Testing the bipolar see-saw with Pa and Nd isotopes in the Argentine Basin

Dramatic changes in past climate on millennial timescales are seen in northern-hemisphere ice-core records from Greenland. Somewhat less dramatic changes are also seen in Antarctica at these timescales, but such changes are out of phase with those in the north. Changes in the north are known to be caused by variation in the rate of deep water formation in the North Atlantic, which alters transport of heat to the region. A widely believed model is that such changes in northern hemisphere deepwater formation are mirrored by antiphase changes in southern hemisphere deepwater formation. Vigorous deep-water formation in only one hemisphere causes that hemisphere to be warm, while the other is cold. This belief is, however, largely untested and the antiphase climate behaviour of the two hemispheres may be caused, not by changes in deep ocean circulation, but by more general redistribution of heat by the oceans, or by changes in atmospheric circulation. Here we propose to reconstruct changes in deep-ocean circulation in the South Atlantic at millennial timescales across an interval which experienced significant climate variation. This reconstruction, when compared to existing records from the North Atlantic will provide a direct test of the suggested antiphase behaviour of deep-water flow, and will provide important new insights into the mechanisms causing abrupt climate change. We will use two new geochemical proxies recorded in ocean sediments - 231Pa/230Th and eNd. The former of these provides information about the rate of water flow, and the latter about the source of water mass that is flowing. Combing proxies that provide such information is an innovative and powerful approach to understanding changes in the past ocean. We will measure these proxies in a range of cores, provided by a colleague in Bremen (Germany), taken from the Argentine Basin. This basin is at a similar latitude in the South Atlantic, as those cores from the North Atlantic which provide some of the most compelling evidence for the role of ocean circulation in millennial climate change (including evidence from the new 231Pa/230Th proxy). They are therefore ideally suited to assess the relationship between deep-water circulation in the two hemispheres. In addition to testing the mechanisms causing millennial climate change, our new proxy records will improve our understanding of changes in ocean circulation on longer timescales, and in other areas. Reconstruction of the water-masses present in the past South Atlantic will, for instance, test the common assertion that significantly less deep-water formed in the North Atlantic during the last glacial period. And assessment of changes in the chemical composition of water masses in the South Atlantic will allow more accurate future application of the 231Pa/230Th and eNd tracers in other ocean basins.