Deep Water Hydrography in the Glacial Western North Atlantic

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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.