234Th balance and implications for seasonal particle retention in the eastern Bering Sea

As part of the Bering Ecosystem Study-Bering Sea Integrated Ecosystem Research Program (BEST-BSIERP), distributions of 234Th in the water column and sediments were measured at approximately 60 stations over the shelf and slope/oceanic regions of the eastern Bering Sea during spring and summer of 2009 and 2010. During this study period, the inventory of sediment excess 234Th (234Thxs) on the shelf was determined to be ∼1/3 of the total 234Th deficit in the overlying water column; the average focusing factor (FFTh=sediment 234Thxs/water column 234Th deficit) was 0.34±0.23. In addition, the export flux of 234Th from the shelf to the slope/oceanic region was determined to be on average ∼30% of the total production of 234Th throughout the shelf. Taken together, these results indicate that 234Th and associated particles are largely retained on the shelf, and that shelf sediments represent a sink for particles. In contrast, the FFTh in the slope/oceanic region was higher, averaging 1.52±1.34. The higher FFTh at the slope is attributed primarily to enhanced scavenging removal and sediment deposition of 234Th associated with periods of high biogenic particle flux at the marginal ice zone during the spring sea-ice retreat. A more general conclusion is that the ocean margin of the eastern Bering Sea may serve as an accumulation area for particles and associated reactive chemicals.