Establishing causes of sea-level change and oceanic anoxia in the Late Cretaceous: regional versus global patterns

The Cretaceous, 146-66 million years ago, experienced high levels of atmospheric CO2, and the warmest climates and highest global sea-levels in the last 300 Ma. On several occasions, the oceans became abruptly depleted in oxygen, so-called oceanic anoxic events (OAEs), when organic matter accumulated in the oceans, producing widespread black shales that now act as oil source rocks. However, the mechanisms that caused these events remain hotly debated. This proposal aims to use a new multi-dynamic approach to better understand the mechanisms that caused the onset, duration and cessation of global carbon burial events during the Late Cretaceous. Burial of organic matter leads to the preferential removal of isotopically light carbon from the oceans, increasing the 13C/12C ratio of seawater and, via the atmospheric CO2 reservoir, the entire Earth surface system. Weathering releases 12C back to the surface carbon cycle. Carbonates and organic matter in rocks preserve changes in C-isotope ratios through time, providing a basis for C-isotope stratigraphy. Major changes are synchronous and global in extent, and we have proposed that C-isotope variation in the Late Cretaceous may be used as a proxy for global sea-level change; this remains to be tested. Osmium, a platinum group element with a short ocean residence time of <40 kyr, also shows isotopic variation in seawater through time, being controlled predominantly by two end-member components: weathering of crust and input from volcanic activity (mantle). These have drastically different ratios, so Os isotopes potentially may provide high-resolution stratigraphic control during times of palaeoenvironmental change, such as episodes of increased weathering or volcanic input. The modern oceans display a uniform Os-isotope ratio, but our new data for Os isotopes through an OAE at ~94 Ma indicate that the Atlantic displayed diachronous shifts in Os isotopes. This offers an exciting potential new tool for studying palaeocean-mixing. However, this OAE may be a unique event with regard to oceanic Os; further regions and OAEs need to be tested. This project will use C-isotope stratigraphy from organic matter to correlate global successions from diverse environments, palaeolatitudes and oceanic settings. The time interval to be investigated, 101 - 83 Ma, was characterized by two OAEs and other significant changes in the carbon cycle. We aim to answer the following: (1) Are secular C- and Os-isotope curves related to sea-level change? (2) Can Os-isotope stratigraphy be used for chemostratigraphy: is it synchronous or diachronous? (3) Do OAEs coincide with Os-isotope excursions, and what was the steady state of the oceans? (4) What are the relationships between sea-level change, climate and ocean anoxia; can we finally identify the key forcing mechanism for widespread ocean stagnation? Sites in Canada, France, Czech Republic, Far East Russia, Ecuador, South Atlantic, and offshore Australia will be studied. The relative sea-level histories for each basin, correlated using C-isotopes, will be used to test relationships between C-isotope stratigraphy and sea-level change. Key stratigraphic time intervals will be characterised for Os isotopes and trace-metals to: establish the evolution of Os isotopes in the Late Cretaceous oceans; evaluate possible regional variation in the Os-isotope composition of seawater; establish levels of seawater oxygenation in the associated water masses; and identify the causes of widespread ocean stagnation. Results from our Cretaceous extreme-greenhouse study will provide unique constraints for modelling interactions between, and the impacts of, sea-level and climate change, and perturbations of the global carbon cycle for an icecap-free Earth; the increasingly likely near-future for our planet. The proposed research will aid in understanding whether periods of ocean stagnation are a likely future consequence of present-day global warming.