Scavenging of 230Th, 231Pa and 10Be in the Southern Ocean (SW Pacific sector): the importance of particle flux, particle composition and advection

The scavenging of ²³⁰Th, ²³¹Pa and ¹⁰Be was studied in the Pacific sector of the Southern Ocean along 170°W using measurements from surface sediments, time-series sediment traps, and the water column. All sediment traps collected an annual flux of ²³¹Pa greater than the integrated rate of production by ²³⁵U-decay (up to 5.5 times greater) and a flux of ¹⁰Be greater than the global-average deposition rate of ¹⁰Be (up to 3.4 times greater). Fluxes of ²³⁰Th were on average close to the production rate in the overlying water column. These large fluxes of ²³¹Pa and ¹⁰Be, and high unsupported ²³¹Pa/²³¹Th and ¹⁰Be/²³⁰Th ratios in the sediments, are not associated with depletion of nuclides in the water column, and therefore are not the result of the classical boundary scavenging mechanism. We estimate that of the ²³¹Pa and ²³⁰Th advected into the Southern Ocean as part of the large-scale overturning circulation, only those nuclides associated with the “upper limb” of this circulation, i.e. those that pass through the surface as part of the wind-driven Ekman flow, are scavenged efficiently. The majority of the nuclides advected into the Southern Ocean and associated with the “bottom water limb” are not scavenged to the sediments of the Southern Ocean, but are returned northward.

Throughout the year, the unsupported ²³¹Pa/²³⁰Th and ¹⁰Be/²³⁰Th ratios of material reaching sediment traps at ∼1000 m at a given site increase with increasing particle flux. This behavior is explained by a conceptual model in which Th, Pa and Be are scavenged reversibly by fine-grained suspended particles at all depths, while Pa and Be are in addition scavenged strongly by diatom-rich aggregates in surface waters. Spatial variability in the annually averaged unsupported ²³¹Pa/²³⁰Th and ¹⁰Be/²³⁰Th ratios of sinking particulate matter reflects primarily the variability in particle composition, and in the ratio and quantity of nuclides upwelled to the mixed layer, rather than variability in particle flux.