Deep Water Hydrography in the Glacial Western North Atlantic

这个项目将调查海洋在最后一个冰河时期（大约2万年前）在气候中的作用，当时北大西洋周围的条件与今天非常不同。海面更冷，大气更有风，大气中的二氧化碳少得多，海平面也低了300英尺。现代北大西洋的运作被比作一条传送带，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。失去热量后，地表水下沉到海底，从北欧海流出，流入北大西洋盆地深处。三十多年来，海洋学家和气候学家已经知道，这条传送带在过去曾多次中断，尤其是在上一个冰河时期。对海洋沉积物岩心中化石壳的化学组成的分析表明，深海的来源不是北大西洋的北部深水，而是南部（南极）的。这种关于北大西洋冰川的观点已经成为一种范例。最近在北大西洋非常深的盆地中发现的岩心挑战了这一传统观点，表明毕竟有一个北部的底水来源。该项目试图通过收集和分析北大西洋最深盆地（5000米）的新沉积物岩心来证实这一发现。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，由于冰川年龄数据被用来测试不同情景下的现代气候模型，因此提出的新研究可能会导致预测未来气候变化的模型的改进。研究考察将有大约8个泊位，供对学习海底测量和采样方法感兴趣的早期职业专业人员使用。收集的长沉积物岩心将被存档，并随时可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小组的国际合作。这项研究的核心部分是对北大西洋西部科德角、马塞诸塞州和亚速尔群岛之间的一次考察，以调查海底深处的新取心位置，并使用不同的设备（多芯仪、重力仪和活塞仪）对它们进行取样。深海环流的变化将利用浮游生物和底栖有孔虫化石的碳同位素比率来追踪。13C/12C比率是深水营养成分含量的代表，在4.2公里左右的核心地点，这一比率表明，在冰川时期，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表面比现在更多地覆盖着海冰，所以北部的水源产生的水更少，要么是因为深海的水流速度更慢，从表面积累了更多的营养物质。现有的14C/12C数据支持在13C/12C中看到的模式，这种示踪剂也被用来确定沉积物的年代。然而，在4200 m以下，这些同位素对的模式相反，随着深度的增加（至5 km），这些同位素对表明更多的北部水源。这一假设将在3公里至5公里的地点进行取心和岸基分析工作。一名德国同事将收集3公里以上新核心地点的数据，这项合作将产生北大西洋西部盆地过去水文变化的第一次高分辨率深度重建。

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.