Resolving Past Changes in Ocean Oxygenation: Utility of Chromium Isotopes

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There is growing concern that a recent reduction in the concentration of dissolved oxygen in parts of the world's oceans signals a change in the Earth's climate. This is because there is lots of evidence in the geological record that links seawater oxygen to climate change- the anoxic (oxygen deficient) oceans at the end of the Permian (251 million years ago), for example, are associated with global warming and massive extinctions. Nevertheless, what we don't know, is exactly what controls the link between the concentration of dissolved oxygen and climate? Answering this question is extremely difficult because direct measurements of past levels of seawater oxygen, as well as climate parameters such as sea-surface temperature, extend back only as far as a few tens of years. Instead, we must rely on indirect, or so-called 'proxy' measurements that are preserved in the geological record. There are a number of 'proxies' for past levels of seawater oxygen. Most of them rely on measurements of the concentration of metals in marine sediments. However, it is now clear that metal concentrations can be affected by a number of different variables (for example, the level of primary productivity in the overlying water column), all of which can vary independently of seawater oxygen. One way to circumvent some of these problems is by analysis of the isotopes of these metals. In recent times, research has focused on the analysis of iron and molybdenum isotopes, but a key problem is that both iron and molybdenum are insensitive to small changes in seawater oxygen. They are only able to distinguish between oxic (oxygen-replete) or euxinic (no oxygen, high levels of hydrogen sulphide) conditions. For all of these reasons, there is a clear need to find new proxies for seawater oxygen. Chromium is an ideal candidate, for a number of reasons. Firstly, it is soluble under oxic conditions, but insoluble (and therefore accumulates in marine sediments) under slightly oxygen deficient conditions. Second, its isotopes behave very differently under oxic vs oxygen deficient conditions, and this can now be detected using state-of-the-art instrumentation. Third, the chemistry of chromium and its isotopes is relatively simple. The overarching aim of this project is to test the utility of chromium isotopes as a tracer of seawater oxygen. To this end, we will: (1) Measure the chromium isotope composition of marine sediments deposited in a range of oxygen conditions. (2) Determine the precise mechanism of chromium uptake into marine sediments, and the potential for movement of chromium after it has been buried. (3) Obtain records of chromium isotopes for a period of dramatic climate change. We expect the results of our research to provide an improved understanding of the relationships between seawater oxygen and global climate change. Such knowledge will be imperative for both the prediction and mitigation of Earth's climate in the future.