Seminar

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 sedimentary records of ²³⁰Th and ²³¹Pa excesses (²³⁰Th_xs and ²³¹Pa_xs) from the Arctic Ocean in relation to environmental changes through glacial-interglacial (stadial-interstadial) cycles. This study improves the understanding of ²³⁰Th_xs and ²³¹Pa_xs in Arctic Ocean sediments from three aspects: (1) driving mechanisms of ²³⁰Th_xs and ²³¹Pa_xs variability, (2) ²³⁰Th and ²³¹Pa budgets in Arctic deep basins, and (3) potential reasons for low ²³¹Pa_xs/²³⁰Th_xs ratios measured in Arctic sediments. Combined analyses of ²³⁰Th, ²³¹Pa and sedimentological characteristics in a sediment core from the western Arctic provide evidence that the alternating pattern of high and low concentrations of ²³⁰Th_xs and ²³¹Pa_xs downcore was mainly controlled by changes in lateral transport of these isotopes. An overall deficit of sedimentary ²³⁰Th (and ²³¹Pa) relative to water column production through the last glacial cycle was revealed. On the other hand, sediment focusing may have occurred systematically in the western Arctic Ocean due to meltwater events. In addition, comparison of sedimentary ²³⁰Th_xs across the Arctic Ocean reveals that variations in ²³⁰Th_xs activities and fluxes in western Arctic cores can be correlated to those in central Arctic cores but differ greatly from those in eastern Arctic cores. The removal of ²³⁰Th in the eastern Arctic might be comparable to that in the other oceans. Last, low ²³¹Pa_xs/²³⁰Th_xs ratios (< production ratio = 0.093) in sediments from deep Arctic basins can be explained by a higher degree of ²³⁰Th-focusing than that of ²³¹Pa-focusing (during meltwater events) and potential boundary scavenging.

Additional information: Mr. XU Qian, oliver_xu@hku.hk