Thesis Defense – Nancy Karas MS

Alkenone sea surface temperature record of the Last Glacial Inception off southern Chile: Implications for regional oceanographic change and atmospheric CO₂

Nancy Karas
MS Candidate
Advisor: Dr. Samantha Bova

Monday, July 31, 2023
12:30 pm in CSL 422
watch Nancy’s talk

Abstract

Many mechanisms have been proposed to explain atmospheric carbon dioxide (CO₂) drawdown during glacial periods. Here, we present an alkenone sea surface temperature (SST) record spanning the Last Glacial Inception (LGI, 118,000 to 105,000 years ago) from the Chilean Margin (Expedition 379T Site J1005, 41° 4.5800’S, 74° 26.7000’W, 807 m water depth) that helps determine the cause of the initial 35 ppmv drawdown of atmospheric CO₂ during the last glacial period. At 41°S along the Chilean Margin, SST cooled across the LGI in sync with Antarctic temperatures and sea ice, but prior to the decrease in atmospheric CO₂, by ~4°C, more than double that observed at other southern hemisphere mid- and high-latitude locations. The magnitude of temperature change suggests cooling at the site was substantially amplified, most likely via an influx of cool sub-Antarctic waters driven to the core site via a northward shift in the Southern Westerly Winds (SWW). A northward shift in the SWW has been hypothesized to contribute to the drawdown of CO₂ at the LGI, via a reduction in Southern Ocean wind-driven upwelling. However, our data suggest Southern Ocean upwelling and atmospheric CO₂ remained unaffected until the SWW had moved far enough north, only reaching this threshold at 115 ka. We therefore suggest the response of Southern Ocean upwelling and carbon balance to latitudinal migrations of the SWWs may be nonlinear, which should be considered both in past and future projections.