A direct link between ocean circulation and abrupt climate change?

Summary The surprising possibility that climate may change by as much as 10 degrees C within just 10-20 years was discovered through examining ice-cores drilled into the Greenland ice sheet. The enormous impact that similar future changes could have on the world's society motivates efforts to understand the possible causes of abrupt climate change and the likelihood of future occurrences. Abrupt oscillations in climate, termed Dansgaard-Oeschger (D-O) events, occurred frequently during an interval 30 - 60 thousand years ago (ka). Reorganisations of the ocean circulation in the Atlantic (the so-called Atlantic meridional overturning circulation, AMOC) have been invoked to play a dominant role in governing the abrupt nature of D-O events. The AMOC is characterised by warm salty surface water flowing northwards to the northern North Atlantic, where it cools and sinks, forming North Atlantic deepwater (NADW) that flows southwards as a deep return flow. It is the movement of warm water to the high latitudes and the subsequent release of heat to the atmosphere that helps regulates the climate of the North Atlantic. Changes in the AMOC have been related to climate shifts in modelling studies. Yet despite intensive study, we still lack unequivocal physical evidence for a systematic and direct role of deep ocean circulation in abrupt climate change. Here we propose to examine ocean sediments at very high temporal resolution in order to test the involvement of deep circulation changes during past abrupt climate change. The flow of NADW (the deep water limb of the AMOC) forms an intense current at depth along the eastern continental margin of North America. We aim to reconstruct changes in the strength of this current by assessing changes in the grain size of seafloor sediments: very simply, faster current flow causes sorting of the sediment and leads to larger average grain size. Previous studies have suggested that cold intervals are related to reduced deep water formation in the northern North Atlantic and a shoaling of NADW. We will test this hypothesis by detecting intervals of stronger shallow flow using sediment cores at 1.8 km and 3 km depth. Previous work has demonstrated that our chosen core sites and sampling intervals will produce records with a resolution of ~100 years and will therefore be capable of reconstructing the multi-centennial scale D-O oscillations. Pilot study data that we have obtained from an earlier time interval provide confidence in our chosen methods and have yielded very promising results, suggesting a tight coupling between ocean circulation changes and climate. This study addresses a clearly identified gap in our understanding of abrupt climate change. Testing the hypothesis that abrupt climate change is directly related to changes in ocean circulation, and further investigating the differences between different cold events, will represent an important advance in the field of paleoclimatology. The results of this study will be of direct benefit to, for example, climate modellers, and hence will be of importance in enhancing our predictive knowledge of future abrupt climate change. Our study will address directly NERC's current challenge to 'Quantify forces and feedbacks that drive the Earth System' within its Earth System Science Theme, and specifically Challenges 8 and 11 ('Ocean processes and their interaction with the Earth System' and 'What do records of past environments reveal about the operation of the Earth System?).