Southern Ocean, Antarctic ice and climate interactions during Neogene cooling

Abstract

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 height while expanding seaward, maintaining a relatively stable volume during the mid-Miocene climatic transition (~14.5 Ma) and verified using ice sheet modelling. The migrations of the subtropical front near Tasmania are closely linked to a high-resolution pCO2 dataset for the Pliocene (2.58–5.3 Ma), which shows a delay of ~10,000 years in pCO2 compared to δ18O during the M2 glaciation and, at least, an elevated pCO2 during glaciation. I proposed that carbon emissions from the deep sea, driven by frontal migrations, was the dominant process in regulating pCO2, rather than physical diffusion or the biological carbon pump. This leads to an higher pCO2 during cold phases because less CO2 is stored in the ocean than during deglaciation. In another crucial geographical area, the Agulhas Plateau, I discovered that migrations of the subtropical front, regulating the inflow of warm water into the Atlantic Ocean, play a significant role in the variability of the Atlantic Meridional Overturning Circulation in the Pliocene. I concluded that there was a connection between frontal migrations and the locality of North Atlantic Deep Water formation, significantly altering heat transport to high northern latitudes and providing insights into understanding future AMOC changes