Pleistocene fluctuations in the western boundary undercurrent on the Blake Outer Ridge

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Abstract
The mean grain size of the detrital silt fraction was analyzed in two piston cores from the crest of the Blake Outer Ridge using electronic particle analysis. The resultant profiles in the two cores are highly correlatable and are interpreted to reflect the paleospeed of the Western Boundary Undercurrent (WBUC) for the past 90 kyr. The record can be divided into three periods. The first encompasses the past 12,000 years and begins with an abrupt acceleration marking the transition from slow, glacial to more vigorous, Holocene conditions. The later Holocene is a period of strong but variable flow. The second period extends from 30 to 12 × 10³ years and is characterized by a steady reduction of WBUC strength through the last ice age. The current continued to decelerate until 12 × 10³ years when the polar front finally retreated northward past the passages connecting the Norwegian Sea to the north Atlantic. The third period, from 30 to about 90 × 10³ years, was a time of strong and variable flow that was independent of the glacial/interglacial signal that is seen in the oxygen isotope record. During this time, the core of maximum flow appears to have shifted up and down the flank of the Blake Outer Ridge and may have been influenced as much by salinity as by temperature variations in the source areas of the WBUC.