Once thought to be devoid of life, the ice-covered parts of Antarctica are now known to be a reservoir of metabolically active microbial cells and organic carbon. The potential for methanogenic ...archaea to support the degradation of organic carbon to methane beneath the ice, however, has not yet been evaluated. Large sedimentary basins containing marine sequences up to 14 kilometres thick and an estimated 21,000 petagrams (1 Pg equals 10(15) g) of organic carbon are buried beneath the Antarctic Ice Sheet. No data exist for rates of methanogenesis in sub-Antarctic marine sediments. Here we present experimental data from other subglacial environments that demonstrate the potential for overridden organic matter beneath glacial systems to produce methane. We also numerically simulate the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model and show that pressure/temperature conditions favour methane hydrate formation down to sediment depths of about 300 metres in West Antarctica and 700 metres in East Antarctica. Our results demonstrate the potential for methane hydrate accumulation in Antarctic sedimentary basins, where the total inventory depends on rates of organic carbon degradation and conditions at the ice-sheet bed. We calculate that the sub-Antarctic hydrate inventory could be of the same order of magnitude as that of recent estimates made for Arctic permafrost. Our findings suggest that the Antarctic Ice Sheet may be a neglected but important component of the global methane budget, with the potential to act as a positive feedback on climate warming during ice-sheet wastage.
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DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Eruption dynamics are sensitive to ash aggregation, and aggregates are commonly found in eruptive deposits. While ash dispersal and associated hazards are sensitive to aggregation, few experiments ...have been conducted on this phenomena using natural materials across the diverse range of conditions expected in volcanic flows. We have isolated two regimes, wet and dry, in which aggregation occurs due to two different forces, electrostatic and hydrodynamic. Using a closed chamber to create a controlled atmosphere, we found that relative humidity, residence time, and kinetic energy are the three variables necessary to define wet and dry flow regimes. A series of process‐based equations defining the behavior of ash particles have been developed. We propose an aggregation model that can be used for ash dispersal forecasts across a range of conditions in an eruptive plume.
Key Points
Ash aggregation is important for eruption dynamics and ash dispersal hazardsTwo flow regimes, wet and dry, are quantified in ash interactionsExperimentally derived expressions describe the physics of ash aggregation
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Eruption dynamics are sensitive to ash aggregation, and ash aggregates (e.g. accretionary lapilli) are commonly found in eruptive deposits, yet few experiments have been conducted on aggregation ...phenomena using natural materials. Here we developed experiments to produce a probabilistic relationship for the efficiency of ash aggregation with respect to collision kinetic energy and atmospheric water vapor in subsaturated conditions. The laboratory experiments were carried out in an enclosed tank designed to allow for the control of atmospheric water vapor. A synthetic ash proxy, ballotini, and ash from the 2006 eruption of Tungurahua, in Ecuador were examined for their aggregation potential. Image data were recorded with a high speed camera and post-processed to determine the number of collisions, energy of collisions and probability of aggregation. Aggregation efficiency was highly dependent on collision kinetic energy and no dependence on atmospheric water vapor was seen in the range of relative humidity conditions tested, 20 to 95%. Electrostatic aggregation was found to be the most plausible mechanism for aggregation and an estimate charge density was determined for the sample. The average minimum charge density necessary for particle aggregation was found to be 3
×
10
−5
C
m
−2, or on the order of 1
×
10
−4
C
kg
−1. Equations governing the relationships between aggregation efficiency and collision kinetic energy were determined for implementation into large scale numerical volcanic models.
► Aggregation efficiency showed little correlation with relative humidity. ► Aggregation efficiency increased as collision kinetic energy decreased. ► Sub-saturated aggregation is driven primarily by electrostatic forces.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Summary
Uncertainty surrounds estimates of microbial cell and organic detritus fluxes from glacier surfaces. Here, we present the first enumeration of biological particles draining from a ...supraglacial catchment, on Midtre Lovénbreen (Svalbard) over 36 days. A stream cell flux of 1.08 × 107 cells m−2 h−1 was found, with strong inverse, non‐linear associations between water discharge and biological particle concentrations. Over the study period, a significant decrease in cell‐like particles exhibiting 530 nm autofluorescence was noted. The observed total fluvial export of ∼ 7.5 × 1014 cells equates to 15.1–72.7 g C, and a large proportion of these cells were small (< 0.5 μm in diameter). Differences between the observed fluvial export and inputs from ice‐melt and aeolian deposition were marked: results indicate an apparent storage rate of 8.83 × 107 cells m−2 h−1. Analysis of surface ice cores revealed cell concentrations comparable to previous studies (6 × 104 cells ml−1) but, critically, showed no variation with depth in the uppermost 1 m. The physical retention and growth of particulates at glacier surfaces has two implications: to contribute to ice mass thinning through feedbacks altering surface albedo, and to potentially seed recently deglaciated terrain with cells, genes and labile organic matter. This highlights the merit of further study into glacier surface hydraulics and biological processes.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Field and remote sensing observations in the ablation zone of the Greenland Ice Sheet have revealed a diverse range of ice surface characteristics, primarily reflecting the variable distribution of ...fine debris (cryoconite). This debris reduces the surface albedo and is therefore an important control on melt rates and ice sheet mass balance. Meanwhile, studies of ice sheet surface biological processes have found active microbial communities associated with the cryoconite debris, which may themselves modify the cryoconite distribution. Due to the considerable difficulties involved with collecting ground-based observations of the ice surface, our knowledge of the physical and biological surface processes, and their links, remains very limited. Here we present data collected at a field camp established in the ice sheet ablation zone at 67° N, occupied for almost the entire melt season (26 May–10 August 2012), with the aim of gaining a much more detailed understanding of the physical and biological processes occurring on the ice surface. These data sets include quadrat surveys of surface type, measurements of ice surface ablation, and in situ biological oxygen demand incubations to quantify microbial activity. In addition, albedo at the site was retrieved from AVHRR (Advanced Very High Resolution Radiometer) remote sensing data. Observations of the areal coverage of different surface types revealed a rapid change from complete snow cover to the "summer" (summer study period) ice surface of patchy debris ("dirty ice") and cryoconite holes. There was significant correlation between surface albedo, cryoconite hole coverage and surface productivity during the melt season, but microbial activity in "dirty ice" was not correlated with albedo and varied widely throughout the season. While this link suggests the potential for a remote-sensing approach to monitoring cryoconite hole biological processes, very wide seasonal and spatial variability in net surface productivity demonstrates the need for caution when extrapolating point measurements of biological processes to larger temporal or spatial scales.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Nitrogen inputs and microbial nitrogen cycling were investigated along a 79 km transect into the Greenland Ice Sheet (GrIS) during the main ablation season in summer 2010. The depletion of dissolved ...nitrate and production of ammonium (relative to icemelt) in cryoconite holes on Leverett Glacier, within 7.5 km of the ice sheet margin, suggested microbial uptake and ammonification respectively. Positive in situ acetylene assays indicated nitrogen fixation both in a debris-rich 100 m marginal zone and up to 5.7 km upslope on Leverett Glacier (with rates up to 16.3 μmoles C2H4 m−2 day−1). No positive acetylene assays were detected > 5.7 km into the ablation zone of the ice sheet. Potential nitrogen fixation only occurred when concentrations of dissolved and sediment-bound inorganic nitrogen were undetectable. Estimates of nitrogen fluxes onto the transect suggest that nitrogen fixation is likely of minor importance to the overall nitrogen budget of Leverett Glacier and of negligible importance to the nitrogen budget on the main ice sheet itself. Nitrogen fixation is however potentially important as a source of nitrogen to microbial communities in the debris-rich marginal zone close to the terminus of the glacier, where nitrogen fixation may aid the colonization of subglacial and moraine-derived debris.
Microbially mediated carbon fluxes on the surface of the Greenland ice sheet (GrIS) were recently quantified by Hodson and others (2010) using measurements of the surface coverage of debris ...(cryoconite) and rates of biological production associated with debris near the ice-sheet margin. We present updated models that do not assume the same spatial uniformity in key parameters employed by Hodson and others (2010) because they make use of biomass distribution and biological production data from a 79 km transect of the GrIS. Further, the models presented here also include for the first time biomass associated with both cryoconite holes and surficial algae. The predicted annual carbon flux for a small (1600 km2) section of ice surrounding the field transect is about four times that estimated using spatially uniform biomass and production in this area. When surficial algae are included, the model predicts about 11 times more carbon fixation via photosynthesis per year than the cryoconite-only models. We therefore suggest that supraglacial carbon fluxes from the GrIS have previously been underestimated by more than an order of magnitude and that the hitherto overlooked surficial algal ecosystem can be the most crucial contributor. The GrIS is shown to be in a relatively stable state of net autotrophy according to our model and so a strong link between bare-ice area and total carbon fluxes is evident. The implication is a biomass feedback to surface albedo and enhanced ablation as a result. Climate predictions for the year 2100 show that greater carbon fixation could also result from climate warming.
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