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New geochemical and isotopic (Sr, Nd, Pb) data are presented for a composite sedimentary record encompassing the past 50 Ma of history of sedimentation on the Lomonosov Ridge in the Arctic Ocean. The sampled sediments encompass the transition of the Arctic basin from an enclosed anoxic basin to an open and ventilated oxidized ocean basin. The transition from anoxic basin to open ventilated ocean is accompanied by at least three geochemical and isotopic shifts and an increase in elements (e.g., K/Al) controlled by detrital minerals highlighting significant changes in sediment types and sources. The isotopic compositions of the sediments prior to ventilation are more variable but indicate a predominance of older crustal contributions consistent with sources from the Canadian Shield. Following ventilation, the isotopic compositions are more stable and indicate an increased contribution from younger material consistent with Eurasian and Pan-African crustal sources. The waxing and waning of these sources in conjunction with the passage of water through Fram Strait underlines the importance of the exchange of water mass between the Arctic and North Atlantic Oceans. [ABSTRACT FROM AUTHOR]

The geochemistry of Re-Os and the recent use of Re as a non-traditional stable isotope both need accurate and precise quantification of 187Re/185Re ratios. This paper reports rhenium isotopic data obtained from the analysis of a standard solution and geological samples by MC-ICP-MS. We show that measured isotopic ratios are modified by matrix effects that cannot be accounted for by the standard solution bracketing technique. The bias resulting from measurements on a spiked (185Re-enriched) sample is shown to alter the apparent Re concentration by several percent. When spiking samples and calibrators with tungsten, simultaneous measurement of tungsten and rhenium isotopes compensates for the matrix-induced modification of mass bias. Rhenium and tungsten are shown to have different fractionation factors. This may be due to the fact that the two elements fractionate in a different but systematic way, or that the reference isotopic ratios used for elemental Re and W are incoherent with one another. The consistency of fractionation through time can be used to obtain an empirical relationship between W and Re measured ratios from a standard solution to obtain a sample's fractionation-corrected 187Re/185Re spiked ratio on samples containing pg g−1 levels of Re, even if some matrix capable of affecting mass bias remains in the final solution. [ABSTRACT FROM AUTHOR]

Abstract: Radiogenic isotopes (Sr, Nd) in Ice Rafted Debris (IRD) sediments from the Lomonosov Ridge and Fram Strait Arctic areas are used to document changes in Arctic sea-ice sources and trajectories since the Last Glacial Maximum (LGM). The two records provide evidence for enhanced sea-ice production in the Beaufort Sea and its subsequent export through Fram Strait during the Younger Dryas (YD) interval. This pattern, exclusive to the YD, followed an LGM–Bølling–Allerød interval, when multiyear and/or reduced sea-ice mobility resulted in sedimentary hiatuses in the Central Arctic. Meanwhile, IRD from Svalbard–Barents ice-sheet margin source were still deposited in the Fram Strait area. The “isotopic excursion” of the YD points to enhanced sea-ice production in the Beaufort Sea, which we link to the Lake Agassiz (?)/Laurentide ice sheet drainage event through the Mackenzie route. Following this event, the Holocene depicts a 5-fold reduction in IRD rates with a shift towards prominent sea-ice production along Russian shelves. From a methodological viewpoint, we demonstrate that radiogenic isotopes must be used in a multi-proxy approach to better constrain IRD and sea-ice sources and routes. From a paleoceanographic perspective, we conclude that an Arctic freshwater/sea-ice export route should now be seen as the most likely mechanism for an AMOC reduction during the YD in accordance with more recent improved model experiment. [Copyright &y& Elsevier]