The Principal Investigator will construct a history of meltwater discharge into the Arctic Ocean through the Mackenzie River system during the most recent deglaciation by developing an AMS-dated chronology of N. pachyderma by d18 O and sediment lithology, and geochemistry for the last 15,000 years which will record the incursion of low- d18 O water melting from the northern Laurentide Ice Sheet. Previous stable isotopic reconstructions show that the effects of meltwater discharge in the central Arctic basin were large and sedimentological studies trace two ice rafted debris events from northern Canada to the Fram Strait during the deglacial period. These results imply that changes in Arctic salinity likely played a major role in the variability of North Atlantic overturning circulation. These meltwater events should show up as minima in N. pachyderma d18 O and the magnitude of the anomalies should be proportional to the size of the meltwater event. The cores were collected in 1989 and archived in the Woods Hole Oceanographic Institution core repository. They are in excellent condition and remain unsampled. Based on color changes, core sedimentation rates (~1 to 3 cm kyr-1) permit the development of a relatively high-resolution record of deglacial meltwater discharge.
Intellectual Merit. This research will provide insight into a very important climate question: how sensitive is the Atlantic overturning circulation to reduced salinity in the Arctic Ocean and subpolar North Atlantic? Accurate future projections of climate require this knowledge, yet the best examples of past changes in Atlantic overturning are in the geological record. Atlantic salinity is changing in ways that may affect circulation and heat transport to the North Atlantic: high latitude locations have been freshening for the last forty years in ways never before documented and reduced Greenland Sea convection may already be underway. However, it is difficult to know how much of a salinity change is needed to alter the circulation significantly. Physical oceanographers are unsure and the numerous numerical simulations of future greenhouse climate predict a full range of outcomes from no change in the overturning to a 15 Sv reduction in the Atlantic. To fully understand how sensitive the circulation is to changing salinity requires study of the geological record and specific knowledge of the Arctic Ocean's role in these changes.
Broader Impacts. Graduate student training is a significant part of this project. Sharon Hoffman, (a second year graduate student) will produce the thorium data for the cores. Additional training will include the research experiences gained by the high school participants who will be involved with sample preparation and analytical procedures for this project. The Principal Investigator regularly speaks before the public in a variety of venues, including public libraries, undergraduate classes, and at annual WHOI gathering of media for Ocean Science Journalism Fellows. Recent interviews with magazines (Discover, Newsweek), newspapers (Boston Globe, Wall Street Journal) and radio (local NPR station) attest to the places where the results of this research are likely to be discussed. The Principal Investigator has presented climate briefings and formal testimony to members of the House of Representatives, the Senate, and their staffs in Washington, DC and at Woods Hole.
