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

当今，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。在今天，由温盐驱动的环流将大约一半的太阳能量从热带地区输送到两极。海洋还吸收温室气体二氧化碳(CO2)。气候是由海洋与地球系统其他部分，即大气、大陆、极地冰盖和生物的复杂物理、化学和生物相互作用所控制的。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中的一个根本挑战。例如，大规模火山喷发将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降水增加和化学风化，这反过来又增加了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。这些在表层海洋中产生的生物质的一部分然后沉入海底，并被消耗氧气的微生物过程分解。在生产力高的时期(即更多的生物量)，深海中的大部分氧气被消耗，使海洋缺氧，并导致巨大的生态影响，如海洋生物的大规模灭绝。然而，在缺氧的海洋中，有机物没有得到有效的分解，这导致富含有机物质的沉积物(黑色页岩)中二氧化碳的埋藏量增加。更多的化学风化也会消耗二氧化碳，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了，但我们不能轻易量化这些缺氧的程度。然而，这种量化对于预测未来海洋对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含氧量，所以我们需要使用沉积记录中的地球化学物来提取有关过去海洋状况的信息。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”的稳定同位素(如过渡金属同位素)。在钼和铬的情况下，分离其同位素的过程(即偏爱较轻或较重的钼和铬同位素)主要是氧化还原过程，这取决于可用的氧气量。例如，含氧的海洋沉积物含有轻的钼同位素，而强烈还原的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，如果氧沉积增加，轻的钼被去除，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(地区性与全球性)。因此，这项研究将调查在风化、河流输送和海洋沉积过程中控制钼和铬同位素组成的过程。一旦了解并量化了这些过程，就可以使用建模来量化海洋氧合作用过去的变化。这项研究最终应能更好地了解大陆风化、海洋化学和氧气作用以及海洋对快速气候变化的反应之间的联系。通过这样做，这将为气候模型的发展提供信息，这些模型可以更好地预测地球气候的演变，以应对人为和自然变化。

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.