Download Possible evidence for a large decrease in seawater strontium

Earth and Planetary Science Letters 282 (2009) 122–130
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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.
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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.
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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. We would like to thank Rob
Franks for his assistance with method development and data analysis,
and Thomas Steuber, Linda Ivany, Louis Derry, and an anonymous
reviewer for comments that improved an earlier version of this
manuscript, as well as Mike Bickle, Albert Galy, Harry Elderfield,
Margaret Delaney, Ed Tipper, Rob Franks, Linda Anderson, Kyger C.
Lohmann, Paul Koch, Rosalind Rickaby, and James Zachos for
comments on this work.
129
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