The Aurora vent field (82°53.83’ N, 6°15.32’ W) is located in the weakly stratified Arctic Ocean under perennial ice cover at the western edge of the ultraslow-spreading Gakkel Ridge, the slowest spreading mid-ocean ridge on Earth. Here, we report data on the dispersal of the proximal hydrothermal plume in this extreme environment. The hydrothermal plume is of unusual dimensions, with a small horizontal, but large vertical extent, which is caused by the hydrography of the Arctic Ocean. Water column parameters such as turbidity and redox potential show a highly variable but horizontally confined non-buoyant plume. Dissolved iron (dFe), manganese (dMn), δ3He, and methane (CH4) all show distinct enrichments in the hydrothermal plume relative to background deep-water, but relatively low peak concentrations due to the dilution over a vertical extent of over 500 m. Plume particle samples exhibit elevated Fe/Al ratios consistent with Fe-oxyhydroxide precipitation close to the vent, whereas particulate Mn/Al ratios do not reveal any complementary pMn enrichments in the proximal plume. Positive correlation between Fe/Al, and several other element/Al ratios (e.g. P, V, As) are consistent with scavenging of these elements onto Fe-hydroxide plume particles and removal into the underlying sediments. Surface sediment samples collected close to Aurora reveal highly elevated concentrations of hydrothermally-sourced elements in the immediate vicinity of the vent-site. For example, proximal surface sediments contained up to 8222 mg kg-1 Cu, whereas Cu concentrations in core tops a few kilometers away from the site were much lower (<50 mg kg-1).

The Filchner-Ronne Ice Shelf (FRIS) is characterized by moderate basal melt rates due to the near-freezing waters that dominate the wide southern Weddell Sea continental shelf. We revisited the region in austral summer 2018 with detailed hydrographic and noble gas surveys along FRIS. The FRIS front was characterized by High Salinity Shelf Water (HSSW) in Ronne Depression, Ice Shelf Water (ISW) on its eastern flank and an inflow of modified Warm Deep Water (mWDW) entering through Central Trough. Filchner Trough was dominated by Ronne HSSW-sourced ISW, likely forced by a recently intensified circulation beneath FRIS due to enhanced sea ice production in the Ronne polynya since 2015. Glacial meltwater fractions and tracer-based water mass dating indicate two separate ISW outflow cores, one hugging the Berkner slope after a two-year travel time, and the other located in the central Filchner Trough following a ~six year-long transit through the FRIS cavity. Historical measurements indicate the presence of two distinct modes, in which water masses in Filchner Trough were dominated by either Ronne HSSW-derived ISW (Ronne-mode) or more locally-derived Berkner-HSSW (Berkner-mode). While the dominance of these modes has alternated on interannual time scales, ocean densities in Filchner Trough have remained remarkably stable since the first surveys in 1980. Indeed, geostrophic velocities indicated outflowing ISW-cores along the trough’s western flank and onto Berkner Bank, which suggests that Ronne-ISW preconditions Berkner-HSSW production. The negligible density difference between Berkner- and Ronne-mode waters indicates that each contribute cold dense shelf waters to protect FRIS against inflowing mWDW.

Environmental dating tracers (3H, 3He, 4He, CFC-12, CFC-11, SF6) and the natural response of spring (hydrochemistry, water temperature, and hydrodynamics) were jointly used to asses mixing processes and to characterize groundwater flow in a relatively small carbonate aquifer with complex geology in South Spain. Results evidence a marked karst behavior of some temporary outlets, while some perennial springs show buffer and delayed responses to recharge events. There is also a general geochemical evolution pattern, from higher to lower altitudes, in which mineralization and the relation Mg/Ca rises, evidencing longer water-rock interaction. The large SF6 concentrations in groundwater suggest terrigenic production, while CFC-11 values are affected by sorption or degradation. The groundwater age in the perennial springs deduced from CFC-12 and 3H/3He point out to mean residence times of several decades, although the difference between both methods and the large amount of radiogenic 4He in the samples indicates a contribution of old groundwater (free of 3H and CFC-12). Lumped Parameter Models and Shape-Free Models were created based on 3H, tritiogenic 3He, CFC-12, and radiogenic 4He data in order to interpret the age distribution of the samples. The resulting groundwater-age distributions evidence the existence of two mixing components, with an old fraction ranging between 160 and 220 years. Some dating parameters derived from the mixing models and their correlation to physicochemical parameters permits to explain the hydrogeochemical processes occurring within the system. All these results prove that large time residence times are possible in small alpine systems with a clear karst behavior when the geological setting is complex, and they highlight the importance of applying different approaches, including groundwater dating techniques, to completely understand the groundwater flow regime within this type of media.