Molecular evidence of palaeoceanographic changes across the Middle-Cenomanian Event 1 at Demerara Rise, Equatorial Atlantic Ocean
Supervisor: Jessica H Whiteside (National Oceanography Centre, Southampton)
How the Earth and life have interacted through time is central to our understanding of the Earth system. Over the course of the coming century, marine anoxia is likely to expand as a direct response of global warming and agricultural nutrient runoff. The Middle Cenomanian Event (MCE) 1 was an episode of large black shale deposit in the proto-North Atlantic due to enhanced marine productivity, subsequent carbon burial and marine anoxia. Biomarker analysis – n-alkanes, pristanes, phytanes, lycopanes, hopanes, steranes and isorenieratene – give attention to three main questions: the biological source of the marine organic matter, the contribution from land material and the expansion of the oxygen minimum zone at the Demerara Rise. An increase in continental material delivery revealed by odd long-chain n-alkanes across MCE 1 may have enhanced marine productivity. The occurrence of isorenieratene, diagenetic products of green sulphur bacteria, demonstrate that the oxygen minimum zone extended to the photic zone. The sulphidic water column favoured the proliferation of the prasinophytes (biological precursors of the C28-steranes). The preservation of lycopane coupled to the pristane/phytane ratio indicate anoxic bottom waters possibly due a decreasing in upwelling rate, and the sporadic formation of warm and saline intermediate-waters: Demerara Bottom-waters. Euxinic conditions sustained by high carbon export and warm temperatures may have enhanced nutrient recycling from the sediment to the water column thus establishing a positive feedback loop supporting marine productivity. Based on geochemical proxies, MCE 1 does not appear as a prelude to the OAE 2 as both event show a different palaeoceanographic origin: increase in stratification during MCE 1 while a break-down of water column stratification during the OAE 2.