Excess sediment Th-230: Transport along the sea floor or enhanced water column scavenging?

The sedimentary inventory of Th-230 is often employed to distinguish between areas which receive excess sediment as the result of along-bottom transport from those which lose sediment as a result of winnowing. This process is referred to as "focusing.'' A case is made here that, at least in the western equatorial Pacific, the excess Th-230 in on-equator sediment is more likely scavenged from the overlying water column by the rain of organic matter than retransported along the bottom. If so, lateral variations in biological productivity may well contribute to the excess Th-230 inventories elsewhere as well.