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Earth and Planetary Science Letters 282 (2009) 122–130 Contents lists available at ScienceDirect Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l Possible evidence for a large decrease in seawater strontium/calcium ratios and strontium concentrations during the Cenozoic Aradhna K. Tripati a,⁎, Warren D. Allmon b, Daniel E. Sampson c a b c Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Paleontological Research Institution and Department of Earth & Atmospheric Sciences, Cornell University, 1259 Trumansburg Road, Ithaca, New York 14850, USA Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA a r t i c l e i n f o Article history: Received 7 June 2008 Received in revised form 3 March 2009 Accepted 4 March 2009 Available online 2 April 2009 Editor: R.W. Carlson Keywords: strontium calcium carbon cycle Paleogene turritella a b s t r a c t Few constraints exist on the major element chemistry of ancient oceans. Turritellid marine snails precipitate aragonitic shells and are abundant in the Cenozoic fossil record, and therefore may be ideal for reconstructing past seawater chemistry. Here we report strontium to calcium (Sr/Ca) ratios of modern and early Cenozoic turritellid shells that still retain an aragonitic mineralogy. By applying partition coefficients determined from modern specimens to data for Paleocene and Eocene shells, we calculate paleo-seawater Sr/Ca values. The high Sr/Ca values recorded in fossil shells may imply that seawater Sr/Ca ratios varied from at least 9.5– 15.2 mmol/mol, much higher than modern average seawater values (8.6 mmol/mol). High paleo-seawater Sr/Ca ratios may indicate that substantial changes in the biogeochemical cycling of Sr have occurred over the past ~ 37 million years. A decrease in seawater strontium concentrations could have arisen from an increase in the proportion of weathered radiogenic silicate rocks relative to carbonates, reducing the riverine flux of strontium to the oceans, or changes in the exposure and erosion of aragonitic carbonates on tropical shelves due to variations in sea level. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The chemical evolution of the oceans is fundamentally tied to climate change throughout Earth history (Berner et al., 1983; Delaney and Boyle, 1988; Kump, 1989; Wilkinson and Algeo, 1989; Clemens et al., 1993; Hodell and Woodruff, 1994; Benson et al., 1995; Hardie, 1996; Stanley and Hardie, 1998; Ravizza and Zachos, 2003; Tripati et al., 2005). Constraining the chemistry of ancient oceans is of interest because the processes that control the elemental composition of seawater (e.g., weathering, seafloor spreading rates, biological productivity) also affect the carbon cycle. Calcium (Ca) is the third most abundant cation in seawater, and strontium (Sr) the fifth most abundant (Broecker and Peng, 1982; de Villiers, 1999). Significant variations in Cenozoic marine 87Sr/86Sr and δ44Ca records reflect large changes in ocean chemistry (Raymo and Ruddiman, 1992; de la Rocha and DePaolo, 2000; Farkas et al., 2007; Sime et al., 2007; Griffith et al., 2008). Interpreting these records has been a long-standing problem in the earth sciences. The dramatic increase in the marine 87Sr/86Sr record over the past 40 million years may reflect increased alkalinity inputs to the oceans driven by uplift of the Himalayan–Tibetan Plateau, which could possibly support a role for chemical weathering changes in carbon dioxide ⁎ Corresponding author. E-mail address: [email protected] (A.K. Tripati). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.03.020 drawdown, although interpretation of the Cenozoic 87Sr/86Sr record is the subject of controversy (Raymo et al., 1988; McCauley and DePaolo, 1997; Bickle et al., 2005). These records remain ambiguous because they can reflect both changes in the fluxes of these ions to the oceans, and changes in the isotopic composition of these fluxes. Sources of Sr and Ca to the oceans include riverine inputs, hydrothermal circulation (Elderfield and Schultz, 1996), and calcium carbonate dissolution. The primary sink for both is calcium carbonate sedimentation, with much greater amounts of strontium incorporated into coralline aragonite (deVilliers et al., 1994, 1995), relative to calcite (Stoll and Schrag, 2000; Rickaby et al., 2002). A secondary flux influencing seawater Sr concentrations on geologically short timescales is celestite (strontium sulfate) production and dissolution. Thus, longterm controls on seawater Sr/Ca ratios are changes in carbonate and silicate weathering, the primary mineralogy of carbonate deposition, carbonate diagenesis, and hydrothermal circulation. In the modern ocean both Sr and Ca have a relatively uniform concentration. In seawater, Sr/Ca ratios spatially vary by less than 2–3% (de Villiers, 1999), and in estuarine settings Sr/Ca ratios remain constant at salinities greater than 10 (Dodd and Crisp, 1982). Seawater inventories of Sr and Ca are high relative to the input and output fluxes (Broecker and Peng, 1982), leading to very long residence times in seawater. Modern residence times are ~2.5–3 Myr for Sr, and 1 Myr for Ca (Broecker and Peng, 1982; Palmer and Edmond, 1989). Seawater Sr/Ca ratios should A.K. Tripati et al. / Earth and Planetary Science Letters 282 (2009) 122–130 only change substantially over timescales greater than a million years (Broecker and Peng, 1982), although glacial–interglacial variations of 3– 4% have been inferred for the Quaternary (Stoll et al., 1999). Few methods exist for directly reconstructing the past ionic composition of seawater, and thus empirical constraints on seawater Sr/Ca ratios are very limited. Partition coefficients for Sr (KD) have been determined for inorganic precipitates (Averyt and Paytan, 2003) and some biogenic carbonates (Lorens, 1981; Delaney et al., 1985; deVilliers et al., 1994, 1995; Lea et al., 1999; Lear et al., 2003), and provide a direct method for inferring the composition of seawater in the past. These studies demonstrate that there is a link between the elemental ratio of seawater, precipitation temperature, and the elemental composition of minerals such as calcium carbonate. However, skeletal growth rate can also exert a primary or secondary influence on Sr/Ca ratios in some biologically-produced carbonates (de Villiers et al., 1995; Klein et al., 1996; Purton and Brasier, 1997; Elderfield et al., 2000; Stoll and Schrag, 2000; Cohen et al., 2002; Sosdian et al., 2006; Gentry et al., 2008), as can diagenetic overprinting (Andreasen and Delaney, 2000). Here we develop the novel method of using the Sr/Ca ratio of aragonitic gastropod mollusks belonging to the family Turritellidae to reconstruct past seawater Sr/Ca ratios. Turritellids are selected because members of this family are abundant in the geologic record, occurring in many shallow marine fossiliferous sediments of the Cenozoic. Fossil shells are found in a wide variety of facies worldwide, and they have numerous modern representatives. The ecological tolerances of modern turritellids are relatively well constrained: they prefer relatively shallow shelf environments and relatively cool, nutrient-rich waters (Allmon, 1988). They precipitate monomineralic aragonite shells that are often well preserved (Allmon, 1996; Andreasson and Schmitz, 1996). Retention of an aragonitic mineralogy and trace metal ratios provides a means for assessing preservation of skeletal chemistry. They are primarily epifaunal, though some burrowing infaunal species are known. Of the ~100 modern species, fewer than five occur in hypersaline environments; none are observed in brackish waters. Our objectives were to estimate Paleocene and Eocene seawater Sr/ Ca ratios using aragonitic fossil turritellid specimens, and examine the implications of these values for the processes that control the biogeochemical cycling of Sr and Ca. The aim of previous published studies that have used Sr/Ca ratios in early Cenozoic aragonitic mollusks has been to assess the preservation of aragonitic mineralogy (Andreas- 123 son and Schmitz, 1996, 1998), or to determine the utility of Sr/Ca ratios as a paleotemperature indicator (Purton et al., 1999). To this end, we needed to study the controls on Sr/Ca ratios using modern specimens of turritellid snails from a range of marine climate regimes. To date, no data exist on the trace metal chemistry of turritellids, and very little data exist documenting the relationship between environmental conditions and the trace metal chemistry of modern mollusks, whether aragonitic or calcitic (Steuber and Veizer, 2002; Sosdian et al., 2006; Gentry et al., 2008). A few studies have examined controls on trace metal partition coefficients in mollusks and these have mainly focused on calcitic bivalves that are abundant in archeological sites (Lorens, 1981; Dodd and Crisp, 1982; Klein et al., 1996). Very little work has been done investigating temperature and “vital effect” controls on Sr/Ca ratios in modern aragonitic mollusks. The existing data (Lorens, 1978; Dodd and Crisp, 1982; Stecher et al., 1996; Sosdian et al., 2006; Gentry et al., 2008) suggest that the incorporation of Sr into modern aragonitic marine mollusk shells is influenced by seawater composition and temperature, and also possibly skeletal growth (calcification) rate. 2. Materials and methods We chose to investigate a large number of modern and Early Cenozoic turritellid species that have been assigned to several different genera in an attempt to encompass as much potential taxonomic variation as possible. The genus-level systematics in the Turritellidae are still unclear (Allmon, 1996, 2005); herein we use a small number of supraspecific assignments that recent phylogenetic analyses have confirmed, but otherwise refer to species all as Turritella sensu lato. Modern specimens (n = 43) of turritellids used in this study were from multiple, widely dispersed localities (Fig. 1). The modern species studied were Turritella exoleta, T. gonostoma, T. variegata, T. leucostoma, T. cingulata, T. banksi, and Maoricolpus roseus. Modern specimens were obtained from the collections in the Department of Invertebrate Zoology and Geology at the California Academy of Sciences, the Paleontological Research Institution, and Kingsborough Community College (Brooklyn, NY). At least two individual specimens of each species were analyzed. A large number of modern individuals belonging to the species Turritella gonostoma, T. leucostoma, and T. banksi were also analyzed to look at the variability within monospecific sample populations. Fig. 1. Map showing modern (filled circles) and Paleocene and Eocene (open circles) localities. Modern specimens of Turritella gonostoma, Turritella rossi, Turritella cingulata, Turritella leucostoma, Turritella banksi, Turritella variegata, Turritella anactor, Turritella communis, Turritella clarionensis, Turritella cooperi, Turritella acicula, and Gazameda iredalei from a dozen localities were used in this study (Fig. 1), and are from the collections of the Department of Invertebrate Zoology and Geology at the California Academy of Sciences. In addition, specimens of Turritella exoleta and Turritella gonostoma, and Maoricolpus roseus were obtained from Kristin Polizzotto (Kingsborough Community College, Brooklyn, NY) and the collections of the Paleontological Research Institution. Individual specimens have all been collected within the past 50 years. Specimens of Haustator rina from the Lisbon Formation and H. carinata from the Gosport Sand, of H. perdita from the Moodys Branch Formation, and of H. alabamiensis from the Clayton Formation were analysed. A specimen of K. pleboides (Vaughan) from the Upper Lisbon Formation, on loan from the Smithsonian Institution's National Museum of Natural History, was analyzed. This shell is from an outcrop of the Gosport Formation and is middle Eocene in age (Allmon, 1996). A second middle Eocene specimen of Turritella cf. imbricataria, is from the Bracklesham Beds of the Hampshire Basin, England (on loan from the University of California Museum of Paleontology's Invertebrate Paleontology Collections. 124 A.K. Tripati et al. / Earth and Planetary Science Letters 282 (2009) 122–130 Table 1 Sr/Ca ratios for some different modern biogenic calcium carbonate. Organism Mineralogy Possible controls on Sr/Ca Turritellid gastropods Coral Mytilus trossulus (bivalve mollusc) Coccoliths Planktonic foraminifera Aragonite Aragonite Calcite Calcite Calcite Benthic foraminifera Calcite Sr/Ca range KD (mmol/mol) Seawater Sr/Ca Growth rate Temperature 1.1–2.7 Seawater Sr/Ca, growth rate, temperature 8.2–9.1 Seawater Sr/Ca, growth/metabolic rate, salinity 1.0–1.6 Seawater Sr/Ca, growth rate, temperature? 1.5–3.2 Seawater Sr/Ca, dissolution, growth rate and 1.2–1.5 other vital effects, temperature, carbonate ion Seawater Sr/Ca, dissolution, growth rate and 0.8–2.0 other vital effects, temperature, carbonate ion 0.13–0.31 0.85–1.06 0.12–0.19 0.17–0.37 0.14–0.17 Source This work de Villiers et al., 1995,1994 Klein et al., 1996 Rickaby et al., 2002; Stoll and Schrag, 2000 Elderfield et al., 2000; Lea et al., 1999 0.09–0.23 Lear et al., 2003; McCorkle et al., 1995; Rathburn and DeDeckker, 1997; Rosenthal et al., 1997 Values for the partition coefficient (KD) are calculated using a mean seawater Sr/Ca ratio of 8.6 mmol/mol. A total of 18 Paleocene and Eocene specimens were also analyzed as part of this study, representing 11 time horizons, with an average resolution of a sample/2.5 Ma (Table 1). In order to minimize vital effects, specimens classified as members of a single putative evolutionary lineage — T. exoleta lineage (Haustator rina group) (Allmon, 1996) — were used (n = 5), including H. alabamiensis, H. rina, H. carinata, and H. perdita. All fossil specimens were obtained from the collections of the Paleontological Research Institution and the National Museum of Natural History, Smithsonian Institution. Preservation of early Cenozoic specimens was determined through analysis of shell microstructure and mineralogy, and visual inspection of specimens. Some samples retained their original luster and none were chalky. Shell microstructures were seen to be pristine using scanning electron microscopy (SEM), and X-ray diffraction (XRD) pattern analysis was used to characterize the mineral phases present in each fossil shell. Inspection of SEM and XRD patterns for the fossil specimens confirmed the shells are completely aragonitic (Tripati et al., 2003). Specimens were scrubbed with a brush, and then rinsed and sonicated in distilled water and methanol to remove any dirt. Powdered samples were milled from the surface of each specimen. Powder from the outermost layer was discarded. A dental drill was used in combination with a microscope to collect samples. Between 2 and 10 powdered samples, ranging from approximately 100 to 400 µg in mass, were collected from each whorl. A total of 20 to 80 samples were taken from each specimen, using a previously described technique (Allmon et al., 1992; Andreasson and Schmitz, 1996). Fewer samples were taken near the apex. Growth rates have been previously discussed for some modern species in the group (Allmon et al., 1992, 1994), and were taken from these studies as well as being qualitatively estimated by comparing whorl number and shell length to variability in stable isotope and Sr/Ca profiles. Elemental concentrations were determined using a Thermo Finnigan Element magnetic-sector inductively-coupled plasma mass spectrometer (ICP-MS) and a Perkin–Elmer ICP optical emission spectrophotometer (ICP-OES) in the Institute of Marine Sciences at the University of California, Santa Cruz. Samples were dissolved in 2% nitric acid, and analyzed for calcium, strontium, barium, and magnesium concentrations. The nitric acid was spiked with a stock solution containing scandium, which was monitored as an internal standard on the ICP-MS. Concentrations were determined using a calibration curve established using 4 synthetic standards created from reagents and reference materials to match the matrix of and span the trace element concentrations in the unknowns. In addition, three consistency standards were measured on a routine basis, typically after every five samples. Cation concentrations and matrices in all standards were similar to sample concentrations. Long-term monitoring of the consistency standards indicates a precision (%RSD) of better than 0.7% for analyzed Sr/Ca and Mg/Ca ratios. Over 1070 sample analyses were performed in total. Two to three replicates were run of over 100 samples; each was run at various dilutions (between 50 and 200 ppm [Ca]). Inclusion of reductive or oxidative cleaning steps did not significantly impact Sr/Ca ratios. Splits of material taken for trace element analyses were analyzed for stable isotope ratios. From 20 to 100 μg of powdered sample was analyzed on either a Micromass Prism or a Micromass Optima gas Fig. 2. Sr/Ca ratios in modern turritellids compared to climatological temperatures and shell δ18O. A) Sr/Ca ratios for modern species compared to Levitus sea surface temperatures (light blue: Turritella exoleta (modern member of early Paleogene lineage utilized in this study), red: T. cingulata, black: T. leucostoma, dark blue: T. banksi, purple: T. variegata, orange: T. gonostoma; green: Maoricolpus roseus). Vertical error bars corresponding to the range of (minimum, maximum, or mean) values observed within each specimen are shown, and horizontal error bars correspond to average ±2 °C uncertainty in minimum, mean, and maximum calcification temperature due to range of possible water depths. Analytical uncertainty is smaller than the size of the symbols. Partition coefficients range from 0.12–0.32, and are calculated by dividing the shell Sr/Ca ratios by the seawater ratio of 8.5 mmol/mol. B) Profiles of Sr/Ca and δ18O (‰; V-PDB) in specimen of Maoricolpus roseus (collected at 3 m depth) showing strong coherence. C) Comparison of Sr/Ca ratios with Levitus temperatures. A.K. Tripati et al. / Earth and Planetary Science Letters 282 (2009) 122–130 125 3. Results The modern shells analyzed have mean Sr/Ca values of 1.5 to 2.2 mmol/mol, with minima and maxima of 1.3 and 2.7 mmol/mol, respectively (Figs. 2 and 3). Turritellid gastropods, similar to other mollusks and fish otoliths, form low-Sr aragonitic skeletons, in contrast to the high-Sr aragonitic skeletons precipitated by corals (Table 1). Most of the modern and fossil turritellid shells we examined exhibit a pronounced cyclicity in skeletal Sr/Ca values (Figs. 2 and 3). In modern specimens, shell Sr/Ca values are negatively correlated with δ18O (Figs. 2B, 3 and 4), and positively correlated with temperature (Fig. 2C). In fossil specimens, a negative correlation Fig. 3. Sr/Ca (filled circles) and Mg/Ca (open circles) profiles of several modern and early Cenozoic specimens. A) Modern specimen of Turritella gonostoma from Baja California. B) Modern specimen of Maoricolpus roseus from Tasmania. C) Modern specimen of M. roseus from New Zealand. (D) Modern specimen of T. exoleta from Tobago. E) Middle Eocene specimen of T. imbricataria from England. F) Middle Eocene specimen of Kapalmerella pleboides. source mass spectrometer with a common acid bath at the University of California, Santa Cruz Stable Isotope Laboratory. Prior to analysis, samples were roasted in vacuo for 1 hour. Stable isotope values are reported as delta values (δ) in per mil (parts per thousand) notation (‰) relative to a Vienna Pee Dee Belemnite (V-PDB) standard. Precision of an in-house standard (which was run repeatedly during every sample run) is ± 0.05‰ for δ13C and ± 0.08‰ for δ18O (1 std. dev.; n = 107). Some stable isotope data for 4 specimens of M. roseus and T. gonostoma has been published (Allmon et al., 1992, 1994) and was used in this study. Fig. 4. δ18O (filled circles) and δ13C (open circles) profiles of several modern and early Cenozoic specimens. Labels are same as in Fig. 3. 126 A.K. Tripati et al. / Earth and Planetary Science Letters 282 (2009) 122–130 Table 2 Sr/Ca ratios in early Cenozoic turritellid specimens from marine facies. Species Age (global stage) Formation Mean (min, max) Sr/Ca mmol/mol Turritella perdita (T. exoleta lineage) Haustator carinata (T. exoleta lineage) H. carinata (T. exoleta lineage) H. carinata (low-res. sampling of 3 specimens) Kapalmerella pleboides H. rina (T. exoleta lineage) T. nasuta T. imbricataria Kapalmerella femina Kapalmerella mortoni postmortoni T. praecincta praecincta T. multilira T. aldrichi H. alabamiensis (T. exoleta lineage) H. alabamiensis (T. exoleta lineage) Late Middle Eocene ~ 37–38 Ma Late Middle Eocene ~ 38 Ma Late Middle Eocene ~ 38 Ma Late Middle Eocene ~ 38 Ma Late Middle Eocene ~ 38 Ma Late Middle Eocene ~ 40 Ma Late Middle Eocene ~ 43 Ma Middle Eocene ~45 Ma Lower Eocene ~54 Ma Upper Paleocene ~55 Ma Upper Paleocene ~56 Ma Upper Paleocene ~58 Ma Lower Paleocene ~60–63 Ma Lower Paleocene ~60–63 Ma Early Paleocene (Danian) ~ 65–64 Ma Moodys Branch Formation Gosport Sand Gosport Sand Gosport Sand Gosport Sand Lisbon Formation Lower Lisbon Formation Bracklesham Beds Bashi Marl Tuscahoma Formation Tuscahoma Formation Nanafalia Formation Porters Creek Formation Porters Creek Formation Clayton Formation 2.7 (2.3, 3.0) 3.3 (3.1, 3.6) 3.4 (2.8, 3.8) 3.6 (3.4, 3.6) 3.3 (3.0, 3.5) 3.1 (2.8, 3.4) 3.5 (3.3, 3.9) 3.5 (2.9, 4.4) 3.0 (2.6, 3.4) 5.0 (4.9, 5.5) 2.8 (2.5, 3.3) 3.6 (3.5, 3.7) 3.9 (3.8, 3.9) 3.4 (3.5, 3.2) 3.3 (2.8, 3.9) For reference, the mean, minimum, and maximum Sr/Ca ratios observed in modern Turritella exoleta are 1.5, 1.4, and 1.8, respectively. between Sr/Ca and δ18O is also observed (Figs. 3 and 4). Our observations are consistent with the intershell Sr/Ca/δ18O and Sr/ Ca/temperature relationship observed in other fossilized aragonitic gastropods (Andreasson and Schmitz, 1996, 1998; Purton et al., 1999), bivalves (Stecher et al., 1996), and otoliths (Bath et al., 2000). We compared Sr/Ca profiles to monthly average sea-surface temperatures from the Levitus Ocean Atlas dataset (Levitus et al., 1994), using climatologically-averaged temperatures from the depth the organism was collected at. The depth range for each species (observed for modern) (Allmon et al., 1992, 1994; Allmon, 1988) was used to estimate a range of temperatures an organism might have experienced throughout its life and a temperature uncertainty for the calibration. Using minimum, maximum, and mean Sr/Ca values from all of the modern turritellid specimens, a Sr/Ca-temperature calibration for the family Turritellidae was calculated (Fig. 2A). A linear regression is the most robust and simple fit to the data over the temperature range observed. These data were used to estimate the temperature effect on the partition coefficient. Partition coefficients range from 0.13–0.31 (Table 1), and are calculated by dividing the shell Sr/Ca ratios by the mean seawater Sr/Ca ratio (Broecker and Peng, 1982). The application of the range of reported seawater Sr concentrations (2–3% in the Atlantic and Pacific Oceans (de Villiers, 1999)) does not make a significant difference to our results. In order to investigate possible vital effects, the species-specific relationships between the partition coefficient and temperature were also determined. We observe that the turritellid Sr/Ca data falls into three clusters (Fig. 2). Slower-growing turritellid species (e.g., T. cingulata, M. roseus — see Allmon et al., 1994) have distinctly lower Sr/Ca ratios than faster-growing turritellid species (e.g., T. gonostoma — see Allmon et al., 1992), supporting a secondary growth rate control on skeletal Sr/Ca values. We speculate the observed clustering in Sr/Ca (and KD) between species may be related to the phylogenetic distance between various species, but this conclusion would need more supporting data before it can be accepted. Despite this clustering, slopes of the general turritellid calibration and species-specific calibrations are similar. For example, the calibration for T. exoleta (Fig. 2) differs by 15% from the general calibration. Both calibrations are applied to fossil shell data in order to reconstruct the Sr/Ca ratio of seawater. The Sr/Ca maxima that occur with “summertime” oxygen isotope minima are associated with periods of high growth rate; in contrast, in some specimens an ontogenetic increase in Sr/Ca and decrease in δ13C occurs after the first or second year of growth, when growth rates are the lowest. These observations indicate that it is unlikely that the cyclical fluctuations in shell Sr/Ca primarily record seasonal changes in growth rate, although growth rate might exert a minor secondary control. The Sr/Ca ratios of the early Cenozoic turritellid specimens analyzed in this study are distinctly higher than those of modern specimens (Table 2). Modern turritellids all have mean Sr/Ca ratios of less than 2.2 mmol/mol, with maximum values of less than 2.7 mmol/mol (Fig. 2A). In contrast, a specimen of Kapalmerella pleboides from the ~38 Ma (mid-Bartonian) Gosport Sand (Table 2) has a mean shell Sr/Ca value of 3.3 mmol/mol, with a range of values from 3.0 to 3.5 mmol/mol. A Lutetian age (~45 Ma) specimen of T. imbricataria has a mean shell Sr/Ca value of 3.5 mmol/mol, and a range of values from 2.9 to 4.4 mmol/mol. Specimens belonging to the T. exoleta lineage have similar values. These high Sr/Ca ratios are also supported by values reported for fossil aragonitic turritellids (and conid) gastropods from the Gulf Coast (N3.0 mmol/mol) (Kobashi et al., 2001), Hampshire Basin (3–4 mmol/mol) (Purton et al., 1999), Paris Basin (Andreasson and Schmitz, 1996) and Nigeria (Andreasson and Schmitz, 1998). 4. Discussion The absolute Sr/Ca ratios for these fossil turritellid shells are higher than values observed in modern specimens, which may imply significantly higher seawater Sr/Ca ratios in the past. Shell length and whorl number, in relation to the δ18O, δ13C, or Sr/Ca profiles, are similar in modern and fossil specimens (Fig. 3–5) suggesting metabolic and calcification rates were not significantly different in the past. Thus the high Sr/Ca ratios of the early Cenozoic turritellids appear to not be attributable to higher overall rates of precipitation. Shell δ18O values (Fig. 4) and other proxy data (Zachos et al., 1994; Pearson et al., 2001; Tripati et al., 2003) indicate temperatures were warmer during the early Cenozoic, and this would contribute to the elevated Sr/Ca ratios observed in these specimens. However, assuming seawater Sr/Ca ratios have not changed over the Cenozoic and the temperature–KD relationship has remained the same, unrealistically warm early Cenozoic shallow marine temperatures of N40 °C are necessary to explain these observed shell Sr/Ca values. If seawater Sr/ Ca ratios were lower in the early Cenozoic, even warmer temperatures are required to explain these data. We estimated a range of early Cenozoic seawater Sr/Ca values (Table 3) using end-member temperatures constrained by shell δ18O profiles from this study (Fig. 4), and from the literature (Zachos et al., 1994; Andreasson and Schmitz, 1996; Purton and Brasier, 1997; Kobashi et al., 2001; Pearson et al., 2001; Tripati et al., 2003), our own temperature calibrations (Fig. 2A), and Sr/Ca values for the fossil shells. For example, using the specimen of K. pleboides, we calculate a range of seawater Sr/Ca values of 12.1–16.2 mmol/mol for ~ 38 Ma, significantly higher than the modern value of 8.6 mmol/mol (Broecker and Peng, 1982; de Villiers, 1999). We estimate a higher ratio of ~ 15.2–25.2 mmol/mol during the Lutetian (~45 Ma) using T. imbricataria from England. Similar seawater Sr/Ca ratios are estimated using the Sr/Ca data for other early Cenozoic turritellids, including members of the T. exoleta lineage. A.K. Tripati et al. / Earth and Planetary Science Letters 282 (2009) 122–130 Fig. 5. Sr/Ca ratios for Eocene turritellid specimens from the Gulf Coast. Sample and whorl number are also shown. A) Haustator rina (member of T. exoleta lineage) from the Lisbon Formation, 33 mm long. B) H. carinata (member of T. exoleta lineage) from the Gosport Sand, 35 mm long. C) Kapalmerella pleboides from the Gosport Sand, 28 mm long. The lowest Sr/Ca values are reconstructed in the late middle Eocene, at ~37–38 Ma. There is evidence for a possible transient maximum in seawater Sr/Ca ratios at ~ 55 Ma, near the Paleocene– Eocene boundary. Excluding this point, the minimum range in seawater Sr/Ca values we calculate for the Paleocene and Eocene is ~ 9.5 (~37–38 Ma) to ~ 15 mol/mol (~45 Ma), with a large uncertainty 127 in our estimates (Fig. 6 and Table 3). Our reconstructed values for the Early Cenozoic are broadly consistent with evidence for high seawater Sr/Ca ratios in the Cretaceous of ~ 13–14 mmol/mol, based on measurements of low-Mg calcite in bivalves (Steuber, 2002). A compilation of measurements for 500–70 Ma using data for brachiopods and mollusks indicates that seawater Sr/Ca ratios may have been at a maximum ~70 Ma (and at 420 Ma) (Steuber and Veizer, 2002). There is also data from aragonitic coral that support values of 9.9 mol/mol in the early Oligocene (Ivany et al., 2004), similar to seawater Sr/Ca ratios estimated from our youngest sample. We find that Mg/Ca ratios of the fossil turritellid specimens are just below the modern range observed (Fig. 3), consistent with seawater Mg/Ca ratios during the Paleocene and Eocene that were slightly lower than today, supporting previous findings based on fluid inclusions in evaporites (Zimmerman, 2000; Lowenstein et al., 2001; Horita et al., 2002), echinoid Mg/Ca ratios (Dickson, 2002), and modeling studies (Wilkinson and Algeo, 1989; Hardie, 1996; Stanley and Hardie, 1998). However, these high early Cenozoic seawater Sr/Ca estimates are significantly different than some published reconstructions that are based on benthic and planktic foraminifera, and bulk carbonate (mainly coccoliths). All of these phases are comprised of calcite, and some authors have proposed that in early Cenozoic deep-sea sediments, their δ18O is influenced by diagenesis (Schrag et al., 1995; Pearson et al., 2001, 2007). If true, the Sr/Ca ratios may also be impacted. A multi-species benthic foraminiferal Sr/Ca record was interpreted as reflecting large changes in oceanic Sr/Ca, with a maximum in the Cretaceous but a minimum in seawater ratios during the Eocene (~7.8 mmol/mol; 34–55 Ma) (Lear et al., 2003). This reconstruction used large species-specific and site-specific adjustments for an unknown effect on calcification that may be due to variations in pressure, carbonate ion concentrations, dissolution, and/ or temperature. A global composite of records utilizing the Sr/Ca ratio composition of bulk carbonate shows very little change throughout the Cenozoic (Andreasen and Delaney, 2000), yet as mentioned above, bulk carbonate records are reported to primarily reflect diagenetic overprinting (Schrag et al., 1995; Andreasen and Delaney, 2000). A long-term Cenozoic record based on planktonic foraminifera shows extensive scatter with little trend in the mean value (Graham et al., 1982), but is based on measurements of mixed-species assemblages which renders it poor for using to infer changes in seawater Sr/Ca ratios given recent work showing large species offsets in planktonic foraminifera (Elderfield et al., 2000). By including a large number of modern specimens, and multiple species, we estimated the natural variability of KD, which should reflect a number of factors including temperature, inter-specific Table 3 Reconstructed seawater Sr/Ca values (mmol/mol) using shell Sr/Ca data in Table 2, and different partition coefficients. Species Age T (°C) Sr/Caa Sr/Cab Sr/Cac Turritella perdita Haustator carinata H. carinata H. carinata Kapalmerella pleboides H. rina T. nasuta T. imbricataria Kapalmerella femina Kapalmerella mortoni postmortoni T. praecincta praecincta T. multilira T. aldrichi H. alabamiensis H. alabamiensis Late Middle Eocene ~37–38 Ma Late Middle Eocene ~38 Ma Late Middle Eocene ~38 Ma Late Middle Eocene ~38 Ma Late Middle Eocene ~38 Ma Late Middle Eocene ~40 Ma Late Middle Eocene ~43 Ma Middle Eocene ~45 Ma Lower Eocene ~54 Ma Upper Paleocene ~55 Ma Upper Paleocene ~56 Ma Upper Paleocene ~58 Ma Lower Paleocene ~60–63 Ma Lower Paleocene ~60–63 Ma Early Paleocene (Danian) ~ 65–64 Ma 25–35 25–35 25–35 25–35 25–35 25–35 25–35 15–25 25–35 25–35 25–35 25–35 25–35 25–35 25–35 10.4–12.4 12.4–16.8 13.1–15.1 12.4–18.4 12.1–16.2 11.7–15.1 13.5–17.8 15.2–25.2 11.7–14.1 19.0–26.5 11.4–13.5 12.8–18.9 13.5–20.6 11.1–18.9 13.5–15.1 9.5–14.0 11.4–18.9 12.0–17.1 11.4–20.7 7.4–23.1 10.0–27.7 9.0–29.2 11.0–27.7 9.7–26.9 9.0–26.2 10.6–30.0 9.4–33.8 8.4–26.2 15.8–42.3 8.1–25.4 11.3–28.5 12.3–30.0 11.3–24.6 9.0–30.0 a Assumes KD between 0.13 and 0.31 (range observed in modern; Table 1). Assumes KD–temperature relationship determined for modern Turritellidae, and temperatures shown above. c Assumes KD–temperature relationship determined for modern T. exoleta, and temperatures shown above. b 10.8–17.1 10.1–21.3 12.4–17.1 128 A.K. Tripati et al. / Earth and Planetary Science Letters 282 (2009) 122–130 Fig. 6. Shell Sr/Ca ratios for Paleocene and Eocene turritellids (black squares), and estimates of seawater Sr/Ca (circles and black dashed line). Black dashed line represents minimum reconstructed seawater Sr/Ca ratios, and vertical black dotted line marks modern seawater Sr/Ca ratio. Dark blue circles are estimates calculated for members of the T. exoleta lineage, using T. exoleta calibration. Light blue circles are estimates calculated for all fossil shells using Turritellidae calibration. Also shown are published estimates of seawater Sr/Ca for the Cretaceous (Steuber, 2002; Steuber and Veizer, 2002) and early Oligocene (Ivany et al., 2004). Seawater Sr/Ca ratios are reconstructed based on measured shell Sr/Ca ratios, partition coefficients (KD) in modern turritellids, and estimates of temperature from δ18O and Mg/Ca (shown in Table 3). North American stage names are also shown. variability, growth rate, and other factors. Mean Sr/Ca ratios of turritellids, as well other aragonitic gastropods and bivalves, have declined over the last 37 Ma. Using the modern partition coefficients (Table 1), we estimate minimum seawater Sr/Ca ratios in the Paleocene and Eocene that range from 9.5 to 15.2 mmol/mol (Fig. 5), consistent with evidence described above for high Sr/Ca ratios in the Cretaceous and values of ~ 9.9 mmol/mol in the Oligocene. A caveat is that controls on strontium partitioning in biogenic carbonates are not yet well understood. However, these reconstructions should be testable with other proxies, such as Sr/Ca ratios in marine barite (Averyt and Paytan, 2003). A decrease in seawater Sr/Ca to modern values would imply that there were large changes in the biogeochemical cycling of calcium and/or strontium in the oceans after the late middle Eocene. Measurements of fluid inclusions in halites indicates that seawater Ca concentrations have declined over the Cenozoic, with estimates of ~15–20 mmol/kg for the Paleocene and Eocene, roughly 1.5–2× modern values (Zimmerman, 2000; Lowenstein et al., 2001; Horita et al., 2002). The Ca isotope composition of marine barites from the last 28 Myr shows changes that may indicate higher seawater Ca concentrations in the Oligocene, relative to present (Griffith et al., 2008). If seawater Ca concentrations were higher throughout the early Cenozoic (or similar to modern), as implied by these proxies, and seawater Sr values remained constant, then the Sr/Ca ratio of seawater should increase through time, the opposite trend to what is seen in our data. Therefore our results can be interpreted to reflect a decrease in seawater Sr concentrations since the middle Eocene. A.K. Tripati et al. / Earth and Planetary Science Letters 282 (2009) 122–130 Seawater Sr concentrations (and Sr/Ca ratios) are controlled by the primary mineralogy of carbonate deposition (aragonite versus calcite), by changes in rates of carbonate and silicate weathering, carbonate diagenesis, and by hydrothermal circulation. Riverine fluxes, hydrothermal input, and carbonate dissolution represent sources with low Sr/Ca ratios; most calcite deposited has very little Sr and has a KD value that is a factor of 10 lower than typical values for coralline aragonite (Table 1). A large decrease in seawater Sr concentrations could arise from an increase in aragonite deposition relative to calcite, or variations in riverine or hydrothermal Sr fluxes. The last major transition from calcite to aragonite seas occurred at the base of the Cenozoic (Horita et al., 2002) and is too early to account for all of the inferred change in seawater Sr, although may account for some of the change. A recent reevaluation of seafloor production rates (Rowley, 2002) argues that spreading rates were relatively constant during the Cenozoic, consistent with Li/Ca data (Delaney and Boyle, 1986) that indicate hydrothermal exchange has not varied by more than 30–40%. In light of these constraints on hydrothermal and carbonate Sr fluxes throughout the Cenozoic, our reconstruction, if taken at face value, would imply that there has been a large decrease in riverine Sr fluxes over the past 37 Ma. If true, this would suggest that a component of the sharp upswing in the marine 87Sr/86Sr isotope curve (Howarth and McArthur, 1997; Ravizza and Zachos, 2003) could be due to a decrease in riverine strontium fluxes to the oceans, possibly arising from an increase in the proportion of weathered silicate rocks relative to carbonates. In that scenario, our findings could be reconciled with the seawater Sr isotope curve if there was a large increase in the 87Sr/ 86 Sr ratio of riverine fluxes over the same interval. It would also be consistent with a role for uplift of the Himalayan–Tibetan Plateau over the past 40 Ma (Rowley and Currie, 2006) and global weathering changes in the transition from early Cenozoic greenhouse to icehouse conditions (Tripati et al., 2005). Alternately, drops in sea level during the early Cenozoic (Late Eocene-Oligocene), associated with the growth of ice sheets (Tripati et al., 2005, 2008), could have led to the exposure of tropical shelves and the weathering of aragonitic shelf carbonates (Delaney and Boyle, 1988; Opdyke and Wilkinson, 1988), providing a source of Sr to the oceans. The riverine flux of Sr associated with the erosion of aragonite may have decreased through time, as tropical shelf area is thought to have been greater in the early Cenozoic (Delaney and Boyle, 1988; Opdyke and Wilkinson, 1988). Therefore this mechanism might also explain a decline in seawater Sr/Ca ratios over the past 37 Ma. Future development of temporally-detailed histories for seawater cation composition from minerals such as aragonite, calcite, and barite (Averyt and Paytan, 2003), if coupled with isotopic records, should allow us to determine how the timing of changes in the biogeochemical cycling of strontium and calcium is related to changes in the carbon cycle and will help to assess the role of uplift and weathering in the Cenozoic decline in atmospheric carbon dioxide (Pearson and Palmer, 2000; Pagani et al., 2005). Acknowledgements Funding for this work was from a Junior Research Fellowship from Magdalene College, a NERC fellowship, a Gates Millennium Scholars Program Fellowship, and an IGPP grant through the Center for the Origin, Dynamics, and Evolution of Planets. These samples were obtained in part through the kind assistance of Thomas Waller, Warren Blow, and Kristin Polizzotto. 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