RADIOGENIC ISOTOPE SIGNATURES (ND, SR) OF MARINE SEDIMENTS FROM THE ARCTIC OCEAN: MODERN VS. LAST GLACIAL CONDITIONS

Radiogenic isotopes (Pb, Nd, Sr) of marine sediments are frequently used to document sedimentary fluxes and water mass histories. However in the Arctic Ocean, major changes in sedimentary sources occurred between glacial and interglacial intervals in relation to ice-sheet development and melt, sea-level oscillations and shelf reduction/extension, opening/closure of straits, river runoff and their relative particulate fluxes. In addition, ice-rafting largely controls the sediment transport and its deposition in most parts of the basin. Under the present geographical conditions, the exceptionally large continental shelves of the Arctic Ocean control ~2/3 of the sedimentary flux to deep basins and ridges through coastal erosion. Hence, isotopic signatures of large river runoffs do not necessarily label sediment sources. Marine surface sediments from the circum-Arctic are more likely to illustrate these sources, at least under the present interglacial conditions. Here, we added ~30 sites to a database from literature, notably data from the East Siberian Sea margins. A few major isotopic shelf "provinces" can be now identified based on Sr and Nd isotope data: the Arctic Canadian shelf (εNd from -13 to -15 and ⁸⁷Sr/⁸⁶Sr from 0.724 to 0.730); the Bering Sea/Bering Strait areas (εNd from -7 to -10 and ⁸⁷Sr/⁸⁶Sr from 0.711 to 0.712) that extends as far as the Chukchi Sea; a near shore Lena river cluster (εNd -15 and ⁸⁷Sr/⁸⁶Sr ~ 0.716) contrasting with its off-shore area (εNd -11 and ⁸⁷Sr/⁸⁶Sr ~ 0.717) and the Svalbard-Barents-Kara Sea margin (εNd ~ -9 and ⁸⁷Sr/⁸⁶Sr ~ 0.719). Ice rafting seems the most efficient mechanism for the dispersal and mixing of these signatures in deeper marine settings. Under full glacial conditions, notably when large ice sheets were present (e.g. Laurentide ice sheet, Eurasian ice sheet), distinct and clearer isotopic "sources" were active (the Canadian Arctic vs Barents-Kara Sea ice margins). It seems thus difficult to link unequivocally isotopic properties of cored-sediments to modern sediment sources, and even more, to modern dissolved Sr and Nd from river catchments.