TROPICAL TEMPERATURES DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM AND EARLY EOCENE CLIMATIC OPTIMUM FROM CARBONATE CLUMPED ISOTOPE THERMOMETRY

The nature of tropical sea surface temperatures during past high CO₂ intervals have been controversial. Although climate model simulations of past high pCO₂ intervals indicate warm tropical sea surface temperatures (SSTs), high δ¹⁸O ratios of planktonic foraminifera tests from open ocean successions have been interpreted to reflect carbonate mineral precipitation in cool waters. One argument for solving this “cool tropics paradox” of past greenhouse intervals is that all planktic foraminiferal tests from such settings are extensively recrystallized. Isotopic modeling has been used to argue for substantial (~50%) amounts of diagenetic calcite in tests from tropical and subtropical oozes, with recrystallization occurring in pore-fluids similar in temperature and δ¹⁸O_w to bottom waters (i.e., cold and ¹⁸O-enriched), although other calculations have yielded different estimates. If recrystallization is this pervasive, it calls into question the preservation of most deep-sea sequences and the utility of their absolute δ¹⁸O values in paleoceanography. But although other studies have utilized a similar mass balance approach to infer a range of other estimates for recrystallization, they yield a range of values due to the indirect constraints being used as all are subject to a number of different assumptions. Here we use electron backscatter diffraction (EBSD) data for 150 foraminifera from two sites in the tropical Pacific to directly test the hypothesis that δ¹⁸O values from carbonate-rich sequences reflect substantial amounts of recrystallization and estimate ocean temperatures. This measurement allows us to quantify the fraction of secondary calcite. Preliminary results support an average of 10% +/- 20% (2 s.d.) recrystallization. Clumped isotope data indicate central Pacific SSTs typically in excess of 30 ^oC, with warmest SSTs during the Early Eocene Climatic Optimum and Paleocene-Eocene Thermal Maximum. We find evidence for warm tropical temperatures that likely led to high rates of evaporation and vapor export, resulting in surface waters in the tropical Pacific that were more ¹⁸O-enriched than previously considered. These findings have significance for Earth systems history, including the utility of deep-sea sediments in studying past climate change in response to changing greenhouse gas forcing.