Ocean Structure and Biological Productivity of the Southern and Atlantic Oceans during the Early to Middle Eocene Climatic Transition

Alexander, Sophie McLean (2023). Ocean Structure and Biological Productivity of the Southern and Atlantic Oceans during the Early to Middle Eocene Climatic Transition. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00015cfc

Abstract

At the onset of the Cenozoic cooling trend, the Earth system transitioned from the early Eocene greenhouse towards Antarctic glaciation. In this thesis, I integrate foraminifera stable isotope records with portable X-Ray Fluorescence (pXRF) and Benthic Foraminiferal Accumulation Rate (BFAR) records to assess changes in the Atlantic and Southern Oceans across the Early-Middle Eocene Transition (EMET). This thesis presents evidence of heterogeneous response of the Southern Ocean during the Eocene, with the Antarctic Zone (AZ) stratifying at ~47 Ma, whilst the Sub-Antarctic Zone (SAZ) remains well mixed, with high productivity. Stratification of the AZ is likely caused by intrusion of a warm surface current sourced from the Pacific into the Southern Ocean. Flow of a shallow current from the Pacific into the Atlantic AZ could occur due to shallow transient (~400 kyr) openings of Drake Passage. The AZ stratification resultant from throughflow caused a reduction in Southern Ocean deep water formation intensity, allowing the dominance of Northern Sourced Waters (NSW) in the Equatorial Atlantic. The closure of Drake Passage via shifting continental blocks reinvigorates deep water formation in the Southern Ocean homogenising the abyss and redistributing nutrients through the Atlantic basin, fuelling export productivity. Finally, this thesis compares the spatial distribution of sedimentary hallmarks of hyperthermals and ocean circulation during two rapid warming events, C21nH2 and ETM-2. From comparing these hyperthermals, I suggest a new mechanism for the cause of hyperthermals. Shoaling of Southern Ocean overturning and a subsequent filling of the Atlantic abyss by NSW results in a strong chemical stratification of the Atlantic water column. The role of eccentricity-modulated changes in ocean overturning producing the sedimentary features of a hyperthermal has previously been overlooked, and the hypothesis of a ‘source’ of carbon can now be discarded.

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