210 Pb– 226 Ra– 230 Th systematics in very low sedimentation rate sediments from the Mendeleev Ridge (Arctic Ocean)This article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network .GEOTOP Publication 2008-0031.

• Published in: Canadian Journal of Earth Sciences
• Main Authors: Not, Christelle, Hillaire-Marcel, Claude, Ghaleb, Bassam, Polyak, Leonid, Darby, Dennis
• Other Authors: Hollings, Pete
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2008
• Subjects: General Earth and Planetary Sciences; Arctic; Arctic Ocean
• Online Access: http://dx.doi.org/10.1139/e08-047; http://www.nrcresearchpress.com/doi/full-xml/10.1139/E08-047; http://www.nrcresearchpress.com/doi/pdf/10.1139/E08-047

U-series isotope behaviour in subsurface sediment of the Arctic Ocean is investigated based on high resolution measurements of natural radionuclides ( 210 Pb, 226 Ra, 230 Th) and a few analyses of anthropogenic 137 Cs in cores collected during the 2005 Healy-Oden Trans-Arctic Expedition (HOTRAX). Cores from the Mendeleev Ridge, representing distinct bathymetric 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 210 Pb ( 210 Pb xs ) over parent 226 Ra content, down to ∼1 cm below core top. This 210 Pb xs distribution is due to shallow mixing by benthic organisms and (or) diffusion from the sediment–water interface, as also indicated by 137 Cs activities. From ∼1 to 7 cm downcore, 210 Pb activities closely follow 226 Ra activities. Below 7 cm downcore, 226 Ra activities are controlled by variable excesses in parent 230 Th ( 230 Th xs ) resulting from its scavenging in the overlying water column. 226 Ra 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 230 Th xs peaks but with lesser fluxes. Both cores show identical 210 Pb profiles despite their 1 km bathymetric difference. This suggests negligible 230 Th and 210 Pb 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 210 Pb xs require precise knowledge of the 226 Ra-supported fraction, which is controlled by 230 Th xs , Ra diffusion, and thus sedimentation rates and porosity.