CENOZOIC VARIATIONS IN SEAWATER SR/CA RATIOS: INSIGHT FROM GASTROPOD SHELLS

Temporal variations in chemical weathering, hydrothermal processes, and carbonate deposition have had large-scale impacts on the global carbon cycle and global climate across Earth’s history. Records of seawater chemistry, specifically changes in the concentrations of Sr and Ca, help constrain the temporal variation in these processes.

To reconstruct seawater Sr/Ca, we performed elemental and stable isotope analysis of 17 fossil mollusk specimens collected from stratigraphic formations from the US Gulf Coast ranging in age from the mid-Eocene to early Miocene. Gastropods are abundant in the Cenozoic fossil record, fast-growing, relatively long-lived, and precipitate aragonitic shells that show systematic relationships between Sr/Ca and temperature. We focus on Conus spp. and turritellids, taxa for which the distribution coefficients have been determined as a function of temperature based on modern specimens (Sosdian et al., 2006; Tripati et al., 2009). Seasonal variations in the Sr/Ca ratios of fossil specimens correlate negatively with oxygen isotopes, indicating a similar temperature sensitivity to modern specimens. Absolute Sr/Ca ratios of fossil specimens are higher than those of modern specimens (2.4 ± 0.2 vs. 1.5 ± 0.2 mmol/mol for Conus spp. and 3.1 ± 0.3 vs. 1.8 ± 0.4 mmol/mol for turritellids) such that application of the modern Sr/Ca-temperature calibration to fossil specimens gives unrealistically warm marine paleotemperatures (>40 °C). We therefore interpret our high fossil Sr/Ca values as reflecting elevated past seawater Sr/Ca.

Using the Sr/Ca distribution coefficients, oxygen isotope data, seawater δ¹⁸O estimates, and the δ¹⁸O paleotemperature equation, we calculate seawater Sr/Ca from the Eocene to the Miocene. Seawater Sr/Ca increased from ~11.5 to 13.9 mmol/mol between the mid-Eocene (42 Ma) and early Oligocene (33 Ma) and subsequently decreased from the mid-Miocene to the present (8.5 mmol/mol). This record suggests that seawater Sr/Ca remained high across the transition from greenhouse to icehouse climate, and well into the Neogene icehouse. The overall Sr/Ca decline in the Neogene toward present indicates substantial changes to the fluxes of Sr and Ca into and out of the oceans, possibly related to greater preservation of aragonitic sediments.