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，科学家们就确定，地球冰河时期循环期间大气中二氧化碳含量与气候相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设长期以来一直存在争议，但事实证明这些假设很难检验。 这里提出的工作将检验一个主要假设，特别是海洋在冰河时期经历了更大的密度分层。 也就是说，随着冰河时期分层程度的加深，以及两极附近形成的寒冷浓水对深水的替代速度较慢，深海将容纳更多的二氧化碳，这些二氧化碳是由有机碳的生物呼吸产生的，这些有机碳不断以死亡生物体和有机碎片的形式从阳光照射的海洋表层下沉到深渊。 为了检验这一假设，我们将通过比较在南极洲周围关键区域的表层海洋和海底生长的生物体的放射性碳年龄来限制最后一个冰河时代的海洋分层程度和深水替代率，大部分深水替代发生在该区域。 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，海洋吸收了燃烧化石燃料产生的约 25% 的二氧化碳。 大部分碳被南大洋（南极洲周围地区）吸收。 未来这种情况将如何改变却鲜为人知。 模型很难准确地反映南大洋的物理条件，从而给未来海洋吸收二氧化碳的预测增加了很大的不确定性。 拟议研究的结果将为测试模型在与当今明显不同的气候条件下模拟海洋过程的能力提供基准，最终导致模型的改进以及对海洋未来吸收二氧化碳的更可靠的预测。 拟议的工作将为罗斯海和新西兰之间地区的南大洋现有科学考察增加一个研究部分，该考察将在西纬约 170° 的太平洋-南极海岭北侧的三到五个地点收集沉积物岩心。 目标是收集自上一个冰河时代高峰期早期以来沉积在每个位置的沉积物。 该地区在南大洋中很不寻常，因为在最后一个冰河时期沉积的沉积物中含有有孔虫（一种具有碳酸钙壳的微小生物），其数量比南大洋其他地区要多得多。 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，居住在表层海洋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。 表面和深层膨胀生物之间的年龄差异将被用来区分冰河时期深水更新的两种拟议机制：南极洲边缘沿海冰间湖的形成，就像今天发生的那样，与远离大陆的深水区域的公海对流形成。 如果后一种机制占主导地位，那么预计地表和深层有孔虫将表现出相似的放射性碳年龄。 在沿海冰间湖中深水形成占主导地位的情况下，人们预计会在两个有孔虫种群中发现非常不同的放射性碳年龄。 在上一个冰河时期海洋分层程度更大的极端情况下，人们甚至预计地表居民似乎比当代海底居民更古老，因为目标核心地点直接位于最古老的深水在经过深海长途迂回后返回地表的区域下方。 这项工作的主要目标是重建上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储存的二氧化碳量的主要因素。 此外，要回收的岩心中存在的有孔虫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养分利用水平，从而限制海洋生物泵的效率，2）应用钕同位素来限制深水团的运输历史，3）应用硼 同位素和硼/钙比率来限制冰河时代海水的 pH 值和无机碳系统参数，4) 利用有孔虫中的金属/钙比率来重建上一个冰河时代南极洲附近深水源头位置的温度 (Mg/Ca) 和营养物含量 (Cd/Ca)。