Sedimentology and chronology of sediment cores from Arctic Ocean Ridges: insights from U-series isotopes

Abstract

U- and Th-series isotopes have been intensely used in ocean science for the documenting of water column and sedimentation processes. In this respect, the geochemistry of these isotopes in the central Arctic Ocean has received a much lesser attention. Here, using sediment cores retrieved during the 2005 HOTRAX expedition over Arctic Ridges, we will pay some attention to the behaviour of a few of these isotopes (^{210}Pb, ^{226}Ra, ^{230}Th, ^{231}Pa, ^{234}U, ^{238}U, ^{232}Th) with special attention to their use for documenting sedimentation rates and the setting of chronostratigraphical benchmarks in this very low sedimentation rate setting. Other chronological methods (e.g., ^{18}O stratigraphy, ^{14}C ages) revealed mostly inconclusive at this sites, but ^{10}Be data provided some constraints on a Ma-scale chronology, leading to consider using U-series isotopes to document the more recent sedimentation history of the ridge. At the study sites, ^{210}Pb-excess activity in the top cm is tightly linked to its scavenging in the water column. Below, its distribution is governed by that of ^{226}Ra: i.e., it follows the Ra-diffusion gradient that characterizes the upper few cm, then both isotopes are linked to parent ^{230}Th behaviour, with some blurring around peaks of ^{230}Th-excesses due to some discrete ^{226}Ra-diffusion on both sides of the peaks. ^{230}Th- and ^{231}Pa-excess profiles present variations linked to the depositional regime, where coarser ice-rafted deposition is shows low ^{230}Th- and ^{231}Pa-excesses (mostly during glacials), whereas finer grain layer are highlighted by higher excesses (interglacial and deglacial intervals). Based on the decay of these excesses, downcore, we proposed an age model for the last two climatic cycles. Correlative studies of a core raised from the Lomonosov ridge revealed significantly higher sedimentation rates (below the Trans-Polar Drift route) and high ^{230}Th fluxes to the sediment pointing to much distinct geochemical and sedimentological conditions over this ridge.