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

Lead Research Organisation: University of Oxford
Department Name: Earth Sciences


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.


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Description The oxygenation state of Earth's oceans is a driver of biological evolution and extinction events as well as climate change. In recent years stable isotope fractionation of redox sensitive elements such as molybdenum (Mo) have been used as quantitative tracers of past redox-conditions in a number of marine environments. However, little is known about the processes controlling the Mo isotope compositions of the riverine inputs to the oceans and their short- and long-term variations.

This study has investigated the behavior of Mo isotopes during weathering of basalt under different conditions. Results from oxic to reducing soil profiles in Hawaii show that redox conditions during soil formation can control Mo isotope compositions in soils. This general isotope behavior is confirmed by results from soil profiles from Iceland.

In oxic profiles a surprisingly strong interaction of Mo with organic matter can be observed producing significant Mo isotope fractionation. This behavior might explain long term retention of Mo in soils besides its high mobility in molybdate form. Mo associated with organic matter is bioavailable and essential for processes like nitrogen fixation.
Exploitation Route The findings of this study can help understand natural versus cultural soil productivity and depletion. This research has shown that Mo has the potential to inform on soil formation processes. For future research, the combination of Mo isotopes with other isotopes-, elemental- and biological tracers might help us to de-convolute complex biological and abiological processes during soil formation with implications for not only the principal biogeochemical cycling of these elements, but also for our understanding of the complex interactions of soils with the hydro- and atmosphere and their consequences for climate change (e.g. CO2 storage).
Sectors Environment