Circulation in the glacial North Atlantic inferred from grain-size measurements

Abstract

RECORDS of nutrient proxies in marine sediments indicate that the nutrient distribution—and hence circulation—of the glacial North Atlantic Ocean was markedly different from that of today^1,2. But these tracers are influenced by several biogeochemical factors unrelated to ocean circulation^3–6, and thus do not provide a direct measure of the vigour of circulation. Distributions of grain size in marine sediments can provide such information, owing to the sorting effects of currents^7-8, if the characteristics of the input sediment flux are known. Here we present a record, inferred from grain-size measurements, of variations in the relative strength of deep and intermediate currents in the eastern North Atlantic over the past 25,000 years. We find that glacial intermediate water flowed rapidly at depths of between 1,100 and 2,000 m. In contrast, deep-water circulation was sluggish during the last glaciation, but increased in strength shortly after the glacial maximum. This would have resulted in increased heat flux to high latitudes, and may have triggered sudden deglaciation. Deep current strengths declined again at the start of termination stage IA and during the Younger Dryas, perhaps as a result of iceberg discharges (the so-called Heinrich events^9,10). A remarkably similar record of grain-size variations has been found in the western North Atlantic¹¹, indicating that these changes in circulation are ocean-wide.