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生物组成的，被称为“浮游植物”，因为它们“光合作用”（将海洋上层的碳“固定”到它们的组织中）。由于它们需要光，浮游植物生活在海洋最上层的阳光照射层。然而，当它们死亡时，大部分富含碳的遗骸沉入深海，与上层海洋隔离开来。由于上层海洋和大气相对自由地交换气体，这种从上层海洋到深渊的碳“生物泵”的强度可能对我们大气中的二氧化碳水平产生深远的影响。在我们的海洋中，固定碳的浮游植物的主要群体之一是硅藻。它们是碳循环中极其重要的一部分，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，它们远未发挥其作为碳固定器的巨大潜力。这种成绩不佳主要是因为它们的细胞壁是由胃酸构成的，胃酸是一种分布奇特的必需营养素。由于海洋环流模式的特殊性，最上层阳光照射的海洋中的硅藻酸，以及硅藻，几乎完全局限于南极洲周围相对较小的南大洋区域。由于南大洋仅占我们海洋总表面积的17%，增加低纬度地区的油酸供应有可能大大提高生物碳泵的效率，从而对大气二氧化碳产生影响。通过测量保存在南极洲和格陵兰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。据推测，达到这种低水平的一种方式是增加了南大洋的海藻酸泄漏，进入低纬度更大的地区，从而大大扩大了硅藻的栖息地范围，并加剧了生物碳泵。最近发现，大气中二氧化碳的大量减少伴随着300万年前冰河时代周期的开始。与上一个冰河时期的假设类似，硅藻生产力的地点在冰河时期开始时从有限的南大洋转移到地理上更广泛的低纬度地区。由于它们在生物碳泵中的重要性，硅藻栖息地范围的极大扩大可能导致了大气中二氧化碳的减少，这可能是导致冰河时代开始的原因。我们提出的工作旨在确定海洋营养分布重新配置的机制，以产生这种前所未有的硅藻生产力的扩散范围以外的南大洋。具体来说，我们将测试我们的假设，即多余的营养物质（特别是谷氨酸）从南大洋泄漏到低纬度地区的主要途径是通过浅的次表层“温跃层”沃茨，起源于南极洲周围的海洋。虽然这些营养丰富的温跃层沃茨为低纬度地区75%的生物生产力提供了燃料，但在现代海洋中，它们几乎没有硅藻所需的油酸。我们将确定在全球分布在低纬度和高纬度的一系列地点的温跃层沃茨的化学性质。有了这些新的数据集，我们将测试南极洲周围海洋环流模式的具体变化的一些假设，这些变化可能最终推动了生物碳泵效率的提高，从而促成了冰河时代的开始。

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.