The distribution of 230Th and 231Pa in the deep-sea surface sediments of the Pacific Ocean

Abstract

In order to determine the areal extent and distribution of 231Pa sinks in the Pacific Ocean, we have analyzed 34 deep-sea surface sediment samples from widely distributed locations for their Pa, Th and U isotopes. While the 230Th xs content varies by more than a factor often, the 231Pa xs content is confined in a relatively narrow range with a mean value of 2.8 ± 1.4 dpm/g. There is a positive correlation between 230Th xs and 231Pa xs and, using the regression line it is possible to calculate the ( 230Th xs 231Pa xs ) activity ratios in samples for which only 230Th data are available. We then constructed a map of ( 230Th xs 231Pa xs ) distribution. Using ( 230Pa xs 231Pa xs ) < 11 (the production ratio in the water column) as a criterion for identifying preferential 231Pa sinks, we have identified an area covering 30–40% of the Pacific Ocean. This area includes most of the Pacific margins, the northwest Pacific, the Antarctic Ocean and perhaps the mid-ocean ridge regions, and is nearly coincident with that of high sediment accumulation rates: this suggests that the paniculate flux is important for the removal of 231Pa from the ocean. Two models for the removal of 231Pa produced in the open ocean are: 1. (1) lateral transport into the vertical flux in regions of high particulate flux ( Damaster, 1979; Anderson et al, 1983a,b) or 2. (2) uptake at the sediment-water boundary: the second model is more likely to be correct. To distinguish unequivocally between the two proposed models, we would need sediment trap data for the western North Pacific.