Sedimentary thorium-230 and protactinium-231 records of paleoenvironmental changes in the Arctic Ocean

Abstract

The long-lived particle-reactive radionuclides 230Th and 231Pa, produced uniformly by uranium decay in the ocean, are useful tracers for past dynamics of both oceanic and sedimentary processes. Despite much work done over large areas of the world ocean, there is still a lack of systematic understanding of 230Th and 231Pa behaviour in the Arctic Ocean. Sedimentary records of 230Th and 231Pa excesses (230Thxs and 231Paxs) from the Arctic Ocean in relation to environmental changes through glacial-interglacial (stadial-interstadial) cycles were investigated in this thesis. Measurable concentrations of 230Thxs(231Paxs) in a new sediment core from the western Arctic Ocean display an alternation of “230Thxs(231Paxs)-rich” and “230Thxs(231Paxs)-poor” sections. Sedimentary characteristics reveal this alternation does not directly correspond to interglacial-glacial cycles. The 230Thxs and 231Paxs concentrations show an inverse relationship to their corresponding sedimentation rates, suggesting the distributions of 230Thxs and 231Paxs are affected by variable dilution with terrigenous material. However, changes in lateral transport of these isotopes may also account for the observed oscillations. Sedimentary cumulative inventories of 230Thxs show evidence for a net deficit of 230Th in the sediments during the last glacial period, possibly due to a lack of scavenging material in general. Comparison of sedimentary 230Thxs from across the Arctic Ocean reveals that variations in 230Thxs activity in cores from the Amerasian Basin can be correlated to those from the Lomonosov Ridge. The similarity of variations in both 230Thxs activity and flux between Amerasian Basin cores indicates the removal of 230Th in the western Arctic is largely controlled by common mechanisms. A deficit of 230Thxs in western Arctic sediments may be responsible for the “extinction age” of 230Thxs downcore that is younger than it would be expected from an exponential decay pattern. On the contrary, the removal of 230Th in the eastern Arctic seems more likely influenced by local conditions. Evidence for a substantial deficit of 230Thxs in eastern Arctic sediments is not found. A systematic investigation on fluxes of 231Paxs and 230Thxs into Arctic deep-sea sediments reveals the variation trends of both fluxes are similar. Boundary scavenging may act as an important sink for both 231Pa and 230Th from the central Arctic Ocean. A stronger degree of boundary scavenging for 231Pa is likely the main cause for low 231Pa/230Th ratios (< 0.093) in Arctic deep-sea sediments.