Oceanographic and Biotic Changes in Australia During Cretaceous Oceanic Anoxic Events

Abstract

This thesis investigates two key periods of ocean deoxygenation during the midCretaceous: Oceanic Anoxic Event 2 (OAE 2, ~94 Ma) and the Mid-Cenomanian Event (MCE, ~96.5 Ma). These events are associated with high global temperatures, elevated CO₂, and increased primary productivity, which is recorded in marine sediments as positive δ¹³C excursions. However, high-latitude Southern Hemisphere records of these events remain scarce, and bulk isotopic analyses often obscure regional differences, complicating global correlations. Moreover, the volcanic sources triggering these events, including potential contributions from various Large Igneous Provinces (LIPs), remain debated.

This project addresses these gaps by studying sediment cores from the Mentelle Basin (SW Australia), a key high (palaeo-)latitude Southern Hemisphere site (~62°S in the mid-Cretaceous). Utilising carbon isotopes from bulk carbonate, organic carbon, and benthic foraminifera species, alongside biostratigraphy and a range of other geochemical proxies, both OAE 2 and the MCE were successfully identified. This work presents the first Southern Hemisphere record of the MCE, confirming its global significance. Species-specific carbon isotope data also allowed for detailed correlations of OAE 2 stages with other global records.

Benthic foraminiferal data indicate sustained marine productivity during and beyond the peak of OAE 2, corroborated by proxies for enhanced nutrient runoff and upwelling. Elevated mercury (Hg/TOC) levels, alongside an increasingly radiogenic signal from young source material indicated by Nd and Sr isotopes, suggest significant volcanic activity from the nearby Kerguelen LIP, particularly before and during OAE 2. These findings suggest that submarine volcanic eruptions from the Kerguelen LIP likely played a key role in triggering environmental changes, including productivity-driven ocean deoxygenation caused by an intensified hydrological cycle delivering enhanced nutrient input to the Mentelle Basin.

This thesis provides new insights into the role of Southern Hemisphere volcanism in driving Cretaceous anoxic events, offering a valuable contribution to the global understanding of mid-Cretaceous environmental change.