Nutrient and Weathering Controls on Cretaceous Oceanic Anoxic Events

Cretaceous OAE2 (~94 million years ago) functions as a unique event in Earth’s past to decipher biogeochemical dynamics of the ocean-atmosphere system under greenhouse climate conditions. The emission of vast amounts of CO2, linked to the emplacement of Large Igneous Provinces, initiated a cascade of biogeochemical processes, in which increasing global temperatures led to continental weathering enhancement, resulting in a higher nutrient supply to the ocean. A long-term sea-level rise and the subsequent increase in epi-continental shelf areas, as well as strong orbital forcing may have further contributed to an increase in the marine nutrient reservoir. Higher rates of production, export and burial of organic matter eventually led to the deposition of organic-rich successions under anoxic conditions. These successions are typically characterized by a positive carbon isotope excursion, indicative of a perturbed carbon cycle. The initiation of OAE2 has been the focus of various studies utilizing a variety of geochemical proxies, biogeochemical modelling, (cyclo-) stratigraphy and different palaeontological techniques. The termination of the event, however, has received little attention, and the factors leading to the recovery from anoxia on a global scale remain unknown.

In this thesis, I present a multidisciplinary approach using multiple geochemical proxies combined with biogeochemical box modelling and a coupled atmosphere-ocean climate model, focussing on the termination of OAE2. Geochemical analyses (iron-sulfur systematics, redox-sensitive trace metal enrichments and phosphorus phase partitioning) were conducted on sedimentary successions from the Tarfaya Basin, a shallow marine shelf setting located in the proto-North Atlantic. Here, the termination of the event is characterized by a gradual recovery from severe water column anoxia coinciding with lower rates of sedimentary phosphorus recycling and a decrease in chemical weathering input. The resulting lower nutrient availability on the shelf may therefore have been a decisive driver during the recovery of the Earth system.

The decrease in chemical weathering, however, does not seem related to a nascent cooling trend, as global temperatures remained high even after the event. Instead, weak orbital forcing caused by an insolation node at the end of OAE2 may have impacted global chemical weathering trends, as observed in conducted climate simulations. The onset of OAE2, on the other hand, shows a clear link between strong orbital forcing, enhanced chemical weathering input and nutrient availability, following the general theory of the biochemical cascade as the initial trigger for OAE2. However, in the applied box modelling, the CO2-driven weathering enhancement alone does not provide a sufficient amount of nutrients to initiate oceanic anoxia. The release of phosphorus (and iron) from the marine deposition of volcanic ash, as well as from alteration processes of volcanic glass during (submarine) volcanism on a large scale is therefore proposed as an additional direct nutrient source to the marine reservoir. As observed in the models, this additional source is required to boost organic matter production and burial, resulting in the expansion of shelf anoxia and a positive carbon isotope excursion.
Overall, this project highlights the importance of nutrient availability during a prominent deoxygenation event under past greenhouse climates, with implications for potential future greenhouse scenarios.