Sources and Cycling of Neodymium and Rare Earth Elements in the Arctic Amerasian Basin and Bering Sea

The isotopic composition of dissolved neodymium (143Nd/144Nd, expressed as εNd) behaves quasi-conservatively in the ocean and has been used as a palaeoceanographic tracer of ocean circulation. The εNd value of water masses is acquired—and often altered—during interaction with sediments along the margins; however, the processes involved in seawater εNd acquisition remains poorly characterized and quantified. Here we report the results of 349 Nd isotope and 354 rare earth element (REE) concentration measurements from 27 water column profiles across the Bering shelf and through the Arctic Amerasian Basin collected during U.S. Arctic GEOTRACES cruise HLY1502 (GEOTRACES section GN01). At the edge of the shelf (4 – 733 m), Bering Sea εNd is not appreciably altered from North Pacific endmember values despite transiting the Aleutian volcanic arc. The εNd values observed are then modified during transit across the eastern shelf; less radiogenic compositions accompany increases in the Nd concentration ([Nd]), particularly in samples below the pycnocline. The spatial pattern of [Nd] and less radiogenic εNd values point to benthic Nd additions to the water column, which is supported by a strong correlation between εNd values and 228Ra activities in sub-pycnocline samples and an enrichment of middle rare earth elements (MREE) The flux and the Nd isotope composition required to produce the observed changes was calculated by mass balance using a box model of the Bering Sea shelf. The results are consistent with Nd derived from the sedimentary authigenic ferromanganese component Near the Bering Strait, the εNd value is closer to the unaltered values at the southern Bering Shelf edge, reflecting the contribution of waters transiting the western shelf in a current system that moves faster and in deeper water compared to the eastern shelf, providing less opportunity for benthic driven alterations to the εNd value. Stations north of Bering Strait (on the Chukchi shelf) indicate the presence of Atlantic Arctic Water (AAW). The surface layer of the Arctic Amerasian Basin is a dynamic region effect by seasonal sea-ice formation and melt and the largest percentage of river flux relative to the basin size of any ocean basin. Moreover, a relatively fresh North Pacific flux from the Bering Sea combines with the other freshwater source to form a strong halocline, isolating the deep basin. The investigation demonstrates that sea-ice melt did not contribute to the Nd isotope and concentration budget. Arctic rivers, on the other hand, had a proportionate effect on the Nd isotopic composition and concentrations relative to their input, where Mackenzie River water shows a 15% contribution to the surface layer of the Canada Basin and a 20% Lena River contribution to the Makarov basin. Interestingly, the relationship between REE concentrations and the rivers deviates from the expected 71% loss due to estuarine scavenging, as seen in other non-Arctic River systems. This deviation may relate to the ratio of organic to inorganic nanoparticles and colloids in the rivers. The deep waters in the arctic ocean are largely derived from North Atlantic inflow through the Fram Strait and the Barents Sea. These waters overflow the Lomonosov Ridge at a depth of ~1700 m, filling the deep Amerasian basin at a rate of ~0.25 Sv. The slow transfer of water and the isolation of the deep Amerasian basin by the Lomonosov ridge and strong surface halocline contribute to the old ventilation age of ~450 yr for deep waters. Given the ‘quasi-conservative’ nature of Nd isotopes, the deep basin should reflect the less radiogenic εNd range of North Atlantic Water masses (εNd = -10.8 - -11.7). Moreover, samples collected from the Amerasian Basin in 2000 reflect these values with an εNd range of -10.7 to -11. However, this investigation found an εNd range of -8.8 to - 10.36. The increase εNd observed here can be attributed to dense brines forming on the East Siberian Shelf and Chukchi Shelf and some contributions from benthic sources. However, the exact cause for the isotopic shift is not clear.

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