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Ocean circulation and iceberg discharge in the glacial North Atlantic: Inferences from unmixing of sediment size distributions

¹ Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands
² Department of Applied Earth Sciences, Delft University of Technology, P.O. Box 5028, 2600 GA Delft, Netherlands
³ Faculty of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, Netherlands
⁴ Geological Survey of Denmark and Greenland, Thoravej 8, 2400 Copenhagen NV, Denmark
⁵ Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands

Variability in iceberg discharge and deep-ocean circulation in the North Atlantic during the last glacial period is inferred from the grain-size distribution and trace elemental composition of terrigenous sediments in a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of the grain-size distributions is used to unmix the signals of varying bottom-current speed and iceberg discharge. The size distribution within the silt fraction appears to be influenced by both factors. Based on Th-Sc-La relationship, we established that during the ice-rafted detritus events, continental material of likely Greenlandic origin increased to 87%, and that bottom-current–derived material contains to 40% mid-oceanic ridge fines, probably of Icelandic origin. Our results have important implications for the use of silt grain size as an indicator for paleocurrent speed in the glacial North Atlantic. We show that reconstructions of variations in bottom-current speed based on the raw grain-size data are opposite to inferences from the unmixed record. The latter indicates that deep-water convection decreased during periods of enhanced iceberg discharge, which is in general agreement with paleoceanographic reconstructions of the North Atlantic.

Key Words: ocean circulation • iceberg discharge • grain-size distribution • end-member modeling • trace element geochemistry

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