Thorium and Uranium Isotopes in Upper Pleistocene Sediments of ODP Sites 645 (Baffin Bay), 646, and 647 (Labrador Sea)

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Concentrations and activity ratios of uranium and thorium isotopes (234U/238U, 230Th/232Th) were determined at about 5-m intervals through the composite top 22-m sequence of Ocean Drilling Program (ODP) Hole 645 in Baffin Bay and, in the Labrador Sea, at 1-m intervals through the top 11 m of Core 84-030-003 (TWC and P) collected by the Hudson during a preliminary survey of Site 647, and also at about 2-m intervals through the composite top 22-m se­ quence of Hole 646. In the Labrador Sea, surficial sediments show unsupported 230Th having a 230Th/234U activity ra­ tio of about 3. At Site 647, a regular decrease in the 230Th/232Th activity ratio was observed downcore from about 1.2 (at 1 mbsf) to about 0.4 (at ~8 mbsf), through a sequence spanning over 180 stages 2 through 8. The correlative tho­ rium/uranium chronology and 180 stratigraphy indicate relatively constant sedimentation rates throughout the se­ quence. At Site 646, down Greenland slope, and at Site 645, in Baffin Bay, highly variable uranium and thorium con­ centrations and isotopic ratios were observed in relation to highly variable sedimentation rates. As a whole, the lower -excess observed in Baffin Bay records is indicative of very high absolute sedimentation rates in comparison with those of the Labrador Sea. These rates are confirmed by the lsO-stratigraphy and a few AMS 14C controls on handpicked foraminifers. At both Labrador Sea sites, a clear indication of an initial 230Th-excess (over the 230Th-rain from the water column) was found.