Antarctic and Southern Ocean Climate Instability during the Middle Eocene "Greenhouse" to "Icehouse" Transition

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

This project will document the temperature evolution of Southern Ocean waters in the late middle Eocene through analyses of the Mg/Ca and oxygen isotope ratios of foraminifera from various sediment cores collected by the Ocean Drilling Program. The middle Eocene interval (~49 to 37 Ma) is possibly the most critical period in Cenozoic climatic evolution. Deep-sea temperatures cooled by as much as 7 degrees C, and small ice sheets appeared on Antarctica as early as ~42-40 Ma. Recent work on Southern Ocean sediment cores, however, suggests that this long-term cooling trend was not monotonic. New high-resolution oxygen isotope records reveal a pronounced warming event at 41.5 Ma, designated as the Middle Eocene Climatic Optimum (MECO). This transient event (~600 k.y. in duration) is hypothesized to represent an important climatic reversal (~4 degrees C warming) in the midst of long-term cooling. This reversal is compelling in light of new evidence of wide swings in pCO2 during the middle Eocene. As such, this event provides a unique opportunity to characterize the sensitivity of high-latitude climate to changes in boundary conditions. This project will document the temperature evolution of Southern Ocean waters in the late middle Eocene through development of high-resolution stable isotope and Mg/Ca datasets. Sites from all three sectors of the Southern Ocean are included in order to address questions of local vs. regional temperature variation and to confirm the presence of regional signals and trends. One additional site in the low-latitude Pacific will provide a comparative framework for the high-latitude sites. In conjunction with a temperature history, these records should also enable the construction of a seawater oxygen isotope and, by proxy, ice-volume curve for the late middle Eocene. By providing a detailed paleoclimatic framework, the broader significance of this study will ultimately lie in its contribution to demonstrating a relationship between greenhouse-gas forcing and global climate change. As details of Eocene pCO2 variability are established, and as climate models become more sophisticated, high-resolution records of Southern Ocean climate and Antarctic ice-sheet history will be essential for assessing the impact of pCO2 variability on global climate. Given that the Eocene may have been most the recent period in which atmospheric pCO2 fluctuated on the scale projected to occur over the next few centuries, it provides a unique opportunity to gauge climate sensitivity/insensitivity to this range of forcing. In terms of broader impacts, this research is relevant to understanding the societal impacts of global climate change. It will also support a graduate student's Ph.D. research and involve undergraduates in hands-on research.