High resolution sampling of the upper 10 m of Hole 722B has provided information on the extent of 234U/230Th disequilibrium in Owen Ridge sediments accumulating over the last 250 k.y. High biological productivity in surface waters is sustained by seasonal upwelling driven by the Southwest Monsoon. Evidence from the Owen Ridge suggests that there has been significant variation in this productivity over the late Pleistocene, and that greater biological production occurred during interglacial episodes. The resulting organic matter flux drives a variety of redox-related reactions in the sediment, including the reduction and fixing of U. High precision measurements of U/Th trace the paleoproductivity record of enhanced organic matter input during the interglacials. Sedimentary 230Th, produced by 234U decay in the water column, is a sensitive indicator of water column scavenging by particulates, and sediment winnowing and focusing. The decay-corrected flux of excess 230Th at the Owen Ridge is substantially less than that recorded at other upwelling margins. The historical record of 230Th flux suggests that erosion and winnowing of fine sediment has occurred at the ridge site by active intermediate water circulation, and that this was most prevalent during mid isotope Stage 5 to Stage 3.

Corrected for salt contribution and dilution. Stratigraphy from Clemens and Prell (pers. comm., March 1989).

Supplement to: Shimmield, Graham; Mowbray, Stephen R (1991): U-series disequilibrium, particle scavenging, and sediment accumulation during the late Pleistocene on the Owen Ridge, Site 722. In: Prell, WL; Niitsuma, N; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 117, 465-472