Particulate fluxes of 230Th, 231Pa, and 10Be in the northeastern Pacific Ocean

Abstract

Measurements of 230Th, 231Pa, and 10Be in sediment trap samples and Holocene sediments collected along a transect in the northeastern Pacific indicate that the removal of these nuclides from the ocean is greatly influenced by boundary scavenging. More than 35% of the 230Th, at least 70% of the 231Pa, and as much as 85% of the 10Be fluxes collected by sediment traps at a nearshore site (located ~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 230Th, ~ 100 y for 231 Pa, and ~ 1000 y for 10Be). Results of this study indicate that particle flux is a principal factor influencing scavenging of 230Th, 231Pa, and 10Be from the water column. The marine geochemistry of 10Be in ocean-margins is fundamentally different from that in the open ocean. Scavenging of 10Be occurs throughout the water column at the margin area in this study, in contrast to the pattern of 10Be being removed only in the surface waters and then remineralized at depth in the open ocean. Aluminosilicate may be a major phase removing 10Be 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 230Th and 231Pa as well as certain other trace elements (such as Ba, Cu, and Zn).