tropical sea-surface temperature reconstruction for the ...jzachos/pubs/tripati_etal_2003.pdf ·...

Tropical sea-surface temperature reconstruction

for the early Paleogene using Mg//Ca ratios

of planktonic foraminifera

Aradhna K. Tripati,1 Margaret L. Delaney,2 James C. Zachos,3 Linda D. Anderson,4

Daniel C. Kelly,5 and Harry Elderfield6

Received 8 June 2003; revised 12 September 2003; accepted 7 October 2003; published 26 December 2003.

[1] To understand the climate dynamics of hypothesized past greenhouse intervals, it is essential to constraintropical sea-surface temperatures (SST), yet existing proxy records give conflicting results. Here we present thefirst Mg/Ca-based study of pre-Quaternary SST and investigate early Paleogene (late Paleocene through latemiddle Eocene; 58.6–39.8 Ma) tropical temperatures, using planktonic foraminifera belonging to the genusMorozovella from Ocean Drilling Program Site 865 on Allison Guyot (western central equatorial Pacific Ocean).Calcification temperatures similar to or warmer than modern tropical SST are calculated using a range ofassumptions regarding diagenesis, temperature calibration, and seawater Mg/Ca. Long-term warming isobserved into the early Eocene (54.8–49.0 Ma), with peak SST between 51 and 48 Ma and rapid cooling of 4�Cbeginning at 48 Ma. These findings are inconsistent with the d18O-based SST previously estimated for thissite. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 1099 Geochemistry: General or miscellaneous; 9355

Information Related to Geographic Region: Pacific Ocean; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: tropical

sea-surface temperatures, Mg/Ca, Ocean Drilling Program Site 865, Early Paleogene, planktonic foraminifera, Morozovella

Citation: Tripati, A. K., M. L. Delaney, J. C. Zachos, L. D. Anderson, D. C. Kelly, and H. Elderfield, Tropical sea-surface

temperature reconstruction for the early Paleogene using Mg/Ca ratios of planktonic foraminifera, Paleoceanography, 18(4),

1101, doi:10.1029/2003PA000937, 2003.

1. Introduction

[2] The climate of the early Paleogene (�65–40 Ma) isdistinguished by some of the warmest temperatures of theCenozoic [Zachos et al., 1994; Greenwood and Wing, 1995;Lear et al., 2000; Zachos et al., 2001; Billups and Schrag,2003] and by long-term global cooling that culminated in theestablishment of a cryosphere [Miller et al., 1987; Zachos etal., 1996; Lear et al., 2000; Zachos et al., 2001; Billups andSchrag, 2003]. In order to understand the dynamics of earlyPaleogene climate it is essential to constrain the geographicdistribution of sea-surface temperatures (SST) and the equa-tor-to-pole temperature gradient, because model simulationssuggest these parameters respond to mechanisms drivingclimate change [Rind and Chandler, 1991; Sloan and Rea,1995; Huber and Sloan, 1999, 2000, 2001; Sloan et al.,

2001]. However, tropical SST during hypothesized green-house periods such as the early Paleogene are an ongoingcontroversy in paleoceanography, with existing proxyrecords yielding conflicting results. The d18O values ofplanktonic foraminifera from deep-sea sediments indicatetropical SST several degrees cooler than modern values[Savin, 1977; Shackleton and Boersma, 1981; Zachos etal., 1994; Bralower et al., 1995], with absolute calcificationtemperatures of 17 to 25�C ± 3�C estimated for the earlyPaleogene [Bralower et al., 1995; Crowley and Zachos,2000]. These calculations assumed no ice volume [Browninget al., 1996] and constant surface water salinity. In addition,subtropical and tropical records exhibit a long-term decreasein d18O between 53 and 42 Ma that suggests a decline intemperature of 7–8�C and/or an increase in surface waterdensity [Bralower et al., 1995]. In contrast, proxies fromother marine settings (outer shelf and coastal) and terrestrialenvironments suggest stable and warm (>26�C) tropical SSTthroughout the early Paleogene [Adams et al., 1990;Graham, 1994; Andreasson and Schmitz, 1998; Pearson etal., 2001; Tripati and Zachos, 2002], similar to modernconditions. Ambiguities associated with interpretingplanktonic foraminiferal d18O records have been invokedto explain the apparent discrepancy between these proxydata for tropical SST, termed the ‘‘cool tropics’’ paradox[Pearson et al., 2001; Zachos et al., 2002]. The d18O ofcalcium carbonate records both water d18O and calcificationtemperature, and therefore variations in regional sea-surfacesalinities can complicate and introduce large uncertaintiesinto temperature reconstructions [Crowley and Zachos,

PALEOCEANOGRAPHY, VOL. 18, NO. 4, 1101, doi:10.1029/2003PA000937, 2003

1Department of Earth Sciences/Godwin Laboratory, University ofCambridge, Cambridge, UK.

2Department of Ocean Sciences/Institute of Marine Sciences, Universityof California, Santa Cruz, California, USA.

3Department of Earth Sciences/Institute of Marine Sciences, Universityof California, Santa Cruz, California, USA.

4Department of Ocean Sciences, University of California, Santa Cruz,California, USA.

5Department of Geology and Geophysics, University of Wisconsin,Madison, Wisconsin, USA.

6Department of Earth Sciences, University of Cambridge, Cambridge,UK.

Copyright 2003 by the American Geophysical Union.0883-8305/03/2003PA000937$12.00

25 - 1

2000]. Diagenesis can also influence the robustness ofd18O-based paleotemperature reconstructions (Table 1). Assuch, the ‘‘cool tropics’’ paradox has been attributed to thecontribution of water d18O and/or diagenetic overprinting[Schrag et al., 1995; Wilson and Opdyke, 1996; Schrag,1999; Pearson et al., 2001; Rudnicki et al., 2001]. Recently,some authors have speculated planktonic foraminifera fromdeep-sea sediments may be composed of as much as 50%secondary calcite [Pearson et al., 2001].[3] Mg/Ca ratios of planktonic foraminifera provide an

alternative method of estimating SST that should havedifferent sensitivities to the factors described above. TheMg content of modern planktonic foraminifera is sensitive tocalcification temperatures [e.g., Nurnberg et al., 1995, 1996;Lea et al., 1999; Elderfield and Ganssen, 2000; Dekens etal., 2002; Anand et al., 2003], probably due to both atemperature effect on the inorganic distribution coefficientand to physiological processes influencing Mg uptake.Empirical Mg/Ca-temperature calibrations have been ap-plied to Mg/Ca records from extant foraminiferal speciesto estimate absolute water temperatures, and combined withforaminiferal d18O values to deconvolve changes in SST andwater d18O [Lea et al., 2000]. To date, Mg/Ca-based studiesutilizing extinct planktonic foraminiferal species have beenlimited because of lack of understanding or quantification ofenvironmental and vital effects in modern species [Delaneyet al., 1985; Lea et al., 1999; Rosenthal et al., 2000] andappropriate temperature calibrations. The recent develop-ment of a multispecies Mg/Ca-temperature calibration fortropical and subtropical planktonic foraminifera [Anand etal., 2003], which matches previous laboratory and core-topcalibrations for single species [e.g., Nurnberg, 1995; Lea etal., 2000;Dekens et al., 2002], enables the application of thisproxy to studies of pre-Quaternary paleoceanography. Theapplication of Mg/Ca paleothermometry to Paleogene taxa islimited to two studies that utilize benthic foraminifera, andapply models of seawater Mg/Ca to these data in order toassess long-term changes in deep-sea temperatures and icevolume [Lear et al., 2000; Billups and Schrag, 2003].[4] Our objectives are to use planktonic foraminiferal Mg/

Ca ratios to (1) estimate absolute calcification temperaturesand long-term changes in SST, and (2) to compare Mg/Ca-based SSTwith foraminiferal d18O values in order to resolvethe ‘‘cool tropics’’ paradox. In this paper we presentmeasurements of the Mg/Ca composition of planktonic

foraminifers belonging to the genus Morozovella fromODP Site 865, Allison Guyot, in sediments ranging from39.8 to 58.6 Ma. In order to estimate calcification temper-atures, we assume the systematics governing the partition-ing of magnesium into extinct foraminiferal taxa is similarto modern taxa. Morozovellids are thought to have beenmixed-layer dwellers inhabiting a niche similar to thatoccupied by modern Globigerinoides sacculifer [Braloweret al., 1995; Rosenthal et al., 2000] and harboring algalsymbionts similar to G. sacculifer [D’Hondt et al., 1994].[5] Complexities in interpreting Mg/Ca data can arise from

dissolution [Lohmann, 1995; Brown and Elderfield, 1996;Rosenthal et al., 2000, Benway et al., 2003], recrystallization[Baker et al., 1982; Delaney, 1989], and changes in seawaterMg/Ca [Lear, 2001; Billups and Schrag, 2002, 2003]. Wediscuss these factors in detail, and present sensitivity analy-ses to the first three parameters in order to provide reasonableconstraints on uncertainties associated with Mg/Ca-basedSST. Diagenetic trajectories are calculated to estimate thepotential effects of recrystallization on Mg/Ca, Sr/Ca, d18O,and d13C ratios given the unique geologic setting of ODP Site865. We then assess the suitability of foraminiferal Mg/Cavalues for reconstructing absolute calcification temperaturesand long-term changes in tropical SST during the earlyPaleogene, and the implications of Mg/Ca-based SST forthe origin of the ‘‘cool tropics’’ paradox. TheMg/Ca record iscompared to the planktonic foraminiferal oxygen isotoperecord to assess the oxygen isotopic composition of tropicalsurface waters at this site. We also compare this record oftropical SST to benthic foraminiferal Mg/Ca data and high-latitude SST records in order to estimate past vertical andequator-to-pole temperature gradients.

2. Methods

2.1. Site Information

[6] ODP Hole 865B (Figure 1) was drilled in the westerncentral equatorial Pacific on Allison Guyot (18�260N,17�330W), part of the Mid-Pacific Mountain chain, at a waterdepth of 1530 m. The recovered Paleogene sedimentarysection consists of pelagic sediments that overlie Cretaceousshallow marine limestones. An age model has been devel-oped [Bralower et al., 1995] that is primarily based oncalcareous nannofossil biostratigraphy [Bralower andMutterlose, 1995] and adapted to the Berggren et al.

Table 1. Effects of Recrystallization on d18O-Based Tropical SST for Several Time Slices Illustrating That if Foraminiferal d18O Values

are Affected by Recrystallization, Then Calculated SST Were Warmer Than Previously Estimateda

Age, Ma d18O-Based SST, �C

10% Recrystallization 30% Recrystallization 50% Recrystallization

End-Member Ab End-Member Bc End-Member Ab End-Member Bc End-Member Ab End-Member Bc

41.0–42.0 17.0 17.1 17.7 17.4 19.7 17.9 23.445.5–46.5 20.6 21.1 21.7 22.6 25.0 25.2 30.851.0 22.9 23.7 24.3 25.9 28.3 29.9 35.555.0 24.8 25.7 26.4 28.5 30.9 33.6 39.357.0–58.0 21.5 22.2 22.8 23.9 26.3 27.1 32.7

aTime intervals are selected to span range of geochemical values observed in foraminifera from ODP Site 865. Diagenetic end-member d18O values usedrepresent reasonable compositions for early diagenetic calcite formed at ODP Site 865 given the geologic setting (Table 3; Section 4.1). d18O-basedtemperatures are calculated using data for the genus Morozovella [Bralower et al., 1995], the equation of Erez and Luz [1983] and assuming a constantseawater d18O of �0.5% (V-SMOW; Zachos et al. [1994]; Bralower et al. [1995]).

bAssuming inorganic calcite with d18O of �0.3% (V-PDB).cAssuming inorganic calcite with d18O of 0.9% (V-PDB).

25 - 2 TRIPATI ET AL.: WARM EARLY PALEOGENE TROPICAL SST FROM FORAM Mg/Ca

[1995] timescale. Paleomagnetic data indicate this site waslocated at or near the equator throughout the Paleocene andEocene [Bralower et al., 1995]. During the late Paleocene,the site is estimated to have been at 2�N, and during the lateEocene at 6�N. The foraminiferal assemblage is relativelydiverse, and indicates warm, stratified, oligotrophic waters[Bralower et al., 1995] at this site. Paleogene fauna present insediments from ODP Site 865 are similar to fauna fromDSDP and ODP Sites 762 (Exmouth Plateau), 577 (ShatskyRise), 689 (Maud Rise), and 525 (Walvis Ridge), and indicatelower bathyal paleodepths between 1300 and 1500 m[Shipboard Scientific Party, 1993; Bralower et al., 1995].[7] This section is unusual in the relative completeness

and unique preservation of Paleocene and Eocene sediments[Bralower et al., 1995; Bralower and Mutterlose, 1995].Burial depths for these sediments are very shallow (�20–130 mbsf) and lithologic data are consistent with earlyPaleogene sediments being above the ooze-to-chalk diage-netic transition [Shipboard Scientific Party, 1993]. Porewater geochemical profiles have constant strontium, mag-nesium, and calcium concentrations downcore, indicatingthat the upper sedimentary column is relatively open toseawater circulation [Shipboard Scientific Party, 1993].Therefore in calculating the effects of diagenesis on fora-miniferal geochemistry, we assume pore fluid compositionand temperatures similar to those of seawater during theearly Paleogene, as they are similar at present. Inspection offoraminifera tests shows thin coatings with minor amountsof secondary calcite, no in-filling calcite, and are consistentwith good to excellent preservation [Bralower et al., 1995].Offsets in carbon and oxygen isotope ratios between plank-tonic foraminifera species from this site also suggest goodpreservation [Bralower et al., 1995; Kelly et al., 1998].

2.2. Sample Preparation

[8] Samples were taken from the interval spanning 39.8 to58.6 Ma (35.20–127.23 mbsf) to generate a Mg/Ca record.

No single species had a range spanning the entirety of theinterval of interest, and therefore the following specieswere analyzed: Morozovella velazcoensis, Morozovellasubbotinae, Morozovella aragonensis, Morozovellaspinulosa, and Morozovella lehneri. We chose examinedspecies from one lineage and selected specimens from the300–355 mm size fraction. Foraminiferal samples consistingof ten to nineteen individuals were weighed, crushed, andcleaned to remove clays, organic matter, and secondarycarbonate (Barker et al. [2003] as modified from Boyle andRosenthal [1996]) in acid-cleaned polystyrene vials. Theseadditional phases can contribute significant amounts ofmagnesium during analysis [Emiliani, 1955; Hastings etal., 1998; Barker et al., 2003].[9] Crushed samples were rinsed and briefly ultrasoni-

cated in ultrahigh quality water (UHQ H2O) five times, inmethanol (Aristar grade) twice, and then in UHQ H2O againto remove clays and fine-grained carbonates. To removeorganic matter, samples were then reacted twice with anoxidizing reagent (buffered hydrogen peroxide) in a hotwater bath, each time for ten minutes with brief intervals ofultrasonication. Samples were then transferred to cleanvials, reacted with a weak acid, and rinsed in UHQ H2O.Cleaned samples were dissolved the day of analysis in300 ml of quartz-distilled 0.075M HNO3 and ultrasonicatedto promote dissolution, and then transferred to a cleanpolystyrene vial to prevent possible leaching from residualparticles. Calcium concentrations were determined using aworking curve, and samples diluted to achieve a solution ofbetween 30 and 60 ppm Ca.

2.3. Determination of Elemental Ratios

[10] High-precision Mg/Ca and Sr/Ca ratios were deter-mined in the Department of Earth Sciences at the Universityof Cambridge on a Varian Vista inductively coupled plasmaoptical emission spectrophotometer (ICP-OES), using themethod outlined by de Villiers et al. [2002]. Fresh acid

Figure 1. Site map showing modern location of ODP Site 865 (Leg 143), Allison Guyot, westerncentral equatorial Pacific Ocean. Map made using GEOMAR make-a-map program (from GEOMARWeb site).

TRIPATI ET AL.: WARM EARLY PALEOGENE TROPICAL SST FROM FORAM MG/CA 25 - 3

blanks and vial blanks were monitored throughout sampleruns. No blank correction was applied as sample Mgintensities are three orders of magnitude greater than blankintensities, and Ca and Sr intensities are four orders ofmagnitude greater. 30, 45, and 60 ppm Ca dilutions ofprimary standards with a range of molar ratios (Mg/Caof 0.5059, 1.289, 2.374, 4.048, 5.139, and 7.507 mmol/mol,Sr/Ca of 0.5057, 0.8083, 1.063, 1.615, 2.088, 2.835, and3.554 mmol/mol) were run at the beginning and end ofeach run to construct intensity ratio calibrations. Mg/Ca andSr/Ca ratios were determined using the 285.213 nm line forMg, the 315.887 nm line for Ca, and the 421.552 nm linefor Sr.[11] Repeat analysis of liquid standards yields a long-term

analytical precision, as defined as ±1s (sample relativestandard deviation, rsd) of ±0.4% for both Mg/Ca andSr/Ca. Solution consistency standards with three differentmolar ratios were analyzed throughout the run, with aprecision of ±0.3% rsd for both Mg/Ca and Sr/Ca. Inaddition, analyses of 10 splits of a core-top foraminiferalstandard (Globigerinoides ruber) have a % rsd of ±2.7% forMg/Ca and ±1.9% for Sr/Ca. Between two and five fora-miniferal samples from seventeen intervals were also sep-arately processed and analyzed in order to assess bothcleaning reproducibility and sample heterogeneity, yieldingan uncertainty of ±2.7% for Mg/Ca and ±2.5% for Sr/Ca.Mn/Ca ratios of samples were monitored as an indicator ofoxyhydroxides and manganese carbonate overgrowths, andsample values (between 0.003 and 0.015 mmol/mol) indi-cate these phases are not present. To assess possiblecontamination by clays, we monitored Fe/Ca, and Fe/Mgratios, and Ti and Al concentrations. Typical planktonicforaminiferal calcite has Fe/Ca ratios of less than 0.10 mmol/mol and Fe/Mg ratios of less than 0.01 to 0.02 mol/mol.Three samples had high iron content, with Fe/Ca and Fe/Mgratios exceeding these criteria, and were excluded from theresults and discussion (Table 2). Also, in order to eliminateinterspecific differences in Mg/Ca and Sr/Ca, species offsetsare calculated for every sample for which there are multiplespecies measured (n = 2–4 samples). An average speciesoffset from M. aragonensis, the most abundant species, isapplied to these data.

3. Results

[12] Measured Mg/Ca ratios in these foraminiferal sam-ples range from 3.6 to 5.3 mmol/mol (Table 2, Figure 2),and Sr/Ca ratios range from 0.84 to 1.10 mmol/mol(Table 2). Mg/Ca ratios are within the range of valuesobserved in modern low-latitude planktonic foraminifera[Elderfield and Ganssen, 2000; Anand et al., 2003]. Sr/Caratios are lower than the 1.25–1.45 mmol/mol reported forlow-latitude planktonic foraminifera by Elderfield et al.[2000], and are consistent with excellent preservation andminimal recrystallization [e.g., Thomas et al., 1999]. Notrend is observed between Mg/Ca and Sr/Ca ratios.[13] Average apparent species offsets relative to

M. aragonensis aresmall (Mg/Ca,�0.20to+0.16mmol/mol;Sr/Ca, �0.03 to +0.04; Table 2), and adjusted values areused in paleotemperature calculations. Similar trendsare observed in the unadjusted Mg/Ca and species-adjusted

Mg/Ca records (Figure 2). Application of species offsets,however, reduces the variability observed in the Mg/Carecord. The species-adjusted values range from 3.5 to5.3 mmol/mol (Table 2; Figure 2). Late Paleocene (54.8–58.6 Ma) planktonic foraminiferal Mg/Caadj ratios rangefrom 3.6 to 4.9 mmol/mol (Table 2; Figure 3). Early Eocene(54.8–48.5 Ma) ratios are the highest observed in the ODPSite 865 record, reaching values of 5.3 mmol/mol between51.2 and 48.5 Ma. Early middle Eocene (48.5–41.3 Ma)Mg/Ca ratios rapidly decrease at 48 Ma and then increaseslightly at 43 Ma. Mg/Caadj ratios of 3.5 to 4.3 mmol/mol arerecorded during the late middle Eocene (41.3–39.8 Ma).

4. Discussion

[14] Before using the Mg/Ca data to reconstruct calcifi-cation temperatures, we discuss the potential errors inapplying the Mg/Ca temperature proxy to this record. First,we assess the preservation of primary test composition andthe impact of diagenesis on shell geochemistry. Second, thetemperature sensitivity of Mg uptake in calcite is described,and different calibrations for modern planktonic foraminif-era from tropical and subtropical settings are compared.Finally, the effects of uncertainties in estimates of earlyPaleogene seawater Mg/Ca on paleotemperatures are eval-uated. Other secondary factors such as water salinity and pHlikely also influence the Mg/Ca record from ODP Site 865;however, these environmental parameters have been dem-onstrated to exert a minor control on foraminiferal Mg/Carelative to temperature effects and interspecies offsets [Leaet al., 1999], and therefore are not discussed.

4.1. Preservation

[15] After burial foraminiferal test composition can beinfluenced by (1) the selective dissolution of Mg-richportions of the test in undersaturated waters near thesediment-water interface [Lohmann, 1995; Brown andElderfield, 1996; Rosenthal et al., 2000], and by (2) theprecipitation of secondary inorganic calcite during diagen-esis. Dissolution makes the Mg/Ca and Sr/Ca of a forami-niferal test decrease and d18O increase, and therefore wouldbias both Mg/Ca and d18O-based temperatures towardcooler values. In order to minimize the influence of disso-lution on paleoceanographic reconstructions, it is importantto assess the paleodepths of the calcite saturation horizonand use foraminiferal tests from sediment cores that weresignificantly above this depth. On the basis of the above,dissolution is unlikely to have influenced the magnesiumcontent of the tests considered here because of the shallowpaleodepths of the site and because of the sedimentburial history [Shipboard Science Party, 1993]. Paleodepthsof 1300–1500 m are reconstructed for ODP Site 865[Shipboard Scientific Party, 1993; Bralower et al., 1995],whereas the equatorial Pacific carbonate compensationdepth is estimated as consistently deeper than �3200 m[Shipboard Scientific Party, 2002].[16] Foraminiferal tests can accumulate secondary inor-

ganic calcite that precipitates from pore fluids. To assess theinfluence of secondary calcite on the elemental and stableisotope composition of foraminifera, it is necessary toconsider the pore fluid composition and the burial condi-


Table 2. Mg/Ca and Sr/Ca Composition of Planktonic Foraminifera From ODP Leg 143, Site 865, Hole B

Core SectionInterval,

cm SpeciesDepth,mbsf

Age,Ma

Mg/Ca,mmol/mol

NormalizedMg/Ca,a

mmol/molSr/Ca,

mmol/mol

NormalizedSr/Ca,b

mmol/mol

4 6 20–22 Morozovella spinulosa 35.20 39.76 3.59 3.53 0.97 0.974 6 20–22 Morozovella spinulosa 35.20 39.76 3.79 3.73 0.93 0.934 6 20–22 Morozovella lehneri 35.20 39.76 3.86 4.02 0.88 0.925 2 70–72 Morozovella lehneri 39.20 40.62 3.99 4.15 0.96 1.005 4 70–72 Morozovella lehneri 42.20 41.27 4.17 4.33 0.96 1.005 5 70–72 Morozovella spinulosa 43.70 41.60 4.65 4.59 0.99 0.995 5 70–72 Morozovella spinulosa 43.70 41.60 4.57 4.51 0.87 0.875 5 70–72 Morozovella lehneri 43.70 41.60 4.20 4.36 0.84 0.885 5 70–72 Morozovella lehneri 43.70 41.60 4.27 4.43 1.00 1.046 1 81–83 Morozovella spinulosa 47.31 42.38 4.49 4.43 0.99 0.996 1 81–83 Morozovella lehneri 47.31 42.38 3.97 4.13 0.93 0.976 1 81–83 Morozovella lehneri 47.31 42.38 4.08 4.24 0.93 0.976 4 70–72 Morozovella aragonensis 51.70 42.96 4.17 4.17 1.06 1.066 4 70–72 Morozovella spinulosa 51.70 42.96 4.08 4.02 0.95 0.957 1 118–120 Morozovella aragonensis 57.18 43.69 3.68 3.68 1.04 1.047 1 118–120 Morozovella aragonensis 57.18 43.69 3.96 3.96 0.94 0.947 1 118–120 Morozovella aragonensis 57.18 43.69 3.70 3.70 0.97 0.977 1 118–120 Morozovella spinulosa 57.18 43.69 3.94 3.88 1.05 1.057 4 66–68 Morozovella aragonensis 61.18 44.22 3.92 3.92 0.95 0.957 4 66–68 Morozovella spinulosa 61.18 44.22 4.05 3.99 1.00 1.007 4 66–68 Morozovella spinulosa 61.18 44.22 3.94 3.88 1.00 1.007 4 66–68 Morozovella spinulosa 61.18 44.22 4.09 4.03 1.01 1.017 4 68–70 Morozovella aragonensis 61.20 44.22 3.74 3.74 1.02 1.028 1 89–91 Morozovella spinulosa 66.39 44.91 4.23 4.17 0.95 0.958 3 70–72 Morozovella aragonensis 69.20 45.65 3.77 3.77 1.08 1.088 4 67–69 Morozovella aragonensis 70.67 46.21 3.91 3.91 1.09 1.098 5 70–72 Morozovella aragonensis 72.20 46.79 3.88 3.88 1.07 1.078 5 70–72 Morozovella aragonensis 72.20 46.79 4.11 4.11 1.05 1.058 5 70–72 Morozovella aragonensis 72.20 46.79 3.80 3.80 1.07 1.078 5 70–72 Morozovella aragonensis 72.20 46.79 3.93 3.93 1.07 1.078 5 70–72 Morozovella aragonensis 72.20 46.79 3.81 3.81 1.08 1.088 6 70–72 Morozovella aragonensis 73.70 47.36 4.36 4.36 1.00 1.009 1 13–15 Morozovella aragonensis 75.13 47.90 5.05 5.05 0.99 0.999 2 20–22 Morozovella aragonensis 76.70 48.50 5.16 5.16 0.99 0.999 2 20–22 Morozovella aragonensis 76.70 48.50 4.88 4.88 1.03 1.039 2 20–22 Morozovella aragonensis 76.70 48.50 4.93 4.93 1.01 1.019 2 20–22 Morozovella aragonensis 76.70 48.50 5.12 5.12 0.99 0.999 3 20–22 Morozovella aragonensis 78.20 49.07 5.06 5.06 0.96 0.969 3 120–125 Morozovella aragonensis 79.20 49.46 4.99 4.99 0.96 0.969 4 10–12 Morozovella aragonensis 79.60 49.60 5.06 5.06 1.01 1.019 5 120–122 Morozovella aragonensis 82.20 51.16 5.27 5.27 0.95 0.959 5 120–122 Morozovella aragonensis 82.20 51.16 5.14 5.14 0.90 0.909 5 120–125 Morozovella aragonensis 82.20 51.16 5.21 5.21 0.91 0.9110 1 83–85 Morozovella aragonensis 85.33 51.68 4.46 4.46 1.00 1.0010 2 120–122 Morozovella aragonensis 87.20 51.99 4.55 4.55 0.98 0.9810 2 120–122 Morozovella aragonensis 87.20 51.99 4.69 4.69 0.97 0.9710 2 120–125 Morozovella aragonensis 87.20 51.99 4.39 4.39 0.98 0.9810 2 120–125 Morozovella subbotinae 87.20 51.99 4.64 4.44 0.99 0.9610 3 4–6 Morozovella subbotinae 87.54 52.05 4.52 4.32 0.99 0.9610 3 4–6 Morozovella aragonensis 87.54 52.05 4.21 4.21 0.94 0.9410 3 60–62 Morozovella subbotinae 88.10 52.14 5.05 4.85 0.96 0.9310 4 60–62 Morozovella velazcoensis 89.60 52.39 4.49 4.41 0.94 0.9110 4 60–62 Morozovella velazcoensis 89.60 52.39 4.39 4.31 0.97 0.9410 4 60–62 Morozovella velazcoensis 89.60 52.39 4.47 4.39 0.94 0.9110 4 60–62 Morozovella velazcoensis 89.60 52.39 4.65 4.57 0.95 0.9210 5 60–62 Morozovella subbotinae 91.10 52.74 4.60 4.40 0.96 0.9311 2 20–22 Morozovella subbotinae 95.70 53.80 4.26 4.06 0.95 0.9211 2 20–22 Morozovella velazcoensis 95.70 53.80 4.18 4.10 0.95 0.9211 2 20–22 Morozovella velazcoensis 95.70 53.80 4.13 4.05 0.96 0.9311 2 20–22 Morozovella velazcoensis 95.70 53.80 4.12 4.04 0.98 0.9511 2 85–87 Morozovella subbotinae 96.35 53.95 4.23 4.03 0.98 0.9511 5 85–87 Morozovella subbotinae 100.85 54.99 4.47 4.27 1.01 0.9811 5 85–87 Morozovella velazcoensis 100.85 54.99 4.30 4.22 1.01 0.9811 6 85–87 Morozovella subbotinae 102.35 55.27 4.38 4.18 1.01 0.9811 6 85–87 Morozovella velazcoensis 102.35 55.27 4.22 4.14 1.01 0.9812 1 0–2 Morozovella velazcoensis 103.50 55.49 4.34 4.26 0.99 0.9612 1 10–12 Morozovella velazcoensis 103.60 55.50 4.97 4.89 1.01 0.9812 1 10–12 Morozovella velazcoensis 103.60 55.50 4.35 4.27 0.98 0.9512 1 10–12 Morozovella velazcoensis 103.60 55.50 4.29 4.21 1.01 0.98


tions to which tests were exposed. Early Paleogene forami-nifera from ODP Site 865 are in a unique preservationalsetting. They occur within relatively unlithified sediments (athin sequence of foraminiferal ooze) at shallow burialdepths (<150 mbsf), and pore fluids are identical in com-position to seawater with respect to Sr, Mg, and Ca content[Shipboard Scientific Party, 1993]. At these conditions, Mg/Ca and Sr/Ca ratios of inorganic calcite will reflect porefluid composition and partition coefficient (which can becontrolled by precipitation temperature), and d18O valueswill depend on fluid composition and precipitation temper-ature. We predict the geochemical composition of inorganiccalcite (Table 3) using a range of effective partition coef-ficients for Mg and Sr observed in deep-sea settings,experimentally derived partition coefficients, estimated frac-tionation factors, and a wide range of interstitial watercompositions and precipitation temperatures (references inTable 3). On the basis of these calculations we estimateinorganic calcite precipitating from interstitial waters duringearly diagenesis at ODP Site 865 would have similar Mg/Caand d13C ratios, lower Sr/Ca ratios, and higher d18O ratios,relative to foraminiferal calcite.[17] To estimate the impact of secondary calcite on the

range of measured ratio and isotope compositions at this site(Foram 1, 2, 3 in Tables 4–6), we calculate the primaryforaminiferal test compositions assuming varying amounts

of secondary calcite and using reasonable values for aninorganic component formed during early diagenesis(Table 3). It is important to note that existing data on theelemental composition of inorganic calcites from deep-seasettings is very limited [Baker et al., 1982; Delaney, 1989;Andreasen and Delaney, 2000] and therefore the composi-tion of secondary calcite is poorly constrained. Reportedeffective partition coefficients for Mg and Sr can be an orderof magnitude different from experimentally derived parti-tion coefficients [Mucci and Morse, 1983; Morse andBender, 1990]. Thus the application of the latter to deep-sea settings may not be appropriate [Baker et al., 1982;Morse and Bender, 1990]. Calculated diagenetic trajectoriesare most consistent with tests being composed of minoramounts of secondary calcite (i.e., less than 20–30%secondary calcite; see Tables 4–5), although greateramounts of recrystallization may be reflected in somesamples. This recrystallization would result in smallchanges in Mg/Ca and d13C, and larger changes in Sr/Caand d18O (Tables 4–6). However, if secondary calcite ischaracterized by high Mg/Ca values (reflecting high DMg)similar to experimentally derived values [Mucci andMorse, 1983], then minor recrystallization would result invery large changes in Mg/Ca (Table 4). In this case, themeasured Mg/Ca ratios from ODP Site 865 can only bereconciled with a very minor diagenetic component (i.e.,

Table 2. (continued)

Core SectionInterval,

cm SpeciesDepth,mbsf

Age,Ma

Mg/Ca,mmol/mol

NormalizedMg/Ca,a

mmol/molSr/Ca,

mmol/mol

NormalizedSr/Ca,b

mmol/mol

12 1 20–22 Morozovella velazcoensis 103.70 55.51 4.02 3.94 0.98 0.9512 1 40–42 Morozovella velazcoensis 103.90 55.52 4.02 3.94 0.99 0.9612 1 70–72 Morozovella subbotinae 104.20 55.54 3.79 3.59 0.96 0.9312 1 70–72 Morozovella velazcoensis 104.20 55.54 3.74 3.66 0.95 0.9212 1 70–72 Morozovella velazcoensis 104.20 55.54 3.91 3.83 0.99 0.9612 1 70–72 Morozovella velazcoensis 104.20 55.54 3.86 3.78 1.00 0.9712 2 20–22 Morozovella velazcoensis 105.20 55.60 4.13 4.05 0.98 0.9512 2 70–72 Morozovella velazcoensis 105.70 55.63 4.18 4.10 0.98 0.9512 3 20–22 Morozovella velazcoensis 106.70 55.69 4.08 4.00 0.99 0.9612 5 20–22 Morozovella velazcoensis 109.70 55.87 4.24 4.16 1.03 1.0012 6 70–72 Morozovella velazcoensis 111.70 55.99 4.00 3.92 1.02 0.9913 1 70–72 Morozovella velazcoensis 113.70 56.11 4.34 4.26 1.01 0.9813 2 70–72 Morozovella velazcoensis 115.20 56.20 4.11 4.03 1.04 1.0113 3 70–72 Morozovella velazcoensis 116.70 56.36 3.69 3.61 1.03 1.0013 3 70–72 Morozovella velazcoensis 116.70 56.36 4.01 3.93 1.02 0.9913 3 70–72 Morozovella velazcoensis 116.70 56.36 3.95 3.87 1.01 0.9813 4 70–72 Morozovella velazcoensis 118.20 56.65 4.19 4.11 1.06 1.0313 5 70–72 Morozovella velazcoensis 119.70 56.94 4.15 4.07 1.06 1.0313 6 70–72 Morozovella velazcoensis 121.20 57.23 4.64 4.56 1.09 1.0613 6 70–72 Morozovella velazcoensis 121.20 57.23 4.63 4.55 1.07 1.0413 6 70–72 Morozovella velazcoensis 121.20 57.23 4.63 4.55 1.07 1.0414 1 20–22 Morozovella velazcoensis 122.70 57.53 4.09 4.01 1.07 1.0414 2 20–22 Morozovella velazcoensis 124.20 57.82 3.92 3.84 1.07 1.0414 2 20–22 Morozovella velazcoensis 124.20 57.82 3.82 3.74 1.10 1.0714 2 20–22 Morozovella velazcoensis 124.20 57.82 3.89 3.81 1.03 1.0014 3 76–78 Morozovella velazcoensis 126.26 58.29 4.15 4.07 1.04 1.0114 4 23–25 Morozovella velazcoensis 127.23 58.60 4.02 3.94 1.08 1.054 6 20–22 Morozovella lehneric 35.20 39.76 4.39 1.006 1 81–83 Morozovella spinulosac 47.31 42.38 4.99 0.9810 4 60–62 Morozovella subbotinaec 89.60 52.39 5.51 0.97

aMg/Ca ratios have been adjusted using the average species offset from M. aragonensis. The offset is +0.16 mmol/mol for M. lehneri, �0.06 mmol/molfor M. spinulosa, �0.20 for M. subbotinae, and �0.08 for M. velazcoensis.

bSr/Ca ratios have been adjusted using the average species offset from M. aragonensis. No offset is observed between M. spinulosa and M. aragonensis.The offset is +0.04 mmol/mol for M. lehneri, �0.03 for M. subbotinae, and �0.03 for M. velazcoensis.

cSample values have been excluded from discussion because of high Fe/Ca (>0.10 mmol/mol) and Fe/Mg (>0.02 mol/mol) ratios.


much less than 10%) that would only have a minor impacton d18O and d13C (Table 6).

4.2. Temperature Calibration

[18] Comparisons of calcification temperature to Mg/Cadata for various species of modern planktonic foraminiferafrom different settings suggest the interspecies variability inmagnesium uptake is quite small. Various calibrationsconverge on a temperature sensitivity of 9–10% per degree[Nurnberg et al., 1995, 1996; Lea et al., 1999; Elderfieldand Ganssen, 2000; Anand et al., 2003]. Although this isless precise than species-specific calibrations developed forG. sacculifer and G. ruber, we choose to apply the multi-species calibration developed for subtropical and tropicalplanktonic foraminifera [Anand et al., 2003] since it is morerobust for application in studies utilizing multiple and/orextinct species, and because interspecies offsets observed inthe record from ODP Site 865 are smaller than intraspeciesdifferences. The relationship between Mg/Ca (mmol/mol)and temperature (�C) defined by Anand et al. [2003] for

their multispecies calibration is Mg/Ca = 0.38 (±0.02) exp0.091 (±0.003) T. For comparison, the temperature sensi-tivities published for species-specific calibrations rangefrom 0.089 to 0.09, and the pre-exponential constants rangefrom 0.3 to 0.39 [Nurnberg et al., 1995, 1996, 2000; Leaand Martin, 1996; Lea et al., 2000; Dekens et al., 2002].Temperature calculations vary by 3�C depending on thecalibration used (Table 7).

4.3. Mg/Ca of Seawater

[19] To evaluate temperatures using foraminiferal Mg/Cavalues, seawater Mg/Ca ratios must be known. Spatially,seawater Mg/Ca is relatively insensitive to large changes inwater salinity, unlike the d18O of seawater [e.g., Klein et al.,1996]. Although seawater Mg/Ca can change over time, thelong residence times of magnesium and calcium (13 and1Ma, respectively) [Broecker and Peng, 1982] result in slowchanges in seawater Mg/Ca, occurring on timescales of tensof millions of years [e.g., Wilkinson and Algeo, 1989;Stanley and Hardie, 1998]. Estimates of early Paleogene

Figure 2. Mg/Ca and Sr/Ca records for ODP Hole 865B of planktonic foraminifera belonging to thegenus Morozovella, from 35.20 to 127.23 m below seafloor (mbsf). Filled symbols indicate valuesnormalized to M. aragonensis (species offsets listed in Table 2 legend). Reported errors are calculatedusing precision of repeat analyses of three liquid consistency standards (±0.4% for Mg/Ca and Sr/Ca),and of foraminiferal samples used in this study (average reproducibility of replicates is ±2.7% for Mg/Ca;1s of replicates is 1.8%; average reproducibility of replicates is ±2.6% for Sr/Ca; and 1s of replicates is2.3%).


seawater Mg/Ca ratios are based on either sedimentaryarchival constraints on the chemistry of ancient oceans[e.g., Lowenstein et al., 2001; Dickson, 2002] or models[e.g., Berner et al., 1983; Wilkinson and Algeo, 1989;Hardie, 1996; Stanley and Hardie, 1998] that include themajor geochemical processes that can influence the marinebudgets of these elements [Broecker and Peng, 1982].Though direct reconstruction of seawater Mg/Ca duringthe early Paleogene is limited in temporal resolution andhas large errors, these do place constraints on bothpast seawater Mg/Ca values and also on the accuracy ofseawater composition derived from models. Sedimentaryarchives indicate seawater Mg/Ca values were less thanmodern (5.1 mol/mol), and likely were in the range of 2 to

4 mol/mol during the Paleocene (echinoderms; Dickson[2002]) and Eocene (fluid inclusions in evaporites;Lowenstein et al. [2001]).[20] Seawater models falling within these constraints

include a model of Wilkinson and Algeo [1989] and thatof Stanley and Hardie [1998] which both take into accountchanges in riverine input, the amount and type of carbonatesedimentation, evaporite precipitation, and hydrothermalactivity. Both models predict a gradual long-term increasein seawater Mg/Ca due to a decrease in overall hydrother-mal activity and seafloor spreading rates, with valuesincreasing from 3.0 to 3.5 mol/mol [Wilkinson and Algeo,1989] and from 1.4 to 1.9 mol/mol [Stanley and Hardie,1998] between 59 and 40 Ma. However, these two seawater

Figure 3. Records of d18O, Mg/Ca, Sr/Ca, and reconstructed paleotemperatures for the early Paleogeneusing planktonic foraminifera of the genus Morozovella from ODP Hole 865B. Oxygen isotope data andpaleotemperatures are from Bralower et al. [1995]. Elemental data are normalized toM. aragonensis, andmean Mg/Ca and Sr/Ca values are shown. Error bars represent average reproducibility of measurementsand also, where multiple values exist, standard deviations of the means.

Table 3. Hypothesized Composition of Inorganic Calcite Formed During Early Diagenesis

Calculated Value Partition Coefficienta Interstitial Water Compositionb Precipitation Temperaturec

Mg/Ca 1.5 mmol/mol 0.0003–0.0005 3.0–5.1 mol/mol4.1 mmol/mol 0.0008–0.001 3.0–5.1 mol/mol20 mmol/mol 0.004–0.007d 3.0–5.1 mol-mol

Sr/Ca 0.44 mmol/mol 0.04–0.06 7.5–12 mol/mol0.61 mmol/mol 0.05–0.08 7.5–12 mol/mol

d18O (V-PDB)e �0.3% �1.2 to �1.5% (V-SMOW) 10–13�Cd18O (V-PDB)e 0.9% �1.2 to �1.5% (V-SMOW) 5–8�Cd13C (V-PDB)f 3%

aFrom Baker et al. [1982], Delaney [1989], Katz [1973], Lorens [1981], Tesoriero and Pankow [1996].bWe assume this was similar in composition to early Paleogene seawater because of the similarity of modern pore fluid profiles to seawater and the

shallow burial depth [Shipboard Science Party, 1993]. Early Paleogene seawater ratios are from Broecker and Peng [1982], Delaney and Boyle [1986],Wilkinson and Algeo [1989], Paytan and Kastner [1997], Andreasen and Delaney [2000], Lear et al. [2000], Lowenstein et al. [2001], Zachos et al. [2001],Dickson [2002], Rowley [2002], Steuber and Veizer [2002], Tripati [2002], and Lear et al. [2003].

cOwing to shallow burial depth, this is assumed to be between 5 and 13�C, similar to early Paleogene bottom-water temperatures [Lear et al., 2000; Lear,2001, Zachos et al., 2001]. The range of Mg/Ca and Sr/Ca values used take into account the temperature sensitivity of Mg and Sr partition coefficients overthis temperature range.

dExperimentally derived partition coefficient [Katz, 1973].eFractionation factors are from McCrea [1950] and O’Neil et al. [1969].fInorganic calcite d13C value used is 3% (V-PDB), as this is the mean value for the ambient bulk (nannofossil) carbonate [Shackleton et al., 1984;

Bralower et al., 1995].


models may overestimate the magnitude of Cenozoic sea-water Mg/Ca changes. A recent reevaluation of seafloorproduction rates [Rowley, 2002] argues that spreading rateshave remained relatively constant, and are consistent withLi/Ca data [Delaney and Boyle, 1986] in suggesting thathydrothermal exchange has not varied by more than 30–40%. An alternate seawater model by Wilkinson and Algeo[1989] assumes invariant spreading rates and therefore alonger residence time of Mg in seawater with respect tohydrothermal circulation, and predicts seawater Mg/Caratios were within 15% of modern values during theCenozoic. If we assume there is an upper limit to tropicalSST [e.g., Pierrehumbert, 1995], then the low seawaterratios of Stanley and Hardie [1998] are unreasonable asthese would yield calcification temperatures for planktonic

foraminifera in excess of 37–40�C when applied to theMg/Ca record from ODP Site 865. This seawater modelalso results in very warm deepwater temperatures whenapplied to benthic foraminiferal Mg/Ca values [Billups andSchrag, 2003]. Therefore we apply both the model ofWilkinson and Algeo [1989], with seawater Mg/Ca ratiosof 3.0 to 3.5 mmol/mol, and modern seawater Mg/Cavalues to reconstruct calcification temperatures from fora-miniferal Mg/Ca data.

4.4. Mg/Ca-Based Paleotemperatures

[21] Using the seawater Mg/Ca history of Wilkinson andAlgeo [1989], calculated paleotemperatures based on ODPSite 865 foraminiferal Mg/Ca data range between 30 and34�C (Figure 3). Late Paleocene temperatures are estimatedto be 31�C, gradually warming to peak temperatures of 33–34�C during the early Eocene, between 51.2 and 48.5 Ma.Mg/Ca-based temperatures rapidly cool during the earlymiddle Eocene at 48 Ma, to 30–31�C, and remain relativelystable through the middle Eocene. Assuming constantCenozoic seawater Mg/Ca ratios we estimate a similartemperature history, and absolute calcification temperaturesthat are 5 degrees cooler, between 25 to 29�C. For compar-ison, modern SST at similar equatorial Pacific sites are 27–28�C, and water d18O values are 0.5 to 0.7 per mil[Bralower et al., 1995]. Thus the Mg/Ca record indicatesrelatively stable and warm calcification temperatures duringthe early Paleogene, similar to reconstructed SST fromshallow-marine sequences [Adams et al., 1990; Andreassonand Schmitz, 1998; Pearson et al., 2001; Tripati andZachos, 2002], and similar or slightly higher than modernSST. These Mg/Ca-based calcification temperatures areseveral degrees warmer than those that have been previouslycalculated using foraminiferal d18O values from this site[Figure 3; Bralower et al., 1996] and other low-latitude sites[Savin, 1977; Shackleton and Boersma, 1981; Zachos et al.,1994].[22] One source of uncertainty in both Mg/Ca and d18O-

based paleotemperatures is the amount of secondary calcitepresent, and the composition of this overgrowth. Recrystal-

Table 6. Reconstructed d18O and d13C of Planktonic Foraminiferal

Calcite Assuming Varying Percentages of Early Diagenetic Calcite,

Demonstrating Sensitivity of Foraminiferal Stable Isotope Ratios

to Recrystallizationa

0% 5% 10% 20% 30% 50%

Inorganic Calcite Has d18O of �0.3%Foram 1 �1.8 �1.9 �2.0 �2.2 �2.4 �3.3Foram 2 �1.5 �1.6 �1.6 �1.8 �2.0 �2.7Foram 3 �1.3 �1.4 �1.4 �1.6 �1.7 �2.3

Inorganic Calcite Has d18O of 0.9%Foram 1 �1.8 �1.9 �2.1 �2.5 �3.0 �4.5Foram 2 �1.5 �1.6 �1.8 �2.1 �2.5 �3.9Foram 3 �1.3 �1.4 �1.5 �1.9 �2.2 �3.5

Inorganic Calcite Has d13C of 3%Foram 1 2.8 2.8 2.8 2.8 2.7 2.6Foram 2 3.3 3.3 3.3 3.4 3.4 3.6Foram 3 2.8 2.8 2.8 2.8 2.7 2.6

aInorganic calcite values are derived from Table 3. Values for ‘‘Foram1–3’’ represent measured range of sample values from this study.

Table 4. Reconstructed Mg/Ca of Planktonic Foraminiferal

Calcite Assuming Varying Percentages of Early Diagenetic

Calcite, Demonstrating Sensitivity of Foraminiferal Mg/Ca to

Recrystallizationa

0% 5% 10% 20% 30% 50%

Inorganic Calcite Has Mg/Ca Ratio of 1.5 mmol/molForam 1 5.2 5.4 5.6 6.1 6.8 8.9Foram 2 4.4 4.6 4.7 5.1 5.6 7.3Foram 3 3.5 3.6 3.7 4.0 4.4 5.5

Inorganic Calcite Has Mg/Ca Ratio of 4.1 mmol/molForam 1 5.2 5.3 5.3 5.5 5.7 6.3Foram 2 4.4 4.4 4.4 4.5 4.5 4.7Foram 3 3.5 3.5 3.4 3.4 3.2 2.9

Inorganic Calcite Has Mg/Ca Ratio of 20 mmol/molForam 1 5.2 4.4 3.6 1.5Foram 2 4.4 3.6 2.7 0.5Foram 3 3.5 2.6 1.7

aInorganic calcite values are derived from Table 3. Values for ‘‘Foram1–3’’ represent measured range of sample values from this study.Boldface Mg/Ca values are outside of range observed in modern tropicaland subtropical planktonic foraminifera. Mg/Ca values in modern species[Anand et al., 2003] are between �1.4 and 5.2 mmol/mol (�2.3–5.2 mmol/mol for spinose species only), although values as high as8.8 mmol-mol have been observed in Orbulina universa [Lea et al., 1999;Anand et al., 2003].

Table 5. Reconstructed Sr/Ca of Planktonic Foraminiferal

Calcite Assuming Varying Percentages of Early Diagenetic

Calcite, Demonstrating Sensitivity of Foraminiferal Sr/Ca to

Recrystallizationa

0% 5% 10% 20% 30% 50%

Inorganic Calcite Has Sr/Ca Ratio of 0.44 mmol/molForam 1 0.91 0.93 0.96 1.03 1.11 1.38Foram 2 1.00 1.03 1.06 1.14 1.24 1.56Foram 3 1.09 1.12 1.16 1.25 1.37 1.74

Inorganic Calcite Has Sr/Ca Ratio of 0.61 mmol/molForam 1 0.91 0.93 0.94 0.99 1.04 1.21Foram 2 1.00 1.02 1.04 1.10 1.17 1.39Foram 3 1.09 1.12 1.14 1.21 1.30 1.57

aInorganic calcite values are derived from Table 3. Values for ‘‘Foram1–3’’ represent measured range of sample values from this study.Boldface Sr/Ca values are outside of range observed in modern tropicaland subtropical planktonic foraminifera. Sr/Ca values in modern species[Brown and Elderfield, 1996; Lea et al., 1999; Elderfield et al., 2000] arebetween �1.15 and 1.55 mmol/mol.


lization would bias temperature estimates toward coolervalues (Tables 1 and 8). Assuming shell geochemistryreflects up to 30% recrystallized calcite and using effectivepartition coefficients calculated at deep-sea sites, Mg/Ca-based temperatures may be biased by as much as 3�C, andd18O-based temperatures up to 6�C. Other known sources ofuncertainty in Mg/Ca-based temperature estimates includeoffsets in pre-exponential constants between different tem-perature calibrations, and the history of seawater Mg/Ca.We use conservative calculations to estimate the sensitivityof Mg/Ca and reconstructed paleotemperatures to thesefactors (Table 7), and find that absolute temperatures varyby 3–5�C. Importantly, all estimates still yield calcificationtemperatures in excess of 25�C (Tables 7–8), and arewarmer than previous d18O-based temperatures (Table 1).Mg/Ca data support warm and stable SST despite theuncertainties discussed above.

4.5. Implications for Interpreting the Planktonic D18O

Record and for Tropical Water Hydrography

[23] At ODP Site 865 there is a discrepancy between (1)temperature histories calculated using d18O and Mg/Ca and(2) absolute calcification temperatures derived from d18O

and Mg/Ca, even assuming foraminiferal geochemistryreflects between 10 and 30% secondary calcite. The firstobservation can be explained if surface water salinity variedtemporally at this site. Assuming the Mg/Ca-based temper-atures are correct, the offset in absolute temperaturescalculated using the two different proxies may arise fromforaminifera d18O values being affected by diagenesis, and/or that water d18O estimates have large uncertainties. Simpleassumptions for the early Paleogene may result in inaccuratewater d18O values being used in paleotemperature recon-structions. d18O-based SST reconstructions for this intervalassume a constant water d18O value that supposes ice-freeconditions, and in the first instance use either a global meanocean d18O or the modern offset observed between thelocation (or paleolatitude) of the site and the global mean[e.g., Zachos et al., 1994]. We calculate surface water d18Ovalues (Table 9) using standard d18O-paleotemperatureequations [Erez and Luz, 1983] and Mg/Ca-based paleo-temperatures. However, the uncertainties in this calculationare large because any recrystallization is likely to haveimpacted shell geochemistry. The 1.5% range in recon-structed water d18O probably reflects these large uncertain-ties, and also some component of surface water salinity and/or ice volume fluctuations in the isotope record. Nonethe-less, these absolute water d18O values are greater thancalculated using either of the approaches typically used.This suggests that during the early Paleogene there mayhave been greater evaporation of surface waters at this site,resulting in higher sea-surface salinities. This hypothesis isconsistent with a more vigorous hydrologic cycle during

Table 8. Mg/Ca-Based Paleotemperature Estimates for Several

Time Slices That Assume Different Diagenetic Histories and

Seawater Mg/Ca Ratiosa

Age, Ma

Seawater Mg/CaRatio of 3.5 mol/molb

Seawater Mg/Ca Ratioof 5.1 mol/molc

Mg/CaSST, �C

DiageneticHistory Ad

Mg/CaSST, �C

DiageneticHistory Be

DiageneticHistory Cf

41.0–42.0 31.0 33.7 26.7 26.8 25.045.5–46.5 30.5 33.1 25.6 25.2 23.451.0 34.4 37.3 29.1 30.0 30.155.0 32.0 34.7 26.7 26.8 26.057.0–58.0 32.9 35.6 27.2 27.5 27.3

aThis illustrates that temperatures similar to modern tropical SST arecalculated for the early Paleogene, even using a range of assumptionsregarding diagenesis and seawater Mg/Ca. Calibration of Anand et al.[2003]. Time intervals are selected to span range of geochemical valuesobserved in foraminifera from ODP Site 865.

bFrom Wilkinson and Algeo [1989].cAssuming constant seawater Mg/Ca during Cenozoic.dAssuming 30% inorganic calcite with Mg/Ca of 1.5 mmol/mol.eAssuming 30% inorganic calcite with Mg/Ca of 4.1 mmol/mol.fAssuming 10% inorganic calcite with Mg/Ca of 20 mmol/mol.

Table 7. Effects of Applying Different Temperature Calibrations and Seawater Mg/Ca Models on Reconstructed Paleotemperatures (�C),Using Range of Mg/Ca Values Observed in Foraminifera From ODP Site 865

Test Mg/Ca,mmol/mol

Seawater Mg/Ca Ratio of 3.5 mol/mola Seawater Mg/Ca Ratio of 5.1 mol/molb

Calibration1c

Calibration2d

Calibration3e

Calibration4f

Calibration5g

Calibration1c

Calibration2d

Calibration3e

Calibration4f

Calibration5g

5.2 33.4 33.7 33.4 33.4 36.4 29.1 29.4 29.1 29.1 32.14.4 31.5 31.8 31.6 31.5 34.5 27.2 27.5 27.2 27.2 30.23.8 29.9 30.2 29.9 29.9 32.9 25.6 25.9 25.6 25.6 28.5

aFrom Wilkinson and Algeo [1989].bAssuming constant seawater Mg/Ca during Cenozoic.cMultispecies [Anand et al., 2003].dG. sacculifer [Dekens et al., 2002].eG. sacculifer [Nurnberg et al., 1995, 1996, 2000].fG. ruber [Dekens et al., 2002].gG. ruber [Lea and Martin, 1996; Lea et al., 2000].

Table 9. Calculated Seawater d18O for Several Time Slices Using

Mg/Ca-Based Temperaturesa

Age, Ma Water d18O, %b Water d18O, %c

41.0–42.0 2.6 1.645.5–46.5 1.7 0.651.0 2.1 0.955.0 1.1 �0.157.0–58.0 2.0 0.8

a(V-SMOW); using equation of Erez and Luz [1983] and usingcalibration of Anand et al. [2003]. Time intervals are selected to spanrange of geochemical values observed in foraminifera from ODP Site 865.

bAssuming seawater Mg/Ca history of Wilkinson and Algeo [1989].cAssuming constant seawater Mg/Ca during Cenozoic.


warm climates [Barron et al., 1989]. There may also besome component of ice volume recorded in foraminiferald18O, though this should amount to a few tenths of a per milat most [Lear et al., 2000; Billups and Schrag, 2003].

4.6. Temperature Gradients

[24] Recent climate model simulations of early Paleogeneequable climates [e.g., Huber and Sloan, 2000, 2001] havefailed to reproduce the equator-to-pole (ETP) and verticalthermal gradients calculated using solely open ocean fora-miniferal d18O data, primarily due to discrepancies intropical SST. However, the warm and stable Mg/Ca-basedtropical SST from ODP Site 865 are similar to what ispredicted by model simulations and should therefore greatlyreduce this discrepancy between simulated and recon-structed ETP and vertical thermal gradients. These gradientscan be estimated by comparing tropical SST to deep-watertemperatures, and by using SST reconstructions for sitesfrom different latitudes. Our revised estimates of tropicalPacific vertical and ETP temperature gradients reduce thediscrepancy observed between model simulated gradientsfor the early Paleogene and open ocean data [e.g., Sloan etal., 1995; Huber and Sloan, 2001; Huber and Caballero,2003], and are consistent with a weaker ETP heat transportrelative to modern.[25] To estimate early Paleogene vertical temperature

gradients we compare Mg/Ca records from planktonic andbenthic foraminifera. A relatively consistent offset of 1.5 to2.5 mmol/mol is observed between planktonic foraminiferafrom ODP Site 865 and benthic foraminiferal data from aglobal composite record [Lear et al., 2000; Lear, 2001].This corresponds to early Paleogene vertical temperaturegradients of between 15 and 20�C. Comparison of Mg/Ca-based temperatures from ODP Site 865 (using seawater Mg/Ca values from Wilkinson and Algeo [1989]) and high-latitude SST from Zachos et al. [1994] indicate gradients of18–26�C. Small gradients of 18–21�C are calculated forthe late Paleocene through early Eocene, with the lowestvalues between 51 and 48 Ma, associated with peak tropicalSST and pronounced global warmth [Zachos et al., 2001].Gradients increase to 22–26�C during the middle Eocenewhen SST at high and middle latitudes cool by severaldegrees [Zachos et al., 1994] and tropical SST remainrelatively warm. If we assume constant Cenozoic seawaterMg/Ca, ETP temperature gradients are reduced by 4�C, andare between 14–22�C. For comparison, ETP temperaturegradients of 25–28�C are observed in the modern ocean.Both calculation of past vertical temperature gradients, anddirect comparison of tropical SST [Adams et al., 1990;Andreasson and Schmitz, 1998; Pearson et al., 2001;Tripati and Zachos, 2002; this study] to high-latitude SSTestimates [e.g., Zachos et al., 1994; Tripati et al., 2001],indicate significantly smaller vertical and ETP thermalgradients during the early Paleogene than at present, andgradients that are larger than previously estimated usingforaminiferal d18O data [Zachos et al., 1994].

5. Conclusions

[26] Mg/Ca ratios of early Paleogene surface-dwellingplanktonic foraminifera tests from ODP Site 865 range

from 3.6 to 5.3 mmol/mol, similar to observations inmodern tropical planktonic foraminifera. Early Eoceneratios are the highest observed, reaching a maximumbetween 51 and 48 Ma. Mg/Ca values decrease during thelate early Eocene, at 48 Ma, and remain relatively stablethrough the middle Eocene. The Mg/Ca record impliesrelatively warm and stable calcification temperatures thatare similar to modern tropical Pacific SST, despite uncer-tainties associated with diagenesis, choice of calibration,and past seawater Mg/Ca ratios. To assess sensitivity offoraminiferal Mg/Ca and Mg/Ca-based temperatures to ourassumptions, and to place reasonable constraints on sourcesof uncertainty, we perform a simple set of calculations andconsider the environmental conditions to which tests wereexposed. We conclude that carbonate dissolution had aminimal impact on foraminiferal Mg/Ca, as paleodepthsof ODP Site 865 are estimated to have been at least 1700 mabove the carbonate compensation depth. Recrystallizationhas probably impacted the geochemistry of foraminifera,and we estimate up to 20–30% secondary calcite may bepresent. Calcification temperatures of 30–34�C are calcu-lated using a published model for seawater Mg/Ca driven bychanges in overall hydrothermal activity and spreadingrates, and temperatures of 25�C to 29�C are calculatedassuming constant seawater Mg/Ca ratios during the Ceno-zoic. The generation of records from other sites wouldminimize these uncertainties, and a consistency in observedtrends in Mg/Ca for different planktonic taxa would alsovalidate our interpretations.[27] Our results have several implications for the origins

of the ‘‘cool tropics’’ paradox, despite the range of possibletemperatures derived from foraminiferal Mg/Ca. Mg/Ca-based paleotemperatures from ODP Site 865 are similar toestimates from low-latitude shallow-marine sequences, andreconstructions based on marine fauna. Comparison offoraminiferal elemental and stable isotope records at ODPSite 865 suggests the absolute SST values inferred from thed18O of mixed-layer planktonic foraminifera in deep-seasediments are biased toward colder values due to(1) secondary calcification, and (2) assumptions aboutsurface water d18O.[28] We interpret planktonic foraminiferal Mg/Ca and

d18O values to record early Paleogene surface water con-ditions that were slightly warmer and more saline than atpresent. Comparison to deepwater temperature records indi-cate there may have been weaker vertical thermal stratifica-tion during the early Paleogene. We also estimate reducedequator-to-pole thermal gradients during the early Paleo-gene, with minimum gradients coincident with peak tropicalwarmth during the early Eocene. Gradients increase duringthe middle Eocene, with tropical SST cooling by 3–4�C andhigh-latitude SST cooling by several degrees. This finding isconsistent with a weaker ocean circulation and heat transportduring the early Paleogene, relative to modern.

[29] Acknowledgments. We would like to thank Adina Paytan and ananonymous reviewer for critical and thoughtful reviews, as well asN. McCave, K. Billups, and C. Lear for comments on an earlier versionof this paper. Special thanks to A. Ravelo and M. Lyle for valuableadvice that helped to improve the manuscript. We would also like toacknowledge J. Morse, R. Norris, M. Wara, S. Schellenberg, and J. Farrellfor providing useful insights; R. Franks, M. Greaves, and S. Farquhar for


their invaluable technical help; and A. Johnson, P. Rumford, and B. Horanfor support. This research used samples and data provided by the OceanDrilling Program (ODP). The ODP is sponsored by the U.S. NationalScience Foundation (NSF) and participating countries under management

of Joint Oceanographic Institutions (JOI), Inc. Funding for this researchwas provided to A. Tripati by the U.S. Science Support Program through aSchlanger ODP Fellowship and by the British Council through a MarshallSherfield Postdoctoral Fellowship.

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��L. D. Anderson, Department of Ocean

Sciences, University of California, 1156 HighStreet, Santa Cruz, CA 95064, USA. ([email protected])M. L. Delaney, Department of Ocean Sciences/

Institute of Marine Sciences, University ofCalifornia, 1156 High Street, Santa Cruz, CA95064-1077, USA. ([email protected])H. Elderfield, Department of Earth Sciences,

University of Cambridge, Cambridge, UK.([email protected])D. C. Kelly, Department of Geology and

Geophysics, University of Wisconsin, 1215 W.Dayton Street, Madison, WI 53706, USA.([email protected])A. K. Tripati, Department of Earth Sciences/

Godwin Laboratory, University of Cambridge,Cambridge, UK. ([email protected])J. C. Zachos, Department of Earth Sciences/

Institute of Marine Sciences, University of Cali-fornia, Santa Cruz, CA 95064, USA. ( [email protected])


tropical sea-surface temperature reconstruction for the ...jzachos/pubs/tripati_etal_2003.pdf ·...

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