Quantifying export production in the Southern Ocean: Implications for the Ba xs proxy

Quantifying export production in the Southern Ocean:Implications for the Baxs proxy

Maria T. Hernandez-Sanchez,1 Rachel A. Mills,2 Hélène Planquette,2 Richard D. Pancost,1

Laura Hepburn,2 Ian Salter,3 and Tania FitzGeorge-Balfour4

Received 3 January 2011; revised 13 September 2011; accepted 10 October 2011; published 21 December 2011.

[1] The water column and sedimentary Baxs distribution around the Crozet Plateau is usedto decipher the controls and timing of barite formation and to evaluate how exportproduction signals are recorded in sediments underlying a region of natural Fe fertilizationwithin the Fe limited Southern Ocean. Export production estimated from preserved,vertical sedimentary Baxs accumulation rates are compared with published export fluxesassessed from an integrated study of the biological carbon pump to determine the validityof Baxs as a quantitative proxy under different Fe supply conditions typical of the SouthernOcean. Detailed assessment of the geochemical partitioning of Ba in sediments and thelithogenic end-member allows appropriate correction of the bulk Ba content anddetermination of the Baxs content of sediments and suspended particles. The upper watercolumn distribution of Baxs is extremely heterogeneous spatially and temporally. Organiccarbon/Baxs ratios in deep traps from the Fe fertilized region are similar to other oceanicsettings allowing quantification of the inferred carbon export based on establishedalgorithms. There appears to be some decoupling of POC and Ba export in the Fe limitedregion south of the Plateau. The export production across the Crozet Plateau inferredfrom the Baxs sedimentary proxy indicates that the Fe fertilized area to the north of thePlateau experiences enhanced export relative to equivalent Southern Ocean settingsthroughout the Holocene and that this influence may also have impacted the site to thesouth for significant periods. This interpretation is corroborated by alternative productivityproxies (opal accumulation, 231Paxs/

230Thxs). Baxs can be used to quantify exportproduction in complex settings such as naturally Fe-fertilized (volcanoclastic) areas,providing appropriate lithogenic correction is undertaken, and sediment focusing iscorrected for along with evaluation of barite preservation.

Citation: Hernandez-Sanchez, M. T., R. A. Mills, H. Planquette, R. D. Pancost, L. Hepburn, I. Salter, and T. FitzGeorge-Balfour(2011), Quantifying export production in the Southern Ocean: Implications for the Baxs proxy, Paleoceanography, 26, PA4222,doi:10.1029/2010PA002111.

1. Introduction

[2] The oceanic biological carbon pump (BCP) is enhancedin regions where the ratio of nutrient consumption to supplyof upwelled nutrients is high [Sigman et al., 2010], but isoften limited in the open ocean by Fe concentrations in thesurface waters [e.g., Martin et al., 1994]. Therefore, themagnitude of the primary production and the BCP arecontrolled by the Fe flux to the surface in many regions of

the world’s ocean [Frew et al., 2001; Gall et al., 2001;Blain et al., 2007; Pollard et al., 2009].[3] Proxies for the BCP track processes such as nutrient

utilization, primary production, remineralization and carbonexport (export production). Reconstructions of the BCPusing traditional proxies - e.g., carbonates, organic carbon(OC), opal - might be hampered in areas where sedimenta-tion rates are low [Pichon et al., 1992], leading to opal dis-solution, or below the lysocline where carbonate dissolutionstarts. Similarly, sedimentary OC content might not reflectexport productivity, because other factors (including O2

availability and matrix protection) affect preservation of OC[Canfield, 1994; Keil et al., 1994] in the sedimentary record.Furthermore, paleoceanographic reconstructions/interpreta-tions are often hindered in areas of high volcanic input,sediment redistribution and suboxic diagenesis (e.g., TheCrozet Plateau).[4] The empirical observation of a correlation between

sedimentary Ba content and overlying production has led to

1Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre,School of Chemistry, University of Bristol, Bristol, UK.

2National Oceanography Centre, Southampton, University of Southampton,Southampton, UK.

3Observatoire Océanologique de Banyuls-sur-mer, Université Pierre etMarie Curie, CNRS-INSU, UMR, Paris, France.

4School of Biological and Chemical Sciences, Queen Mary Universityof London, London, UK.

Copyright 2011 by the American Geophysical Union.0883-8305/11/2010PA002111

PALEOCEANOGRAPHY, VOL. 26, PA4222, doi:10.1029/2010PA002111, 2011

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http://dx.doi.org/10.1029/2010PA002111

the wide use of Ba as a proxy for export production inpalaeoceanographic studies [Dymond et al., 1992; Francoiset al., 1995]. Excess Ba (Ba concentration corrected for Baassociated with the lithogenic phase: Baxs) fluxes have beenshown to correlate with carbon export [Dymond et al., 1992;Francois et al., 1995; Dymond and Collier, 1996; Pfeiferet al., 2001] and biogenic barite fluxes (Babar) also corre-lates with carbon export [Paytan et al., 1996; Eagle et al.,2003] in sediment core tops [Paytan et al., 1996; Pfeiferet al., 2001; Eagle et al., 2003] and sediment traps [Dymondet al., 1992; Francois et al., 1995; Dymond and Collier,1996]. Attempts to develop transfer functions, linkingdeep Baxs fluxes to export production have been made bymeasuring Ba and OC fluxes in sediment trap material indifferent oceanic regions (e.g., Equatorial Pacific, NorthPacific, North Atlantic [Dymond et al., 1992], Atlantic,Pacific and Indian Oceans [Francois et al., 1995]). Dymondet al. [1992] first proposed an algorithm based on Baxs andOC fluxes measured in sediment trap material deployed inthe Pacific and Atlantic Oceans, which was subsequentlysimplified by Francois et al. [1995]. These equations havebeen used widely in different ocean settings to quantita-tively determine or infer export productivity variationsusing Baxs accumulation rates [e.g., Rutsch et al., 1995;Dean et al., 1997; Nürnberg et al., 1997; Bonn et al., 1998;Bains et al., 2000].[5] Several mechanisms by which barite crystals precipi-

tate in undersaturated waters in the twilight zone underlyingthe euphotic zone have been proposed, [Monnin et al., 1999;Rushdi et al., 2000] including (1) precipitation of barite inmicro environments of decaying organic matter due to eitherenhanced sulphate concentrations sourced through organicsulphur oxidation [Chow and Goldberg, 1960; Dehairset al., 1980; Bishop, 1988] or enhanced barium concentra-tions [Paytan and Griffith, 2007] sourced from marineorganisms to the micro-environment [Fisher et al., 1991],(2) precipitation in microenvironments as a result ofBa-enriched celestite (SrSO4) dissolution in acantharia[Bernstein et al., 1992, 1998; Bernstein and Byrne, 2004],and (3) precipitation by living organisms [Levin, 1994;Gonzalez-Muñoz et al., 2003]. Laboratory studies with cul-tured phytoplankton have demonstrated that barite canprecipitate as a result of enhanced Ba concentrations releasedfrom phytoplankton during decomposition [Ganeshramet al., 2003]. Celestite dissolution and direct precipitationhave been observed in some environments [Bernstein et al.,1992, 1998] and the processes leading to barite formationare not fully understood. Besides the uncertainty associatedwith barite formation, and in addition to its utility as a proxyfor carbon export, Baxs has recently been proposed as a proxyfor twilight zone remineralization [Cardinal et al., 2005].[6] A number of issues lead to uncertainty in the applica-

tion of the Baxs productivity proxy. First, the relationshipbetween particulate Ba and OC export fluxes measured inmoored sediment traps is not constant [Dymond and Collier,1996]; Dymond et al. [1992] found that sinking particles hadhigher Corg/Babio ratios in the equatorial Pacific than in thewestern Atlantic and attributed this difference to Ba con-centration in seawater (higher in Pacific intermediatewaters). Francois et al. [1995] subsequently proposed thathigh Corg/Babio ratios in deep traps are the result of refractory

organic carbon addition in some locations. Dehairs et al.[2000] speculated that faster carbon export with shortertime for barite to synthesize might also lead to higherCorg/Babio ratios in ocean margin systems. However, thefactors controlling the flux of biogenic Ba to the seafloorare not yet fully understood.[7] Second, a range of methods are employed for quanti-

fication of export production and no common framework fordefining depth of export or integration times exists. Oftencarbon export fluxes are poorly constrained either spatiallyor temporally and are dependent on a number of crudeassumptions about the functioning of the BCP. Third, themeasured Baxs flux depends on sedimentary redox status,because barite (the main carrier of Ba in seawater which isapproximated by the Baxs pool) is soluble when sulphatereduction is active [Dymond et al., 1992; Paytan andKastner, 1996; Eagle et al., 2003] and sediment composi-tion [Dymond et al., 1992; Gingele and Dahmke, 1994]because large lithogenic correction introduces significanterrors.[8] The Southern Ocean exhibits variations in physico-

chemical properties that stimulate biological activity andresult in significant productivity gradients [Arrigo et al.,1998; Moore et al., 1999] that are a major component ofthe ocean BCP. Paleoceanographic reconstructions usingBaxs are difficult in the Southern Ocean because of the sig-nificant lithogenic input to the sediments and variable sedi-ment redistribution under the high energy sedimentaryconditions that both require correction.[9] The Crozet Plateau and region experiences a seasonal

Fe-fertilized phytoplankton bloom to the north of the CrozetIslands each year [Pollard et al., 2007a; Venables et al.,2007], and carbon export fluxes have been well con-strained as a result of the Crozet Natural Iron Bloom andExport Experiment (CROZEX) [Pollard et al., 2007a,2009]. This makes the area ideal to investigate the effect ofFe supply and a range of productivity and export regimes onthe Baxs proxy. Here, we assess the distribution of Baxs inthe water column (spatially and temporally) and sedimentcore tops across the significant productivity and export gra-dients in the region. This study includes evaluation of thecontrols and timing of Baxs generation in the water columnand accumulation of Baxs in the sedimentary record. Addi-tionally, we assess the quantitative use of the Baxs proxy asan indicator of carbon export and reconstruct (with addi-tional export proxies) Holocene carbon export around theCrozet Plateau. The accuracy of Baxs as an estimate of thebiogenic barite in sediments is also assessed.

2. Methodology

2.1. Oceanographic Setting

[10] The Crozet Plateau is located in the Indian sectorof the Southern Ocean (45–47°S, 49–51°E; Figure 1),�700 km north of the Polar Front (PF). In this area, theeastward flowing Antarctic circumpolar current (ACC) issignificantly diverted to the north by the bathymetry andflows anticyclonically round the del Caño rise [Pollard et al.,2007a]. Consequently, there is weak circulation betweenthe sub Antarctic front (SAF) and the Crozet Islands, creatinga hydrographically constrained area [Pollard et al., 2007b]

HERNANDEZ-SANCHEZ ET AL.: BAXS CALIBRATION IN THE SOUTHERN OCEAN PA4222PA4222

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which is fertilized by iron from the Crozet Islands[Planquette et al., 2007]. Nutrients are not consumed duringwinter due to light limitation, but stratification in summerleads to the development of a phytoplankton bloom eachyear to the north of the Plateau. A bloom as such does notoccur south of the Plateau, but relief of light limitationbrings about an increase in productivity from late Decemberto January [Venables et al., 2007]. Water column and sed-iment samples were collected during RRS Discovery cruisesD285 and D286 (November 2004 to January 2005 [Pollardet al., 2007a]) and D300 (December 2005 to January 2006)north (M10, M3, M8 and M9; Figure 1), east (M5; Figure 1)and south (M6 and M2; Figure 1) of the Crozet Plateau. Thenorthern stations and eastern site experience an annual Feenhanced bloom (+Fe hereafter [Pollard et al., 2009]),whereas the southern sites constitute a high nutrient lowchlorophyll (HNLC) area (�Fe hereafter [Pollard et al.,2009]).

2.2. Suspended Particle Samples

2.2.1. One mm Particles[11] Particles larger than 1 mm were collected from the

surface mixed layer at different stations within the PolarFrontal Zone ((PFZ); M10, M3, M5 and M6; Figure 1)during austral summer 2004 (Cruise D286) and 2005 (CruiseD300). Challenger Oceanic Stand Alone Pump Systems(SAPS) were deployed and set to pump for 0.5–2 h, nor-mally filtering around 500 L. 1 mm nucleopore filters wererinsed with Milli Q water and stored at �20°C until analysis.Filters were micro-wave digested in 20 ml of HNO3 at 175°C

for 60 min and subsequently cooled overnight. Metals (Fe,Ca, Ti, Mn, Fe, Zn and Ba) in solution were further diluted(20 to 200 times) with Milli Q water and analyzed byInductively Coupled Plasma Mass Spectrometry (ICP-MS)at the University of Portsmouth. External analytical preci-sion was better than 6% for Al, Ti and Ba. Data for allsuspended samples are expressed as micrograms per liter offiltered water. Re-precipitation of barite after digestion ispossible when using this procedure when the solution iscooled down prior to dilution. Thus, our data provide min-imum estimates of the total Ba content in the sample.However, we are confident that sediment trap material(section 2.2.2) and particles larger than 53 mm (section 2.3)were fully digested by comparison with the analysis ofcertified reference materials that contain similar order ofmagnitude Ba content (e.g., MAG1 contains 480 ppm Ba)to the samples measured. Moreover, the Baxs content ofparticles larger than 1 mm is similar to that of the largersinking particles (section 3.3), which gives us confidencethat the digestion was effective.[12] Chlorophyll a was measured for two samples col-

lected from the same depths as suspended particles werecollected. Briefly, pre-ashed GF/F filters (22cm; 0.7mm poresize) were used in each SAPS deployment, wrapped in alu-minum foil and immediately frozen (�80°C). Samples werelyophilized (�60°C; 10�2T) and weighed prior to extraction.Acetone (90%) was added (6mL) to the lyophilized filters,sonicated for 30 s and centrifuged at 3000 rpm for tenminutes. The extract was passed through a (0.2 mm) Nyalomembrane filter (Gelman) prior to analysis to remove any

Figure 1. Bathymetry and stations sampled during the CROZEX project: M10, M3, M8E, M8W, M7,M5, M2 and M6. Crozet Plateau (CP) and Del Caño Rise (DCR) are also indicated. Black lines illustratethe circulation pattern around the Plateau described by Pollard et al. [2007b]; the thick lines represent theAgulas Return Current (ARC) and the Sub Antarctic Front (SAF), with thinner lines marking transienteddies. The thin lines with the arrows illustrate the very weak circulation in the Polar Frontal Zone(PFZ) between the Sub Antarctic Front (SAF) and the Crozet Plateau.


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remaining small particles. Pigments were then separatedusing ion paired reverse phase HPLC and quantified (ng L�1)according to the method of Barlow et al. [1993].2.2.2. The 53 mm Particles[13] Larger particles were collected from the +Fe area

(stations M10, M3, M8E, M8W, M7 and M5; Figure 1) andfrom the �Fe area (stations M6 and M2; Figure 1). Filterswere mounted with a 53 mm nylon mesh monofilamentscreen (300 mm diameter) and the SAPS were set up topump for 1.5 h, filtering between 1000 and 2400 L. Imme-diately after recovery, excess water in the housing wasdrawn off in a laminar flow hood, and swimmers wereremoved. Subsequently, filters were frozen at �20°C untilanalysis. At the same time, samples for determination of234Th, Particulate Organic Carbon (POC) [Planquette et al.,2009] Particulate Organic Nitrogen (PON) [Planquette et al.,2009] and biogenic silica ((bSi) 53 mm mesh [Planquetteet al., 2009]) were taken using another SAPS deployed atthe same depth [Morris et al., 2007]. The two SAPS weredeployed simultaneously during austral summer 2004/2005(D285/6). In austral summer 2005/2006 (D300) sub-samplesfor POC and PON were taken from the same screens as fortrace metal analyses. Particles were rinsed from the screenusing sub-boiling distilled water and collected onto pre-weighed 0.4 mm polycarbonate filters. The filters were thenfreeze-dried and digested using a two step modified aceticacid and HF digestion procedure [Planquette et al., 2009]and trace metals in solution were subsequently measured byICP-MS at the University of Portsmouth. External analyticalprecision was better than 2% for Al and Ti and 1% for Badata. Accuracy was assessed through analysis of a range ofinternational certified standards (JA-2; JB-2; MAG-1;SGR-1; BIR-1, TORT-2 and NIES) and was within 4% and8% of the certified values (for Ba and Al respectively).

2.3. Sediment Trap Material

[14] A set of McLane sediment traps (21 cups) wasdeployed during austral summer 2004–2005 (D285) at M10(2000 m), M5 (3000 m) and M6 (3000 m). The sedimenttrap moorings were recovered during austral summer 2005–2006 (D300). The traps consisted of large funnels (0.5 m2

surface area) with a baffle at the top and a narrow opening atthe bottom, through which the particles could fall into indi-vidual cups. These cups (250 ml) were filled with preser-vative solution [Salter, 2007]. Prior to analyses, swimmers,fish debris and scales were removed. It is worth noting that afew of the samples (M10) were heavily contaminated by fishdebris (Table 1) but these data are included here to provide afull assessment of the seasonal variation in export produc-tion. The 250 ml samples were split into 8 equal aliquotsusing a rotary splitter. Not all samples had enough materialand only those presented in Table 1 were analyzed for Baxs.[15] Each split of sample material was freeze-dried over-

night and weighed (to within 0.00001 g) into individualTeflon digestion vials. The digestion procedure was basedon that described by Dehairs et al. [2000]. 3.5 ml of a 4:2:1mixture of HNO3, HCl and HF (4:2:1, v:v:v) were added toeach sample and refluxed on a hotplate for 2–3 days at80°C. At this stage, any undigested samples were drieddown on a hotplate and re-treated with aqua regia (HCl:HNO3; 1:3) at 140°C. Once fully digested, all samples weredried to incipient dryness and diluted ca. 2000-fold for ICP-MS analysis in 2% HNO3. External analytical precision wasbetter than 2% for Al, Ti and Ba data. Accuracy wasassessed through analysis of international rock standards(BHVO2, JA2, JB-1a, JB-3 and JGb-1) and was within 6%of the certified values.

2.4. Sediment Samples

2.4.1. Extraction of Biogenic Ba[16] Short mega-cores (which recovered overlying water

and the sediment-seawater interface) were recovered fromstations M5 (4269 m) and M6 (4268 m) during australsummer 2004/05. An additional mega-core was collectedfrom M10 during austral summer 2005 (3227 m). Mega-cores were sliced into 1 to 2 cm slices and frozen afterrecovery. Sediments recovered at M6 are characterized bythe presence of a turbidite deposit between 6 and 26 cmbsf[Marsh et al., 2007]. Sediment samples were digested using3 different methods including (1) a total digestion procedureand (2) two sequential extraction procedures (sections 2.4.1.2and 2.4.1.3 and Table 2).2.4.1.1. Total Digestion Procedure[17] About 100 mg of dried grounded sediment were

subjected to a combined aqua regia, HF and HClO4 totaldigestion. Five ml of aqua regia were added to each sampleand refluxed in a hot plate overnight at 80°C. Sampleswere subsequently dried down and 5.25 ml of HF:HClO4

(3:2.25, v:v) were added to each sample and refluxed in ahot plate overnight at 150°C. Samples were dried down and2 ml of HClO4 were added; subsequently, samples (fullydigested) were dried to incipient dryness. Metals in solution(Ti, Al, Ca and Ba) were measured by Inductively CoupledPlasma Optical Emission Spectroscopy (ICP-OES). Mea-sured Ba and Ti were used to calculate Baxs (section 2.5).External analytical precision, monitored by repeat analyses

Table 1. Baxs and Fluxes Measured in Deep Sediment Trapsa

LocationOpen Dateat 12:00 h

JulianDay

SamplingInterval(days)

Baxs Flux(mg m�2 d�1)

M5 28/12/04 �4.5 12 0.088M5 9/01/05 8.5 14 0.096M5 6/02/05 36.5 21 0.087M5 23/01/05 53.5 14M5 27/02/05 57.5 28 0.098M5 24/04/05 85.5 28 0.091M5 18/12/05 113.5 2 0.081M10 21/12/04 �5.5 12 0.15M10 2/01/05 8.5 14 0.11M10 16/01/05 22.5 14 0.099M10* 30/01/05 40 21 0.054M10* 20/02/05 64.5 28 0.034M10* 20/03/05 92.5 28 0.021M6 05/01/05 5.5 11 0.11M6 16/01/05 15.5 14M6 30/1/05 29.5 14M6 13/02/05 36.5 21 0.09M6 06/03/05 65.5 28 0.01M6 03/04/05 92.5 28 2.49M6 01/05/05 119.5 28 0.19M6 29/05/05 149.5 28 0.06M6 26/06/05 185.5 28 0.03

aFish contamination is indicated by asterisks. Julian day refers to middeployment day (relative to 1 January 2005).


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of certified standards over the period of analyses was betterthan 3%. Accuracy was assessed through analysis of certi-fied reference material (MAG-1) and was within 4% of thecertified value.2.4.1.2. Sequential Procedure[18] In addition, different splits of the same sediment

samples were subjected to a modified [Rutten and de Lange,2002; Reitz et al., 2004] sequential extraction procedure,where 125 mg of dried grounded sediment was successivelyleached with different solvents (Table 2). This sequentialextraction technique starts with the extraction of barite (firstleachate; Babar; Table 2) and constitutes a separation ofbarite from other Ba-phases in the sediment [Rutten and deLange, 2002]. Metals (Mg, Al, Ti, Mn, Fe and Ba) in solu-tion were then measured by ICP-OES. External analyticalprecision for Ba, monitored by repeat analyses of certifiedstandards over the period of analyses was better than 5%.2.4.1.3. Sequential Procedure[19] Babar was extracted from six sediment samples, cor-

responding to the upper and lower layers of three mega-cores (M5, M6 and M10) by using a sequential procedure[Eagle et al., 2003] (Table 2), where barite was extracted inthe last leaching step. Ten grams of dried grounded sedimentwere successively leached with different solvents and baritewas extracted in the last step after checking for purity underScanning Electron Microscopy Energy dispersive X-rayspectroscopy (SEM - EDS). The residue (digested in the laststep) mainly comprised barite crystals and Ti Fe oxides(possibly illmenite). Metals (Al, Ti and Ba) in solution weremeasured by ICP-OES. External analytical precision, moni-tored by repeat analyses of certified standards over theperiod of analyses was better than 3%.2.4.2. Determination of Total Organic Carbon,Carbonate and bSi[20] Total organic carbon (TOC) content in sediments was

determined using a Carlo Erba AE1108 elemental analyzer.Carbonate content was estimated from ICP-OES Ca dataafter correction for lithogenic Ca, whereas biogenic silicawas estimated by difference assuming a three componentmixing model [Marsh et al., 2007]; where the total sedimentcomposition is equal to the sum of lithogenic, carbonate andbSi phases. This approach has been shown to be robust inSouthern Ocean sediments [Marsh et al., 2007].2.4.3. Preserved Accumulation Rates[21] Radiocarbon analysis was performed for freeze-dried

and wet sediment samples. Samples were digested in 2MHCl (80°C; 8 h), washed with deionized water and subse-quently dried and homogenized. The total carbon in a known

weight of pre-treated sample was recovered as CO2 byheating with CuO in a sealed quartz tube. The gas wasconverted to graphite by Fe/Zn reduction. Radiocarbon dateswere obtained by Accelerator Mass Spectrometry (AMS) atthe Scottish Universities Environmental Research Centre(SUERC).[22] Sediments are often subjected to the effect of strong

bottom currents which leads to significant sediment focusingand winnowing at different sites [Dezileau et al., 2000].Thus TOC, bSi, Babar and Baxs accumulation rates have beencorrected by measurement of 230Thxs [Francois et al., 2004]and are presented as preserved vertical fluxes of each com-ponent. 200 mg of dried, ground homogenized sedimentwere completely digested using HF/HClO4 at the NationalOceanography Centre Southampton (NOCS). U, Th and Paisotope composition of NOCS sediment digests was deter-mined at the University of Oxford. The fully digested sam-ple was spiked with a mixed 229Th/236U spike and 233Pamilked from 237Np (precision of spike calibration after decayto secular equilibrium = 2.3% 2s SE). U, Th and Pa wereseparated using Dowex Bio-Rad AG1-X8 100–200 meshanion exchange resin [Thomas et al., 2006]. Pa and Th weremeasured on a Nu Instruments Multi Collector-InductivelyCoupled plasma Mass Spectrometer (MC-ICP-MS) usingmultiple ion counting channels. Blanks were monitored andwere <0.005% of the 231Pa measured. The unsupported, age-corrected 230Thxs

0 and 231Paxs0 were estimated by assuming

that the end-member lithogenic fraction had a Th content of6.2 ppm [Gunn et al., 1970] and that U-series isotopes are insecular equilibrium with a lithogenic fraction with a U/Thactivity ratio of 0.5 [Francois et al., 1993; Chase et al.,2003]. Sediment age was determined by 14C AMS con-verted to Calendar age by applying reservoir correctionsappropriate for the region. Data from Marsh et al. [2007]are included here, with the 231Paxs

0 /230Thxs0 ratio recalcu-

lated using measured 14C ages.

2.5. Lithogenic Correction

[23] Crozet sediments contain a significant lithogeniccomponent derived from the nearby volcanic Crozet archi-pelago [Marsh et al., 2007]. Crozet basalts are ocean islandbasalts and therefore enriched in Ti (�1.5%). There is sig-nificant evidence that the sedimentary Al/Ti ratio isenhanced in areas of high productivity because of scaveng-ing of Al during particle formation and sinking [Murray andLeinen, 1996; Dymond et al., 1997], therefore the ideal ele-ment for lithogenic correction is Ti. Unfortunately Ti anal-ysis by ICP-MS at Portsmouth was not successful for some

Table 2. Sequential Extraction Procedures

Step Ba Phase Extracted Solution Used

Reitz et al. [2004] Method1 Barite 8 � 25 ml 2 M (NH4Cl), pH 72 Ba associated with Fe/Mn oxides 25 ml solution of 0.15 M Na-Citrate and 0.5 M NaHCO3 (pH 7.6) plus 1.125 g of Na-dithionate3 Ba detrital Aqua Regia, HF and HClO4 total digestion (section 2.4.1.1)

Eagle et al. [2003] Method1 Ba associated with carbonates 50 ml of 4N acetic acid2 Ba associated with organic matter 40 ml of 5% ClNaO3 Ba associated with Fe and Mn oxyhydroxides 50 ml of 0.2 N hydroxyl amine in 25% acetic acid4 Ba associated with aluminosilicates HF: HNO3 (1:2, 1:1 and 2:1)5 Barite HNO3:HCl:HF (2:1:2)


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upper water column samples (SAPS filters), and therefore Alis used for lithogenic correction for these samples, while Tiis used for the rest of the sample set. Because these sampleshave low lithogenic content and are from the upper watercolumn the error associated with using Al is minor.[24] Measured Ba, Ti and Al concentrations were used to

calculate Baxs as follows:

Baxs ¼ Batotal � X* Ba=Xð Þend member

� � ð1Þ

where X represents Al in the case of SAPS filters and Ti forthe deep traps and sediments. The choice of end-memberratio is important for sediment samples with significantlithogenic content. Therefore, we have examined differentBa/Ti ratios in order to assess the sensitivity of these on theBaxs proxy. The basalt lithogenic input to Crozet sedimentshas been well constrained [Marsh et al., 2007] and approx-imates Ankaramite samples from the Isle de L’Est [Gunnet al., 1970]. We estimate sedimentary Baxs content usingthe lowest and highest Ba/Ti ratios found in Crozet basalts(0.01502 and 0.0189 respectively [Gunn et al., 1970](Table 3)). In addition, we estimate sedimentary Baxs con-tent using the Ba/Ti ratio in the most lithogenic turbiditesample from M6 [Marsh et al., 2007], which has no mea-surable biogenic content (Ti = 1.52%, Ba = 411 ppm,Ba/Ti = 0.0269, Ba/Al = 0.0052 [Marsh et al., 2007]).Similarly, we have estimated the Baxs content in suspendedmaterial using Crozet basalt Ba/Al ratios (0.0019 and 0.0026respectively [Gunn et al., 1970] (Table 3)) and the Ba/Alratio from the most lithogenic turbidite sample from M6(0.0058).

2.6. Core Top Accumulation of Baxs[25] Dissolution of barite and sediment reworking occur at

the sediment-water interface such that Fagel et al. [2002]have previously argued that the use of sedimentary Baxs asa proxy for export requires the conversion of preservedaccumulation rates into dissolution-corrected vertical rainrates. Dymond et al. [1992] proposed an algorithm relatingthe preservation of biogenic Ba and sediment Mass Accu-mulation Rate (MAR) as follows:

FBaxs ¼ Baxs-acc= 0:209 log MARð Þ � 0:213½ Þ� ð2Þ

where FBaxs is the dissolution-corrected Baxs flux to theseafloor. Baxs-acc is the preserved Baxs accumulation rate andMAR (in mg cm�2 yr�1) is the bulk sediment mass accu-mulation rate [Marsh et al., 2007]. Dissolution correctedaccumulation rates have been estimated using the approachof Dymond et al. [1992].

3. Results

3.1. Surface Mixed Layer Particle Distribution

[26] A portion of the 1mm filters collected from the surfacemixed layer was examined under SEM (aiming to establishif biogenic barite mineral phases are present in the upperwater column) and euhedral mineral phases were identifiedas BaSO4 using SEM - EDS. Photomicrographs show thepresence of biogenic barite in the filters collected at M5(Figures 2a and 2g), M6 (Figures 2b–2e) and M3 (Figure 2f;samples collected from other stations were not analyzed).Additionally diatom frustules, coccoliths and some litho-genic fragments are present. Photomicrographs are notavailable for deeper 53 mm SAPS.

3.2. Barium in Suspended Particles

[27] Total barium abundances in the upper water columnrange from 0.0000084 to 0.87 nmol L�1 and are highlyvariable; the highest values are observed in the center of thebloom at M8E (Table 4). Baxs concentrations are often sig-nificantly higher than that of Balithogenic, constituting 25 to99% of the total Ba measured (Table 4). Baxs content alsoshows no trend with depth and it is extremely variable,presenting high and low values for similar depths (Figure 3).The highest and most variable values generally occur deeperthan 90 m.

3.3. Baxs Fluxes in the Water Column

[28] Baxs fluxes in deep sediment traps (2000 and 3000 m)range from 0.009 to 0.15 mg m�2 d�1 (Table 1), the highestvalues are observed in the +Fe region. Temporal trends aresimilar at M5 and M10, decreasing from late December toMarch and increasing from then until May (only at M5;Figure 4). In the �Fe region, mass flux was very low for theperiod between January 2005 and January 2006, with sig-nificant accumulation only present in two collection cups[Salter, 2007]. Baxs analysis was only possible for one col-lection cup (January 2005; Table 1), which absolute fluxvalue was similar to those measured in the +Fe area. How-ever, integrated Baxs fluxes (with respect to the total timeperiod sediment traps were deployed) are about 10 timeshigher in the +Fe area (10–16 mg m�2 yr�1) compared to the�Fe area (1.2–2.1 mg m�2yr�1).

3.4. Core Top Preserved Baxs, TOC and bSiAccumulation Rates and 231Paxs

0 /230Thxs0 Ratios

Around the Crozet Plateau

[29] Holocene 230Th-corrected Baxs, TOC and bSi accu-mulation rates vary across the Plateau (744 to 1804 mg Baxscm�2 kyr�1, 1.78 to 12.8 mg C cm�2kyr�1 and 0.33 to 1.17 gSi cm�2kyr�1 respectively). Preserved accumulation ratesare described in detail in Table 4.[30] Mid to late Holocene 231Paxs

0 /230Thxs0 activity ratios

within the +Fe region are significantly higher (0.15–0.18)

Table 3. Ba/Ti and Ba/Al Ratios Estimated From Crozet BasaltsBa, Ti and Al Contentsa

Crozet Basalts Ba (ppm) Ti (%) Al (%) Ba/Ti Ba/Al

O 186.2 1.12 5.62 0.0164 0.00207A1 250.8 1.45 7.14 0.0172 0.00220A2 257.1 1.41 7.13 0.0182 0.00226A3 242.4 1.54 7.04 0.0156 0.00215A4 238.3 1.53 7.63 0.0155 0.00195A5 287.5 1.52 8.27 0.0189 0.00218A6 248.1 1.55 8.02 0.0159 0.00194A7 259.1 1.72 8.07 0.0150 0.00201B1 272 1.70 8.34 0.0159 0.00204B2 326.2 2.003 9.84 0.0162 0.00207L1 279 1.82 8.003 0.0152 0.00218L2 354 2.05 8.41 0.0172 0.00264L3 356 2.07 8.48 0.0171 0.00263

aBa, Ti and Al content are from Gunn et al. [1970]. Ba, Ti and Al contentwas determined in oceanites (O), ankaramites (A1 to A7), basalts (B1 andB2) and lavas (L1 to L3) collected from East Island, Crozet Archipelago.


6 of 19

than the water column production ratio (0.09). In the �Fearea, the ratios are ca. 0.13 and 0.15 for Holocene sediments;231Paxs

0 /230Thxs0 ratios for the underlying turbidite deposit are

0.0815–0.0817 which is slightly lower than the water col-umn production ratio (Table 5).

3.5. Total Ba, Babar and Baxs Contentin Sediment Samples

[31] Total barium (Batotal), Babar and Baxs content of sedi-ment samples range from 465 to 1419 ppm, 117 to 1292 ppm,and 103 to 1304 ppm respectively around the Crozet Plateau.Batotal, Babar and Baxs (estimated using different Ba/Ti

Figure 2. SEM photomicrographs from the 1mm nucleopore SAPS filters collected in surface watersaround the Crozet Plateau. Shown are photomicrographs of filters collected during December 2004(Figure 2a) and December 2005 (Figure 2g) for station M5. Shown are photomicrographs of filters col-lected in January 2005 (Figures 2b and 2c) and December 2005 (Figures 2d and 2e) for station M6.Shown is a photomicrograph of the filter collected in January 2005 (Figure 2f) for station M3. In all filters,2–4 mm ovoid-shape minerals have been identified by SEM EDS as BaSO4 and are similar to previouslyidentified biogenic barite in the water column [Dehairs et al., 1980].


7 of 19

Tab

le4.

Total,Lith

ogenic

andBa x

sCon

centratio

nsMeasuredin

Sinking

andSuspend

edParticlesa

Statio

nDate

Depth

(m)

Ba lith

(nmol

l�1)>53

mmBa x

s

(nmol

l�1)>53

mmBa lith

(nmol

l�1)>1mm

Ba x

s

(nmol

l�1)>1mm

Ba total

(nmol

l�1)

Ba a

canthariab

(%)

Ba a

canthariac

(%)

Ba x

s

(%)

M3

31/12/04

100

0.00

1–0.00

30.17

2–0.17

40.17

50.15

06–0

.152

31.21

6–1.23

098–99

M3

10/01/05

800.00

017

00.00

017

0M3

12/01/05

800–0.00

032

00.00

065

0M3

06/01/05

700.00

2–0.00

60.06

69�0

.067

50.06

7798–99

M3

06/01/05

300.00

780

0.00

078

0M3

13/11/04

225

0.00

2–0.00

40.19

6–0.19

80.20

0.84

–0.85

6.8–6.9

98–99

M3

18/11/04

155

0.00

1–0.00

40.24

0–0.24

30.24

40.14

5–0.14

71.17–1.18

98–99

M3

25/11/04

200

0.00

01–0

.000

50.00

12–0.001

80.00

1913

4–20

110

87–163

063–94

M3

22/12/04

180

0.00

012–

0.00

053

0.00

042–0.00

083

0.00

0956

5.16

–10.2

41.7–82.4

43–87

M3

06/01/06

340

0–0.02

0.13

3–0.13

50.13

598–99

M10

20/12/04

0.00

0008

40

0.00

0008

40

M8E

30/11/04

200

0.00

1–0.00

30.87

3–0.87

50.87

60.25

43–0

.254

92.05

4–2.05

899

.6–99.8

M8W

01/12/04

150

0.00

0302

0�0.00

0033

0.00

0302

0M9

03/12/04

00.00

016

0.00

016

100

M9

19/12/04

00.00

0049

0.00

0049

100

M7

25/11/04

150

0.00

018

00.00

018

0M5

24/12/05

100

0.00

05–0

.001

80.07

89–0.080

20.08

070.76

8–0.78

16.20

9–6.31

197–99

M5

27/12/04

700.00

01–0.000

60.04

13–0

.041

80.04

1998–99

M5

11/12/05

800.00

1–0.00

40.08

64–0

.089

50.09

0595–98

M5

27/12/04

125

0.00

0025–0

.000

302

0–0.00

0077

0.00

0302

0–25

M5

11/12/05

140

0.00

1–0.00

20.55

6–0.55

70.55

80.07

11–0

.071

20.57

4–0.57

599

.6–99.8

M6

22/11/04

200

0–0.00

031

0.31

9–0.32

00.32

0.43

8–0.43

93.53–3.54

99.6–99.9

M6

03/01/05

120

0–0.00

008

0–0.00

010.00

018

0–55

M6

04/01/06

110

0.00

0023

00.00

0023

0M6

28/12/05

600.00

05–0

.002

0.09

43–0

.095

80.09

6398–99

M6

04/01/05

700.00

01–0.000

40.07

67–0.077

0.07

7199

.4–99.8

M2

20/11/04

150

0.00

0027

00.00

0027

0M2

06/01/05

160

0.00

01–0.000

40.09

29–0.093

20.09

3399

.5–99.8

a Blank

cells

indicatestations/dates

notsam

pled.B

a acanthariarepresentsthepo

rtionof

Ba x

sprod

uced

byacantharia:S

rconcentrationin

collected

particleswas

multip

liedby

theBa/Srmolar

ratio

ofSou

thernOcean

acantharia

[Bernstein

etal.,19

98;Jacquetet

al.,20

07].The

resulting

Ba(associatedwith

acantharia)contentwas

dividedby

Ba x

sandmultip

liedby

100.

bBernstein

etal.[199

8].

c Jacqu

etet

al.[200

7].


8 of 19

ratios; section 2.5) content of sediment samples aredescribed in detail in Table 6.

4. Discussion

4.1. Assessment of Analytical Determination of Baxs[32] A variety of methods are used to approximate the

biogenic Ba content of marine sediments and we comparethe three most common techniques here. The sequentialleach techniques used provide an alternative estimate of theBabar content of the sediments and allow assessment of thevalidity of the Baxs approach. Babar content estimated usingthe Eagle et al. [2003] method are very low compared withvalues elsewhere in the Southern Ocean and other regions ofthe ocean [Fagel et al., 2002; Eagle et al., 2003], constitut-ing only 0.064 to 12% of the Baxs measured (Figure 5a).Eagle et al. [2003] only observed Babar in sediments withBaxs content at least between 10 and 100 ppm, howeverCrozet sediments have a Baxs content of �1000 ppm sug-gesting that there should be no problem with determinationof Babar in these samples.[33] Acid decalcification is the first step of the Eagle et al.

[2003] procedure and it risks pre-dissolution of marine barite[Reitz et al., 2004]. There is evidence of barite dissolution indilute acids such as 0.5N HNO3, 0.3 N HF or 0.4 N HCl[Cardinal et al., 1999], suggesting that barite could dissolvein the early steps of the Eagle et al. [2003] procedure.Alternatively, there may be sample loss of the fine grained

(<5 mm) barite during sample handling. Certainly the Babarestimated by application of the Eagle et al. [2003] methodon Crozet sediments suggests that this method is inappro-priate in such setting, vastly underestimating the baritecomponent of the sediment. Thus, this method should beused with caution in Southern Ocean and similar settings.[34] The lithogenic correction of the sediment (estimated

as Batotal-Baxs; Table 6) ranges from 1.6 to 43%, 3.6 to 77%and 2.1 to 54% of the total Ba content when the lowest Ba/Tiratio from Gunn et al. [1970], the highest Ba/Ti ratio fromGunn et al. [1970] and the Ba/Ti ratio from the most litho-genic sample from M6 turbidite are used, respectively. Babarhas also been estimated using the Reitz et al. [2004] method,which effectively separates barite from other Ba-phases (Feoxides and lithogenic bound Ba) in the sediment (99.4%)[Rutten and de Lange, 2002]. Generally Baxs content is verysimilar to Babar content when using Ba/Ti ratios from Gunnet al. [1970] and the most lithogenic sample from the tur-bidite at M6, and there is a good correlation between thesetwo parameters - r2 = 0.97 (n = 41), the highest Gunn et al.[1970] ratio; r2 = 0.9701 (n = 41), the lowest Gunn et al.[1970] ratio (Figure 5); and r2 = 0.9667 (n = 41), M6 tur-bidite Ba/Ti ratio. Three samples gave anomalously lowBabar content (Table 6) suggesting that sample handling maylead to loss of fine grained Babar and underestimation of thebarite content. Baxs content estimated using the lowest Gunnet al. [1970] Ba/Ti ratio reproduces most accurately Babarbut deviates from Babar content by 2 to 129 ppm (Table 6).We converted Babar and Baxs (using the Gunn et al. [1970]lowest Ba/Ti ratio) sedimentary fluxes to OC export usingthe Francois et al. [1995] algorithm to assess if that devia-tion significantly affects OC export estimates. Baxs derivedOC export only deviates from OC export (Babar estimated)by 0.2 to 0.8 gCm�2yr�1 (and up to 4g Cm�2yr�1 within theturbidite at M6). This difference is not significant given thefact that OC export ranges from �20 to �140 g cm�2 ka�1

(e.g., Figure 6e) and therefore the well constrained litho-genic end-member from the Crozet Archipelago (using thelowest Ba/Ti ratio from Gunn et al. [1970]), sampled froman Ile de l’Est ankaramite allows the use of Baxs as a proxyfor biogenic barite in this region despite the large and vari-able lithogenic input in this area.

4.2. Bloom Progression and Particulate BaxsConcentration in the Surface Mixed Layer

[35] Both the 53 and the 1 mm SAPS filters collect sus-pended barite from the upper water column. The 1 mm filterscollect barite as individual grains (Figure 2) and in aggre-gates whereas the particles recovered from 53 mm SAPSfilters constitute only aggregates, as the small (�2–4 mm)individual grains will pass through the mesh in these SAPSdeployments. Thus, both filter sizes provide informationabout barite formation in surface waters with generallyhigher concentrations of Baxs found in the >53 mm fractionconfirming that biogenic barite (as Baxs) is forming andaggregating in the water column.[36] Total Ba and Baxs concentrations are highly variable

spatially, temporally and with depth (Figure 3) at all sitesstudied. Moreover, total Ba concentrations are either veryhigh (0.13 to 0.87 nmol L�1) or very low (0.0000084 to0.090 nmol L�1). When low concentrations occur, most ofthe barium is of lithogenic origin (i.e., Ba/Al = 0.00147). In

Figure 3. Baxs content of particulate matter within thewater column. Circles represent small particles (>1mm) andcrosses refer to large particles (>53 mm).


9 of 19

contrast, high concentrations are associated with high Baxsvalues suggesting a biogenic origin.[37] Chl-a and Baxs concentration are weakly correlated in

small particles (>1 mm; r2 0.63; n = 5; Figure 7a); however,there is no correlation observed for larger particles (>53 mm;Figure 7b). Additionally, Baxs and OC content (proxy forbiomass of phytoplankton) and Baxs and bSi (proxy for

diatom biomass) content are poorly correlated in these filters(Figures 7c and 7d). This suggests that barite formationmight only be related to primary production in the upperwater column (represented by filters collecting smallparticles, >1 mm) and this relationship breaks down duringparticle aggregation and sinking/remineralization. If there isa relationship between chl-a (representing PP) and Baxs in

Figure 4. Measured Baxs fluxes and POC fluxes from Pollard et al. [2009] in deep sediment traps. Baxsfluxes are represented by star symbols whereas squares represent POC fluxes. Chl-a concentrations [fromHernandez-Sanchez et al., 2010] are represented by solid line and shaded areas indicate mass fluxes.


10 of 19

Tab

le5.

HoloceneBa x

s,bS

i,andTOC

230ThCorrected

Accum

ulationRates

and

231Pa x

s0/230Th x

s0RatiosforSedim

entCores

a

Site

Depth

(cmbsf)

Ba x

s

(mgcm

�2ky

r�1)

Ba x

sDissolutio

n-Corrected

(mgcm

�2ky

r�1)

232Th

(dpm

g�1)

230Th

(dpm

g�1)

231Pa

(dpm

g�1)

231Pa x

s0/230Th x

s0

bSi

(gcm

�2ky

r�1)

TOC

(mgcm

�2ky

r�1)

CalendarAge

(yrBP�

1s)

+Fe

0.5

1790

�14

3763

�10

80.24

2�

0.00

15.26

5�

0.03

10.88

8�

0.00

90.18

5�

0.00

31.15

12.84

2.5+

0.16

7�

0.00

33.5

1774

�14

3739

�97

0.63

1�

0.00

612

.51�

0.14

1.05

9�

0.01

91.11

10.13

502�

195.5

1689

�13

3550

�10

11.13

9.80

7.5

1668

�14

3506

�10

30.23

9�

0.00

15.25

�0.02

60.95

�0.01

20.17

5�

0.00

21.13

10.74

9.5

1626

�12

3418

�97

1.15

7.85

1297

�18

11.5

1582

�12

3325

�95

1.17

8.18

13.5

1644

�12

3498

�98

1.13

8.18

1516

11�

1233

86�

961.15

40.5+

0.35

3�

0.00

65.54

�0.12

31.18

6�

0.02

00.13

43�

0.00

472

02�

41�F

e1.5

1470

�12

3092

�90

0.44

9�

0.00

28.18

�0.04

21.18

6�

0.02

00.15

6�

0.00

20.88

4852

�23

2.5+

0.40

9�

0.00

58.98

�0.15

1.13

2�

0.00

70.13

5�

0.00

23

1105

�9

2323

�68

0.79

5.18

*5.5

1181

�33

2483

�12

10.82

8�

0.00

64.00

1�

0.02

00.29

6�

0.00

40.08

2�

0.00

31.02

5.87

*9.5

926�

2623

74�

522

0.82

4.73

*11

.569

9�

2014

70�

730.66

5.15

*13

.586

8�

2518

25�

910.77

5.12

*15

.548

5�

1410

20�

210.62

3.35

*17

.542

2�

1288

7�

440.45

3.98

*19

.553

2�

1511

16�

552.26

5.99

*21

.552

8�

1511

10�

550.55

2.99

*23

.531

0�

965

4�

320.41

3.83

*25

.537

4�

1078

8�

350.39

4.46

a Turbiditeho

rizons

aremarkedby

asterisks.Add

ition

alsamples

where

231Pa x

s0/230Th x

s0ratio

shave

been

measuredcorrespo

ndto

gravity

cores(m

arkedby

crosses)

collected

atthesamesites[M

arsh

etal.,20

07].


11 of 19

deeper water [e.g., Cardinal et al., 2005], that relationshipmight require the confluence of certain conditions not con-strained in this study. We have identified barite crystals insurface waters (Figure 2). However, phytoplankton areknown to carry labile Ba [Fisher et al., 1991; Sternberget al., 2005] and thus, Ba associated with plankton cells islikely contributing (in addition to Babar) to the Baxs pooland variability in the results, especially when total Ba islow (i.e., our filters have not collected any barite crystals).Alternatively, although large particles (>53mm) are consid-ered to be responsible for exporting carbon and other ele-ments from surface waters [Bishop, 1988], it is likely thatbarite crystals are smaller than 53mm [Dehairs et al., 1980;Jacquet et al., 2007; Sternberg et al., 2008] and were undersampled by deeper SAPS deployments.

[38] Another mechanism proposed to cause oversaturationof Ba and sulphate in seawater, and to ultimately lead toprecipitation as BaSO4, is the dissolution of Ba-enrichedcelestite (SrSO4) [Bernstein et al., 1992]. We have measuredSr content in particles larger than 53 mm and calculated Baxssourced from acantharia. For this, we have applied a Ba/Srmolar ratio of 2.6 � 10�3 [Bernstein et al., 1998] and2.1 � 10�2 (Southern Ocean [Jacquet et al., 2007]) foracantharia. In most of the samples, acantharian Ba does notexceed 7%, (Table 4), suggesting that BaSO4 is not the resultof dissolution of celestite from acantharia and rather occursdue to decaying aggregates. Only three samples give sig-nificantly high acantharian Ba values (M3 12/01/05, M3 25/11/04 and M3 22/12/04 when using a molar ratio of

Table 6. Batotal, Babar, and Baxs Content and Estimates of Ba Associated With the Lithogenic Fraction in Sediments Recovered From theCrozet Plateaua

StationDepth(cmbsf)

Batotal(ppm)

Babar(ppm)

Baxs(M6 Turbidite Ratio)

(ppm)

Baxs(Low Ratio)

(ppm)

Baxs(High Ratio)

(ppm)L1

(%)L2

(%)L3

(%)Baxs-Babar

(ppm)

M5 2 1407 1245 1223 1304 1277 9.1 7.3 13 59M5 4 1232 1143 1066 1140 1116 9.4 7.5 13 �2.8M5 6 1180 1145 1036 1099 1079 8.5 6.8 12 �45M5 8 1218 1223 1065 1133 1111 8.7 6.9 12 �90.5M5 10 1291 1116 1134 1204 1181 8.5 6.7 12 87M5 12 1419 1292 1248 1324 1299 8.4 6.7 12 31M5 14 1365 1237 1196 1270 1246 8.6 6.9 12 33M5# 16 1136 212 1000 1060 1041 8.3 6.6 11 848M5 18 1085 1035 959 1015 997 8.1 6.5 11 �20M5 20 1167 1273 1029 1090 1070 8.3 6.6 11 61M5# 22 1105 199 990 1041 1024 7.3 5.83 10 842M5 24 983 810 884 930 916 6.8 5.43 9.7 119M5 25 1432 1259 1304 1361 1342 6.3 5.01 8.9 101M5 34 1275 1098 1163 1213 1196 6.1 4.9 8.7 114M5 45 1249 1065 1072 1150 1124 9.9 7.9 14 85M10 0.5 1150 1054 1089 1116 1107 3.7 2.9 5.3 62M10 3.5 1141 1061 1079 1107 1098 3.7 3.01 5.4 45M10 5.5 7083 1066 1028 1052 1044 3.5 2.8 5.1 �13M10 7.5 1074 992 1019 1044 1036 3.6 2.8 5.1 51M10 9.5 1048 1006 994 1018 1010 3.6 2.8 5.1 11M10 11.5 1019 905 967 990 983 3.5 2.8 5.09 84M10 13.5 1059 985 1005 1029 1021 3.5 2.8 5.08 43M10 15 1039 976 984 1009 1001 3.7 2.9 5.2 32M10 40.5+ 1055 909 983 1015 1004 4.8 3.8 6.8 105M10 67 927 704 733 819 791 14 11 20.9 115M10 74 772 612 632 694 673 12 10.1 18 81M10# 84 734 117 573 644 621 15 12 21 644M6 1.5 1222 1059 1092 1149 1142 7.4 5.9 10.5 90.3M6 3.5+ 988 811 820 895 885 11 9.4 16 83M6* 5.5 683 435 394 522 505 29 23 42 86M6* 9.5 625 351 309 448 431 35 28 50.5 97M6* 11.5 566 247 233 380 361 41 32 58 132M6* 13.5 605 309 289 429 411 36 29 52 119M6* 15.5 488 210 161 306 287 46 37 66 95M6* 17.5 502 180 140 300 280 50 40.1 71 120M6* 19.5 525 214 177 331 311 46 36 66 116M6* 21.5 508 165 176 323 304 45 36 65 157M6* 23.5 465 172 103 263 243 54 43 77 91M6* 25.5 500 136 125 290 269 52 41 75 153M6 27 1336 987 1285 1307 1305 2.7 2.1 3.8 320M6 30 1024 992 993 1007 1005 2.1 1.6 3.03 14M6 32 1143 1160 1105 1122 1120 2.3 1.8 3.2 1122M6 34 1170 1016 1127 1146 1143 2.5 2.06 3.6 129

aTurbidite horizons are marked by asterisks and samples which gave anomalously low Babar values are marked with hashes. The lithogenic correction(L1, L2 and L3) has been calculated as the difference between Batot and Baxs, estimated using the Ba/Ti ratio from the most lithogenic sample from M6turbidite (L1), and the lowest (L2) and highest (L3) Ba/Ti ratios from Gunn et al. [1970]. The difference between biogenic Ba (Baxs) and barite contentin the sediment is also shown in this table.


12 of 19

2.1 � 10�2), possibly due to very low Baxs values in thesesamples (Table 4).

4.3. The 230Th Normalization of Baxs Fluxesin Sediments

[39] To further investigate the behavior of the Baxs proxywe compare fluxes measured in deep sediment traps andcore tops north and south of the Crozet Plateau, which rep-resent the integrated signal of this proxy over months andyears-millennia respectively. Additionally, we comparePOC fluxes and other proxies for export production. Thecore collected at M10 (Figure 1) from the center of the +Fearea best represents sediment accumulation under the influ-ence of Fe enhanced export to the seafloor. The core col-lected from the M6 site (Figure 1) records sedimentation

under the HNLC conditions prevalent south of the Plateau(�Fe).[40] The core top 14C age at the +Fe site is 920 � 35 14C

years which is consistent with relatively fast (2 g cm�2

kyr�1) accumulation of volcanogenic/biogenic sediment onthe northern region of the Plateau [Marsh et al., 2007].

Figure 5. Correlation between Baleach (first leach from theReitz et al. [2004] method, representing Babar) and Baxs insediments estimated using the lowest Ba/Ti ratio of fromCrozet Basalts (circles [Gunn et al., 1970]) and Babar dataobtained using the Eagle et al. [2003] method (squares).Note that these data points have not been taking into accountfor the correlation.

Figure 6. Meridional distribution of 230Th-corrected accu-mulation of preserved (a) Baxs corrected for dissolution,(b) bSi, (c) organic carbon, and (d) 231Pa/230Th ratios. Thehorizontal line in Figure 6c represents the production ratiofor 231Pa/230Th in the water column. Additional Baxs data(open circles) in Figure 6a are from Fagel et al. [2002].Additional bSi, and 231Paxs

0 /230Thxs0 data (open circles) are

from Dezileau et al. [2003] and Pondaven et al. [2000].Organic carbon results are compared with data from thepacific sector of the Southern Ocean (open circles [Chaseet al., 2003]). MD-84-527 refers to data from Francoiset al. [1993]. (e) Southern Ocean seasonally integrated OCfluxes ( = export) at 100 m are shown for comparison; opencircles are interpolated fluxes at 100 m from sediment trapdata (Indian sector [Miquel et al., 1998; Trull et al., 2001])using Martin Curve. Solid circles represent the upper andlower seasonally integrated 324Th derived carbon fluxes at100 m estimated by Pollard et al. [2009] for the +Fe and�Fe regions.


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Within the �Fe area, TOC radiocarbon ages at the core topsare relatively old (4877 � 38 14C years) which is attributedto lateral input of older organic matter and bioturbation ofolder carbon from deeper in the sediment at slow accumu-lation rates [Mollenhauer et al., 2005; Bard, 2001]. Inaddition, the presence of a subsurface turbidite deposit[Marsh et al., 2007] indicates episodic downslope transportat this site. Under these circumstances, it is possible that230Th normalized Baxs fluxes are overestimating real fluxesin the �Fe region. By normalizing to 230Thxs, we assumethat the radionuclide comes from the overlying water col-umn [Francois et al., 2004]; however downslope transportcould lead to overestimation of preserved vertical fluxes[Francois et al., 2004; Mollenhauer et al., 2005]. Themagnitude of the effect of this process on the 230Thxs0 nor-malized sediment accumulation rate is shown in Figure 8.An input of 10–15% of sediment transported downslopefrom 1000 m depth with no significant interaction withseawater decreases the calculated flux by �0.9 (1.2 timeslower than those obtained assuming no lateral input). Any

Figure 7. Correlation between Baxs and chl-a, OC and bSi in suspended particles. (a) The correlationbetween Baxs and chl-a in small particles (>1 mm), where gray squares represent chl-a determined in thisstudy for the same SAPS where small particles were collected; black squares refer to chl-a data determinedby Seeyave et al. [2007] at the same depths as SAPS filters were collected. (b) The correlation betweenBaxs and chl-a measurements [Seeyave et al., 2007] in large particles (>53 mm). (c and d) The correlationbetween Baxs and OC content [Planquette et al., 2009] and Baxs and bSi content (respectively) in largeparticles (>53 mm) collected from the upper water column.

Figure 8. Calculated effect of downslope sediment trans-port (from 1000 m) on 230Th normalized sediment accumu-lation rates at the �Fe site (M6).


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errors associated with differential uptake of sedimentarycomponents during sediment redistribution are within ana-lytical uncertainty for Holocene sediments. Thus, in thisstudy, any uncertainties associated with the use of 230Thxsnormalization are significantly smaller than the differencesin fluxes discussed in (section 4.3.1).4.3.1. Comparison of Baxs in Sediment Trap Materialand Surface Sediments[41] Baxs and POC fluxes in sediment trap material in

the +Fe area decrease from December 2004 (Julian day 5) toMarch 2005 (Julian day 92; Table 1 and Figure 4). Thebloom peaked in surface waters in September/October 2004(Figure 4) [Hernandez-Sanchez et al., 2010], and particlesare estimated to sink at a speed of 50–100 m d�1 [Salter,2007], and thus, the sediment traps were likely deployedafter the POC export flux peaked [Pollard et al., 2009](Figure 4). We have estimated Pnew (= export) at 100 m(using Sarnthein et al.’s [1988] predictive equation; Table 7)from sediment trap POC values and the estimates are half ofthose derived from seasonally integrated 234Th measure-ments [Pollard et al., 2009]. This suggests that under sam-pling of the integrated seasonal flux occurred at the +Fesite because of late deployment of the 2000 m trap. Inte-grated POC fluxes in sediment traps (300 mg m�2 yr�1

[Pollard et al., 2009]) are still �2.3 times higher thancore top 230Thxs

0 normalized POC accumulation rates(120 mg m�2 yr�1) because significant remineralization ofOC takes place between deep waters and sediments, andburial efficiency is generally between 10 and 20% [Burdige,2007].[42] The integrated Baxs flux collected by sediment traps

(7.5 mg m�2 yr�1), however, represents only half of thevertical preserved flux recorded in sediment core tops(18 mg m�2 yr�1) and a smaller proportion of the dissolu-tion corrected core tops fluxes (36 mg m�2 yr�1). The coretop Baxs inventory, which represents the integrated signalover several millennia is consistent with the assessment ofPollard et al. [2009] that the +Fe trap deployment missedabout half of the deep export flux.[43] Within the �Fe region only one sediment trap cup

was analyzed (10–31 January 2005; Julian day 20). Theremaining cups were either contaminated by fish or had toolittle mass flux to be processed. This single sample repre-sents the major mass and POC flux event at this site [Pollardet al., 2009] and Pnew estimates using Sarnthein et al. [1988]

algorithm agree with 234Th export estimates [Pollard et al.,2009]. This suggests that the single trap captured themajority of the annual flux of carbon at the �Fe site. POCfluxes at 3000 m (85 mg m�2 yr�1 [Pollard et al., 2009])are similar to preserved vertical core top POC fluxes(60 mg m�2 yr�1). As stated above, remineralization takesplace between sinking and burial and thus, the similar sed-iment trap and core top POC fluxes suggest that the 2004/05export event must be lower than the mean POC fluxes overthe late Holocene. The sediment trap Baxs flux only con-stitutes a tenth (1.28 mg m�2 yr�1) of the vertical pre-served Baxs flux measured in the sediment core top(15.4 mg m�2 yr�1) and is 24 times lower than dissolutioncorrected core top flux (31 mg m�2 yr�1). It is possible thatsignificant Baxs flux was present in the low flux cups and itappears that the timing of the export of these two phases(POC and Baxs) is decoupled. This is discussed moreextensively in section 4.3.2. The mismatch between the Baxsflux recorded by our single trap sample and preserved Baxsfluxes in the �Fe sediment core agree with Holocene meanconditions being different to the annual 2004–2004 flux assuggested above (from POC data).4.3.2. Proxy Assessment[44] The application of the Baxs proxy is based on the

observation that OC and Baxs export are related according toan empirically derived relationship [e.g., Francois et al.,1995]. In the Crozet region, the OC/Baxs ratio of trap

Table 7. Inferred and Measured Carbon Fluxes Around the CrozetPlateaua

Export Fluxes (g C m�2 yr�1) +Fe (Fertilized) �Fe (HNLC)

234Th at 100 m [Morris et al., 2007;Pollard et al., 2009]

11.5–15 3.4–4.8

Pnew predicted by Sarnthein et al.[1988] equation. Sediment traps

5.7 3.4

Sediment traps 2.2 0.11Dissolution-corrected vertical

rain rates12.1–13.1 9.2–10

Preserved vertical rain rates 4.4–4.5 3.3–3.4

aThe last three rows represent carbon export derived from integratedsediment trap Baxs fluxes, sedimentary Baxs dissolution-corrected verticalrain rates, and preserved vertical rain rates using the Francois et al.[1995] algorithm.

Figure 9. OC/Baxs ratios for sediment trap material (opencircles). The gray line represents the Francois et al. [1995]OC/Baxs trend.


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material collected north (M10) and east (M5) of the Plateauunder the influence of Fe fertilization (+Fe [Pollard et al.,2009]) fall on the Francois et al. [1995] trend for deeptraps (Figure 9), suggesting that this algorithm is appropriatefor the setting. The OC/Baxs ratio observed in the singlesouthern (M6; �Fe) trap sample is anomalously high sug-gesting that this trap sample is affected by alternative pro-cesses during export. POM export in the �Fe area occurs asa single pulse [Pollard et al., 2009], which might be fastenough to minimize the synthesis of biogenic barite(reflected by the low Baxs content in sediment traps at the�Fe area), resulting in anomalously high Corg/Baxs ratios ashas been previously suggested by Dehairs et al. [2000]. Newproduction (e.g., carbon export) has been calculated usingthe Francois et al. [1995] algorithm:

Pnew ¼ 1:95 FBaxsð Þ1:41 ð3Þ

Pnew represents new production (e.g., carbon export;gC m�2 yr�1) and FBaxs is the dissolution corrected verticalflux of Baxs to the seafloor (mg cm�2 kyr�1). We estimateexport from dissolution corrected accumulation rates, as it isstandard treatment when estimating carbon export fromsedimentary barite [Fagel et al., 2002]. When correcting fordissolution, it is assumed that barite has gone through thebenthic processing cycle. Calculated Pnew values are com-pared with carbon export at 100 m estimated from 234Thmeasurements across the Crozet region [Morris et al., 2007;Pollard et al., 2009] in Table 6.[45] Although the OC/Baxs ratios from sediment trap

material collected at the +Fe site in this study suggest thatthe Francois et al. [1995] algorithm is appropriate, estimatedexport from sediment traps underestimates the integrated234Th based export by an order of magnitude (Table 6). Thismost probably reflects under sampling of the seasonal fluxas suggested by Pollard et al. [2009] and in section 4.3.1.Pnew (= export) estimated from core tops using the Francoiset al. [1995] algorithm (12.1–13.1 g C m�2 yr�1) repro-duces the 100m 234Th-estimated export production (7.5–15 g C m�2 yr�1) [Pollard et al., 2009] (Table 6). Thus,under nutrient replete conditions and once sediment focus-ing and dissolution is corrected the proxy reproduces theestimated export quantitatively. In the �Fe region, carbonexport estimates from sediment Baxs (9.8–10.6 g Cm�2 yr�1)are two to three times higher than those from 234Th estimates(3.4–4.8 g C m�2 yr�1) [Pollard et al., 2009]. Thus, at the�Fe area, the proxy does not reproduce export estimated bythe 234Th method. This likely reflects the 2004/2005 exportevent being different to mean carbon export fluxes over thelate Holocene as suggested in sections 4.3.1 and 4.3.3.[46] The Francois et al. [1995] algorithm estimates Pnew at

the base of the euphotic zone. This depth varies dependingon the location and season at which sampling takes place.In contrast, export estimates using the 234Th method[Pollard et al., 2009] are determined at 100 m water depth(Table 7). Seeyave et al. [2007] measured the photosyn-thetic available radiation (PAR) around Crozet and deter-mined the depth of the euphotic layer (delimited by the 1%light depth) to be �80 m in the –Fe region and �60 m inthe +Fe region during austral summer 2004/2005 but thesevalues are inferred to be extremely variable with space andtime in this region. Under these circumstances export

estimates (at 100 m) are appropriate and comparable withother estimates.4.3.3. Assessment of Dissolution Correctionto Sediment Accumulation of Baxs[47] Accurate determination of sedimentary MARs is dif-

ficult in the �Fe area due to the presence of tephra inputsand slumping [Marsh et al., 2007] and estimates of MAR areonly accurate in the +Fe area, where linear sediment accu-mulation is determined by 14C data. Thus, we used the samedissolution factor for both the �Fe and +Fe areas, assumingthe MAR in the �Fe area is the same as in the +Fe area(2 g cm�2 ky�1). However, export production at the �Feregion is 3 times lower [Pollard et al., 2009] and lowerMARs are expected under these conditions, resulting inmore extensive dissolution of Baxs. In order to test thesensitivity of this assumption, we estimate dissolution fac-tors using MARs four times higher (20 cm�2 ky�1) andlower (0.2 g cm�2 ky�1) than within the +Fe region. Theseproduce dissolution factors only 1.7 times higher and1.4 times lower respectively. This indicates that the disso-lution correction is not very sensitive to variable MARs andshows that the assumed MARs do not generate significantuncertainty in the estimated Baxs fluxes.[48] Baxs accumulation within sediments will be compro-

mised if there is active sulphate reduction occurring inCrozet sediments. There is no evidence for sulphate reduc-tion from solid phase [Marsh et al., 2007] or pore water[Homoky et al., 2009] data. The cores are suboxic below thesurface mixed layer with evidence for Fe and Mn cycling[Homoky et al., 2009].[49] Dissolution corrected core top Baxs fluxes are com-

pared with data for the Indian Sector of the Southern Ocean[Fagel et al., 2002] in Figure 6a to assess the impact of theFe input from the Crozet islands by comparison with moretypical HNLC environments. Most of the sector exhibits lowBaxs accumulation and enhanced Baxs accumulation isobserved both at the +Fe and �Fe regions (Figure 6a).Similar enhancement of Baxs accumulation is observed southof 50°S at SAF sites [Fagel et al., 2002] and a site under-lying the Fe fertilized Kerguelen Plateau (55°S) [Francoiset al., 1997]. This suggests that the �Fe area experiencesenhanced export production relative to other PFZ sites andsimilar to the +Fe area for significant periods during theHolocene despite being an HNLC area over the period ofsatellite observations (1978 – present).[50] Waters to the north of the Plateau have been in con-

tact with the Islands or the Plateau and thus are likely to havehigh Fe content [Planquette et al., 2007]. In contrast, waterslocated in the �Fe area are largely free of the island’sinfluence [Pollard et al., 2007b]; this water is expected tohave low iron concentrations as it has not been in contactwith the Plateau [Planquette et al., 2007]. Thus, it is sur-prising to see similar Baxs accumulation both north andsouth of the Plateau. Traditionally, it has been assumed thatthe ACC comprises three strong permanent fronts [Orsiet al., 1995]. However, based on satellite images, it seemsthat the ACC is composed of filamentary structures thatcoalesce and shift over time [Dong et al., 2006; Sokolov andRintoul, 2007]. These jets are highly dynamic on timescalesof weeks to months [Thompson, 2010]. There is evidence forthis dynamic behavior around the Crozet Plateau, with highFe content waters bathing the south of the Plateau on


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occasions [Pollard et al., 2007b]. Thus, the enhanced exportproduction inferred from sediment proxies in the �Fe regionsuggests that the current HNLC conditions may not haveconsistently occurred throughout the Holocene and differentwater masses or jets could have bathed this area over time.[51] Similar Baxs accumulation rates at the +Fe and –Fe

areas could alternatively arise as a result of uncertaintiesassociated with (1) the effect of the lithogenic correction or(2) the effect of 230Th normalization in the –Fe area whichis subject to more significant downslope turbidite flows.The effect of the lithogenic correction has been assessed(section 4.1) and it is does not lead to significant uncertaintyin the Baxs proxy in this setting. The effect of significantdownslope transport at the –Fe site, however, could biasBaxs accumulation rates. However, significant downslopeinputs (70 to 90% of total sediment accumulation; Figure 8)is needed to generate a 2–3 fold error in the Baxs accumu-lation rates. Such a high downslope transport is unlikely aswe have carefully avoided sampling ash and turbidite layers.[52] To further evaluate and verify the conclusions

reached from the Baxs proxy we have measured the core top,age corrected 231Paxs/

230Thxs ratio. This proxy has been usedextensively to trace changes in productivity in Southern Oceansettings [Lao et al., 1992; Francois et al., 1993; Kumar et al.,1993]; it is not impacted by downslope transport and/oraccumulation rate/preservation artifacts. 231Paxs/

230Thxs ratiosare similar at the +Fe and �Fe areas over the Holocene andconsistent with OC, Baxs and bSi mass accumulation fluxes inthat the Crozet region supports significantly enhanced exportproduction compared with the HNLC Indian sector(Figures 6a–6d). Enhanced preserved fluxes observed at the�Fe region are inferred to be the result of enhanced exportsouth of the Plateau over the Holocene that must arise fromsoutherly transport of Fe fertilized export production for sig-nificant periods of the late Holocene. Thus, the impact ofvolcanic island fertilization of HNLC regions is variable on arange of spatial and temporal scales ranging from seasonal tomillennia and the impact on the sedimentary record can besignificant.

5. Conclusions

[53] Comparison of Baxs and Babar content of sedimentsallows evaluation of the extent that Baxs is representative ofbiogenic barite in Southern Ocean volcanoclastic sediments.In this volcanogenic, siliceous setting from the SouthernOcean PFZ, Baxs content generally accounts well for bio-genic barite in sediments if the lithogenic end-member iswell constrained by analysis of the dominant lithogenic inputto the region.[54] Our data for the upper water column show that Baxs

concentrations are extremely variable with space, time anddepth and suggest that this is not the result of precipitationfrom Ba-enriched celestite and rather occurs due to decayingorganic aggregates. Baxs fluxes collected by deep sedimenttraps correlate with OC fluxes within the +Fe area and seemto be decoupled either in space or time in the �Fe area southof the Plateau.[55] The OC/Baxs ratio in sediment trap material at the +Fe

area fall on the Francois et al. [1995] trend, and this algo-rithm seems (a priori) to be appropriate to quantify carbonexport in this setting. In addition, carbon export estimated

from dissolution corrected vertical Baxs fluxes quantitativelyreproduces export estimated from seasonally integrated234Th export in the +Fe region. The overestimation of carbonexport within the �Fe region likely results from differencesin sediment accumulation of carbon export over the Holo-cene. Second order compromises to the Baxs proxy includethe lack of a framework to define depth of export anddecoupling of POC and Baxs export (�Fe). However, thedistribution of the core top Baxs proxy across the Indiansector of the Southern Ocean is consistent with 231Paxs/230Thxs ratios for Holocene sediments (that are independentof dissolution, variable sediment accumulation), suggestingBaxs is a good export proxy in this Southern Ocean setting,and therefore, that the Baxs proxy provides a long-termintegrated assessment of export in areas where paleoceano-graphic reconstruction is hindered by high volcanic input,sediment redistribution and suboxic diagenesis.[56] Holocene Baxs fluxes are enhanced in the +Fe and

�Fe regions relative to other PFZ sites. These data, coupledwith other proxy data, suggest that (1) export production hasbeen enhanced within the +Fe region during the Holoceneand that (2) the present HNLC conditions may not haveprevailed throughout the Holocene south of the Plateau(�Fe) due to a dynamic behavior of the ACC resulting indifferent water masses/jets (possibly Fe enriched) bathingthis site over time. Our data indicate that the impact of Fefertilization on the HNLC Southern Ocean can besignificant.

[57] Acknowledgments. The authors would like to thank DarrylGreen and Matt Cooper for analytical support at NOC and for GideonHenderson and Alex Thomas for supporting U-series measurements atthe University of Oxford. We also gratefully acknowledge the NERCfunded Crozet natural Iron bloom Export Experiment (CROZEX) andBenthic Crozet Programme (NER/A/S/2003/00576) for the collection ofsamples. Additionally we would like to thank NERC for funding radiocar-bon analyses (SGM/1255.1007). We also thank Raymond Pollard and thescientists, officers, crew and technical support on RRS Discovery CruisesD285, D286 and D300 and the EU for funding a Ph.D. studentship (MTH)via the sixth Framework (FP6) Marie Curie Actions (MEST-CT-2004-514262). Finally, we are particularly grateful to William Homoky for car-rying out ICP-MS analysis, Hugh Venables for chl-a estimates and MarcusBadger for software support.

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Centre, Southampton, University of Southampton, European Way,Southampton SO14 3ZH, UK.M. T. Hernandez-Sanchez and R. D. Pancost, Organic Geochemistry Unit,

Bristol Biogeochemistry Research Centre, School of Chemistry, University ofBristol, Bristol BS8 1TS, UK. ([email protected])I. Salter, Observatoire Océanologique de Banyuls-sur-mer, Université

Pierre et Marie Curie, CNRS-INSU, UMR, Paris F-7621, France.


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Quantifying export production in the Southern Ocean: Implications for the Ba xs proxy

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