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

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Chase, Z, Anderson, RF, Fleisher, MQ, Kubik, PW
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon-Elsevier Science Ltd 2003
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Southern Ocean; Pacific
• Online Access: https://doi.org/10.1016/S0967-0645(02)00593-3; http://ecite.utas.edu.au/101461

The scavenging of 230 Th, 231 Pa and 10 Be was studied in the Pacific sector of the Southern Ocean along 170W using measurements from surface sediments, time-series sediment traps, and the water column. All sediment traps collected an annual flux of 231 Pa greater than the integrated rate of production by 235 U-decay (up to 5.5 times greater) and a flux of 10 Be greater than the global-average deposition rate of 10 Be (up to 3.4 times greater). Fluxes of 230 Th were on average close to the production rate in the overlying water column. These large fluxes of 231 Pa and 10 Be, and high unsupported 231 Pa/ 231 Th and 10 Be/ 230 Th 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 231 Pa and 230 Th 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 231 Pa/ 230 Th and 10 Be/ 230 Th ratios of material reaching sediment traps at ∼1000m 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 231 Pa/ 230 Th and 10 Be/ 230 Th 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.