COLLABORATIVE RESEARCH: Constraining Tertiary Temperatures, Salinities, and Ocean Chemistry: An Isotopic and Trace-metal Study of Serially-sampled Mollusks

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ABSTRACTConstraining Tertiary Temperatures, Salinities, and Ocean Chemistry: An Isotopic and Trace-metal Study of Serially-sampled MollusksE. Grossman, Y. Rosenthal, and C. Lear The Tertiary Period (55 to ~2 million years ago) presents our best opportunity to study the transition from an ice-free "greenhouse world" to today's "icehouse world" and to understand the response of present and future climates to high atmospheric carbon dioxide levels. This proposal will take advantage of the superb fossil record of the US Gulf Coast and two independent measures of ancient ocean temperatures, oxygen isotope (18O/16O) and strontium/calcium ratios in fossil shells, to characterize this dramatic change in global climate. These results will address a longstanding controversy of early Tertiary climate, the "cool tropics paradox". Oxygen isotope measurements of planktonic foraminifera indicate cool tropical sea-surface temperatures for a time when high latitudes experienced unusual warmth. This contradicts model simulations of greenhouse world climates that predict that both polar and tropical temperatures must have been significantly warmer than today. A common explanation for the discrepancy is that paleotemperature data from planktonic foraminifera have been modified by post-depositional chemical alteration (diagenesis). By analyzing pristine mollusk shells, this study will obviate this limitation of previous studies. A second objective is to improve our understanding of the nature and causes of the climatic transition near the Eocene-Oligocene boundary associated with rapid Antarctic glaciation and faunal overturn. Recent work using oxygen isotope ratios in fish otoliths from US Gulf Coastal Plain sediments has linked faunal overturn to increased seasonality, in particular colder winters in this region. However, interpretation of such data is limited by poorly constrained estimates of global ice volume and local salinity variations. This study will use paired measurements of mollusk Sr/Ca and 18O/16O to examine changes in low-latitude sea-surface temperatures and seasonality across this event. Lastly, modern mollusk samples will be collected with temperature, salinity, and seawater 18O/16O data from various sites in the Gulf of Mexico and Eilat, Israel. These samples will be analyzed for stable isotopes and trace metals (especially Sr/Ca) to calibrate trace metal-temperature relations.