Role of Ocean Biogeochemical Reorganisation in the Intensification of Northern Hemisphere Glaciation

海洋中的大多数植物都是由微小的微型植物组成的，这种植物之所以被称为“浮游植物”，是因为它们进行“光合作用”(将上层海洋中的碳“固定”到它们的组织中)。因为它们需要光，浮游植物生活在海洋中阳光充足的最上层。然而，当它们死亡时，它们的大部分富含碳的遗骸沉入深海，远离上层海洋。由于上层海洋和大气的气体交换相对自由，这种从上层海洋向深渊排放碳的生物泵的强度可能会对我们大气中的二氧化碳水平产生深远的影响。海洋中固定碳的浮游植物的主要类群之一是硅藻。它们是碳循环中极其重要的部分，因为它们负责高达90%的生物泵介导的碳转移到深渊。然而，值得注意的是，在今天的海洋中，它们远未实现其作为碳固定者的巨大潜力。这在很大程度上是因为它们的细胞壁是用硅酸建造的，硅酸是一种重要的营养物质，分布奇怪。由于海洋环流模式的特殊性，阳光照耀的最上层海洋中的硅酸以及硅藻几乎完全限于南极洲附近相对较小的南大洋区域。由于南大洋只占我们海洋总表面积的17%，增加对低纬度地区的硅酸供应有可能极大地提高生物碳泵的效率，从而对大气中的二氧化碳产生影响。通过测量保存在南极洲和格陵兰岛冰盖内微小气泡中的冰河时代大气的组成，科学家们知道，冰河时代伴随着大气二氧化碳水平的大幅下降。有人假设，实现这些低水平的一种方式是增加硅酸从南大洋泄漏到更大面积的低纬度地区，从而极大地扩大了硅藻的栖息地范围，并为强化的生物碳泵提供了燃料。最近发现，随着三百万年前冰河时代周期的开始，大气中的二氧化碳大幅下降。与上一次冰河时代的假设类似，在冰河时代开始时，硅藻的生产地从受限制的南大洋转移到地理范围更广的低纬度地区。由于硅藻在生物碳泵中的重要性，硅藻的栖息地范围大大扩大，可能是大气中二氧化碳下降的原因之一，而大气二氧化碳可能是引发冰河时代的原因。我们拟议的工作旨在确定海洋营养物质分布被重新配置以在南大洋范围外产生这种前所未有的硅藻生产力激增的机制。具体地说，我们将检验我们的假设，即多余的营养物质(特别是硅酸)从南大洋泄漏到较低纬度的主要途径是通过起源于南极洲周围海洋的浅层次表层‘温跃层’水域。虽然这些营养丰富的温跃层水域为低纬度地区75%的总生物生产力提供了燃料，但在现代海洋中，它们几乎缺乏硅藻所需的硅酸。我们将确定全球分布在低纬度和高纬度地区的温跃层水的化学成分。有了这些新的数据集，我们将测试一些假设，以确定南极洲周围海洋环流模式的具体变化，这些变化最终可能推动生物碳泵效率的提高，从而促成冰河时代的开始。

The majority of plant life in the ocean is made up of tiny microscopic plants, termed 'phytoplankton' because they 'photosynthesise' ('fix' carbon from the upper ocean into their tissues). Because of their need for light, phytoplankton live in the uppermost sunlit layers of the ocean. However, when they die most of their carbon-rich remains sink into the deep ocean, locked away from the upper ocean. Because the upper ocean and atmosphere exchange gases comparatively freely, the intensity of this 'biological pump' of carbon from the upper ocean into the abyss can have a profound impact on levels of carbon dioxide in our atmosphere. One of the main groups of carbon-fixing phytoplankton in our oceans are the diatoms. They are an extremely important part of the carbon cycle because they are responsible for up to 90% of the biological pump-mediated carbon transfer to the abyss. Yet remarkably, in today's oceans they are far from achieving their enormous potential as carbon fixers. This underachievement is largely a consequence of the fact that they build their cell walls from silicic acid, an essential nutrient that has a curious distribution. Owing to peculiarities in ocean circulation patterns, silicic acid in the uppermost sunlit ocean, and hence diatoms too, are almost entirely restricted to the relatively small area of the Southern Ocean around Antarctica. Because the Southern Ocean represents only 17% of the total surface area of our oceans, increasing the supply of silicic acid to lower latitudes has the potential to greatly increase the efficiency of the biological carbon pump, with consequences for atmospheric carbon dioxide. By measuring the composition of ice age atmospheres preserved in tiny gas bubbles within Antarctica and Greenland's ice sheets, scientists know that the ice ages were accompanied by large reductions in atmospheric carbon dioxide levels. It has been hypothesised that one way in which these low levels could have been achieved was increased leakage of silicic acid out of the Southern Ocean and into the much larger area of the lower latitudes, thereby greatly expanding the habitat range of diatoms and fuelling an intensified biological carbon pump. It has recently been discovered that a substantial drop in atmospheric carbon dioxide accompanied the onset of ice age cycles three million years ago. In a similar fashion to the hypothesis for the last ice age, the locus of diatom productivity switched from the restricted Southern Ocean to the more geographically extensive lower latitudes during the onset of the ice ages. Because of their importance in the biological carbon pump, this greatly expanded habitat range of diatoms may have contributed to the observed drop in atmospheric carbon dioxide that was likely responsible for initiating the ice ages. Our proposed work aims to determine the mechanism by which oceanic nutrient distributions were reconfigured to produce this unprecedented proliferation of diatom productivity outside the confines of the Southern Ocean. Specifically, we will test our hypothesis that the primary route by which excess nutrients (especially silicic acid) leaked out of the Southern Ocean to lower latitudes was via shallow sub-surface 'thermocline' waters that originate in the ocean around Antarctica. While these nutrient-rich thermocline waters fuel 75% of total biological productivity in lower latitudes, they are, in the modern ocean, almost devoid of the silicic acid required by diatoms. We will determine the chemistry of thermocline waters across an array of globally distributed sites at lower and higher latitudes. With these new datasets, we will test a number of hypotheses for specific changes in the ocean circulation patterns around Antarctica that may have ultimately driven increased efficiency of the biological carbon pump and thereby contributed to the onset of the ice ages.