Episodic reductions in bottom-water currents since the last ice age

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Past changes in the freshwater balance of the surface North Atlantic Ocean are thought to have influenced the rate of deep-water formation, and consequently climate1,2. Although water-mass proxies are generally consistent with an impact of freshwater input on meridional overturning circulation3, there has been little dynamic evidence to support this linkage. Here we present a 25,000 year record of variations in sediment grain size from south of Iceland, which indicates vigorous bottom-water currents during both the last glacial maximum and the Holocene period. Together with reconstructions of North Atlantic water-mass distribution, vigorous bottom currents suggest a shorter residence time of northern-source waters during the last glacial maximum, relative to the Holocene period. The most significant reductions in flow strength occur during periods that have been associated with freshening of the surface North Atlantic. The short-term deglacial oscillations in bottom current strength are closely coupled to changes in Greenland air temperature, with a minimum during the Younger Dryas cold reversal and a maximum at the time of rapid warming at the onset of the Holocene. Our results support a strong connection between ocean circulation and rapid climate change.

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Figure 1: Schematic map of the North Atlantic circulation32 with the location of ODP Site 984 (6125 N, 2404 W, 1.65 km).
Figure 2: The δ18O of benthic foraminifera plotted on an inverted axis with the sediment-size data from Site 984.
Figure 3: GISP2 Greenland ice core δ18O record34 plotted with grain size and carbon isotopes from Site 984.


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This study was improved by assistance and technical support from M. Jeglinski.

Support for this research was provided in part by the US-NSF, the WHOI-OCCI and the Comer Science and Research Foundation.

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Correspondence to Summer K. Praetorius.

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Supplementary table and figure S1 (PDF 296 kb)

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Praetorius, S., McManus, J., Oppo, D. et al. Episodic reductions in bottom-water currents since the last ice age. Nature Geosci 1, 449–452 (2008) doi:10.1038/ngeo227

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