Variations in the strontium isotope composition of seawaterduring the Paleocene and early Eocene from ODP Leg 208(Walvis Ridge)

David A. Hodell and George D. KamenovDepartment of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, Florida 32611, USA(hodell@ufl.edu)

Ed C. HathorneDepartment of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK

Now at DFG Research Center for Ocean Margins (RCOM), Bremen University, Leobener Strasse, D-28359 Bremen,Germany

James C. ZachosEarth Sciences Department, University of California, Santa Cruz, Earth and Marine Sciences Building, Santa Cruz,California 95064, USA

Ursula Rohl and Thomas WesterholdCenter for Marine Environmental Sciences (MARUM), Bremen University, Leobener Strasse, D-28359 Bremen,Germany

[1] We refined the strontium isotope seawater curve for the Paleocene and early Eocene by analysis ofsamples recovered from the Walvis Ridge during Ocean Drilling Project (ODP) Leg 208. The highest87Sr/86Sr values occurred in the earliest Paleocene at �65 Ma and generally decreased throughout thePaleocene, reaching minimum values between 53 and 51 Ma in the early Eocene before beginning toincrease again at �50 Ma. A plausible explanation for the 87Sr/86Sr decrease between 65 and 51 Ma isincreased rates of hydrothermal activity and/or the eruption and weathering of large igneous provinces(e.g., Deccan Traps and North Atlantic). Strontium isotope variations closely parallel sea level and benthicd18O changes during the late Paleocene and early Eocene, supporting previous studies linking tectonicreorganization and increased volcanism to high sea level, high CO2, and warm global temperatures.

Components: 8671 words, 9 figures, 3 tables.

Keywords: strontium isotopes; Paleogene; volcanism.

Index Terms: 4912 Paleoceanography: Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805);

1030 Geochemistry: Geochemical cycles (0330); 4948 Paleoceanography: Paleocene/Eocene thermal maximum.

Received 13 February 2007; Revised 5 June 2007; Accepted 27 June 2007; Published 7 September 2007.

Hodell, D. A., G. D. Kamenov, E. C. Hathorne, J. C. Zachos, U. Rohl, and T. Westerhold (2007), Variations in the strontium

isotope composition of seawater during the Paleocene and early Eocene from ODP Leg 208 (Walvis Ridge), Geochem.

Geophys. Geosyst., 8, Q09001, doi:10.1029/2007GC001607.

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1. Introduction

[2] The history of seawater 87Sr/86Sr variation iswell known for the Oligocene and Neogene whenmarine 87Sr/86Sr was increasing at a relativelyrapid rate. Comparably less effort has been devotedto developing a high-resolution 87Sr/86Sr record forthe Paleocene and Eocene because the rate of87Sr/86Sr change was low and therefore of limitedstratigraphic value. Strontium isotope data areespecially sparse between 45 and 64 Ma [McArthuret al., 2001]. In an effort to refine the 87Sr/86Srseawater curve for this period, we measured sam-ples from the continuous Paleocene to middleEocene sections recovered during ODP Leg 208to the Walvis Ridge (Figure 1).

[3] The strontium isotope composition of seawateris controlled by variations in the flux and isotopicratio of Sr to the ocean from continental weathering,hydrothermal circulation, and carbonate dissolu-tion. Consequently, the strontium isotope sea-water curve has been used to infer changes inglobal tectonics related to orogenic uplift andrifting [Brass, 1976]. The late Paleocene-earlyEocene includes some of the lowest 87Sr/86Srvalues of the Cenozoic and may be significantgeochemically because one possible cause for thelow values is increased hydrothermal activity and/or weathering of volcanic rocks. The early Paleo-gene was a time noted for increased seafloorhydrothermal activity [Owen and Rea, 1985; Reaet al., 1990], formation of the North Atlanticigneous province [Thomas and Shackleton,1996], and increased explosive volcanism in theCaribbean [Bralower et al., 1997a]. Furthermore, ithas been proposed that metamorphic and volcanicoutgassing of CO2 may have been an importantsource to the atmosphere during the late Paleoceneand earliest Eocene (�60 to 52 Ma). Boron-basedpH reconstructions suggest that pCO2 concentra-tions were more than 2,000 ppm [Pearson andPalmer, 2000]. High atmospheric CO2 contributedto an enhanced greenhouse effect resulting in thewarmest temperatures of the Cenozoic during theearly Eocene climatic optimum (EECO) [Zachos etal., 2001].

2. Chronology

[4] During Ocean Drilling Program Leg 208, sixsites were drilled at water depths between 2500 and4770 m to recover complete Paleogene sequenceson the northeastern flank ofWalvis Ridge (Figure 1)[Zachos et al., 2004]. One of the main objectives

was to establish a cyclostratigraphy for the upperMaastrichtian through lower Eocene because thisperiod lacked an orbitally tuned timescale. Toobtain samples from the best-dated Paleogenesections possible for strontium isotope analysis,we identified a composite section made up ofseveral Leg 208 sites spanning the time from�65 to 45 Ma (Table 1). These sections weredetermined to be the most promising for develop-ing a Paleogene cyclostratigraphy for their respec-tive time intervals.

[5] The site-to-site shifts in the composite sectionoccur at easily recognizable events in the sedimentrecord (Table 1). Floating orbital timescales weredeveloped for the Paleocene and most of the earlyEocene up to 53.2 Ma for Site 1262 and 1267(Rohl et al. [2006] and U. Rohl et al. (manuscriptin preparation, 2007) for the Paleocene – EoceneThermal Maximum (PETM); Westerhold et al.[2007] for the C24r/C25n interval; T. Westerholdet al. (The first comprehensive orbital chronologyfor the Paleocene: Implications for the Geomag-netic Polarity Time Scale and the age of the K/Pgboundary, submitted to Palaeogeography, Palae-oclimatology, Palaeoecology, 2007) for the Paleo-cene). Ages from 55 through 65 Ma were orbitallytuned and tied to an assumed age of 55 Ma for thePaleocene-Eocene Thermal Maximum [Zachos etal., 2005]. For Sites 1265 and 1263, we relied uponthe shipboard age-depth models for the intervalyounger than 53 Ma [Zachos et al., 2004]. Giventhe long residence time of strontium in the oceans(�3 Ma), the relatively minor adjustments tosample ages that will result from orbital tuningand future definition of absolute ages will notaffect the conclusions of this paper.

3. Analytical Methods

[6] Mixed species of planktonic foraminifera werepicked from the >150 mm fraction of the sediment.Foraminifer test chambers were gently crushed andfine-grained carbonate and clay removed by ultra-sonic cleaning in deionized water. Samples weredissolved in 1–2 mL of 1M HCl, centrifuged, andthe supernatant was evaporated to dryness underlaminar flow conditions. Samples were then dis-solved in 100 ml of 3.5 M HNO3, and loaded on tocation exchange columns consisting of strontium-selective crown ether resin (Eichrom Technologies,Inc.) to separate Sr from other ions [Pin andBassin, 1992]. Following Sr separation, all forami-niferal samples were loaded onto degassed tung-sten filaments and 87Sr/86Sr was measured with a

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Micromass Sector 54 thermal ionization massspectrometer in the Department of GeologicalSciences at the University of Florida (UF)(Table 2). Each sample was run for 200 ratios at1.5 V whenever possible, and the isotope ratioswere normalized to 86Sr/88Sr = 0.1194. The ana-lytical precision of the 87Sr/86Sr measurement inmultidynamic mode is ±0.00001 (internal preci-sion) and repeated analysis of NBS-987 yielded amean 87Sr/86Sr ratio of 0.710240 with a 2s uncer-tainty of ±0.000015 (external precision). Porewater samples were also eluted through Sr-selectiveresin and 87Sr/86Sr was measured using a Nu

Plasma MC-ICP-MS by time resolved analysis[Kamenov et al., 2006] (Table 3).

4. Results

4.1. Foraminifera

[7] Because of the composite nature of the record,results are presented versus depth (Figure 2) intheir respective Leg 208 sites and time (Figure 3)using cyclostratigraphic and shipboard age models.The highest 87Sr/86Sr values (�0.70792) of thePaleocene and Eocene occur at the base of the

Figure 1. Site locations versus water depth for ODP Leg 208 on the Walvis Ridge [Zachos et al., 2004].

Table 1. Cyclostratigraphic Composite Section From ODP Leg 208 Sites Used for Strontium Isotope Analysisa

Event Marker Approximate Age, Ma Site Water Depth, m Top, mcd Bottom, mcd

K/Pg to PETM 65 to 55 1262 4755 140 217PETM to Elmo 55 to 53.2 1267 4355 205 232Elmo to X event 53.2 to 52.2 1265 3060 257 278X event to ‘‘50 Ma’’ 52.2 to 50 1263 2717 235 27550 Ma to emEocene 50 to 45 1265 3060 227 235

aThe site-to-site shifts in the composite section occur at easily recognizable events in the sediment record: K/Pg, Cretaceous/Paleogene boundary

(�65 Ma); PETM, Paleocene Eocene Thermal Maximum (�55 Ma, same as EETM1) [Zachos et al., 2005]; Elmo horizon (�53.2 Ma; same asETM2) [Lourens et al., 2005]; X event (same as ETM3); 50 Ma, 50 million years before present; emEocene, early middle Eocene boundary; mcd,meters composite depth.

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Table 2. Strontium Isotope Results of Foraminifera From Leg 208 Sites

Leg Site Hole Core Type SectionInterval(Top)

Interval(Bottom)

Depth,mbsf

Depth,mcd

Age,Ma 87Sr/86Sr

Error(% 1s) 86Sr/88Sr

%CaCO3

208 1265 C 2 H 5 135 137 201.85 227.50 47.08 0.707827 0.0007 0.1204 93.9208 1265 C 2 H 6 35 37 202.35 228.00 47.34 0.707842 0.0009 0.1201 94.9208 1265 C 2 H 6 85 87 202.85 228.50 47.58 0.707803 0.0008 0.1198 94.9208 1265 C 2 H 7 35 37 203.35 229.00 47.82 0.707794 0.0008 0.1205 92.6208 1265 A 22 H 1 142.5 144.5 200.93 229.51 48.06 0.707835 0.0010 0.1192 95.7208 1265 A 22 H 2 42.5 44.5 201.43 230.01 48.28 0.707797 0.0009 0.1201 95.0208 1265 A 22 H 2 92.5 94.5 201.93 230.51 48.49 0.707822 0.0008 0.1201 95.1208 1265 A 22 H 2 142.5 144.5 202.43 231.01 48.69 0.707823 0.0009 0.1200 94.1208 1265 A 22 H 3 92.5 94.5 203.43 232.01 49.07 0.707810 0.0008 0.1201 93.2208 1265 A 22 H 3 142.5 144.5 203.93 232.51 49.24 0.707813 0.0009 0.1204 93.1208 1265 A 22 H 4 142.5 144.5 205.43 234.01 49.70 0.707798 0.0008 0.1202 95.3208 1265 B 23 H 1 55 57 204.75 234.50 49.83 0.707817 0.0013 0.1204 93.8208 1263 B 19 H 1 140 142 202.40 235.00 49.94 0.707770 0.0008 0.1201 65.1208 1263 A 23 H 3 67.5 69.5 205.46 238.01 50.24 0.707759 0.0008 0.1204 94.1208 1263 A 24 H 5 125 127 215.35 250.00 51.15 0.707738 0.0009 0.1203 94.4208 1263 A 24 H 6 125 127 216.85 251.50 51.22 0.707733 0.0008 0.1197 88.4208 1263 B 21 H 3 117.5 119.5 218.57 254.49 51.35 0.707728 0.0007 0.1204 93.1208 1263 C 7 H 3 30 32 224.75 259.02 51.55 0.707719 0.0010 0.1205 94.7208 1263 A 26 H 3 105 107 226.65 263.50 51.76 0.707725 0.0008 0.1199 94.7208 1263 A 26 H 6 105 107 231.15 268.00 51.99 0.707722 0.0009 0.1206 94.2208 1263 B 22 H 6 127.5 129.5 232.68 271.01 52.14 0.707710 0.0009 0.1203 95.7208 1265 A 24 H 4 105 107 224.05 257.01 52.28 0.707773 0.0014 0.1199 95.3208 1265 B 25 H 2 50 52 225.04 258.49 52.39 0.707742 0.0008 0.1202 95.1208 1265 B 25 H 5 50 52 229.54 262.99 52.68 0.707737 0.0013 0.1192 94.8208 1265 A 25 H 4 80 82 233.30 267.49 52.93 0.707722 0.0009 0.1199 94.0208 1265 A 25 H 5 80 82 234.80 268.99 53.00 0.707775 0.0009 0.1193 94.2208 1265 B 26 H 3 10 12 235.80 270.49 53.07 0.707741 0.0008 0.1199 93.6208 1267 A 20 H 3 105 107 183.95 205.00 53.195 0.707740 0.0008 0.1200 60.0208 1267 B 21 H 3 7.5 9.5 186.68 209.49 53.507 0.707730 0.0009 0.1204 93.4208 1267 B 21 H 7 7.5 9.5 192.68 215.49 53.879 0.707747 0.0008 0.1203 93.1208 1267 A 21 H 6 25 27 196.45 218.50 54.065 0.707760 0.0012 0.1194 90.2208 1267 B 22 H 2 110 112 195.70 220.01 54.156 0.707747 0.0010 0.1194 92.4208 1267 B 22 H 3 110 112 197.20 221.51 54.254 0.707749 0.0009 0.1199 89.4208 1267 B 22 H 4 110 112 198.70 223.01 54.352 0.707790 0.0013 0.1192 91.9208 1267 B 22 H 5 110 112 200.20 224.51 54.447 0.707799 0.0008 0.1197 93.2208 1267 B 22 H 6 110 112 201.70 226.01 54.529 0.707797 0.0008 0.1198 93.2208 1267 A 22 H 4 75 77 203.52 227.50 54.630 0.707761 0.0009 0.1205 96.5208 1267 A 22 H 5 75 77 205.02 229.00 54.752 0.707758 0.0009 0.1198 92.0208 1267 B 23 H 2 57 59 204.67 230.50 54.854 0.707800 0.0008 0.1194 88.6208 1267 B 23 H 3 144.5 146.5 206.15 231.98 55.022 0.707832 0.0008 0.1197 89.0208 1262 B 15 H 3 133 135 128.43 140.75 55.087 0.707830 0.0010 0.1198 88.5208 1262 B 15 H 5 60 62 130.70 143.02 55.319 0.707811 0.0009 0.1198 91.6208 1262 B 15 H 5 132.5 134.5 131.43 143.74 55.377 0.707756 0.0009 0.1195 91.2208 1262 B 15 H 6 57.5 59.5 132.18 144.49 55.436 0.707766 0.0009 0.1203 92.5208 1262 A 14 H 2 102.5 104.5 126.03 145.24 55.497 0.707765 0.0008 0.1201 91.4208 1262 A 14 H 3 30 32 126.80 146.02 55.559 0.707761 0.0010 0.1196 90.5208 1262 A 14 H 3 102.5 104.5 127.53 146.74 55.617 0.707768 0.0011 0.1196 93.1208 1262 A 14 H 4 27.5 29.5 128.27 147.49 55.676 0.707761 0.0010 0.1201 92.2208 1262 A 14 H 5 30 32 129.80 149.02 55.797 0.707758 0.0014 0.1192 90.8208 1262 A 14 H 5 102.5 104.5 130.52 149.74 55.855 0.707758 0.0011 0.1190 89.9208 1262 A 14 H 6 27.5 29.5 131.27 150.49 55.919 0.707765 0.0007 0.1204 89.4208 1262 B 16 H 3 147.5 149.5 138.38 151.24 55.980 0.707780 0.0008 0.1199 90.8208 1262 B 16 H 4 75 77 139.15 152.01 56.043 0.707820 0.0009 0.1194 87.8208 1262 B 16 H 4 147.5 149.5 139.88 152.74 56.103 0.707770 0.0013 0.1195 91.7208 1262 A 15 H 2 110 112 135.60 155.76 56.332 0.707780 0.0010 0.1195 88.6208 1262 A 15 H 3 35 37 136.35 156.51 56.388 0.707769 0.0009 0.1205 91.4208 1262 A 15 H 3 110 112 137.10 157.26 56.449 0.707779 0.0009 0.1195 86.5208 1262 A 15 H 4 110 112 138.60 158.76 56.609 0.707774 0.0008 0.1201 88.9208 1262 A 15 H 5 35 37 139.35 159.51 56.684 0.707780 0.0009 0.1197 88.1208 1262 B 17 H 3 22.5 24.5 146.63 160.24 56.754 0.707800 0.0008 0.1201 85.4208 1262 B 17 H 4 97.5 99.5 148.88 162.49 56.948 0.707806 0.0008 0.1201 92.1

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Paleocene, just above the Cretaceous/Paleogeneboundary that occurs at a depth of �216 meterscomposite depth (mcd) in Site 1262 (Figures 2and 3). Values decrease between 65 and 63 Maduring the early Paleocene and remain steady from63 to 58.6 Ma. During this interval, two samples at59.9 Ma (184.2 mcd) and 60 Ma (185.7 mcd) showhigh values at Site 1262 (Figure 3). Strontiumisotope values generally decrease from 58.6 to55.2 Ma, but a temporary increase straddles thePaleocene/Eocene boundary at �55 Ma (Figure 3).

[8] The high values begin below the P/E boundaryin the two uppermost samples at Site 1262 and thelowermost sample at Site 1267 (Figure 2). Severalelevated 87Sr/86Sr values also occur above theboundary at Site 1267. The lowest 87Sr/86Sr valuesoccur in the early Eocene between �53 and 51 Ma(Figure 3). This is followed by a trend towardincreasing values between 51 and 47 Ma.

4.2. Pore Water 87Sr/86Sr

[9] Pore water 87Sr/86Sr values are equal to modernseawater values (0.709172 measured at UF) nearthe surface at Site 1262 and decrease down core at

Sites 1262, 1267 and 1265, reaching minimumvalues of 0.70806 at �350 mcd (Figure 4). Porewater 87Sr/86Sr values overlap one another andfollow the same trend at Sites 1262 and 1267,whereas values at Site 1265 are lower than those atSites 1262 and 1267 for the same burial depth.

5. Discussion

5.1. Comparison With Other Records

[10] We compare our results with the most recentcompilation ofMcArthur andHowarth [2004] using‘‘Look-Up Table Version 4: 08/03’’ (Figure 5). TheMcArthur seawater curve for the Paleocene andearly Eocene is based upon data from Denison et al.[1993] and DePaolo and Ingram [1985]. Denisonet al. [1993] measured shell and whole-rock sam-ples from outcrops of shelf sediments exposed onthe U.S. Gulf Coastal Plain, whereas DePaolo andIngram [1985] reported results from Paleocene-Eocene macrofossils from France, the UnitedStates (Alabama) and Denmark.

[11] The trends in 87Sr/86Sr from Leg 208 aregenerally similar to the McArthur curve but the

Table 2. (continued)

Leg Site Hole Core Type SectionInterval(Top)

Interval(Bottom)

Depth,mbsf

Depth,mcd

Age,Ma 87Sr/86Sr

Error(% 1s) 86Sr/88Sr

%CaCO3

208 1262 A 16 H 1 130 132 143.80 164.76 57.131 0.707782 0.0008 0.1201 88.8208 1262 A 16 H 2 127.5 129.5 145.27 166.24 57.276 0.707784 0.0009 0.1202 86.8208 1262 A 16 H 4 55 57 147.55 168.51 57.620 0.707798 0.0008 0.1203 86.2208 1262 B 18 H 5 50 52 159.40 174.49 58.607 0.707831 0.0023 0.1201 82.1208 1262 A 17 H 3 135 137 156.35 176.74 58.993 0.707818 0.0008 0.1201 74.3208 1262 A 17 H 4 135 137 157.85 178.24 59.216 0.707812 0.0009 0.1193 79.7208 1262 B 19 H 2 50 52 164.40 180.49 59.489 0.707808 0.0008 0.1197 88.4208 1262 B 19 H 2 125 127 165.15 181.24 59.582 0.707805 0.0024 0.1200 85.0208 1262 B 19 H 3 50 52 165.90 181.99 59.666 0.707847 0.0011 0.1197 84.8208 1262 B 19 H 3 125 127 166.65 182.74 59.738 0.707833 0.0012 0.1192 84.0208 1262 B 19 H 4 125 127 168.15 184.24 59.883 0.707880 0.0009 0.1193 87.4208 1262 C 10 H 2 131 133 170.81 185.73 60.044 0.707893 0.0010 0.1195 83.5208 1262 C 10 H 3 56.5 58.5 171.57 186.49 60.159 0.707836 0.0008 0.1204 84.7208 1262 C 10 H 4 59.5 61.5 173.10 188.01 60.389 0.707841 0.0009 0.1201 81.9208 1262 B 20 H 2 101 103 174.41 191.74 60.832 0.707830 0.0008 0.1205 84.1208 1262 B 20 H 4 27 29 176.67 194.00 61.092 0.707836 0.0008 0.1202 83.4208 1262 B 20 H 5 27 29 178.17 195.50 61.270 0.707829 0.0009 0.1201 69.6208 1262 C 11 H 2 72 74 179.72 196.24 61.363 0.707834 0.0010 0.1198 77.5208 1262 C 11 H 2 146.5 148.5 180.46 196.99 61.456 0.707816 0.0009 0.1195 76.9208 1262 C 11 H 3 147 149 181.97 198.49 61.646 0.707829 0.0008 0.1204 75.4208 1262 B 21 H 1 88.5 90.5 182.29 202.23 62.163 0.707837 0.0009 0.1197 73.1208 1262 B 21 H 2 17.5 19.5 183.07 203.01 62.262 0.707835 0.0009 0.1196 72.8208 1262 C 12 H 2 96 98 186.46 205.24 62.691 0.707838 0.0010 0.1203 73.8208 1262 C 12 H 4 96.5 98.5 189.46 208.24 63.256 0.707851 0.0011 0.1199 77.1208 1262 C 12 H 6 96 98 192.46 211.24 63.729 0.707925 0.0011 0.1193 55.5208 1262 B 22 H 1 12 14 191.02 212.01 63.837 0.707875 0.0010 0.1203 76.7208 1262 B 22 H 2 60 62 192.50 213.49 64.113 0.707897 0.0008 0.1201 54.1208 1262 B 22 H 3 134.5 136.5 194.01 214.99 64.412 0.707928 0.0007 0.1200 48.4

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Table 3. Strontium Isotope Results of Leg 208 Pore Water Samples

Leg Site Hole Core Type SectionInterval(Top)

Interval(Bottom)

Depth,mbsf

Depth,mcd 87Sr/86Sr

Error(% 1s)

208 1265 B 19 H 5 140 150 173.6 196.08 0.70830 0.00106208 1265 B 20 H 4 140 150 181.6 206.19 0.70830 0.00078208 1265 A 21 H 5 140 150 197.4 223.05 0.70829 0.00085208 1265 A 22 H 5 140 150 206.9 235.48 0.70828 0.00099208 1265 A 23 H 4 140 150 214.9 245.38 0.70827 0.00113208 1265 B 25 H 5 140 150 230.44 263.89 0.70824 0.00106208 1265 B 26 H 4 140 150 238.6 273.29 0.70823 0.00120208 1265 A 35 X 3 140 150 306.1 351.88 0.70806 0.00099208 1267 A 16 H 4 140 150 147.8 164.2 0.70868 0.00099208 1267 A 17 H 4 140 150 157.3 174.11 0.70860 0.00113208 1267 A 18 H 5 140 150 168.3 186.47 0.70862 0.00078208 1267 A 19 H 5 140 150 177.36 196.83 0.70856 0.00099208 1267 A 23 H 5 140 150 215.8 240.72 0.70843 0.00092208 1267 A 25 H 5 140 150 234 264.73 0.70835 0.00106208 1267 A 26 X 5 140 150 243.5 275.23 0.70837 0.00106208 1267 A 27 X 4 140 150 250.8 284.03 0.70835 0.00092208 1267 A 28 X 3 140 150 258.9 291.7 0.70832 0.00085208 1267 A 32 X 4 140 150 298.8 334.81 0.70824 0.00106208 1267 A 33 X 5 140 150 309.88 346.83 0.70821 0.00106208 1262 B 1 H 3 145 150 4.45 4.45 0.70917 0.00092208 1262 A 1 H 5 145 150 7.45 13.49 0.70908 0.00099208 1262 A 2 H 5 142 147 16.92 16.92 0.70902 0.00078208 1262 A 5 H 5 145 150 45.45 54.41 0.70894 0.00085208 1262 A 6 H 5 140 150 54.9 65.65 0.70892 0.00078208 1262 A 9 H 5 145 150 83.45 98.66 0.70879 0.00085208 1262 B 11 H 5 145 150 93.85 103.43 0.70870 0.00092208 1262 A 11 H 5 140 150 102.4 118.45 0.70871 0.00092208 1262 A 14 H 5 140 150 130.9 150.12 0.70864 0.00078208 1262 A 15 H 5 140 150 140.4 160.56 0.70860 0.00169208 1262 A 16 H 5 140 150 149.9 170.86 0.70856 0.00120208 1262 B 19 H 3 140 150 166.8 182.89 0.70861 0.00078208 1262 C 11 H 5 140 150 184.9 201.42 0.70852 0.00078208 1262 C 12 H 5 140 150 191.4 203.68 0.70852 0.00099208 1262 B 23 H 5 140 150 207.8 230.98 0.70846 0.00113

Figure 2. Strontium isotope results from Leg 208 sites (black, 1262; green, 1267; red, 1265; blue, 1263) versusdepth (meters composite depth) in the respective sites. Records are arranged stratigraphically with the events taken asthe splice tie points indicated by arrows (Table 1).

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absolute values differ, especially for the periodsfrom �65 to 64 Ma and from 50 to 47 Ma. At thesetimes, 87Sr/86Sr values measured on Leg 208samples are significantly higher than the LOW-ESS-fit curve [Howarth and McArthur, 1997](Figure 5). Either the McArthur curve is too lowduring these periods or the 87Sr/86Sr values mea-sured on Leg 208 samples are too high. Given thescarcity and scatter of measurements used to definetheMcArthur seawater curve between 47 and 65Ma,it is difficult to choose between these options.

[12] We also compared our results to data fromHess et al. [1986] which were not used in theMcArthur compilation (Figure 6). Hess et al.[1986] used SEM, Sr/Ca, and pore water 87Sr/86Srto evaluate whether foraminifer strontium isotopevalues had been affected by diagenesis. Most ofthe Paleogene samples were unaltered or showedminor evidence of recrystallization. In the lattercase, pore water 87Sr/86Sr ratios were close tothose of coexisting foraminifera and minor diagen-esis should not significantly alter foraminiferal

Figure 3. Strontium isotope results from Leg 208 sites (black, 1262; green, 1267; red, 1265; blue, 1263) versus age(Ma).

Figure 4. Strontium isotope results of foraminifera (open symbols) and pore waters (closed symbols) and strontiumconcentration of interstitial waters at Leg 208 sites (black, 1262; blue, 1267; red, 1265) versus burial depth in meterscomposite depth (mcd). 87Sr/86Sr values of pore water are higher than 87Sr/86Sr of Paleocene-Eocene foraminiferameasured at the same sites.

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87Sr/86Sr ratios [Hess et al., 1986]. Our resultsfrom Leg 208 sites are in excellent agreement withthose of Hess et al. [1986], providing confidencethat the trends in 87Sr/86Sr accurately reflect varia-tions in the strontium isotopic composition ofseawater.

5.2. Diagenetic Alteration of PlanktonicForaminifera

[13] The burial depths of the Leg 208 sites used inthis study range from 140 to 280 m (Table 1).Partial recrystallization of foraminiferal tests ispossible at such depths. For example, Sexton etal. [2006a, 2006b] showed evidence of micron-scale secondary calcite, dissolution of primarycalcite and broad etching of pores in foraminiferaltests from DSDP Sites 522 and 523 on the northernflank of the Walvis Ridge. We found similarevidence of diagenetic alteration as that describedby Sexton et al. [2006a, 2006b] on the tests ofplanktonic foraminifera at Site 1267 (Figures 7and 8). Abundant neomorphic calcite occurs asrounded crystallites that form mainly on interporeridges, thereby obscuring the original surface walltexture (Figure 7). Diagenetic alteration is notlimited to the test surface and SEM images of

broken wall cross sections also show evidence ofweathering/dissolution and neomorphic replace-ment of calcite within the test interior (Figure 8).This type of diagenetic alteration of Paleogeneforaminifera is apparently ubiquitous in deep-seasediments for specimens that appear ‘‘frosty’’under reflected light [Sexton et al., 2006b].

5.3. The 87Sr/86Sr of Pore Water andSecondary Calcite

[14] The source of pore water Sr is the dissolutionof carbonate in the upper part of the sedimentcolumn and diffusion of this strontium into thepore waters of the sediments above and below thezone of maximum dissolution [e.g., Baker et al.,1982]. For Site 1262, pore water strontium con-centrations are low and suggest that some carbon-ate dissolution has occurred in the upper �75 mcdof the sediment column (Figure 4) [ShipboardScientific Party, 2004a]. At Site 1267, strontiumconcentrations are higher than Site 1262 and indi-cate carbonate dissolution between �50 and111 mcd [Shipboard Scientific Party, 2004b]. AtSite 1265, the strontium pore water profile indi-cates a source of strontium to the interstitial watersbelow 148.7 mcd and diffusion of this strontiuminto the sediments above [Shipboard ScientificParty, 2004c].

Figure 6. Comparison of 87Sr/86Sr values from Leg208 sites (gray crosses) with 87Sr/86Sr results of Hess etal. [1986] (blue squares). Sample ages were convertedto the GTS2004 timescale by comparison of the ages ofmagnetic reversal boundaries between the Berggren etal. [1985] and GTS2004 timescales [Ogg and Smith,2004]. 87Sr/86Sr values were corrected to NBS-987 =0.710240 by adding 0.00002 to the data of Hess et al.[1986].

Figure 5. Comparison of 87Sr/86Sr values from Leg208 sites (crosses with error bars) with the 87Sr/86Srseawater curve of McArthur and Howarth [2004] using‘‘Look-Up Table Version 4: 08/03’’ (bold gray line).From 45 to 65 Ma, the McArthur seawater curve isbased on 87Sr/86Sr results of Denison et al. [1993] (bluetriangles) and DePaolo and Ingram [1985] (redsquares). Bold red line represents a weighted curve fitthrough the lower 87Sr/86Sr values at Leg 208 sitesbecause of the potential for diagenesis to raise 87Sr/86Srvalues. All data are corrected to 87Sr/86Sr for NBS-987 =0.710240.

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[15] Because pore water Sr is derived from thedissolution of carbonate that is younger than theperiod studied, the 87Sr/86Sr values of interstitialwater are higher than ratios measured on Paleocene-Eocene foraminifera from the same sites (Figure 4).This implies that secondary calcite will likely havea greater 87Sr/86Sr ratio than the original forami-niferal test. As a result, secondary calcite over-growths will tend to raise the 87Sr/86Sr of alteredforaminifera above the Paleocene-Eocene seawatervalues.

[16] The presence of some secondary calcite doesnot necessarily mean that 87Sr/86Sr ratios have beenaltered significantly because the magnitude of thechange will depend upon the depth of diageneticalteration in the sediment column and the amountand Sr concentration of secondary calcite added. Forexample, if neomorphic recrystallization occurredearly in a near-closed system, then the secondarycalcite will have a similar 87Sr/86Sr as the originalforaminiferal shell. Numerical modeling indicatesthat carbonate diagenesis occurs predominantly atan early stage during shallow burial [Rudnicki etal., 2001]. In addition, the Sr content of recrystal-

lized calcite is significantly lower than originalforaminiferal calcite because the distribution coef-ficient is much less for inorganic than biogeniccalcite [Baker et al., 1982]. Agreement amongstrontium isotope records from many deep-sea sites(Figure 6) suggests that diagenesis has notobscured the major trends in the strontium isotopecomposition of seawater. In addition to diagenesis,leaching of strontium from clays, which may nothave been completely removed during vigoroussonic cleaning of foraminiferal tests, can alsopotentially raise the 87Sr/86Sr of samples abovePaleocene-Eocene seawater.

[17] To test whether clay-rich sediments yieldedhigher 87Sr/86Sr values compared to carbonate-richsediment, we measured weight percent CaCO3

(Table 2). The deepest Site 1262 has the lowest%CaCO3 values (as low as 50%) and is the only sitethat shows a significant negative correlation (r =�0.79) between 87Sr/86Sr and %CaCO3. The mostclay-rich samples occur just above the K/Pg bound-ary and give the highest 87Sr/86Sr values. Samplesat the other sites generally have %CaCO3 values

Figure 7. SEM images of surface wall texture of selected planktonic foraminifera from Hole 1267B. (a) Acarinasoldadoensis in sample 1267B-22H-4 110–112 cm. (b) A. soldadoensis in Sample 1267B-22H-3 110–112 cm.(c) Globigerina inequispira in Sample 1267B-23H-5 144.5–146.5 cm. (d) Morozovella subbotina in Sample 1267B-22H-5 110 cm. Scale bar is 10 m. Note evidence for abundant secondary calcite overgrowths mainly occurring on theinterpore ridges.

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greater than 85% and show no significant correla-tion with 87Sr/86Sr.

5.4. Paleocene-Eocene 87Sr/86Sr SeawaterCurve

[18] Because of the potential for both diagenesisand unremoved clay to raise the 87Sr/86Sr abovePaleocene-Eocene values for some samples, weapproximated the strontium isotope seawater curveby fitting a weighted line through the lower87Sr/86Sr values (Figure 5). A reasonable agree-ment is obtained with the long-term features of theMcArthur seawater curve [McArthur et al., 2001]

and excellent agreement is achieved with the dataof Hess et al. [1986] (Figure 6).

[19] In addition to long-term trends, several periodsof particularly high 87Sr/86Sr values (i.e., ‘‘spikes’’)are recorded in the Leg 208 data between 65 and45 Ma. These values are suspect because they arein the direction expected from diagenetic alterationor clay leaching and occur on timescales signifi-cantly shorter than the residence time of Sr in theoceans (�3 Ma). Two high 87Sr/86Sr values (withan intervening lower value) occur in Site 1262during the earliest Paleocene just above the K/Pgboundary (Figures 2 and 3). High values have been

Figure 8. SEM images of broken wall cross sections of selected planktic foraminifera from Hole 1267B. (a and b)A. soldadoensis in Sample 1267B-22H-3 110–112 cm. (c) Globigerina inequispira in Sample 1267B-22H-5 110–112 cm. (d) A. soldadoensis in Sample 1267B-22H-5 110–112 cm. (e and f) Morozovella subbotina in Sample1267B-22H-5 110 cm. (g) Globigerina inequispira in Sample 1267B-23H-2, 57–59 cm. (h) A. soldadoensis inSample 1267B-23H-2, 57–59 cm. Note the etched appearance of wall cross section and large (�1 m) neomorphiccalcite on broken wall surfaces and interpore ridges projecting above the test surface. Scale bar is 10 m.

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reported near the K/Pg boundary in other sections[Hess et al., 1986; Martin and Macdougall, 1991;Vonhof and Smit, 1997; MacLeod et al., 2001], butMcArthur et al. [1998] have argued these spikesresult from analytical and diagenetic effects. Twosamples with high 87Sr/86Sr also occur at 60 Ma inthe middle Paleocene that rise above neighboringsamples (Figure 3).

[20] A cluster of high 87Sr/86Sr values straddle thePaleocene-Eocene boundary at Sites 1262 and1267, but the increase in 87Sr/86Sr precedes theP/E boundary at both sites. At Site 1262, the twouppermost samples (140.75 and 143.02 mcd) mea-sured with elevated 87Sr/86Sr clearly occur belowthe PETM as defined by the abrupt decrease incarbonate content and bulk d13C at � 140.16 mcd[Zachos et al., 2005]. At Site 1267, the lowermostsample (231.98 mcd) with high 87Sr/86Sr also fallsbelow the P/E boundary at�231.54 mcd (Figure 2).Increased continental weathering has been associ-ated with the PETM on the basis of an increase inosmium isotopes [Ravizza et al., 2001] and a peakin kaolinite abundance [Kelly et al., 2005], butthese changes occur above the boundary whereasthe increase in 87Sr/86Sr values begins below.Furthermore, two relatively low 87Sr/86Sr values(229 and 227.5 mcd) at Site 1267 are interspersedwithin the cluster of high values near the PETM(Figure 2). Because the increase in 87Sr/86Sr valuesstraddling the PETM at Sites 1262 and 1267 maybe the result of diagenesis, determining whetherthis a genuine feature of the seawater curve mustawait high-resolution strontium isotope results overthe PETM in other sections [Griffith et al., 2006].

5.5. Causes of 87Sr/86Sr Variation

[21] Variations in the strontium isotope composi-tion of seawater are controlled by the relativeinputs of Sr to the ocean from continental weather-ing (rivers), hydrothermal circulation, and carbonatedissolution on the seafloor [Brass, 1976; Palmerand Edmond, 1989]. Continental silicate rocksexhibit high 87Sr/86Sr values (averaging 0.716)whereas mantle-derived volcanic rocks have low87Sr/86Sr values (averaging 0.704). Consequently,continental weathering generally acts to raise the87Sr/86Sr of seawater, whereas hydrothermalexchange and weathering of volcanic rocks gener-ally tend to lower it. The dissolution of carbonateson the seafloor acts as a buffer by adding old marinestrontium to the oceans whose 87Sr/86Sr is not verydifferent from seawater.

[22] The decrease in 87Sr/86Sr values during thePaleocene and early Eocene (�65 to 51 Ma) maybe explained by a decrease in the mass flux ofriverine Sr to the ocean, a decrease in the globalaverage riverine 87Sr/86Sr value, and/or an increasein the mass flux of hydrothermal strontium frommid-ocean ridges. Changes in the global averageriverine 87Sr/86Sr value are controlled mainlyby the relative 87Sr/86Sr of exposed rocks under-going weathering on the continents (e.g., ‘‘granitic’’versus ‘‘basaltic’’) [Brass, 1976] and global weath-ering kinetics [Li et al., 2007]. Strontium isotopedata alone do not allow distinction between thesepotential causes, but comparison of strontium iso-tope variation with other indicators of hydrother-mal, weathering, and climatic processes can helpconstrain plausible explanations [Lear et al., 2003].

[23] Several studies have proposed marked changesin volcanic and hydrothermal activity during theearly Paleogene [Eldholm and Thomas, 1993;Bralower et al., 1997a; Rea et al., 1990; Thomasand Bralower, 2005]. The emplacement of theDeccan Traps near the K/T boundary and theirsubsequent weathering may have contributed to thePaleocene 87Sr/86Sr decline between 65 and 55 Ma[Vonhof and Smit, 1997; Dessert et al., 2001;Ravizza and Peucker-Ehrenbrink, 2003; Das etal., 2006]. In addition, massive volcanism associ-ated with the emplacement and weathering of theNorth Atlantic Igneous Province (NAIP) may havealso contributed to decreasing 87Sr/86Sr values ofseawater. NAIP magmatism occurred in two dis-tinct phases. The first phase occurred in the middlePaleocene (62–58 Ma) when a large volume offlood basalts were erupted [Saunders et al., 1997;Knox, 1996, 1998]. The second phase started nearthe Paleocene-Eocene boundary and continued forca. 1.5 to 2 m.y. during the early Eocene (�56–53 Ma) [Saunders et al., 1997; Knox, 1996, 1998;Jolley et al., 2002].

[24] Rea et al. [1990] hypothesized enhanced sea-floor hydrothermal activity occurred during ChronC24R (53.81–56.67 Ma) associated with a majortectonic reorganization. Although spreading ratehas often been used to estimate the intensity ofmid-ocean ridge hydrothermal activity, tectonicreorganization, such as ridge jumps or changes inridge orientation, may substantially increasehydrothermal activity by fracturing oceanic crustand providing seawater access to deep-seated heatsources [Lyle et al., 1987]. Such processes candecrease the 87Sr/86Sr of seawater even in the

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absence of a large increase in spreading rate, whichmay not have varied significantly during the Ceno-zoic [Rowley, 2002; Cogne and Humler, 2006].

[25] Slightly greater rates of ocean crust productionduring the late Paleocene and early Eocene havebeen suggested as the cause for long-term sea levelvariations during this period [Miller et al., 2005].Rising sea level between 65 to 52 Ma has beenattributed to increases in ridge length, tectonicreorganization, and extrusion of 1–2 � 106 km3

of basalts of the Brito-Arctic province [Miller etal., 2005]. Alternatively, Cogne and Humler[2006] found that spreading and production ratesat ocean ridges have been relatively constant forthe past 180 Ma. If so, then sea level variationsmay be related to climate change rather than aresponse to changes in oceanic crustal production.Nonetheless, long-term changes in sea level and87Sr/86Sr variations closely parallel one anotherduring the Paleocene and early Eocene (Figure 9).From 65 to 52 Ma, there is a clear relationshipbetween rising sea level and decreasing strontiumisotope values. At �51–52 Ma, sea level begins tofall as 87Sr/86Sr starts to increase.

[26] We propose that the 87Sr/86Sr decrease between65 and 51 Ma was related to increased rates ofhydrothermal activity and/or the eruption andweathering of large igneous provinces (e.g., theDeccan Traps andNorth Atlantic Igneous Province).

The mechanism is similar to that invoked for partsof the Cretaceous when low 87Sr/86Sr values ofseawater have been linked to increased ocean crustproduction associated with the eruption of largeigneous provinces [Ingram et al., 1994; Braloweret al., 1997b; Jones and Jenkyns, 2001].

[27] Volcanic processes may have also affectedclimate through atmospheric CO2 feedbacks. Overlong timescales, atmospheric CO2 is controlled byvolcanic and metamorphic outgassing and con-sumption of CO2 during silicate weathering andorganic carbon burial [Berner et al., 1983; Bernerand Kothavala, 2001]. Mantle outgassing of CO2

during tectonic reorganization and the eruption ofthe NAIP has been proposed to explain the long-term warming trend that began at �59 Ma andpeaked at �51 Ma [Rea et al., 1990; Zachos et al.,2001; Thomas and Bralower, 2005; Miller et al.,2005].

[28] The high sea levels and low 87Sr/86Sr valuesof the late Paleocene and early Eocene are alsoassociated with the decreasing benthic foraminif-eral d18O values, indicating increasing deep-watertemperatures that culminated in the warmest periodof the Cenozoic during the EECO [Zachos et al.,2001]. Oxygen isotopes and 87Sr/86Sr closely par-allel one another from 59 to 52 Ma when bothrecords decrease reaching minimum values duringthe early Eocene when sea level is at a maximum

Figure 9. Comparison of Leg 208 87Sr/86Sr seawater curve (filled circles and blue line), long-term sea level (redline) [Miller et al., 2005], and benthic d18O compilation (green line) [Zachos et al., 2001]. Also shown are times oflarge igneous province emplacement and proposed increased hydrothermal activity. EECO, early Eocene climaticoptimum; NAIP-I and NAIP-II, two phases of eruption of the North Atlantic Igneous Province (62–58 Ma and 56–53 Ma); C24R, Chron 24r tectonic event (53.81–56.67 Ma).

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(Figure 9). This 1.5% decrease in d18O between 59and 52 Ma represents the most pronounced warm-ing trend of the Cenozoic [Zachos et al., 2001]. At�51 Ma, benthic d18O begins to increase indicat-ing cooling at the same time as 87Sr/86Sr starts toincrease and sea level begins to fall (Figure 9).

[29] The close correspondence between sea level,87Sr/86Sr, and benthic d18O changes during the latePaleocene and early Eocene is consistent withprevious studies relating tectonic reorganizationand increased volcanism to high sea level, highCO2, and warm global temperatures [Rea et al.,1990; Zachos et al., 2001; Thomas and Bralower,2005; Miller et al., 2005]. The exact nature of thelink between volcanic activity, such as the eruptionof a large igneous provinces (LIPs), and globalclimate change remains uncertain, however. Forexample, Self et al. [2005] argued that the amountof CO2 released during a flood basalt eruption istoo small to have had a significant impact onatmospheric CO2. In the case of the NAIP, how-ever, intrusion of voluminous mantle-derived meltsinto carbon-rich sedimentary strata may have led tothe release of significant amounts of greenhousegases [Svensen et al., 2004; Storey et al., 2007].

[30] Kastings and Richardson [1985] pointed outthat the BLAG model predicts only a weak con-nection between hydrothermal activity and atmo-spheric CO2, but Eocene warming could beexplained if the rate of carbonate metamorphismincreased proportional to total mid-ocean ridgelength. The largest change in ridge length of theCenozoic occurred between �60 and 50 Ma[Cande and Kent, 1992] associated with a signif-icant global reorganization of spreading ridges andinitiation of seafloor spreading in the Norwegian-Greenland Sea during Chron 24r [Tsikalas et al.,2002].

6. Conclusions

[31] Multiple hypotheses have been put forth toexplain the long-term increase in temperature dur-ing the early Paleogene that culminated in theEECO (�55 to 52 Ma). One mechanism isenhanced rates of volcanic activity associated withtectonic reorganization and eruption of the NAIP,which provided a source of atmospheric CO2 thatled to global warming [Rea et al., 1990]. Ourstrontium isotope results are correlated with bothlong-term variations in sea level and benthic d18Oduring the Paleocene-early Eocene (Figure 9). Ifsea level rise in the late Paleocene-early Eocene is

caused by changes in ocean basin volume related toincreased seafloor spreading rates and/or oceanridge lengths [Miller et al., 2005], then a plausibleexplanation for decreasing strontium isotope valuesis increased hydrothermal activity and/or increasedweathering of volcanic terrains between 65 and51 Ma. Increased rates of ocean-crust productionand emplacement of the NAIP may have led toincreased rates of volcanic-CO2 outgassing andcontributed to the long-term warming trend thatbegan at �59 Ma and peaked at�51Ma [Rea et al.,1990; Zachos et al., 2001; Thomas and Bralower,2005; Miller et al., 2005].

Acknowledgments

[32] This research used samples provided by the Ocean

Drilling Program (ODP). The ODP is sponsored by the U.S.

National Science Foundation (NSF) and participating

countries under management of Joint Oceanographic Institu-

tions (JOI), Inc. Funding for this research was provided by a

U.S. Science Support Program grant to D. A. Hodell, by

NERC grant NER/A/S/2003/00411 to E. A. Hathorne, and

by a grant from the Deutsche Forschungsgemeinschaft (DFG)

to U. Rohl and T. Westerhold.

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