| Summary: | The partial pressure of atmospheric carbon dioxide (pCO2) has increased from 280 to 420 ppm (parts per million) since the industrial revolution due to anthropogenic emissions. As a result, the Earth's atmosphere, ocean, and cryosphere are undergoing changes due to increased radiative forcing, leading to a warming planet, loss of ice sheets and sea ice, sea level rise, ocean heat redistribution, and fluctuations in ocean circulation, as predicted in scenarios up to the year 2100. Nevertheless, future changes in these components remain highly uncertain—better understanding of these processes is crucial for human communities. To better comprehend the interactions between the Southern Ocean, the Antarctic ice sheet (AIS), pCO2, and climate in the future, I have studied sea surface temperatures, ocean front migrations and deep-sea temperatures during climate transitions in the geological past, specifically the Neogene (2.58–23.04 million years ago, Ma), which experienced pCO2 levels as high as, and sometimes higher than, the current levels. A distribution model of modern dinocysts was developed based on newly collected surface sediment samples near Antarctica to reconstruct the positions of oceanic fronts in the Southern Ocean in the past (Chapter 2). Subsequently, I demonstrated a significant long-term cooling of ocean surface and northward migrations of ocean fronts during the Neogene near Tasmania. The substantial cooling at mid-latitudes is not caused by solar radiation combined with polar amplification but is attributed to ocean frontal migrations and the northward expansion of the polar sea. Additionally, I identified a substantial deep-ocean cooling, which nearly completely explains the increase in benthic δ18O, leaving little room for an ice volume effect. The relatively stable ice volume during climate cooling seems counterintuitive given the northward migrations of ocean fronts and other geological evidences suggesting the AIS advancing. Hence, I proposed a hypothesis that the AIS gradually decreased in ...
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