Understanding short term changes in seawater redox during Cretaceous Oceanic Anoxic Events using combined isotopic tracers

Understanding the processes capable of driving parts of the ocean from oxic (oxygen containing) to either anoxic (no oxygen) or euxinic (no oxygen and containing toxic sulfide) conditions is critical for identifying possible scenarios for the future ocean on a warmer Earth. The mid Cretaceous is well-known as a time of high global temperatures, and linked to this was the widespread deposition of black shales (organic-rich sediments commonly deposited during ocean anoxic events). The detailed chemical nature of these black shale units can potentially document a remarkable record of the driving factors and response of the Earth System to rapid climate change, and they have therefore become a focal point for extreme climate research. Recent short time-scale (millenial and shorter) sediment records have demonstrated that the Cretaceous ocean was characterized by repeated rapid changes between oxic and anoxic depositional conditions, which were particularly extreme during black shale deposition. Understanding these short-term cycles is essential for improving our understanding of how predictions of future rapid swings in climate may effect ocean chemistry, with subsequent impacts on ecosystem stability and climate feedback mechanisms. While most studies have focussed on making a simple distinction between oxic or anoxic conditions, this approach offers a measure for different levels of oxygen depletion, by applying a number of well established and novel redox indicators. The focus will be to use the isotope tracers (Mo, Cr, S) to assess the spatial extent of marine oxygen depletion and to recognise relationships between local, regional, and global mechanisms, since it is the local and regional effects of global warming that are predicted to cause major uncertainties in the future. The proposed research uses a combination of cutting-edge analytical techniques to develop high resolution geochemical records by combining novel isotope tracers with well-established techniques. Specific expected outcomes of the proposal include: 1. A better understanding of Cr isotope systematics during chemical transformations. This is essential for applying the Cr isotope system as a robust redox proxy. The approach includes a series of experimental measurements of Cr isotope fractionations upon sorption to and co-precipitation with different Fe (oxyhydr)oxides, under oxic, anoxic (non-sulphidic) and sulphidic conditions. 2. High resolution records of the specific nature of oxygen depletion (e.g. oxic versus anoxic versus euxinic), of several well defined, short time-scale cycles, by using multiple geochemical techniques. 3. Insights into short term changes in the local and global redox state of the ocean by pairing high resolution Mo and Cr isotope analyses. 4. An evaluation of the mechanisms involved in driving changes in ocean redox levels, utilizing detailed analyses of C-S-Fe systematics, geochemical and isotopic data. The investigation of redox cycles and short-term redox transitions, where existing data highlight differences in the timing and phase relationships of geochemical features, will reveal which pathways could have led to the observed redox changes.