210Pb-226Ra-230Th systematics in very low sedimentation rate sediments from the Mendeleev Ridge (Arctic Ocean)

Abstract

U-series isotope behaviour in subsurface sediment of the Arctic Ocean is investigated based on high resolution measurements of natural radionuclides (210Pb, 226Ra, 230Th) and a few analyses of anthropogenic 137Cs in cores collected during the 2005 Healy-Oden Trans-Arctic Expedition (HOTRAX). Cores from the Mendeleev Ridge, representing distinct bafhymetric settings, are analyzed in more detail as a means to assess the dating potential of such radionuclides at sites characterized by very low sedimentation rates (∼3 mm ka-1). The sediment consists of variable proportions of fine-grained carbonates, clays, and ice-rafted debris and shows excesses in 210Pb (210Pbxs) over parent 226Ra content, down to ∼ 1 cm below core top. This 210Pbxs distribution is due to shallow mixing by benthic organisms and (or) diffusion from the sediment-water interface, as also indicated by 137Cs activities. From ∼ 1 to 7 cm downcore, 210Pb activities closely follow 226Ra activities. Below 7 cm downcore, 226Ra activities are controlled by variable excesses in parent 230Th (230Thxs) resulting from its scavenging in the overlying water column. 226Ra diffusion is observed towards the water column occuring from the upper ∼7 cm of sediment below the seafloor (with a flux of ∼ 0.043 disintegrations per minute (dpm) cm-2 a-1) and deeper in the sediment below 230Thxs peaks but with lesser fluxes. Both cores show identical 210Pb profiles despite their 1 km bafhymetric difference. This suggests negligible 230Th and 210Pb scavenging below water depths of ∼1.6 km, i.e., the bathymetry of the shallower core. In such settings where sedimentation rates are very low and vertical particle rain is the major sediment source, estimates of the actual 210Pbxs require precise knowledge of the 226Ra-supported fraction, which is controlled by 230Thxs, Ra diffusion, and thus sedimentation rates and porosity. © 2008 NRC.