Particulate Fluxes of Th-230, Pa-231, and Be-10 in the Northeastern Pacific-Ocean

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Geochimica Et Cosmochimica Acta
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Measurements of Th-230, Pa-231, and Be-10 in sediment trap samples and Holocene sediments collected along a transect in the northeastern Pacific indicate that the removal of there nuclides from the ocean is greatly influenced by boundary scavenging. More than 35% of the Th-230, at least 70% of the Pa-231, and as much as 85% of the Be-10 fluxes collected by sediment traps at a nearshore site (located approximately 120 km off shore) are supplied by lateral transport of the dissolved nuclides from the open ocean. These findings are consistent with the hypothesis that boundary scavenging is proportional to the oceanic residence times of the nuclides (10-40 y for Th-230, approximately 100 y for Pa-231, and approximately 1000 y for Be-10). Results of this study indicate that particle flux is a principal factor influencing scavenging of Th-230, Pa-231 and Be-10 from the water column. The marine geochemistry of Be-10 in ocean-margins is fundamentally different from that in the open ocean. Scavenging of Be-10 occurs throughout the water column at the margin area in this study, in contrast to the pattern of Be-10 being removed only in the surface waters and then remineralized at depth in the open ocean. Aluminosilicate may be a major phase removing Be-10 from the water column in the open ocean but not at-ocean-margins. Nuclide accumulation rates in the underlying Holocene sediments are all higher than the corresponding fluxes collected by sediment traps. Several possibilities that could account for this observation include: downslope sediment transport; deployment of the traps during a period when the particle flux was not representative of long-term average conditions; and scavenging of the nuclides from near bottom waters. The high accumulation rate of authigenic Mn in some Holocene sediments, which can be attributed to redox cycling of Mn, may have caused near-bottom scavenging of Th-230 and Pa-231 as well as certain other trace elements (such as Ba, Cu, and Zn).


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