CENOZOIC SEASONALITY AND PALEOCHEMISTRY RECORDED IN SR/CA RATIOS OF SERIALLY-SAMPLED MOLLUSK SHELLS

Trace-metal studies of deep-sea benthic foraminifera reveal Cenozoic cooling of bottom waters and high latitudes surface waters, confirming long-held views based on fossil and oxygen-isotope evidence. Furthermore, trace-metal measurements of these foraminifera provide records of the chemical evolution of seawater associated with this climate change. To examine climate change in low latitudes, we performed trace-element and isotopic analyses on 20 mollusk shells from 11 stratigraphic units in Mississippi, Alabama, and Florida. Fossil specimens range in age from Early Eocene (~54 Ma) to middle Miocene (~17 Ma). For calibration, six additional specimens were collected live from the Gulf of Mexico and analyzed. Specimens of the gastropods Conus and Turritella and the bivalve Venericardia were serially-sampled in the direction of growth. More than 1800 Sr, Mg, and Ca analyses were performed using either ICP-MS or ICP-EOS; additional trace metals (Li, Na, Cd, Mn, Ba, U) were measured by ICP-MS on nearly half the samples. All samples were also analyzed for carbon and oxygen isotopes.

Sr/Ca ratios in modern Conus shells correlate positively with temperature and negatively with oxygen isotopic compositions. Fossil gastropod Sr/Ca ratios also show an inverse correlation with δ¹⁸O and presumably temperature. No other trace-metal/Ca ratio shows a systematic correlation with δ¹⁸O. Sr/Ca ratios vary with taxa, with the highest values in Venericardia (4.3 – 5.0 mmol/mol, N = 4) and lowest values in Conus (1.1 – 3.1, N = 14). Focusing on Conus, Sr/Ca values are between 2.1 and 3.1 mmol/mol in Eocene through Miocene specimens, then decrease to 1.1 – 1.9 mmol/mol in modern specimens. Attributed solely to temperature change, this would imply 10°C warmer temperatures as recently as the Miocene. Oxygen isotope data show that Gulf Coast Eocene temperatures were only 3°C warmer than modern temperatures, suggesting that seawater Sr/Ca ratios during much of the Cenozoic may have been higher than modern values. Negative correlations between Sr/Ca and δ¹⁸O in most gastropod shells support a temperature control on intraannual Sr/Ca variations. Using the temperature dependence of Sr/Ca ratios in modern shells, we can estimate changes in temperature seasonality record in ancient shells. Our first attempt suggests an increase of between 1 and 4°C from the Eocene to the Oligocene.