Bivalve skeletons record sea-surface temperature ... - GeoScienceWorld

Bivalve skeletons record sea-surface temperature and d18O via Mg/Ca and 18O/16O ratios Robert T. Klein Kyger C Lohmann University of Michigan, Department of Geological Sciences, Ann Arbor, Michigan 48109-1063

Charles W. Thayer University of Pennsylvania, Department of Geology, Philadelphia, Pennsylvania 19104-6316 ABSTRACT Paleotemperatures have been widely deduced from skeletal 18O/16O ratios, but these are also dependent on salinity. Without an independent measure of salinity, 18O/16O ratios cannot provide accurate data on past temperature and climate. We grew marine mollusks (bivalves) in the field while real-time data on local seawater temperature and chemistry (Mg, Ca, d18O, and salinity) were gathered. Here we show that for Mytilus trossulus, skeletal Mg/Ca ratios provide an accurate measure of temperature and that weekly sea-surface temperatures may be estimated with an apparent accuracy of approximately 61.5 &C. Thus, with analyses of both Mg/Ca and d18O from the same specimen, it is possible to determine seawater temperature and d18O.

INTRODUCTION Boundary conditions, such as sea-surface temperature data reported by CLIMAP (1981), have provided the basis for general circulation models that attempt to simulate global interactions between the atmosphere, ocean, and land since the last glacial maximum (e.g., Kutzbach and Ruddiman, 1993). Crowley et al. (1986) suggested that seasonality may be the single most important boundary condition for maintenance of permanent ice cover; however, precise seasonal temperature data are lacking because appropriate oxygen isotope records (e.g., marine mollusks) are often overprinted by the participation of fresh-water runoff (compared to seawater, fresh-water runoff is more depleted in d18O). Consequently, owing to salinity effects (fresh water–seawater mixing), paleotemperatures inferred from oxygen isotope compositions of marine skeletons may incorrectly be biased toward warm temperatures. To separate these effects, we grew marine mussels in their natural habitat while simultaneously collecting the temperature, chemistry (Mg, Ca, d18O, and salinity), and isotopic composition of the seawater in the same habitat. Although other studies have calibrated skeletal oxygen isotope composition to seawater temperature (Arthur et al., 1983; Weidman et al., 1994), no one has previously performed such an extensive field experiment on modern marine bivalves. Our measurements of elemental and isotopic composition of accretionary growth bands in mussels are used to provide paleotemperature estimates with high temporal resolution. Marine bivalves secrete their shells nearly

in isotopic equilibrium with seawater (e.g., Mook and Vogel, 1968; Killingley and Berger, 1979). Hence, owing to their extensive geographic distribution, shallow-water habitat, and geologic range (Triassic to Recent), marine mussels are especially well suited for extraction of paleoclimate data (e.g., Krantz et al., 1987). For example, the marine mussel Mytilus trossulus (Koehn, 1991, formerly M. edulis) lives attached to the surface of hard substrata where its exposure and relative immobility (Harger, 1968) cause it to record changes in ambient seawater. In addition, because M. trossulus deposits daily accretionary growth bands in the outer prismatic calcite layer of its shell, the shell contains a continuous record of seawater temperature and chemical conditions. SAMPLE COLLECTION AND ANALYSIS The mussels analyzed in this study are from Squirrel Cove (508069N, 1248559W), a shallow, tidally mixed inlet on Cortes Island, at the northern end of the Strait of Georgia, British Columbia, Canada. The experimental site is a lagoon in which tidal amplitude is buffered by reversing rapids. These conditions made it possible to achieve maximum temperature variation (very shallow water) without growth cessation during tidal emersion. The shells of M. trossulus were notched at the ventral margin and returned to their subtidal growth position in August 1992. One year later, the live mussels were collected for analysis. Approximately 160 growth bands are visible on the outer surface of mussel A between the notch and the

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ventral margin. Similarly, approximately 180 growth bands were counted between the notch and the ventral margin of mussel B. Radial thick sections from each shell were microsampled with a drill bit located over a motorized, computer-controlled X-Y-Z micropositioning stage (Dettman and Lohmann, 1995). Sample paths crosscut individual growth bands such that each sample (80 –100 mg) represented approximately one day to two weeks of shell growth. All samples were roasted in vacuo for 1 h at 380 8C to remove volatile organic matter. Each sample was split so that d18O and Mg/Ca could be analyzed on aliquots of the same sample (Table 1). Sample powders for isotopic analyses were individually reacted with anhydrous phosphoric acid at 73 8C in an automated Kiel device directly coupled to an isotope-ratio mass spectrometer. Precision of the data was maintained at better than 0.1‰ (1s) by daily analysis of powdered carbonate standards. Splits of powder splits were individually dissolved in 1.0 mL of concentrated hydrochloric acid, and the solutions were analyzed for minor element content in an inductively-coupled-plasma– atomic-emission spectrometer (ICP-AES) for Ca and Mg. Owing to small sample size, it was not possible to make accurate measurements of the weights of carbonate powder splits. Therefore, raw solution (mg/L) data were converted to Mg/Ca ratios by assuming that all Ca and Mg were present as carbonate-bound cations. Analytical precision was better than 62% for Ca and 66% for Mg. Two seawater samples were collected from Squirrel Cove at approximately monthly in415

Figure 1. Seawater chemistry at Squirrel Cove, British Columbia, Canada. (A) Variation in seawater Mg/Ca content is minor over the observed salinity (S) range. Mg content covaries with salinity. Error bars for Mg concentration are approximately the size of symbols. The equation is for the principal axis of variation. (B) d18O (relative to SMOW, standard mean ocean water) vs. salinity defines mixing trend between freshwater (S 5 0‰, d18O 5 213.5‰) and seawater (S 5 35‰, d18O 5 0‰) end members. Equation is for the principal axis of variation.

tervals between August 1992 and August 1993 for chemical analysis (Table 2). One sample was reserved for isotopic analysis and the other for elemental analysis. Analysis of oxygen isotope compositions were made by using the conventional CO2-H2O equilibration method. Precision of the data was maintained at better than 0.1‰ (1s) by analysis of seawater standards. One millilitre of the seawater was diluted to 100 mL with doubly deionized water and analyzed for Ca and Mg on an ICP-AES with an analytical precision of better than 62%. Chlorinity was determined on a 0.5 mL aliquot by potentiometric titration with an analytical precision of 60.5%. Seawater temperature was recorded continuously from October 1, 1992, to June 15, 1993. Because of mechanical failure of the temperature probe on June 15, 1993, only monthly temperature readings were taken at 416

Squirrel Cove after June 15. A continuous temperature record for Squirrel Cove after June 15, 1993, was reconstructed in the following manner. Sea-surface temperature was continuously recorded 200 km to the south at a National Oceanographic and Atmospheric Administration recording site at Friday Harbor, Washington. Linear regression of the 1993 temperature record at Friday Harbor against the 1993 temperature record at Squirrel Cove (correlation coefficient, r 5 0.98) allowed us to infer a temperature record for Squirrel Cove from June 15, 1993, through August 26, 1993. The inferred temperatures after June 15, 1993, are accurate because of the linear relationship and high correlation coefficient between sea-surface temperatures at the two sites. Salinity variation (S 5 23.5‰–29.0‰) from riverine discharge accounts for most of the variation in chemical compositions of

seawater observed throughout the year (Fig. 1A). Conservative mixing between fresh-water and seawater components resulted in a proportional dilution of all elements in the mixture. Similarly, two-component mixing resulted in a linear relationship between salinity and d18O of the mixture (Fig. 1B), and extrapolation of the trend to 0‰ salinity yields a fresh-water end member with a d18O value of 213.5‰. The linear relationship between salinity and d18O of the freshwater-seawater mixture at Squirrel Cove suggests that the d18O of the freshwater component was fairly constant for the duration of this study. The weighted average of d18O of precipitation at Victoria, Canada, is 210.3‰ (Rozanski et al., 1993), and glacial runoff from the nearby Columbia icefield via the Fraser River most likely represents a fresh-water source with an even lower d18O value. Thus, a fresh-water end member with an estimated d18O value of 213.5‰ is reasonable for this site. Weekly salinity and d18O values of the mixture were estimated by interpolating between monthly salinity and d18O values (Fig. 1B). SHELL DATING Predicted d18O values (Fig. 2) for skeletal calcite were calculated by using interpolated GEOLOGY, May 1996

Figure 2. Measured skeletal d 18O values for mussels A and B and predicted d18O (calcite) (relative to PDB, Peedee belemnite) based on measured seawater temperatures and d18O at Squirrel Cove. Mussel B added no shell between late November and early February, indicating a growth hiatus below a temperature of ;6 °C. After notching, mussel A did not continue shell growth until after spring temperatures rose above ;6 °C.

temperature values, d18O values of the fresh-water–seawater mixture, and the calcite-water fractionation factor of Friedman and O’Neil (1977). Oxygen isotope values of mussel B vary by 5.2‰ (25.5‰ to 20.3‰, relative to the Peedee belemnite [PDB]) and compare favorably to the predicted range of shell compositions (25.3‰ to 0.4‰). However, the highest predicted shell values are 0.7‰ greater than the highest observed values, indicating that growth either ceased or was greatly reduced during the winter months. It has been suggested that some bivalves cease shell growth when temperature drops below a certain threshold (Dettman and Lohmann, 1994). If shell growth did cease during the winter, then the highest d18O values would not be recorded in shell carbonate. Measured oxygen isotope values of mussels A and B were dated by comparison with predicted isotopic compositions (Fig. 2). Dates were assigned to each sample by first correlating maxima and minima of observed values with those of predicted values and then by correlating individual values of shell d18O to predicted shell d18O values. This method of dating is similar to the one employed by Beck et al. (1992). Because each shell d18O value was matched with a predicted shell value, error that would result from assuming a constant growth rate was minimized. Temperature and salinity conditions were then assigned to each sample, enabling calibration of measured isotopic and chemical compositions of shells to salinity and temperature conditions. SKELETAL Mg/Ca RATIOS These data suggest that the Mg regulatory mechanism in skeletal calcite of M. trossulus is influenced in a predictable manner by seawater temperature (Fig. 3). The covariation of Mg/Ca and temperature is highly significant (r2 5 0.74) and is represented by the following expression: GEOLOGY, May 1996

T 5 2.50~60.36! z @~ Mg/Ca) 3 1000 # 2 2.07 ~62.35!,

(1)

where T is estimated temperature (in 8C). Equation 1 includes 95% confidence limits (in parentheses). These limits reflect both the degree of analytical precision and the accuracy with which skeletal Mg/Ca records seawater temperature. Between 5 and 25 8C, the standard error of a temperature estimate (given an exact Mg/Ca ratio) is less than 61.4 8C. However, the average absolute difference between temperature estimated from Mg/Ca and the observed daily sea-surface temperature for the entire record is approximately 61.9 8C. For a typical sample size of ;100 mg of calcite, the average uncertainty of the skeletal Mg/Ca was 66.5%, an uncertainty that translates into a possible analytical precision for temperature of approximately 61.0 8C. DISCUSSION There has long been interest in the relationship between skeletal Mg/Ca ratios of marine organisms and environmental conditions (Chave, 1954; Dodd, 1965; Lorens

and Bender, 1980). Factors other than temperature have little influence on skeletal Mg/Ca ratios. For example, the Mg/Ca ratio of seawater is little changed by fresh-water dilution from a salinity (Fig. 1A) of 35‰ down to 18‰; this reduction in salinity changes the Mg/Ca ratio from 5.2 to 5.0, based on global averages for seawater (Drever, 1988) and the Fraser River (Livingstone, 1963), the major fresh-water source for the study area. This corresponds to less than a 4% change in the Mg/Ca ratio of the seawater mixture. Lorens and Bender (1980) showed that skeletal Mg/Ca ratios in Mytilus decreased linearly when solution Mg/Ca ratios were lowered below the normal seawater ratio. Using the relationship of Lorens and Bender and the one defined in equation 1, it can be shown that a 4% decrease in the seawater ratio gives rise to an error in estimated temperature of less than 0.5 8C. Lorens and Bender (1980) also showed that skeletal growth rate does not influence the skeletal Mg/Ca ratio of M. edulis. If our mussels had been collected without prior knowledge of temperature and seawater chemistry at Squirrel Cove, and if we had assumed that they grew in normal seawater (i.e., constant salinity at 35‰), isotopic temperature estimates would be ;10 –18 8C higher than observed temperatures (Fig. 4). Perhaps the excess warmth deduced for glacial epochs (e.g., Rind and Peteet, 1985) reflects similar biases. In addition, the estimate of temperature seasonality (Tmax 2 Tmin 5 26 8C) is 7 8C greater than the observed temperature range of 19 8C. This overestimation of temperature seasonality results from the fact that, owing to the high volume of summer fresh-water runoff, d18O of the seawater–fresh water mixture was lower in the summer than in the winter. Integrated isotopic and minor element

Figure 3. Measured skeletal Mg/Ca ratio and temperature of seawater mixture. Dotted field delimits 95% confidence limits for this trend. The equation defines the principal axis of variation through data for mussels A and B.

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Figure 4. Comparison of paleothermometer techniques. Temperatures based on Mg/Ca ratios were calculated by using Equation 1 and measured skeletal Mg/Ca ratios. Temperatures based on d18O were calculated from measured skeletal d18O values under the assumption that the shells grew in normal seawater (S 5 35‰). Salinity variation at Squirrel Cove causes temperature estimates based on d18O (solid circles) to be 10 –18 °C higher than observed seawater temperatures (solid line), whereas temperature estimates based on Mg/Ca (open circles) are accurate despite salinity variation.

analyses of ancient mussel shells collected from temperate coastal regions have the potential to provide records of mean annual temperature, temperature seasonality, and the d18O of seawater. Currently, it is not possible to solve uniquely for salinity by using skeletal Mg/Ca and 18O/16O ratios; however, we will address the relation between salinity and Sr compositions in Mytilus in a forthcoming paper. Because residence times of Mg and Ca are on the order of 106 yr and complete ocean mixing occurs approximately once every 103 yr, only small shifts in the global seawater Mg/Ca ratio are expected for time scales on the order of 105 yr. In theory, this enables the recovery of paleo–sea-surface temperature records from mussels that lived in the past 105 yr. This time scale is appropriate to the last interglacial, for which seasonal temperature data are largely lacking. Finally, for the case where 18O and Mg paleotemperature estimates do not overlap, the difference in estimated temperatures most likely results from salinity fluctuations caused by freshwater input. Application of combined isotopic and chemical proxies to studies of fossil mussels provides powerful constraints for reconstructing paleoclimates and paleosalinities for coastal marine settings. CONCLUSIONS Mussels were collected live from a nearshore marine site, where seawater temperature and chemistry were monitored the year prior to specimen collection, to make an empirical calibration of shell chemistry to environmental conditions (temperature and salinity). Skeletal Mg/Ca ratios of the calcite skeleton of the mussel Mytilus trossulus vary as a function of sea-surface temperature but are little affected by large changes in salinity. In contrast, skeletal d18O varies with both salinity and temperature. Paired elemental and isotopic analysis on the same sample 418

can be used to estimate seawater d18O. The difference between calculated and standard seawater oxygen isotope composition then provides a record of seawater d18O variation, such as might be caused by glacial melting. ACKNOWLEDGMENTS Klein thanks the Scott Turner Endowment (University of Michigan) and Geological Society of America for providing funds for analytical expenses incurred during this research. This work was partially funded by National Science Foundation grant OPP93-18212 to Lohmann, and Petroleum Research Fund (grant 24940-AC), administered by the American Chemical Society, to Thayer, who also thanks R. Aiello, D. Becker, G. Brown, J. Murray, S. Remple, and Friday Harbor Laboratories. We thank G. Kennedy, B. Wilkinson, J. O’Neil, M. Bender, L. Pratt, and an anonymous reviewer for reviews of this manuscript. ICP-AES analyses were conducted in the University of Michigan’s Experimental and Analytical Laboratory under the supervision of L. Walter. Use of these facilities and the analytical expertise of T. Huston are greatly appreciated. REFERENCES CITED Arthur, M. A., Williams, D. F., and Jones, D. S., 1983, Seasonal temperature-salinity changes and thermocline development in the mid-Atlantic Bight as recorded by the isotopic composition of bivalves: Geology, v. 11, p. 655– 659. Beck, J. W., Edwards, R. L., Ito, E., Taylor, F. W., Recy, J., Rougerie, F., Joannot, P., and Henin, C., 1992, Sea-surface temperature from coral skeletal strontium/calcium ratios: Science, v. 257, p. 644– 647. Chave, K. E., 1954, Aspects of the biogeochemistry of magnesium: 1. Calcareous marine organisms: Journal of Geology, v. 62, p. 266–283. CLIMAP Project Members, 1981, Seasonal reconstruction of the earth’s surface at the last glacial maximum: Geological Society of America Map and Chart Series MC-36. Crowley, T. J., Short, D. A., Mengel, J. G., and North, G. R., 1986, Role of seasonality in the evolution of climate during the last 100 million years: Science, v. 231, p. 579–584. Dettman, D. L., and Lohmann, K. C, 1994, Seasonal change in Paleogene surface water

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GEOLOGY, May 1996