Deep Water Hydrography in the Glacial Western North Atlantic

北大西洋冰川西部的深水水文学

• 批准号: 1558307
• 负责人: Lloyd Keigwin
• 金额: $ 56.21万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-09-15 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1558307&HistoricalAwards=false
• 关键词: Deep; Water; Hydrography; Glacial; Western

This project will investigate the ocean's role in climate during the last ice age (about 20,000 years ago), a time when conditions around the North Atlantic were very different from today. The surface ocean was colder, the atmosphere was windier, there was far less carbon dioxide in the atmosphere, and sea level was 300 feet lower. Operation of the modern North Atlantic Ocean has been likened to a conveyor belt, where warm salty waters of Gulf Stream origin enter the Norwegian Sea and release heat to the atmosphere that keeps northern Europe warm. After losing their heat, surface waters sink to the bottom and flow out of the Nordic Seas and into the deep North Atlantic basin. For over three decades oceanographers and climatologists have known that this conveyor belt was interrupted at times in the past, especially during the last ice age. Instead of deep northern water in the North Atlantic, analysis of the chemical make-up of fossil shells in marine sediment cores shows that the deep waters had a southern (Antarctic) source. This view of the glacial North Atlantic has become a paradigm. Recent findings from cores in the very deep basins of the North Atlantic have challenged that conventional wisdom, suggesting that there was a northern source of bottom water after all. This project seeks to confirm that finding by collecting and analyzing new sediment cores from the deepest basins of the North Atlantic (5000 meters). Results could significantly change, or even overturn, our understanding of how the glacial North Atlantic distributed heat, salt, nutrients, and CO2. Furthermore, because glacial age data are used to test modern climate models under different scenarios, the proposed new research could lead to refinement of models that predict future climate changes. The research expedition will have about eight berths for early career professionals who are interested in learning methods of seafloor surveying and sampling, and the long sediment cores to be collected will be archived and readily available for sampling by many diverse scientists. The project will also involve international collaboration with a German research team.The centerpiece of the research is an expedition to the western North Atlantic between Cape Cod, MA and the Azores to survey the very deep seafloor for new coring locations and to sample them with different devices (multicorers, gravity corers, and piston corers). Changes in deep ocean circulation will be traced using carbon isotope ratios in shells of fossil planktonic and benthic foraminifera. The 13C/12C ratio is a proxy for deep water nutrient content, and at core sites around 4.2 km this ratio shows that during glaciation the mixture of waters from the Arctic and Antarctic was different than today and consistent with current thinking. Most likely, either the northern source produced less water because the surface in the Nordic seas was more covered with sea ice than today, or the deep water traveled more slowly and accumulated more nutrients from the surface. Existing 14C/12C data support the pattern seen in 13C/12C, and this tracer is also used to date the sediment deposits. However, below 4200 m, the pattern of these isotope pairs reverses and indicates more northern source waters with increasing depth (to 5 km). This hypothesis will be tested with coring and shore-based analytical work on sites from between 3 km and 5 km. Data from new core sites above 3 km will be collected by a German colleague, and the collaboration will result in the first high resolution depth reconstructions past hydrographic changes in the western basin of the North Atlantic.

该项目将调查上一次冰河时期(约2万年前)海洋在气候中的作用，当时北大西洋周围的情况与今天截然不同。表面的海洋更冷，大气更多风，大气中的二氧化碳要少得多，海平面降低了300英尺。现代北大西洋的运作被比作传送带，起源于墨西哥湾流的温暖咸水进入挪威海，向大气释放热量，使北欧保持温暖。在失去热量后，表层水沉入海底，从北欧海流出，进入北大西洋深水盆地。三十多年来，海洋学家和气候学家都知道，这条传送带在过去有时会中断，特别是在上一个冰河时代。对海洋沉积物岩心中化石贝壳的化学组成的分析表明，深水来自南方(南极)，而不是北大西洋的北部深水。这种对北大西洋冰川的看法已经成为一种范例。最近对北大西洋超深盆地岩心的发现挑战了这一传统观点，表明底水毕竟来自北方。该项目试图通过从北大西洋最深的盆地(5000米)收集和分析新的沉积岩芯来证实这一发现。结果可能会显著改变，甚至颠覆我们对北大西洋冰川如何分布热量、盐、营养物质和二氧化碳的理解。此外，由于冰川时期的数据被用来在不同的情景下测试现代气候模型，拟议的新研究可能会导致预测未来气候变化的模型的改进。研究探险队将为对学习海底测量和采样方法感兴趣的职业早期专业人员提供大约8个泊位，将收集的长沉积物岩芯将被存档，供许多不同的科学家进行采样。该项目还将涉及与一个德国研究团队的国际合作。研究的核心是前往马萨诸塞州科德角和亚速尔群岛之间的北大西洋西部进行探险，调查海底深处的新取心位置，并用不同的设备(多角机、重力取心器和活塞式取心器)进行取样。将使用浮游和底栖有孔虫化石贝壳中的碳同位素比率来追踪深海环流的变化。~(13)C/~(12)C比值是深水营养物质含量的一个指标，在4.2公里左右的核心位置，这个比值表明，在冰川作用期间，来自北极和南极的水的混合物与今天不同，与目前的想法一致。最有可能的原因是，要么是因为北欧海域的海冰覆盖面积比今天更大，导致北方源头产水量减少，要么是因为深水的移动速度更慢，从表面积累了更多的营养物质。现有的14C/12C数据支持13C/12C中的模式，该示踪剂也用于确定沉积物的年代。然而，在4200米以下，这些同位素对的模式发生了逆转，表明随着深度的增加(到5公里)，更多的北方水源。这一假设将在3公里至5公里范围内的取心和岸基分析工作中得到验证。一位德国同事将从3公里以上的新核心地点收集数据，这一合作将导致首次高分辨率深度重建北大西洋西部盆地过去的水文变化。