Reconstruction of Ocean Structure and Circulation during the Extreme Warmth of the early-middle Eocene

McIntyre, Andrew James (2022). Reconstruction of Ocean Structure and Circulation during the Extreme Warmth of the early-middle Eocene. PhD thesis The Open University.



The Atlantic meridional overturning circulation (AMOC) is a major component of global ocean circulation, globally distributing heat, salt, and nutrients, exerting a fundamental influence on global and regional climates. Yet, there is a poor understanding of how unstable the AMOC will be, or its sensitivity to future anthropogenic climate change. To tackle this, the climatic warmth of the Eocene offers an opportunity to investigate AMOC operation under acute warmth. There is limited knowledge to determine whether an active AMOC existed during the Eocene, let alone its structure or stability. Thus, the main objective of this thesis is to identify the structure and dynamism of ocean circulation during the early-middle Eocene.

To reconstruct the early-middle Eocene AMOC, carbon (δ13C), oxygen (δ18O), and neodymium (εNd) proxies were utilised to determine ocean ventilation state, temperature and salinity, and deep water mass flow pathways. Additionally, high-resolution age models based on inter-site correlations of bulk carbonate δ13C underpin the proxy records enabling valuable insight into the deep ocean structure and circulation.

Here, I present the first reconstruction of the early-middle Eocene AMOC, which comprises of a three cell-structure with bi-polar deep-water formation, contrasting the modern two cell structure, particular in the Southern Ocean. Bi-polar deep-water formation during the ice-free early-middle Eocene has distinct implications for the stability of the future AMOC. During the transient warming event C21n.H2 hyperthermal, the southern deep cell weakened and shoaled, causing deep Atlantic ocean stratification. This shows marked Southern Ocean sensitivity to climatic warmth, which has significant implications for the future Southern Ocean. Outside of the hyperthermal, no major changes in AMOC structure were found, but obliquity driven seasonal contrasts in high-latitude insolation regulated surface ocean temperature, the hydrological cycle and high-latitude surface water density, thus modulating deep-water formation and overturning rates during the early-middle Eocene.

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