Fish gut carbonates and the control of ocean alkalinity

目前，全球海洋吸收了大约30%的人为二氧化碳排放。调节这种海洋-大气二氧化碳交换的碳循环，以及与之相关的影响海洋吸收能力的溶解碳和碱度的垂直分布，取决于几个过程。它们被描述为一系列相互作用的“泵”：一个物理/化学溶解度泵；一个生物“软组织”泵；以及一个碳酸钙泵。了解这三个泵，它们如何相互作用，以及它们对大气二氧化碳的反馈，对于准确预测未来海洋碳循环和全球气候将如何变化尤为关键。碳酸钙是一种白色的白垩质矿物，由一系列海洋生物产生。重要的是，当它溶解时，它会增加海水的碱度，这可以将海水中的二氧化碳浓度降低到大气二氧化碳水平以下，并从大气中吸收人为的二氧化碳。因此，准确地知道它在哪里溶解(离海洋表面有多近)是理解碳酸钙泵在调节海洋化学和大气二氧化碳中所起作用的关键。碳酸钙泵的运行不仅取决于产量，还取决于海洋生物产生的碳酸盐矿物的类型、它们下沉的速度，以及这些碳酸盐矿物溶解的速度。大多数海洋碳循环模型都假设碳酸盐的生产主要由浮游生物和球藻(微型藻类)主导。然而，我们现在知道非常大量的碳酸盐是由海洋硬骨鱼(硬骨鱼)排泄的。这种碳酸盐，我们现在也知道，根据鱼类物种的不同，它在矿物学上是多样化的，在鱼类的肠道中不断产生，并作为废物排出。这一过程对海洋CaCO3泵的潜在意义在由Pi Wilson(科学，2009)领导的初步建模工作中得到了认可，该工作保守地提出，鱼类可能至少占全球海洋CaCO3总产量的3-15%，实际情况是高达45%。自第一次建模以来，这一过程背后的科学已经取得了巨大的进步。作为一个群体(通过其他人的工作)，我们现在知道，鱼类产生各种各样的碳酸盐矿物类型，现有的知识表明，这些矿物的溶解速度应该非常不同。因此，最初的建模工作中的假设，即鱼产生统一的和相对可溶的碳酸盐类型不再有效。虽然我们已经可以解决一些知识差距，但关于占全球鱼类生物量约94%的科的鱼类的数据很少或根本没有--包括仅占鱼类生物量至少60%的中上层鱼类的数据。据推测，它们巨大生物量的每日垂直迁移将驱动一种新型的“向上碱度泵”，这可能会对其他已有过程驱动的碱度向下输送提供重要的补偿。此外，我们现在有很好的证据表明，鱼类的产量随代谢率而变化(这在全球重要的活跃上层鱼类中最大)，重要的是还取决于饲料和饮食(特别是饲料中的钙含量)。因此，早期模型中所有鱼类都以相同的速率产生碳酸盐的必要假设不再适用于建模。除了这些问题，关于这些碳酸盐在海洋中下沉或溶解的速度，我们也很少或根本没有数据。因此，该项目的目的是提供关于鱼类碳酸盐产量、矿物学、溶解度和沉降率的新的经验数据，以提供第一个空间和矿物学解析的全球产量估计，从而使我们能够将评估鱼类在当前条件下和未来气候变化情景下对海洋碳循环的贡献的模型参数化。

The global oceans currently absorb ~30% of anthropogenic CO2 emissions. The carbon cycle that regulates this ocean-atmosphere CO2 exchange, and the associated vertical distribution of dissolved carbon and alkalinity that influences the ocean's absorption capacity, depends on several processes. These are described as a series of interacting "pumps": a physical/chemical solubility pump; a biological 'soft tissue' pump; and a calcium carbonate pump. Understanding these three pumps, how they interact, and their atmospheric CO2 feedbacks is especially critical for accurate predictions of how the marine carbon cycle and global climate will change in the future. Calcium carbonate is a white, chalky mineral produced by a range of marine organisms. Importantly, when it dissolves it increases the alkalinity of seawater, which can reduce the seawater CO2 concentration below atmospheric CO2 levels and 'suck' anthropogenic CO2 from the atmosphere. Knowing exactly where it dissolves (how near the ocean surface) is therefore key to understanding the role this calcium carbonate pump plays in regulating ocean chemistry and atmospheric CO2. The operation of the calcium carbonate pump not only depends on the production rate but also the types of carbonate minerals that are produced by marine organisms, the rate at which they sink, and how rapidly these carbonate minerals then dissolve. Most ocean carbon cycle models make the assumption that carbonate production is dominated by the plankton and coccolithophores (microscopic algae). However, we now know that very large amounts of carbonate are excreted by marine bony fish (teleosts). This carbonate, which we now also know is mineralogically diverse depending on the fish species, is continuously produced in the intestines of fish and excreted as waste. The potential significance of this process to the marine CaCO3 pump was recognised in an initial modelling exercise led by PI Wilson (Science, 2009) which conservatively suggested that fish may account for at least 3-15% of total marine CaCO3 production globally, and realistically as much as 45%. Since that first modelling exercise the science behind this process has advanced hugely. As a group (and through the work of others) we now know that fish produce a hugely diverse range of carbonate mineral types, which existing knowledge would suggest should dissolve at very different rates. As a result, the assumptions in the first modelling efforts that fish produce uniform and relatively soluble carbonate types are no longer valid. Whilst we can already address some of the knowledge gaps, there is little or no data for fish from families that comprise ~94% of global fish biomass - including almost no data for mesopelagic fish that alone account for at least 60% of fish biomass. The daily vertical migration of their immense biomass is hypothesised to drive a novel "upward alkalinity pump", which may provide an important offset to the downward transport of alkalinity driven by other established processes. Also, we now have good evidence to show that production rates by fish vary with metabolic rate (which is greatest in the globally significant active epipelagic fishes), and importantly also depending upon feeding and diet (especially the calcium content of the diet). Thus, again, necessary assumptions in early models that all fish produce carbonate at the same rate are no longer realistic to use for modelling. Over and above these issues we also have little to no data on the rates at which these carbonates sink in the oceans or dissolve. The aim of this project is therefore to deliver new empirical data on fish carbonate production, mineralogies, solubilities and sinking rates to inform the first spatially- and mineralogically-resolved global production estimates, thus enabling us to parameterise models assessing fish contributions to the marine carbon cycle both under present day conditions, and for climate change scenarios in the future.