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

海洋在调节全球气候方面发挥着重要作用，大约23亿年前，海洋的氧化作用被认为导致了地球上生命的“爆炸”。海洋吸收温室气体二氧化碳。浮游植物在进行光合作用时吸收二氧化碳，死亡的有机物质落到海底，二氧化碳被锁在深海沉积物中。气候还受到海洋与地球系统其他部分（即大气、大陆和生物）的复杂相互作用的控制。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中的一个基本挑战。例如，大型火山爆发会在大气中产生高浓度的温室气体。由此导致的全球变暖导致大陆降水和风化增加，这反过来又增加了河流向海洋输送的营养物质。这些营养物质对海洋浮游植物的生长至关重要。表层海洋产生的部分生物质随后沉入海底，并被消耗氧气的微生物过程分解。在高生产力时期，深海中的大部分氧气被消耗，使海洋缺氧，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，有机物不能有效分解，导致富有机质沉积物中二氧化碳的埋藏增加。风化作用也消耗二氧化碳，这种温室气体的联合减少导致全球气温下降和地球气候的自我调节。这些相互作用（过去）的程度不能直接量化。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是必不可少的。因为我们不能直接测量过去海洋的氧含量，所以我们需要使用沉积记录中的地球化学代理来提取过去条件的信息。金属同位素分馏（如铁和钼）最近被用作地球化学的代用物。就钼而言，将其同位素分馏（即选择较轻或较重的钼同位素）的过程主要是氧化还原过程，这取决于可用氧的量。例如，含氧海洋沉积物含有轻Mo同位素，而强还原性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时期海水的含氧量。例如，如果氧沉积增加，轻Mo被去除，残留的海水中的Mo同位素变得更重。然而，没有得到很好限制的其他过程正在影响海洋中的这种平衡。我们知识中的一个主要空白是流入海洋的河流的钼同位素组成。生物分馏（例如通过微生物和植物）可能是控制其组成的一个关键因素。表征自然界Mo生物分馏的问题之一是同位素信号经常与非生物过程混合或被非生物过程掩盖。因此，我们将研究冰岛一个湖泊的Mo同位素化学，该湖泊的特殊地质和生物特性将非生物过程对Mo同位素化学的影响降到最低。这将使我们能够分离出生物信号，并将其大小与湖中的生物生产力联系起来。它还将使我们开始了解Mo同位素分馏所涉及的生物过程。这些结果可以用来了解过去河流中的Mo同位素组成和海洋中的生物分馏。这一认识将有助于建立过去和未来海洋氧化和气候的模型。这些结果也将有助于了解早期地球上生命的进化。

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.