminifera and corals show many similarities eventhough the extent of research in the latter groups is clearly muchgreater. Techniques for geochemical analysis such as laser ablationICP-MS, NanoSIMS, and electron probe microanalysis (EPMA),which are state of the art methods in paleoceanography, providegreat potential for application in ostracod research, as they offerhigh-resolution data and thus the ability to show elemental vari-ation associated with shell microstructures (Shirai et al., 2008). Animportant question is, which proxies can be transferred from themarine to the lacustrine environment and hence from foraminiferaor coral chemistry to ostracods. If similar processes control theelement/calcium partitioning in ostracods and, for example, fora-minifera, it may be possible to transfer the fundamental basis of theproxies from marine to lacustrine environments. In addition, theassessment of elemental detection limits will determine what iseven possible technically.

Transferring existing proxies from marine to lacustrine envi-ronments is complicated as the factors that control environmentalconditions differ. Differences also occur between open and closedlake basins, respectively. Thus the applicability of a new proxy hasto be tested case-by-case. Li/Ca ratios as recorder of carbonatesaturation and temperature, as well as paleonutrient proxies suchas Cd/Ca may be used in closed lake basins. Closed lake systems areprimarily influenced by precipitation-to-evaporation ratios andthus climate. In open lake systems the water balance is additionallydependent on inflowing and outflowing water masses. The po-tential of redox sensitive trace elements providing informationabout bottom water oxygenation should be exploited in all marineand continental waters.

Please cite this article in press as: Börner, N., et al., Ostracod shell chemist(2013), http://dx.doi.org/10.1016/j.quaint.2013.09.041

11. Future challenges

Summarizing all existing techniques shows that ostracod shellchemistry provides an important paleoenvironmental proxy.However, environmental variables such as salinity, alkalinity, con-tinental weathering, and atmospheric circulation cannot beassessed entirely by a single proxy. More work is required to betterunderstand existing proxies and their interactions. Factors con-trolling shell chemistry of various genera are very complex, andunderlying chemical and biological processes of most of these newproxies are not well understood, even for living ostracods. Futurestudies should aim at better constraining the sensitivity of differentproxies for different ostracod species. In order to assess paleo-environmental signals, paired trace elemental and stable isotopeproxies should be standard.

Acknowledgments

We wish to thank the Deutsche Forschungsgemeinschaft (DFG)for sponsoring our work within the priority program 1372 “TibetanPlateau: Formation e Climate e Ecosystems (TP)” (SCHW 671/14-1). We thank Andrei Izmer for his help with the evaluation of U/Thmapping. We express our thanks to Javier Marco-Barba and oneanonymous reviewer for very constructive criticism on a previousversion of this paper.

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