Water Mass Structure and Bottom Water Formation in the Ice-age Southern Ocean

科学家早在30多年前就确定，在地球的冰期周期中，大气中二氧化碳水平与气候相关的变化是由海洋调节的。关于海洋如何调节大气二氧化碳的假说一直存在争议，但事实证明这些假说很难验证。这里提出的工作将验证一个主要假设，即海洋在冰河时期经历了更大的密度分层。也就是说，在冰河期更大的分层和极地附近形成的寒冷稠密的水取代深水的速度较慢，深海将容纳更多的二氧化碳，二氧化碳是由有机碳的生物呼吸产生的，这些有机碳以死亡生物和有机碎片的形式不断地从阳光照射的海洋表面下沉到深渊。为了验证这一假设，最后一个冰河期的海洋分层程度和深水替代的速度将受到限制，方法是比较在南极周围一个关键区域的表层海洋和海底生长的生物的放射性碳年龄，大部分深水替代发生在南极周围的一个关键区域。完成这项工作将有助于改进未来气候变化的模型。气候科学家依靠模型来估计未来海洋将吸收的化石燃料二氧化碳的数量。目前，海洋吸收了大约25%的燃烧化石燃料产生的二氧化碳。这些碳大部分被南大洋（南极洲周围地区）吸收。这种情况在未来会如何变化，目前还不得而知。模式难以准确地表示南大洋的物理条件，因此对未来海洋吸收二氧化碳的预估增加了很大的不确定性。拟议研究的结果将提供一个基准，以测试模型在与今天发生的气候条件截然不同的情况下模拟海洋过程的能力，最终导致模型的改进和对未来海洋吸收二氧化碳的更可靠的预测。在罗斯海和新西兰之间的南大洋地区，现有的科学考察将在太平洋-南极脊北侧约170°W的三到五个地点收集沉积物岩心，拟议的工作将为其增加一个研究组成部分。目的是在每个地点收集自上一个冰河时代高峰早期以来沉积的沉积物。这个地区在南大洋是不寻常的，因为在最后一个冰河时期沉积的沉积物中含有有孔虫，这是一种带有碳酸钙外壳的微小生物，比南大洋的其他地区要丰富得多。有孔虫被广泛用作过去海洋条件的几种地球化学示踪剂的档案。在拟议的工作中，居住在海洋表面的有孔虫的放射性碳年龄将与生长在海底的当代标本的年龄进行比较。表层生物和深海膨胀生物年龄的差异将被用来区分冰河期深水更新的两种机制：一种是在南极洲边缘的沿海冰融区形成的，就像今天发生的那样；另一种是在远离大陆的深水区由公海对流形成的。如果后一种机制盛行，那么预计地表和深部有孔虫将表现出相似的放射性碳年龄。在沿海多冰群中深水地层占主导地位的情况下，人们期望在两个有孔虫种群中发现非常不同的放射性碳年龄。在最后一个冰河期海洋分层加剧的极端情况下，人们甚至预计，表层生物似乎比同时代的底层生物更古老，因为目标核心地点就在最古老的深水经过漫长的深海循环后返回表面的区域正下方。这项工作的主要目标是重建最后一个冰河时期南大洋的水团年龄结构，这反过来又是控制深海中储存的二氧化碳量的主要因素。此外，有孔虫在岩心中的存在为许多其他古海洋学应用提供了宝贵的资源，例如：1)应用氮同位素来限制南大洋的营养利用水平，从而限制海洋的效率？2)应用钕同位素约束深水块的输运历史，3)应用硼同位素和硼/钙比值约束冰期海水的pH和无机碳系统参数，4)利用有孔虫金属/钙比值重建末次冰期南极附近深水源附近的温度（Mg/Ca）和营养成分（Cd/Ca）。

Scientists established more than 30 years ago that the climate-related variability of carbon dioxide levels in the atmosphere over Earth's ice-age cycles was regulated by the ocean. Hypotheses to explain how the ocean regulates atmospheric carbon dioxide have long been debated, but they have proven to be difficult to test. Work proposed here will test one leading hypothesis, specifically that the ocean experienced greater density stratification during the ice ages. That is, with greater stratification during the ice ages and slower replacement of deep water by cold dense water formed near the poles, the deep ocean would have held more carbon dioxide, which is produced by biological respiration of the organic carbon that constantly rains to the abyss in the form of dead organisms and organic debris that sink from the sunlit surface ocean. To test this hypothesis, the degree of ocean stratification during the last ice age and the rate of deep-water replacement will be constrained by comparing the radiocarbon ages of organisms that grew in the surface ocean and at the sea floor within a critical region around Antarctica, where most of the replacement of deep waters occurs. Completing this work will contribute toward improved models of future climate change. Climate scientists rely on models to estimate the amount of fossil fuel carbon dioxide that will be absorbed by the ocean in the future. Currently the ocean absorbs about 25% of the carbon dioxide produced by burning fossil fuels. Most of this carbon is absorbed in the Southern Ocean (the region around Antarctica). How this will change in the future is poorly known. Models have difficulty representing physical conditions in the Southern Ocean accurately, thereby adding substantial uncertainty to projections of future ocean uptake of carbon dioxide. Results of the proposed study will provide a benchmark to test the ability of models to simulate ocean processes under climate conditions distinctly different from those that occur today, ultimately leading to improvement of the models and to more reliable projections of future absorption of carbon dioxide by the ocean. The proposed work will add a research component to an existing scientific expedition to the Southern Ocean, in the region between the Ross Sea and New Zealand, that will collect sediment cores at three to five locations down the northern flank of the Pacific-Antarctic Ridge at approximately 170°W. The goal is to collect sediments at each location deposited since early in the peak of the last ice age. This region is unusual in the Southern Ocean in that sediments deposited during the last ice age contain foraminifera, tiny organisms with calcium carbonate shells, in much greater abundance than in other regions of the Southern Ocean. Foraminifera are widely used as an archive of several geochemical tracers of past ocean conditions. In the proposed work the radiocarbon age of foraminifera that inhabited the surface ocean will be compared with the age of contemporary specimens that grew on the seabed. The difference in age between surface and deep-swelling organisms will be used to discriminate between two proposed mechanisms of deep water renewal during the ice age: formation in coastal polynyas around the edge of Antarctica, much as occurs today, versus formation by open-ocean convection in deep-water regions far from the continent. If the latter mechanism prevails, then it is expected that surface and deep-dwelling foraminifera will exhibit similar radiocarbon ages. In the case of dominance of deep-water formation in coastal polynyas, one expects to find very different radiocarbon ages in the two populations of foraminifera. In the extreme case of greater ocean stratification during the last ice age, one even expects the surface dwellers to appear to be older than contemporary bottom dwellers because the targeted core sites lie directly under the region where the oldest deep waters return to the surface following their long circuitous transit through the deep ocean. The primary objective of the proposed work is to reconstruct the water mass age structure of the Southern Ocean during the last ice age, which, in turn, is a primary factor that controls the amount of carbon dioxide stored in the deep sea. In addition, the presence of foraminifera in the cores to be recovered provides a valuable resource for many other paleoceanographic applications, such as: 1) the application of nitrogen isotopes to constrain the level of nutrient utilization in the Southern Ocean and, thus, the efficiency of the ocean?s biological pump, 2) the application of neodymium isotopes to constrain the transport history of deep water masses, 3) the application of boron isotopes and boron/calcium ratios to constrain the pH and inorganic carbon system parameters of ice-age seawater, and 4) the exploitation of metal/calcium ratios in foraminifera to reconstruct the temperature (Mg/Ca) and nutrient content (Cd/Ca) of deep waters during the last ice age at a location near their source near Antarcitca.