Central Equatorial Pacific Sea Surface Temperatures Over the Last Glacial Cycle

The tropical Pacific is a dominant influence on global climate, from interannual to glacial-interglacial timescales. How this system will respond to anthropogenic greenhouse gas forcing, however, remains an area of large uncertainty. In order to better our understanding of how the tropical Pacific ocean-atmosphere system has responded to greenhouse gas forcing in the past, I have generated new reconstructions of sea surface temperature (SST) in the central equatorial Pacific over the last ~150,000 years. First, I investigate Pacific SSTs during the Last Glacial Maximum (LGM, 19,000-23,000 years before present), the most recent interval in earth history when atmospheric CO2 was significantly different than modern. Our new Mg/Ca records show that the central equatorial Pacific cooled by an average of ~2 °C during the LGM, providing the first geochemical records of LGM SST in this region and resolving decades-old disagreement between proxies and climate models. To further improve our understanding of the glacial surface ocean, I present a Mg/Ca record from VM19-74 in the southeastern tropical Pacific. This site suggests no glacial SST change, a puzzling finding given our understanding of glacial climate. We explore possible geochemical explanations for this result, and show that a nearby marine sediment core was evaluated for use, but was ultimately not suitable SST reconstruction. This study highlights the need for continued work to improve our spatial understanding of past SST patterns. Finally, I extend our SST reconstructions to the Last Interglacial (LIG), a period between 115,000-130,000 years before present which has been studied as a possible analogue for future climate change. Here we show that the central equatorial Pacific was ~1.2 °C warmer during the LIG, in agreement with other geochemical proxy records. Notably, climate models do not simulate this magnitude of tropical LIG SST change. Our data underscore the importance of disentangling the components of LIG warming driven by greenhouse gas and orbital forcings.Ph.D

Central Equatorial Pacific Cooling During the Last Glacial Maximum

Establishing tropical sea surface temperature (SST) during the Last Glacial Maximum (LGM) is important for constraining equilibrium climate sensitivity to radiative forcing. Until now, there has been little data from the central equatorial Pacific in global compilations, with foraminiferal assemblage‐based estimates suggesting the region was within 1°C of modern temperatures during the LGM. This is in stark contrast to multi‐proxy evidence from the eastern and western Pacific and model simulations which support larger cooling. Here we present the first estimates of glacial SST in the central equatorial Pacific from Mg/Ca in Globigerinoides ruber. Our results show that the central Pacific cooled by about 2.0°C during the LGM, in contrast with previous global compilations but in agreement with models. Our data support a larger magnitude of tropical LGM cooling, and thus a larger equilibrium climate sensitivity, than previous studies which relied on foraminiferal assemblages in the central tropical Pacific