Deglacial Atlantic Ocean Ventilation Rates

This research project addresses a scientific issue that is of first-order importance, deglacial ocean ventilation rates. Ocean ventilation rates describe the time elapsed since a water mass last 'saw' the atmosphere. This age information is important because ocean ventilation rates are intimately linked to climate change through the formation of deep waters at high latitudes. Most of the carbon in the coupled ocean-atmosphere system is located in the deep ocean, which exerts an important influence on climate via the greenhouse gas connection. Small changes in the rate of deep water formation are likely to have a large impact on the atmospheric carbon budget. Geochemical analyses of marine sediments cores show that many such changes in ocean circulation have happened in the geological past. The most recent large amplitude changes occurred during the last deglaciation (~20 to 10 thousand years ago), a time when the large ice sheets in North America and Northern Europe were retreating. Measurements of the radiocarbon content of deep-sea corals indicate that during the last deglaciation, the ocean was flipping back and forth between different modes of operation (i.e., different water mass distributions and different flow rates). However, it is impossible to convert the radiocarbon contents of water mass masses directly into ventilation rates, if we do not know the mixing proportions of water masses derived from high northern and southern latitude sources. This hurdle can be overcome by measuring the neodymium (Nd) isotopic composition in deep-sea corals from the western North Atlantic Ocean. Deep-sea corals are reliable recorders of the Nd isotopic composition of the water mass in which they grow. The Nd isotopic composition of the water mass in turn, is closely tied to the age of the continents in its formation area, leading to very different Nd isotopic signatures for high northern versus southern latitude waters in the Atlantic Ocean. These distinct signatures enable us to 'un-mix' the composition of waters in the western North Atlantic during abrupt climate events of the last deglaciation. Applying this knowledge of water mass mixtures to the existing radiocarbon data set on the same, absolutely dated samples, we can unravel how rapid and from where ventilation of the Atlantic Ocean occurred during the last deglaciation. Information such as this has not been obtained before, and has the potential to revolutionise our understanding of the ocean's role in rapid climate change.