Deep oceans can help us understand our climate

A view of Hout Bay in Kommetjie, South Africa

There is approximately 60 times more carbon in the deep ocean than in the atmosphere. Therefore, understanding the deep ocean is of critical importance if we are to fully comprehend the role the oceans have on climate. The formation of deep water masses occurs when cold dense waters at high-latitudes sink and subsequently flow at depth around the globe before later upwelling to the surface. This phenomenon occurs in the North Atlantic and the Southern Ocean and this maintains a series of important planetary-scale oceanic flows known as the Atlantic meridional overturning circulation (AMOC). The AMOC distributes heat, salt, nutrients, carbon and oxygen around the globe, thus exerts a fundamental influence on Earth’s climate. Yet, the future/current stability of the AMOC is uncertain. With the ongoing global warming of the surface waters and increasing freshwater inputs from the polar ice sheets, the density of the surface waters will decrease; reducing overturning and potentially lead to a collapse in AMOC in the future. Therefore, understanding the stability of AMOC during warmer than modern times would be very valuable for understanding the future of the AMOC. So, we turn to the past and the geological record for potential clues and this is where my research comes in.

Chart shows deep ocean salinity measurements

Early-middle Eocene Oceans 

My PhD will look into reconstructing the structure and circulation of Atlantic and Southern Ocean during an interval of time known as the early-middle Eocene, approximately 45 to 50 million years ago. The early-middle Eocene was a time of extreme warmth, with the deep oceans at least 12°C warmer and the atmosphere had approximately 2-4 times as much carbon dioxide compared to present. A north-south transect spanning from the North Atlantic to the Southern Ocean will be used to assess deep ocean circulation and determine the evolution and stability of the AMOC during the extreme ‘greenhouse’ warmth of the early-middle Eocene. To reconstruct ocean circulation, geochemical and micropalaeontological techniques will be used to extract seawater signals recorded in fossil benthic foraminifera and fish teeth. The fossils preserved will be analysed for carbon isotopes to understand deep ocean ventilation state and neodymium isotopic compositions of the fish teeth will identify main deep ocean water mass flow pathways. Therefore, studying the early-middle Eocene will provide an opportunity to assess our knowledge of deep ocean circulation during warmer climate states and provide further insight into how deep ocean circulation and AMOC may operate in a warmer climate in the future.

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