EVALUATING DIAGENESIS IN EOCENE TROPICAL PLANKTONIC FORAMINIFERA

Mismatch between latitudinal temperature gradients produced by models and those observed in stable isotope reconstructions of ancient greenhouse climates from the Cretaceous and early Paleogene has been the subject of great debate. This discrepancy results from tropical sea surface temperature reconstructions that underestimate temperatures predicted by models. Debate about the cool tropics paradox has centered on the validity of oxygen isotope-based paleotemperatures in the tropics, where vertical temperature gradients in the ocean are at their greatest and where the potential for early diagenetic recrystallization on the sea floor is consequently at its highest. Because the degree of alteration in foraminiferal calcite has remained difficult to quantify, this limitation has hindered our ability to progress this debate.

We are exploring several methods to better characterize and evaluate the nature and extent of carbonate diagenesis in planktonic foraminifera. We are using several techniques to assess geochemical and textural preservation at the micro- to nano-scale level, including: ion probe mapping, laser ablation elemental analysis across the thickness of chamber walls, SEM, and AFM imaging. This study seeks to compare the geochemical and textural features of Eocene tropical planktonic foraminifera recovered from deep-sea cores with glassy specimens recovered from neritic, clay-rich environments. Specifically, we will test the hypothesis that the presence of recrystallized rims of calcite on deep-sea planktonic foraminifera is responsible for “cool” temperature signals in the tropics during ancient greenhouse climate episodes.