Geochemical signatures of sediments documenting Arctic sea-ice and water mass export through Fram Strait since the Last Glacial Maximum

Abstract

Elemental (Ca, Zr, Th, etc.) and radiogenic isotope (Pb, Nd, Sr) measurements in leachates and residues from deep-sea sediments of core MC16 (WarmPast Program) in central Fram Strait were used to document the geochemical signatures of outflowing Arctic water masses and ice rafted debris (IRD) since the Last Glacial Maximum. In addition, the elemental distribution among the three main sedimentary fractions (terrigenous, biogenic and authigenic) was quantified. Elements dominated by the terrigenous fraction display a change at ∼13 ka assigned to an early Younger Dryas (YD) event. In the authigenic fraction, migration of the redox front, perhaps spurred by discontinuous delivery of organic matter to the sediment, has led to the mobility of elements such as Mn. Fe contents display lesser variability within the sediment suggesting that Fe experienced only minor redox-related redistribution. Authigenic Pb and Nd, thought to be hosted primarily by Fe-oxyhydroxides, also show little evidence of mobility, suggesting that their isotopic compositions should reliably record the isotopic compositions of past bottom water.We have broadly identified the isotopic signatures of the three major source areas of IRD, the Russian, Canadian and Greenland margins. The elemental and isotopic residue records from core MC16 display distinct trends prior to and after the YD. The pre-YD interval, with ɛNd values between −10.1 and −13.2, and 87Sr/86Sr ratios from 0.715 to 0721, reflects a mixture of IRD from the Russian and Canadian margins. The YD episode stands out with sediments originating mostly from the Canadian end-member, displaying the lowest ɛNd values and highest 87Sr/86Sr ratios. This suggests enhanced sea-ice production and/or drifting along the Beaufort Gyre at that time. The post-YD interval, i.e. the Holocene, is characterized by a less variable mixture of IRD material, with ɛNd values and 87Sr/86Sr ratios centred at −12.2 and 0.718 respectively. This material was derived from several sources including northwestern Canada (Mackenzie River), the western Arctic, the East Siberian/Chukchi Seas, and the more proximal Greenland margin.The isotopic compositions of leachates are mostly linked to boundary exchange processes near major meltwater and freshwater source areas. Due to its longer residence time relative to Pb, Nd preserves the isotopic signature of more distal areas where high-density particulate fluxes may occur. In the studied core, Nd-isotope leachate data illustrate the influence of the western Russian margins prior to the YD event and that of the East Siberian and Chukchi Sea margins following this event. This study illustrates that complementary information on IRD sources and water-mass histories can be obtained from isotopic analyses of inherited (residual) and exchangeable (leachable) fractions in deep Arctic Ocean sediments. Higher resolution cores are now needed to illustrate more rapid variations of paleo sea-ice and water mass circulation.