Assessing the role of ocean circulation in rapid climate change through the novel integration of high-resolution proxy records

The role of ocean circulation during global climate changes that occur during glacial-interglacial cycles and on millenial timescales has been the subject of intense research. Ocean circulation redistributes heat on the earth's surface and therefore has the potential to amplify climate change. Furthermore, the deep ocean is the largest reservoir of carbon dioxide and therefore has an important role in glacial-interglacial changes controlling in atmospheric carbon dioxide concentration. As such, rapid shifts between circulation regimes have been suggested as a possible mechanism to trigger climate change, including glacial-interglacial transitions in atmospheric carbon dioxide concentrations and sea level, and as a possible amplifier of climate changes that were started by other processes. Marine proxies provide our only constraints on past ocean conditions during periods of climate change. Most marine proxies are generally used to trace one variable in the ocean, typically watermass location and mixing, but are affected by a number of climate-modulated processes. Most published records of ocean circulation are based on measurements of the carbon isotope composition of fossilized deep-dwelling protozoa that occur in ocean sediments. However, the carbon isotope composition of these creatures is not just affected by changes in circulation, but also by several other processes including respiration of organic matter in the deep ocean. The role of ocean circulation in driving glacial-interglacial climate change and millenial-scale climate variability is still debated because there isn't enough information to address if circulation changs occurred at the same time, before, or after changes in deep ocean carbon chemistry and climate. We are proposing to develop new records that will let us determine the 'sequence of events' during the last two hundred thousand years, the last two glacial-interglacial cycles. As it turns out, the neodymium isotope composition of iron-manganese oxide minerals in marine sediments can be used to directly reconstruct ocean circulation. Neodymium isotopes are not sensitive to the same process as carbon isotopes. We are also going to compare neodymium and carbon isotope measurements on the same marine sediment samples to determine the non-ocean circulation component of the carbon isotope record. We are then going to compare this with a set of nutrient proxies, the trace metal composition of fossilized deep-dwelling foraminifera, in order to separate the regeneration component from the other processes that affect carbon isotopes. This approach will allow us to study the phasing relationships between two different parts of the ocean-climate system, the physically controlled, and the biologically controlled, during these periods of climate change.