Description: It has been suggested that increasing freshwater discharge to the Arctic Ocean may also occur as submarine groundwater discharge (SGD), yet there are no direct observations of this phenomenon in the Arctic shelf seas. This study tests the hypothesis that SGD does exist in the Siberian-Arctic shelf seas but its dynamics may be largely controlled by complicated geocryological conditions such as permafrost. The field-observational approach in the southeast Laptev Sea used a combination of hydrological (temperature, salinity), geological (bottom sediment drilling, geoelectric surveys) and geochemical (224Ra, 223Ra and 222Rn) techniques. Active SGD was documented in the vicinity of the Lena River delta with two different operational modes. In the first system, groundwater discharges through tectonogenic permafrost talik zones was registered in both wintertime and summertime seasons. The second SGD mechanism was cryogenic squeezing out of brine and water-soluble salts detected on the periphery of ice hummocks in the wintertime season. The proposed mechanisms of groundwater transport and discharge in the arctic land-shelf system is elaborated. Through salinity versus 224Ra and 224Ra/223Ra diagrams, the three main SGD-influenced water masses were identified and their end-member composition was constrained. Further studies should apply these techniques to a broader scale with the objective to reach an estimate of the relative importance of the SGD transport vector relative to surface freshwater discharge for both the water balance and aquatic components such as dissolved organic carbon, carbon dioxide, methane, and nutrients.

Description: The Arctic Ocean is a small ocean basin surrounded by wide and shallow shelves receiving large river inputs. The radium quartet are four tracers of contact with sediments that give information on transport processes ranging in time scale from days to hundreds of years. 228Ra (5.8 y halflife) is a good tracer for surface water circulation. New 2015 GEOTRACES (sections GN01+GN04) 228Ra data show the first full Barents Sea to Bering Strait transect with maximum activities in the Transpolar Drift. We compare the 2015 section in the central Arctic with earlier Polarstern sections in 1987/1991, 2007 and 2011 and discuss reasons for increases in maximum 228Ra activities (Kipp et al., 2017). The penetration of 228Ra and its daughter 228Th to waters of intermediate depths (up to about 1500m) shows the exchange of these waters with shelf and slope sediments on the time scale of 228Ra decay. Input from the seafloor causes an enrichment of 228Ra and 226Ra in bottom waters. In deep waters (〉2000m) of the Eurasian and Makarov basins 226Ra (1600 y halflife) accumulates to values around 16 dpm/100L or about twice the surface water concentration. It has been argued that export production increases with decreasing ice cover in the Arctic (Arrigo et al., 2008). Natural radionuclides can help to quantify export production rates, but the fluxes found may depend on the time scale of the tracer used. The most commonly used tracer 234Th/238U (24 d halflife) has a memory much shorter than a season. The 210Po/210Pb (138 d; Roca-Martí et al., 2016) and 228Th/228Ra ratios (1.9 y) are alternatives on a full seasonal time scale. Arrigo, K.R. et al., 2008. Geophys. Res. Lett. 35, L19603 Kipp, L. et al., 2017. ASLO 2017 OSM abstracts Roca-Martí, M. et al., 2016. Journal of Geophysical Research: Oceans 121, 5030-5049