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

目前，海洋在调节全球气候方面发挥着重要作用。海洋在许多方面与气候有关。在当今的温盐驱动的环流运输约一半的太阳能量从热带到两极。海洋还吸收温室气体二氧化碳（CO2）。气候受海洋与地球系统其他部分，即大气、大陆、极地冰盖和生物体之间复杂的物理、化学和生物相互作用的控制。然而，理解这些相互作用是困难的，量化它们对海洋化学的影响仍然是气候研究的一个基本挑战。例如，大型火山爆发会在大气中产生高浓度的温室气体。由此产生的全球变暖导致大陆上的降水和化学风化增加，这反过来又增加了通过河流向海洋输送的营养物质，如磷、氮、铁和其他金属。这些金属和营养物质对海洋中浮游植物的生长至关重要。在海洋表面产生的部分生物质然后沉入海底，并通过消耗氧气的微生物过程分解。在高生产力（即更多的生物量）时期，深海中的大部分氧气都被消耗，使海洋缺氧，并造成巨大的生态影响，例如海洋物种的大规模灭绝。然而，在缺氧的海洋中，有机物不能有效地分解，这导致二氧化碳在富含有机物的沉积物（黑色页岩）中的埋藏增加。增加的化学风化也消耗二氧化碳，这种温室气体的综合减少导致全球气温下降和地球气候的自我调节。然而，尽管我们从地质记录中知道这些海洋缺氧事件确实发生过，但我们无法轻易量化这些缺氧的程度。然而，这种量化对于预测海洋对未来不断变化的环境条件的反应至关重要。因为我们不能直接测量过去海洋的氧含量，我们需要使用沉积记录中的地球化学代用品来提取有关过去海洋条件的信息。近年来，分析技术的改进使得除了氧或氮等较轻元素外，还可以精确测量“重”稳定同位素（例如过渡金属同位素）。就钼和铬而言，分馏其同位素的过程（即优先选择较轻或较重的钼和铬同位素）主要是氧化还原过程，这取决于可用氧气的量。例如，含氧海洋沉积物含有轻钼同位素，而强还原沉积物含有海水的同位素组成。我们可以利用这些特性来确定同期海水的含氧量。例如，如果氧化沉积增加，轻钼被去除，残留的海水中的钼同位素变得更重。然而，其他过程可能会影响这些同位素组成。因此，目前很难精确地限制缺氧的空间范围（区域与全球）。因此，本研究将调查过程中，控制钼和铬的同位素组成的风化，河流运输和沉积在海洋中。一旦这些过程被理解和量化，建模可以用来量化过去海洋氧化的变化。这项研究最终将使人们更好地了解大陆风化、海洋化学和氧化作用以及海洋对快速气候变化的反应之间的联系。这样做将为开发气候模型提供信息，这些模型可以更好地预测地球气候在人为和自然变化下的演变。

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.