Ion fractionation in young sea ice from Kongsfjorden, Svalbard

The fractionation of major sea-water ions, or deviation in their relative concentrations from Standard Mean Ocean Water ratios, has been frequently observed in sea ice. It is generally thought to be associated with precipitation of solid salts at certain eutectic temperatures. The variability found in bulk sea-ice samples indicates that the fractionation of ions depends on the often unknown thermal history of sea ice, which affects the structure of pore networks and fate of solid salts within them. Here we investigate the distribution of ions in Arctic sea ice that is a few weeks old with a reconstructible thermal history. We separate the centrifugable (interconnected) and entrapped (likely disconnected) contributions to the ice salinity and determine their ion fractionation signatures. The results indicate that differential diffusion of ions, rather than eutectic precipitation of cryohydrates, has led to significant ion fractionation. The finding emphasizes the role of coupled diffusive–convective salt transport through complex pore networks in shaping the biogeochemistry of sea ice.

Molybdenum isotope fractionation in pelagic euxinia: Evidence from the modern Black and Baltic Seas

Here we present stable isotope data for vertical profiles of dissolved molybdenum of the modern euxinic water columns of the Black Sea and two deeps of the Baltic Sea. Dissolved molybdenum in all water samples is depleted in salinity-normalized concentration and enriched in the heavy isotope (δ98Mo values up to + 2.9‰) compared to previously published isotope data of sedimentary molybdenum from the same range of water depths. Furthermore, δ98Mo values of all water samples from the Black Sea and anoxic deeps of the Baltic Sea are heavier than open ocean water. The observed isotope fractionation between sediments and the anoxic water column of the Black Sea are in line with the model of thiomolybdates that scavenge to particles under reducing conditions. An extrapolation to a theoretical pure MoS42− solution indicates a fractionation constant between MoS42− and authigenic solid Mo of 0.5 ± 0.3‰. Measured waters with all thiomolybdates coexisting in various proportions show larger but non-linear fractionation. The best explanation for our field observations is Mo scavenging by the thiomolybdates, dominantly — but not exclusively — present in the form of MoS42−. The Mo isotopic compositions of samples from the sediments and anoxic water column of the Baltic Sea are in overall agreement with those of the Black Sea at intermediate depth and corresponding sulphide concentrations. The more dynamic changes of redox conditions in the Baltic deeps complicate the Black Sea-derived relationship between thiomolybdates and Mo isotopic composition. In particular, the occasional flushing/mixing, of the deep waters, affects the corresponding water column and sedimentary data. δ98Mo values of the upper oxic waters of both basins are higher than predicted by mixing models based on salinity variations. The results can be explained by non-conservative behaviour of Mo under suboxic to anoxic conditions in the shallow bottom parts of the basin, most pronounced on the NW shelf of the Black Sea.