Marine Anoxia During Past Warm Climates

Abstract

Sedimentological and geochemical evidence indicate that during particular periods of Earth history, oceanic anoxia expanded and became widespread. These time intervals are associated with expansion of organic-rich black shale in the ocean. The extent, intensity and ecological effects of oceanic deoxygenation, however, differed amongst these events, and what caused those differences is still under debate. The strong association of anoxia with past warm periods, provides an appropriate platform to explore past anoxic systems and the formation of hydrocarbon source rocks, as well as a unique opportunity to investigate the interaction between the two major components, i.e. ocean oxygenation and a greenhouse climate. This thesis explores the processes that led to the spread of marine anoxia during the Cretaceous and across the Palaeocene Eocene Thermal Maximum (PETM), using organic biomarkers and a bulk geochemical approach. Reconstruction of the intensity and persistence of redox conditions and associated biomarker proxies for ecological change, in the proto South Atlantic during the Early Cretaceous suggests that during the initial stages of basin development, when the basin was restricted, anoxic and euxinic conditions were prevalent and occasionally extended into the upper water column. However, strong cyclicity in organic geochemical parameters imply that the anoxia/euxinia was not a persistent state. High amplitude fluctuations in redox conditions accompany fluctuations in marine productivity, likely resulted from the astronomically-controlled variations in the delivery of biolimiting nutrients from terrestrial runoff. This indicates that basin restriction, per se, did not cause anoxia but instead preconditioned the basin for oxygen depletion during episodes of enhanced organic matter production and export. Following a detailed characterisation of the Cretaceous anoxic system and black shales in the South Atlantic, the extent and persistence of PETM anoxia and photic zone euxinia were investigated within the Peri-Tethys margins. Organic biomarker proxies indicate an expansion of anoxia and photic zone euxinia during the PETM, but only in restricted marginal and epicontinental basins that amplified nutrient trapping and not in open marginal settings.

Date of Award	19 Mar 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Rich D Pancost (Supervisor) & B D A Naafs (Supervisor)