Description: Glacial meltwater discharge from Antarctica is a key influence on the marine environment, impacting ocean circulation, sea level, and productivity of the pelagic and benthic ecosystems. The responses elicited depend strongly on the characteristics of the meltwater releases, including timing, spatial structure and geochemical composition. Here we use isotopic tracers to reveal the time-varying pattern of meltwater during a discharge event from the Fourcade Glacier into Potter Cove, northern Antarctic Peninsula. The discharge is strongly dependent on local air temperature, and accumulates into an extremely thin, buoyant layer at the surface. This layer showed evidence of elevated turbidity, and responded rapidly to changes in atmospherically-driven circulation to generate a strongly pulsed outflow from the cove to the broader ocean. These characteristics contrast with those further south along the Peninsula, where strong glacial frontal ablation is driven oceanographically by intrusions of warm deep waters from offshore. The Fourcade Glacier switched very recently to being land-terminating; if retreat rates elsewhere along the Peninsula remain high and glacier termini progress strongly landward, the structure and impact of the freshwater discharges are likely to increasingly resemble the patterns elucidated here.

Description: Iron (Fe) fluxes from reducing sediments and subglacial environments are potential sources of bioavailable Fe into the Southern Ocean. Stable Fe isotopes (δ56Fe ) are considered a proxy for Fe sources and reaction pathways, but respective data are scarce and Fe cycling in complex natural environments is not understood sufficiently to constrain respective δ56Fe “endmembers” for different types of sediments, environmental conditions, and biogeochemical processes. We present δ56Fe data from pore waters and sequentially extracted sedimentary Fe phases of two contrasting sites in Potter Cove (King George Island, Antarctic Peninsula), a bay that is affected by fast glacier retreat. Sediments close to the glacier front contain more easily reducible Fe oxides and pyrite and show a broader ferruginous zone, compared to sediments close to the icefree coast, where surficial oxic meltwater streams discharge into the bay. Pyrite in sediments close to the glacier front predominantly derives from eroded bedrock. For the high amount of easily reducible Fe oxides proximal to the glacier we suggest mainly subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes (likely pyrite oxidation or dissimilatory iron reduction, DIR). Our strongest argument for a subglacial source of the highly reactive Fe pool in sediments close to the glacier front is its predominantly negative δ56Fe signature that remains constant over the whole ferruginous zone. This implies in situ DIR does not significantly alter the stable Fe isotope composition of the accumulated Fe oxides. The nonetheless overall light δ56Fe signature of easily reducible Fe oxides suggests pre-depositional microbial cycling as it occurs in potentially anoxic subglacial environments. The strongest 56Fe-depletion in pore water and most reactive Fe oxides was observed in sediments influenced by oxic meltwater discharge. The respective site showed a condensed redox zonation and a pore water δ56Fe profile typical for in-situ Fe cycling. We demonstrate that the potential of pore water δ56Fe as a proxy for benthic Fe fluxes is not straight-forward due to its large variability in marine shelf sediments at small spatial scales (- 2.4‰ at the site proximal to oxic meltwater discharge vs. -0.9‰ at the site proximal to the marine glacier terminus, both at 2 cm sediment depth). The controlling factors are multifold and include the amount and reactivity of reducible Fe oxides and organic matter, the isotopic composition of the primary and secondary ferric substrates, sedimentation rates, and physical reworking (bioturbation, ice scraping). The application of δ56Fe geochemistry may prove valuable in investigating biogeochemical weathering and Fe cycling in subglacial environments. This requires, however (similarly to the use of δ56Fe for the quantification of benthic fluxes), that the spatial and temporal variability of the isotopic endmember is known and accounted for. Since geochemical data from subglacial environments are very limited, further studies are needed in order to sufficiently assess Fe cycling and fractionation at glacier beds and the composition of discharges from those areas.

Description: The glacier melt of the Western Antarctic Peninsula and its surrounding islands influences biogeochemical processes in the water column and the marine sediment by changing the flux of mineral particles and nutrients (e.g. Fe) into the ocean. Sediment and pore water samples were collected at King George Island (South Shetland Islands) to unravel how the vicinity of ice-covered and -uncovered terrestrial environment affects redox zonation and diagenetic processes in the coastal sediments. The post-depositional dissolution of Fe-minerals and the stable Fe isotope signatures of pore water and specific Fe minerals were of special interest since changing Fe supplies - as reactive particles via melting icebergs or meltwater streams or dissolved via diffusion from the sediment into the bottom water - might not only impact local biogeochemical cycles but most likely also impact productivity in the Southern Ocean. Sediment cores of up to 45 cm length were retrieved in Potter Cove, Marian Cove, and Maxwell Bay. In vicinity to the glaciers the sediments showed an extended redox zonation. The post-oxic zone with Fe2+ concentrations of up to 300 μM ranged from 1 to 25 cm depth. Most probably, microbial activity in sediments close to the glaciers is sluggish due to low input of organic matter (OM). More condensed redox zones prevailed in troughs where OM from terrestrial or marine sources accumulates and in vicinity to research stations. The upward directed diffusive Fe2+ fluxes as inferred from pore water profiles range between 0 and ~1050 μM m-2 d-1. However, the correlation to the intensity of diagenesis is not straightforward. Fe isotopes of specific minerals were used to assess the intensity of Fe cycling. With ongoing Fe-oxide dissolution, the residual Fe pool becomes enriched in 56Fe, whereas dissolved Fe and secondary Fe-oxides become enriched in 54Fe. Thus, easily reducible Fe oxides show lowest !56Fe values at the top of the sediment column. We suggest that the retreat of the glaciers indirectly results in higher OM fluxes to shelf areas fueling diagenetic processes/nutrient recycling.

Description: Polar regions are critical for future climate evolution, and they experience major environmental changes. A particular focus of biogeochemical investigations in these regions lies on iron (Fe). This element drives primary productivity and, thus, the uptake of atmospheric CO2 in vast areas of the ocean. Due to the Fe-limitation of phytoplankton growth in the Southern Ocean, Antarctica is a key region for studying the change of iron fluxes as glaciers progressively melt away. The respective climate feedbacks can currently hardly be quantified because data availability is low, and iron transport and reaction pathways in Polar coastal and shelf areas are insufficiently understood. We show how novel stable Fe isotope techniques, in combination with other geochemical analyses, can be used to identify iron discharges from subglacial environments and how this will help us assessing short and long term impacts of glacier retreat on coastal ecosystems. Stable Fe isotopes (δ56Fe) may be used to trace Fe sources and reactions, but respective data availability is low. In addition, there is a need to constrain δ56Fe endmembers for different types of sediments, environments, and biogeochemical processes. δ56Fe data from pore waters and sequentially extracted solid Fe phases at two sites in Potter Cove (King George Island, Antarctica), a bay affected by fast glacier retreat, are presented. Close to the glacier front, sediments contain high amounts of easily reducible Fe oxides and show a dominance of ferruginous conditions compared to sediments close to the ice-free coast, where surficial oxic meltwater discharges and sulfate reduction dominates. We suggest that high amounts of reducible Fe oxides close to the glacier mainly derive from subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering. A strong argument for a subglacial source is the predominantly negative δ56Fe signature of reducible Fe oxides that remains constant throughout the ferruginous zone. In situ dissimilatory iron reduction (DIR) does not significantly alter the isotopic composition of the oxides. The composition of the easily reducible Fe fraction therefore suggests pre-depositional microbial cycling as it occurs in subglacial environments. Sediments influenced by oxic meltwater discharge show downcore trends towards positive δ56Fe signals in pore water and reactive Fe oxides, typical for in situ DIR as 54Fe becomes less available with increasing depth. We found that a quantification of benthic Fe fluxes and subglacial Fe discharges based on stable Fe isotope geochemistry will be complicated because (1) diagenetic processes vary strongly at short lateral distances and (2) the variability of δ56Fe in subglacial meltwater has not been sufficiently well investigated yet. However, isotope mass balance models that consider the current uncertainties could, in combination with an application of ancillary proxies, lead to a much better quantification of Fe inputs into polar marine waters than currently available. This would consequently allow a better assessment of the flux and fate of Fe originating from the Antarctic Ice Sheet. Henkel et al. (2018) Diagenetic iron cycling and stable Fe isotope fractionation in Antarctic shelf sediments, King George Island. GCA 237, 320-338.