Molybdenum and Chromium Isotope Behaviour during Weathering and Sedimentation: Tracing Changing Oxygen Levels in the Oceans

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At the present-day the oceans play a major role in regulating global climate. The oceans are linked to climate in a number of ways. At the present-day thermohaline driven circulation transports about half of the Suns energy from the tropics to the poles. The oceans also absorb the greenhouse gas carbon dioxide (CO2). Climate is controlled by complex physical, chemical and biological interactions of the oceans with other parts of the Earth system, i.e. the atmosphere, the continents, the polar ice caps and living organisms. However, understanding these interactions is difficult and quantifying their impact on ocean chemistry remains a fundamental challenge in climate research. For example, large volcanic eruptions will produce high concentrations of greenhouse gases in the atmosphere. The resultant global warming then causes increased precipitation and chemical weathering on the continents, which, in turn, increases the delivery of nutrients, such as phosphorous, nitrogen, iron and other metals, to the oceans by rivers. These metals and nutrients are essential to the growth of phytoplankton in the oceans. Part of this biomass produced in the surface oceans then sinks to the ocean floor and is decomposed by microbial processes that consume oxygen. In times of high productivity (i.e. more biomass), most of the oxygen in the deep oceans is consumed, making the oceans anoxic and resulting in dramatic ecological effects, such as mass extinctions of marine species. However, in an anoxic ocean, organic matter is not decomposed effectively which leads to increased burial of CO2 in organic-rich sediments (black shale). Increased chemical weathering also consumes CO2 and this combined drawdown of greenhouse gases results in cooling of global temperatures and self-regulation of Earth's climate. However, although we know from the geological record that these Ocean Anoxic Events have indeed occurred, we cannot easily quantify the extent of these anoxia. Such quantification, however, is essential for the prediction of the oceanic response to changing environmental conditions in the future. Because we cannot measure the oxygen content of past oceans directly we need to use geochemical proxies in the sedimentary record to extract information about the conditions of the past oceans. In recent years analytical improvements have made possible precise measurement of 'heavy' stable isotopes (e.g. transition metal isotopes) in addition to those of lighter elements such as oxygen or nitrogen. In the case of molybdenum and chromium, the processes that fractionate their isotopes (i.e. prefer either lighter or heavier Mo and Cr isotopes) are mainly redox-processes that are depending on the amount of oxygen available. For example, oxic marine sediments incorporate light Mo isotopes whereas strongly reducing sediments incorporate the isotope composition of ocean water. We can use these properties to determine the oxygen levels of contemporaneous seawater. For example, if oxic sedimentation increases, light Mo is removed and the residual ocean water becomes heavier in Mo isotopes. However, other processes may influence these isotope compositions. Consequently, at present it is difficult to precisely constrain the spatial extent of anoxia (regional vs. global). This study will therefore investigate the processes that control the isotope composition of Mo and Cr during weathering, river transport and sedimentation in the oceans. Once these processes are understood and quantified, modeling can be used to quantify past changes in the oxygenation of the oceans. This research should ultimately provide a better understanding of the links between weathering of the continents, ocean chemistry and oxygenation and the response of the oceans to rapid climate change. In doing so this will provide information for the development of climate models that can better predict the evolution of the Earth's climate in response to both man-made and natural changes.