FLUCTUATIONS IN DEEP WESTERN NORTH ATLANTIC CIRCULATION ON THE BLAKE OUTER RIDGE DURING THE LAST DEGLACIATION

Variations in the mean grain size of the detrital silt fraction from four piston cores along a depth transect on the Blake Outer Ridge were used to examine paleocirculation patterns and intensity between 2650 and 3818 m for the late Pleistocene and Holocene. Prior to 12 kyr, circulation was focused on the upper part of the ridge above 2700 m and may have corresponded to enhanced levels of intermediate water production in the North Atlantic. Deepwater circulation during deglaciation underwent a two-step decrease to minimum speeds from 15.5 to 13.5 kyr, followed by a three-step increase to maximum speeds at 10 kyr. Minima in circulation at 13.5 and 12 kyr can be directly associated with oxygen isotopic evidence for advection of reduced salinity surface water to the North Atlantic. Additional circulation minima at 15 and 10.5 kyr may also be the result of reduced salinity from high-latitude meltwater discharge. Evidence for a drop to moderate circulation intensity during the Holocene is supported by isolated geochemical records but could also be explained by decoupling of ocean ventilation rates from bottom circulation or a change in sediment provenance at the end of deglaciation.