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

冰期南大洋的水团结构和底层水的形成

• 批准号: 1542962
• 负责人: Robert Anderson
• 金额: $ 27.85万
• 依托单位: Columbia University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-06-01 至 2020-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1542962&HistoricalAwards=false
• 关键词: Water; Mass; Structure; Bottom; Formation

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.

30多年前，科学家们证实，地球冰河时期大气中二氧化碳水平与气候有关的变化是由海洋调节的。解释海洋如何调节大气二氧化碳的假说长期以来一直存在争议，但事实证明，这些假说很难检验。这里提出的工作将检验一个主要的假设，特别是海洋在冰河时代经历了更大的密度分层。也就是说，在冰河时期，随着更多的层化，深水被两极附近形成的冷密水取代的速度变慢，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，有机碳不断以死亡有机体和从阳光照耀的表层海洋下沉的有机碎片的形式降到深渊。为了验证这一假设，将通过比较生长在表层海洋和南极洲周围关键区域海底的生物的放射性碳年龄，来限制上一个冰期海洋分层的程度和深水置换的速度，南极周围的关键区域是大多数深水置换发生的地方。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，海洋吸收了燃烧化石燃料产生的大约25%的二氧化碳。这些碳的大部分被南大洋(南极洲周围地区)吸收。这种情况在未来将如何改变，我们知之甚少。模型很难准确描述南大洋的物理条件，从而增加了对未来海洋二氧化碳吸收的预测的很大不确定性。拟议研究的结果将提供一个基准，以测试模型在与今天截然不同的气候条件下模拟海洋过程的能力，最终导致改进模型和更可靠地预测未来海洋吸收二氧化碳的情况。拟议的工作将在罗斯海和新西兰之间区域现有的南大洋科学考察队中增加一个研究部分，该科考队将在太平洋-南极海脊北侧西经约170°的三至五个地点收集沉积物岩芯，目标是收集自上一个冰河时代高峰期早期以来每个地点沉积的沉积物。这一区域在南大洋是不寻常的，因为在上一个冰期沉积的沉积物中含有有孔虫，这是一种具有碳酸钙外壳的微小生物，比南大洋其他区域丰富得多。有孔虫被广泛用作过去海洋状况的几种地球化学示踪物的档案。在这项拟议的工作中，居住在表层海洋的有孔虫的放射性碳年龄将与生长在海床上的当代标本的年龄进行比较。表层生物和深膨胀生物之间的年龄差异将被用来区分冰河时代提出的两种深水更新机制：在南极洲边缘附近的沿海冰川形成，很像今天发生的那样，以及在远离大陆的深水区域通过公海对流形成。如果后一种机制占上风，那么预计表层有孔虫和深海有孔虫将表现出类似的放射性碳年龄。在沿海有孔虫中深水地层占主导地位的情况下，人们预计在两个有孔虫种群中会发现非常不同的放射性碳年龄。在上一次冰河时期海洋分层加剧的极端情况下，人们甚至预计表层居民似乎比当代海底居民年龄更大，因为目标核心地点就在该地区的正下方，那里是最古老的深水在漫长迂回地穿过深海后返回表面的地方。拟议工作的主要目标是重建上一次冰期期间南大洋的水团年龄结构，而这反过来又是控制深海中储存的二氧化碳数量的主要因素。此外，在待恢复的岩心中有孔虫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？S生物泵，2)应用钕同位素来限制深水团的运移历史，3)应用硼同位素和硼/钙比来限制冰期海水的pH和无机碳系统参数，4)利用有孔虫中的金属/钙比值，重建末次冰期南极附近深水的温度(Mg/Ca)和营养盐含量(Cd/Ca)。