Biological Fractionation of Mo isotopes and Primary Productivity: Lake Myvatn, Iceland

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The oceans play a major role in regulating global climate and their oxygenation around 2.3 billion years ago is thought to have led to the 'explosion' of life on Earth. The oceans absorb the greenhouse gas carbon dioxide (CO2). Phytoplankton absorbs CO2 when it photosynthesizes, and dead organic material falls to the ocean floor where the CO2 is locked up in deep-sea sediments. Climate is also controlled by complex interactions of the oceans with other parts of the Earth system, i.e. the atmosphere, the continents 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 weathering on the continents, which, in turn, increases the delivery of nutrients to the oceans by rivers. These nutrients are essential to the growth of phytoplankton in the oceans. Part of the 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, 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. Weathering also consumes CO2 and this combined drawdown of greenhouse gases results in cooling of global temperatures and self-regulation of Earth's climate. The (past) extent of these interactions cannot be quantified directly. 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 on past conditions. Fractionation of metal isotopes (e.g. Iron and Molybdenum) has recently been used as geochemical proxy. In the case of molybdenum, the processes that fractionate its isotopes (i.e. prefer either lighter or heavier Mo 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 that are not well constrained are influencing this balance in the oceans. One of the major gaps in our knowledge is the Mo isotope composition of rivers entering the oceans. Biological fractionation (e.g. through microorganisms and plants) may be one key factor controlling that composition. One of the problems in characterizing Mo biological fractionation in nature is that the isotope signal is often mixed with or masked by abiological processes. We will therefore investigate the Mo isotope chemistry of a Lake in Iceland where the special geological and biological properties of the lake minimize the impact of abiological processes on the Mo isotope chemistry. This will allow us to isolate a biological signal and to link its magnitude to biological productivity in the lake. It will also allow us to start understanding the biological processes involved in Mo isotope fractionation. These results can then be used to understand the Mo isotope composition of rivers and biological fractionation in the oceans in the past. This understanding will facilitate models of past and future ocean oxygenation and climate. The results will also help to understand the evolution of life on the early Earth.