Jemma Wadham

Stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high Arctic glacier

Glacier beds may be host to a range of microbial communities, which drive oxic waters towards anoxia along certain hydrological flowpaths. Chemical and isotopic signatures in meltwaters from Finsterwalderbreen, a polythermal glacier on sedimentary bedrock in Svalbard, show clear evidence for anoxia at the glacier bed. Increases in δ34S and δ18O of sulphate indicate that microbial sulphate reduction has resulted in significant decreases in sulphate concentration. The δ13C of the dissolved inorganic carbon (DIC) is isotopically light (δ13C=−8‰), which is consistent with the use of bedrock kerogen and/or the necromass of sulphide oxidising bacteria as organic substrates for the sulphate-reducing bacteria. Calculated rates of organic carbon mineralisation correspond to ∼10% of the total annual DIC flux of the glacial meltwaters. This microbial ecosystem is chemoautotrophically based, ultimately being sustained by the kerogen and/or bacterial necromass and sulphides in the bedrock. This work suggests that glacier beds can be refugia for life when climatic and/or atmospheric conditions are otherwise inclement and also supports the contention that microbial life is present in subglacial Lake Vostok.

Subsurface ice as a microbial habitat

We determine the physicochemical habitat for microorganisms in subsurface terrestrial nice by quantitatively constraining the partitioning of bacteria and fluorescent beads n(1–10 m) between the solid ice crystals and the water-filled veins and boundaries around nindividual ice crystals. We demonstrate experimentally that the partitioning of spherical nparticles within subsurface ice depends strongly on size but is largely independent of nsource particle concentration. Although bacteria are shown consistently to partition to the nveins, larger particles, which would include eukaryotic cells, become trapped in the crystals nwith little potential for continued metabolism. We also calculate the expected concentrations nof soluble impurities in the veins for typical bulk concentrations found in natural nice. These calculations and scanning electron microscope observations demonstrate a concentrated nchemical environment (3.5 M total ions at 10 C) in the veins, where bacteria nwere found to reside, with a mixture of impurities that could sustain metabolism. Our ncalculations show that typical bacterial cells in glacial ice would fit within the narrow nveins, which are a few micrometers across. These calculations are confirmed by microscopic nimages of spherical, 1.9-m-diameter, fluorescent beads and stained bacteria in nsubsurface veins. Typical bacterial concentrations in clean ice (102–103 cells/mL) would nresult in concentrations of 106–108 cells/mL of vein fluid, but occupy only a small fraction nof the total available vein volume (0.2%). Hence, bacterial populations are not limited nby vein volume, with the bulk of the vein being unoccupied and available to supply energy nsources and nutrients.

Groundwater hydrochemistry in the active layer of the proglacial zone, Finsterwalderbreen, Svalbard

Abstract Glacial bulk meltwaters and active-layer groundwaters were sampled from the proglacial zone of Finsterwalderbreen during a single melt season in 1999, in order to determine the geochemical processes that maintain high chemical weathering rates in the proglacial zone of this glacier. Results demonstrate that the principle means of solute acquisition is the weathering of highly reactive moraine and fluvial active-layer sediments by supra-permafrost groundwaters. Active-layer groundwater derives from the thaw of the proglacial snowpack, buried ice and glacial bulk meltwaters. Groundwater evolves by sulphide oxidation and carbonate dissolution. Evaporation- and freeze-concentration of groundwater in summer and winter, respectively produce Mg–Ca-sulphate salts on the proglacial surface. Re-dissolution of these salts in early summer produces groundwaters that are supersaturated with respect to calcite.There is a pronounced spatial pattern to the geochemical evolution of groundwater. Close to the main proglacial channel, active layer sediments are flushed diurnally by bulk meltwaters. Here, Mg–Ca-sulphate deposits become exhausted in the early season and geochemical evolution proceeds by a combination of sulphide oxidation and carbonate dissolution. At greater distances from the channel, the dissolution of Mg–Ca-sulphate salts is a major influence and dilution by the bulk meltwaters is relatively minor. The influence of sulphate salt dissolution decreases during the sampling season, as these salts are exhausted and waters become increasingly routed by subsurface flowpaths.

Suspended sediment fluxes in a high-Arctic glacierised catchment: implications for fluvial sediment storage

Suspended sediment fluxes from the 68 km2 Finsterwalderbreen catchment in Svalbard were monitored intensively during the 1999 and 2000 melt seasons, at proximal and distal ends of a 4.2 km2 proglacial area, which has been deglacierised during the twentieth century. Measured distal sediment fluxes correspond to total catchment denudation rates of 2700±710 t km−2 year−1 (1999) and 1800±350 t km−2 year−1 (2000). Hourly net sediment flux time series (distal flux minus proximal flux, isolating change within the proglacial area itself) reveal that the proglacial area serves as both a source and a sink of sediment during different periods of the melt season, and that the majority of sediment evacuation from the area occurs during discrete episodes of enhanced meltwater discharge. The mean net flux from the proglacial area itself was −690±230 t km−2 year−1 (1999) and +3800±1700 t km−2 year−1 (2000). Therefore, in 1999 there was a net increase in sediment storage in the proglacial area (aggradation), and in 2000 there was a net decrease (denudation). The pattern of sediment storage change appears to be driven by the runoff regime, with net storage occurring during a year of relatively episodic sediment transport in which relative supply exhaustion occurs, and net release in a year of more sustained sediment transport when relative supply exhaustion is absent. Many more years monitoring would be required for any trend to emerge from the large interannual variability in sediment yield.

Detection of superimposed ice on the glaciers Kongsvegen and Midre Lovénbreen, Svalbard, using SAR satellite imagery

Abstract Superimposed ice forms when meltwater refreezes onto a sub-freezing glacier surface. The accumulation zones of many Arctic glaciers include large areas of superimposed ice, which for mass-balance purposes have to be distinguished from the ablation zone consisting of glacier ice. We examine the ability of synthetic aperture radar (SAR) satellite sensors to detect superimposed ice on the glaciers Kongsvegen and midre Lovénbreen on Svalbard. Structural analysis of ice cores as well as surface observations from these glaciers in 1999 and 2000 provide a spatial record of superimposed ice. Winter SAR images show three distinct zones, which correspond closely to areas of glacier ice, superimposed ice and firn. This is seen very clearly on Kongsvegen, but not as clearly on the much smaller midre Lovénbreen. One possible explanation for the contrasting SAR signal may relate to the differing air-bubble content of firn, superimposed ice and glacier ice. Thin layers of winter-formed superimposed ice (510 cm) in some places are not seen on the SAR images, indicating that a certain thickness is needed for detection. The equilibrium-line altitude cannot be detected since the SAR cannot differentiate old superimposed ice, superimposed ice formed currently in the accumulation area in summer and superimposed ice formed currently in the ablation zone in autumn and winter.

Enhancement of glacial solute fluxes in the proglacial zone of a polythermal glacier

Annual proglacial solute fluxes and chemical weathering rates at a polythermal high-Arctic glacier are presented. Bulk meltwater chemistry and discharge were monitored continuously at gauging stations located at the eastern and western margins of the glacier terminus and at the Outlet, 2.5 km downstream where meltwaters discharge into the fjord. Fluxes of non-snowpack HCO 3 - , SO 4 2- , Ca 2+ and Mg 2+ increase by 30-47% between the glacier terminus and the Outlet, indicating that meltwaters are able to access and chemically weather efflorescent sulphates, carbonates and sulphides in the proglacial zone. Smaller increases in the fluxes of non-snowpack-derived Na + , K + and Si indicate that proglacial chemical weathering of silicates is less significant. En3hanced solute fluxes in the proglacial zone are mainly due to the chemical weathering of active-layer sediments. The PCO 2 of active-layer ground-waters is above atmospheric pressure. This implies that solute acquisition in the active layer involves no drawdown of CO 2 . The annual proglacial chemical weathering rate in 1999 is calculated to be 2600 meqΣ + m -2 This exceeds the chemical weathering rate for the glaciated part of the catchment (790 meqΣ + m -2 ) by a factor of 3.3. Hence, the proglacial zone at Finster-walderbreen is identified as an area of high geochemical reactivity and a source of CO 2 .

Modelling the impact of superimposed ice on the mass balance of an Arctic glacier under scenarios of future climate change

Abstract A surface-energy/mass-balance model with an explicit calculation of meltwater refreezing and superimposed ice formation is applied to midre Lovénbreen, Spitsbergen, Svalbard. The model is run with meteorological measurements to represent the present climate, and run with scenarios taken from global climate model predictions based on the IS92a emissions scenario to represent future climates. Model results indicate that superimposed ice accounts for on average 37% of the total net accumulation under present conditions. The model is found to be highly sensitive to changes in the mean annual air temperature and much less sensitive to changes in the total annual precipitation. A 0.5˚C decade–1 temperature increase is predicted to cause an average mass-balance change of –0.43 ma–1, while a 2% decade–1 increase in precipitation will result in only a +0.02 ma–1 change in mass balance. An increase in temperature results in a significant decrease in the size of the accumulation area at midre Lovénbreen and hence a similar decrease in the net volume of superimposed ice. The model predicts, however, that the relative importance of superimposed ice will increase to account for >50% of the total accumulation by 2050. The results show that the refreezing of meltwater and in particular the formation of superimposed ice make an important positive contribution to the mass balance of midre Lovénbreen under present conditions and will play a vital future role in slowing down the response of glacier mass balance to climate change.