On the role of the Southern Ocean in the glacial-interglacial cycles of the past 460,000 years: Changes in wind-driven upwelling and ocean front position revealed by reconstructed surface ocean nutrient conditions and temperatures

Abstract

The Southern Ocean, where the vertical circulation drives exchange between the voluminous ocean interior and the atmosphere, is proposed to play an important role in the Pleistocene glacial-interglacial cycles by redistributing heat and carbon between ocean and atmosphere. However, the mechanisms behind the Southern Ocean’s impact on global climate remain debated.This dissertation reconstructs changes in the Southern Ocean over the past 460,000 years by measuring the nitrogen isotopic (15N-to-14N) ratio of trace organic matter preserved in diatom microfossils, which reflects surface nutrient conditions, and the abundances of archaeal membrane lipids isolated from sediments, which reflect upper ocean temperature. First, data from the Indian sector of the Antarctic Zone (AZ), the more polar domain of the Southern Ocean, suggest that the upwelling driven by the Southern Westerly Winds responds to three modes of change: global mean climate, the northern-to-southern hemisphere temperature difference, and Earth’s axial tilt. The third mode can explain the lag of CO2 behind climate during glacial inceptions and deglaciations. Second, data from the Pacific sector of the AZ extending back 460,000 years show that the nutrient conditions of Antarctic surface waters were continuously correlated with the volume of land ice sheets, possibly through the effect of ice sheet size on Southern Ocean surface water conditions or the regional winds. Third, temperature reconstructions at different latitudes are used to reconstruct the latitudinal displacements of the Antarctic Polar Front (APF) in the last glacial-interglacial cycle, and the data show in general a more southward(northward) APF in warmer(colder) southern hemisphere climates, with a more southward APF than currently during the penultimate interglacial and at the end of the last deglaciation. Fourth, after correction for APF migration, the nutrient conditions at different latitudes of the Southern Ocean as reconstructed with the N isotopes of diatoms and deep-sea corals yield a consistent history for the increase in Southern Ocean upwelling at the end of the last ice age. These findings strengthen the case for the Southern Ocean’s vertical circulation as a cause of glacial-interglacial changes in atmospheric carbon dioxide, supporting the hypothesis of a central role for Southern Westerly Wind-driven upwelling.