Award: OCE-1260514

Intellectual Merit: Ocean circulation is a major means of distributing heat around the Earth, and from the tropics to high latitudes. Changes in ocean circulation play a critical role in changes in global climate, and there is a lot of effort among scientists to understand the relationships between ocean circulation and climate. Because old seawater cannot be measured directly, in order to determine how the oceans operated during different climate states in the past, scientists measure "proxies" for ancient seawater, that is, element concentrations or element ratios, or isotope ratios, that trace different aspects of the oceans in the past, derived from deep sea sediments (the approach requires figuring out the time when the sample formed). In order to be able to trace changes in ocean circulation through time, it is critical to understand how ocean circulation proxies behave in the oceans in the present-day. The isotopes of the rare earth element ?neodymium? (chemical symbol Nd) have been increasingly used as a proxy to trace past ocean circulation, based on observations in the modern ocean that its composition varies by location and depth in a manner that enables the ?fingerprinting? of water masses, and because over long distances the Nd proxy appears to trace water mass mixing. Over the past decade, studies have increasingly focused on the impacts of processes that would alter the true water mass mixing signal. As a consequence, the assumption that the Nd proxy traces ocean circulation in the past is increasingly challenged. This is because Nd could be added locally by sediment eroded from the continent, and local additions would interfere with the global ocean circulation signal. In order to apply any ocean circulation proxy, it is important to know its limitations. The project critically tested the behavior of the Nd proxy in the oceans in a manner that will clarify its value as an ocean circulation tracer. The southwest Atlantic is the best place to perform this test, based on the configuration of water masses there. This project focused on testing the integrity of the Nd proxy in a north-south transect in the southwest Atlantic from the equator to the Malvinas/Falkland Islands, collected on a cruise by the British research vessel James Cook. The seawater samples include the major water masses involved in the global ocean circulation, where in the water masses the Nd proxy is distinct. The project tested how well the Nd proxy in individual water samples reflects the mixture of the water masses contributing to the sample. The results show that the Nd proxy robustly traces the flow and mixing of major water masses in the southwest Atlantic, and thus has the potential to be a major paleo-ocean circulation proxy for climate change studies. Broader Impacts: Understanding the role of deep ocean circulation will be particularly important for our understanding of how the climate system works. A key element in paleo-ocean circulation research is knowing how the proxies behave in the oceans. Our results are important findings that serve as a foundation for applying an important climate proxy. This project trained a Ph.D. student, and contributed to the career of an early career scientist, who is now a professor. Last Modified: 12/10/2017 Submitted by: Steven L Goldstein