Processes controlling the authigenic 10Be/9Be ratio in the Arctic Ocean

The authigenic 10 Be/ 9 Be ratio in marine sediments is a promising proxy for reconstructing Arctic paleoceanography, particularly the mixing of riverine and marine inputs which differ in their 10 Be/ 9 Be by an order of magnitude. This study examines the modern spatial variability of authigenic 10 Be/ 9 Be ratios in surface sediments from the Kara Sea, the Laptev Sea, and the adjacent deep Eurasian Basin, tracing the changes from river mouths and estuaries to the open ocean. Our results show that sedimentary authigenic 10 Be/ 9 Be reflects different inputs: low ratios of riverine origin dominate on the shelves, while values increase toward the deep sea under the influence of waters of North Atlantic origin. However, unlike other pelagic oceanic settings, where authigenic sedimentary 10 Be/ 9 Be generally matches the dissolved 10 Be/ 9 Be of the water column, Arctic deep-sea sediments display a clear offset, characterized by lower authigenic 10 Be/ 9 Be ratios ([1.5 – 3] × 10 -8 ) than expected from the overlying water column ([5–8] × 10 -8 ). We hypothesize that this offset originates from the preservation of preformed Fe-Mn oxides carrying a low riverine 10 Be/ 9 Be signal which are efficiently transported via sea-ice to the central Arctic basins. Additional processes, such as depth dependent scavenging efficiencies in the water column and benthic beryllium fluxes from sediments may also contribute to this offset. Our study thus highlights the contribution of preformed phases to the reactive 10 Be/ 9 Be in Arctic Ocean sediments. We demonstrate that the relevance of preformed phases is not limited to near-coastal settings ( Wittmann et al. 2017 ), likely due to large fractions of the sediment being transported by sea-ice. These processes likely also apply to other isotope systems measured in authigenic coatings of Arctic Ocean sediments such as radiogenic Nd isotopes.