U- and Th-series isotopes in deep Baffin Bay sediments: Tracers of detrital sources and of contrasted glacial/interglacial sedimentary processes

Uranium (U) and thorium (Th) series have been used to document the sedimentary sources, processes and fluxes of the last ~ 115 ka in central Baffin Bay, a major gateway between the Arctic Ocean and the North Atlantic Ocean, which has been fed throughout the last ice age by sediments and meltwaters from the northeastern Laurentide Ice Sheet, the Innuitian Ice Sheet and the western Greenland Ice Sheet. Measurements have been made with a ~ 1.6 ka time resolution in sedimentary core HU2008-029-016PC (70°27′N, 64°39′W; 2063 m water depth). The lack of correlation between U-concentration, 234U/238U activity ratio ((234U/238U)AR), organic carbon and redox sensitive elements suggests minimum early diagenetic U-uptake if any, likely due to very low organic matter sedimentary fluxes in such a subarctic environment. Throughout the last glaciation, U- and Th-concentrations and their mass ratio, as well as (234U/238U)AR, do not show either any unequivocal linkage with detrital dolomitic carbonate and/or detrital silicate fluxes. Data rather suggest sedimentary supplies with singular U- and Th-series signatures linked to specific glacial margin/ice streaming events and highlighting the diversity of sedimentary sources surrounding this small oceanic basin. Large surplus of 230Th ((230Thxs)0), well above the production from dissolved-U decay in the overlying water column, are observed throughout the last glacial interval in response to sediment focusing and high-sedimentation rates in the deepest part of the Bay. During this interval, high-frequency fluctuations in (230Thxs)0 indicate either a similar variability in sedimentation rates, not captured by the age model used, and/or grain size or mineralogical-dependent 230Thxs-scavenging rates. In the much lower sedimentation rate Holocene core-top section, the (230Thxs)0-inventory falls below production in the overlying water column, despite evidence for some primary productivity revival as suggested by the higher organic carbon content. 230Thxs-losses through lateral advection via Baffin Bay water-masses overflowing through Davis Strait might account for this pattern, pointing to ocean circulation drastically differing from that of the glacial interval.