CO2-CarbonCycle-Climate-Interactions (C4I)

海洋占地球表面的70%，支持着巨大的生物多样性，并为人类提供主要的食物资源。自工业革命以来，海洋吸收了大约50%的化石燃料燃烧和水泥生产产生的二氧化碳，限制了全球变暖的程度。二氧化碳溶解在海水中形成碳酸，降低环境pH值，这种现象被称为“海洋酸化”。海洋酸化的一个重要方面是海洋中碳酸盐离子浓度的下降，当向海水中加入二氧化碳时，碳酸盐离子是一种溶解碳的形式，在酸化反应中被耗尽。这是至关重要的，因为许多海洋生物的外壳和骨骼是由碳酸钙（CaCO3）构成的，碳酸钙在低碳酸盐离子浓度（称为“欠饱和”条件）下溶解。然而，早在欠饱和条件实际发生和海洋生物外壳开始溶解之前，破坏的生态阈值就可能被跨越，因为随着碳酸盐离子浓度的下降，碳酸钙外壳和骨骼将需要更多的代谢能量来维持它们的厚度。实验室和野外的实验已经证明了这种效应，并发现在酸性更强的条件下，被称为球石藻的钙化藻类通常产生更少的CaCO3外壳材料。减缓生活在海洋表面的藻类产生碳酸钙的速度可能会产生“有益”的影响，因为它有助于中和化石燃料中的二氧化碳，但也会产生有害的影响，因为生态系统会受到破坏。这也可能对海底生物的有机碎屑供应产生严重影响，因为人们怀疑这种食物供应在很大程度上依赖于碳酸钙颗粒，以使蓬松的有机物质下沉并帮助其下沉。海洋酸化的其他影响可能包括海洋中产生的有机物质的数量和营养成分的变化，以及营养物质的损失：将硝酸盐转化为强大的温室气体一氧化二氮。海洋碳循环是如何“工作”的，CaCO3颗粒对蓬松的有机物是否真的很重要，以及像球石藻这样的藻类对海洋化学变化的反应究竟如何，这些都存在很大的不确定性。这意味着我们将无法知道单个计算机模型对未来的预测是否正确。在这个项目中，我们将直面这个不确定性的问题——成百上千次地运行海洋碳循环和气候的计算机模型，看看未来哪些影响是可能的，哪些是不可能的。通过使用描述现代海洋“看起来”是什么样子的大量数据集（根据沉积物中记录的营养物分布和模式）来约束大量模型，从而使它们在一开始就对现代海洋的样子达成一致，这将极大地帮助我们。我们工作的结果首先是更好地理解现代海洋碳循环，这是在担心未来之前正确认识海洋碳循环的关键。我们还将预测未来海洋碳和营养循环的变化范围，以及海洋如何通过排放更多或更少的温室气体（如二氧化碳和一氧化二氮）来影响未来变暖的程度。

Oceans represent 70% of Earth's surface, supporting vast biodiversity and providing major food resources for humankind. Since the industrial revolution, the oceans have restricted the extent of global warming by taking up approximately 50% of the CO2 from fossil fuel burning and cement manufacture. CO2 forms carbonic acid when dissolved in seawater and lowers ambient pH in a phenomenon known as 'ocean acidification'. An important facet of ocean acidification is a decline in the concentration of carbonate ions in the ocean, a form of dissolved carbon that is depleted in the acidification reaction when CO2 is added to seawater. This is critical, because the shells and skeletons of many marine organisms are made of calcium carbonate (CaCO3) which dissolves at low carbonate ion concentrations (known as 'under-saturated' conditions). However, ecological thresholds of disruption may be crossed long before conditions of under-saturation actually occur and marine organisms' shells start dissolving around them, because calcium carbonate shells and skeletons will require more metabolic energy to maintain their thickness as carbonate ion concentrations fall. Experiments in the laboratory and field have already demonstrated this effect and find that calcifying algae called coccolithophorids generally produce less CaCO3 shell material in more acidic conditions. Slowing the rate of production of CaCO3 by algae living in the ocean surface may have a 'beneficial' impact by helping neutralize fossil fuel CO2, but a detrimental impact as ecosystems are disrupted. There may also be serious implications for the supply of organic detritus to organisms on the seafloor, as it is suspected that this food supply depends heavily on particles of CaCO3 to weigh down the fluffy organic matter and help it sink. Other impacts of ocean acidification may include changes in the amount and nutritional content of organic matter produced in the ocean, and loss of nutrients: converting nitrate to the powerful greenhouse gas nitrous oxide. How the ocean carbon cycle 'works' and whether CaCO3 particles are really important to weighting down fluffy organic matter as well as how exactly algae like coccolithophorids will respond to changing ocean chemistry are subject to significant uncertainties. This means that we would have no way of knowing whether a single computer model prediction for the future is correct or not. In this project we will tackle this question of uncertainty head-on - running out computer models of ocean carbon cycling and climate hundreds and hundreds of times to see what future impacts are possible and what are not. We will be greatly helped in this by using vast datasets describing what the modern ocean 'looks' like (in terms of the distributions of nutrients and patterns recorded in the sediments) to constrain the swarm of models so that they all agree on what the modern ocean looks like to begin with. The outcome of our work will firstly be a better understanding of the modern ocean carbon cycle, which is essential to get right before worrying about the future. We will also make predictions about the range of changes in ocean carbon and nutrient cycles we can expect in the future and how the ocean may affect the degree of future warming by emitting more or less greenhouse gases such as carbon dioxide and nitrous oxide.