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
Abstract
The Greenland Ice Sheet harbours a wealth of microbial life, yet the total biomass stored or exported from its surface to downstream environments is unconstrained. Here, we quantify ...microbial abundance and cellular biomass flux within the near-surface weathering crust photic zone of the western sector of the ice sheet. Using groundwater techniques, we demonstrate that interstitial water flow is slow (~10
−2
m d
−1
), while flow cytometry enumeration reveals this pathway delivers 5 × 10
8
cells m
−2
d
−1
to supraglacial streams, equivalent to a carbon flux up to 250 g km
−2
d
−1
. We infer that cellular carbon accumulation in the weathering crust exceeds fluvial export, promoting biomass sequestration, enhanced carbon cycling, and biological albedo reduction. We estimate that up to 37 kg km
−2
of cellular carbon is flushed from the weathering crust environment of the western Greenland Ice Sheet each summer, providing an appreciable flux to support heterotrophs and methanogenesis at the bed.
During past periods of advance, Arctic glaciers and ice sheets overrode soil, sediments, and vegetation and buried significant stores of organic matter (OM); these glaciers are now shrinking rapidly ...due to climate warming. Little is known about the biogeochemical processing of the OM buried beneath glacier ice which makes the processes associated with deglaciation difficult to predict. Subglacial sediments exposed at receding glacier fronts may represent a legacy of past biogeochemical processes. Here, we analyzed sediments from retreating fronts of 19 Arctic glaciers for their mineralogical and elemental composition, contents of major nutrients, OM biomarkers (aliphatic lipids and lignin‐derived phenols), 14C age, and microbial community structure. We show the character of the sediments is mostly determined by local glaciation history and bedrock lithology. Most subglacial sediments offer high amounts of readily bioavailable phosphorus (i.e., loose, labile, and Fe/Al P fractions) but lack readily accessible carbon substrates. The subglacial OM originated mainly from overridden terrestrial vascular plants. The results of OM biomarker analysis and 14C dating suggest the OM substrates degrade in the subglacial environment and are reworked by the resident microbial communities. We argue the biogeochemical legacy of the perishing subglacial environments is an important determinant for the early processes of proglacial ecological succession.
Plain Language Summary
The rapid melting of Arctic glaciers leaves behind large areas of sediments that have been covered by glacier ice and isolated for long periods of time. These sediments form the base of the new proglacial ecosystem; however, little is known about the legacy of subglacial biogeochemical processes. We comprehensively analyzed sediments from 19 Arctic glaciers to gain insight into the origin and fate of the present organic matter, nutrient content, and microbial communities. We propose that the character of the sediments is determined by local glaciation histories and also affected by the bedrock composition. We found that organic material in the sediments originates mainly from higher plants, overridden during past (Holocene and/or Late Pleistocene) periods of glacier advance. It degrades beneath the ice and is used as a substrate by microorganisms living at the glacier beds. While readily accessible carbon compounds are likely depleted in the exposed sediments, the macronutrient phosphorus, vital for the development of the new proglacial ecosystem, is available in high concentrations. We argue that the biogeochemical characteristics of the disappearing environments of glacier and ice sheet beds is an important factor for the early development of the expanding proglacial areas in the Arctic.
Key Points
Retreating Arctic glaciers are exposing sediments that have been isolated under the ice for thousands to tens of thousands of years
Extracted biomarkers indicates an overridden vegetation origin and significant degradation of organic matter in the subglacial environment
The biogeochemical legacy of subglacial sediments defines the initial stages of ecological succession in proglacial areas
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Abstract
Surface melt from the Greenland Ice Sheet (GrIS) collects particulate organic carbon (POC) as it drains into subglacial environments and transports it downstream where it serves as a ...microbial substrate. We hypothesized that older POC is entrained by meltwaters as the subglacial drainage network expands upglacier over the summer. To test this, POC samples were collected from a meltwater river exiting the GrIS over an ablation season and
14
C dated. Resulting values were compared with meltwater hydrochemistry and satellite observations of the catchment area. We found that POC ages increased from ~5000 to ~9000 years B.P. until peak discharge and catchment size. Afterward, significant fluctuations in POC age were observed, interpreted to result from periods of high and low subglacial hydrological pressure and sediment supply and subsequent exhaustion. These observations suggest a seasonal progression in the source of POC exported from the GrIS and provide evidence for a seasonally evolving subglacial drainage system.
Key Points
A seasonal progression was observed in
14
C ages of POC exported from beneath the GrIS
We propose that patterns result from the seasonal expansion and evolution of the subglacial drainage system
Future changes in glacial melt may affect downstream productivity via shifts in the bioavailability of carbon
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, 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.
Arctic glacier surfaces harbour abundant microbial communities consisting mainly of heterotrophic and photoautotrophic bacteria. The microbes must cope with low concentrations of nutrients and with ...the fact that both the dissolved and debris-bound nutrient pools are dominated by organic phases. Here we provide evidence that phosphorus (P) is deficient in the supraglacial environment on a Svalbard glacier, we quantify the enzymatic activity of phosphatases in the system and we estimate the contribution of the microbes to the cycling of the dominant organic P in the supraglacial environment. Incubation of cryoconite debris revealed significant phosphatase activity in the samples (19–67 nmol MUP g−1 h−1). It was inhibited by inorganic P during incubations and had its optimum at around 30°C. The phosphatase activity measured at near-in situ temperature and substrate concentration suggests that the available dissolved organic P can be turned over by microbes within ~3–11 h on the glacier surface. By contrast, the amount of potentially bioavailable debris-bound organic P is sufficient for a whole ablation season. However, it is apparent that some of this potentially bioavailable debris-bound P is not accessible to the microbes.
The Greenland Ice Sheet is losing mass at a remarkable rate as a result of climatic warming. This mass loss coincides with the export of dissolved organic matter (DOM) in glacial meltwaters. However, ...little is known about how the source and composition of exported DOM changes over the melt season, which is key for understanding its fate in downstream ecosystems. Over the 2015 ablation season, we sampled the outflow of Leverett Glacier, a large land‐terminating glacier of the Greenland Ice Sheet. Dissolved organic carbon (DOC) concentrations and DOM fluorescence were analyzed to assess the evolution of DOM sources over the course of the melt season. DOC concentrations and red‐shifted fluorescence were highly associated (R2 > 0.95) and suggest terrestrial inputs from overridden soils dominated DOM early season inputs before progressive dilution with increasing discharge. During the outburst period, supraglacial drainage events disrupted the subglacial drainage system and introduced dominant protein‐like fluorescence signatures not observed in basal flow. These results suggest that subglacial hydrology and changing water sources influence exported DOC concentration and DOM composition, and these sources were differentiated using fluorescence characteristics. Red‐shifted fluorescence components were robust proxies for DOC concentration. Finally, the majority of DOM flux, which occurs during the outburst and postoutburst periods, was characterized by protein‐like fluorescence from supraglacial and potentially subglacial microbial sources. As protein‐like fluorescence is linked to the bioavailability of DOM, the observed changes likely reflect seasonal variations in the impact of glacial inputs on secondary production in downstream ecosystems due to shifting hydrologic regimes.
Key Points
Dissolved organic carbon concentrations can be estimated in glacial outflow from the fluorescence intensity of red‐shifted fluorescence
Optical tracers can be used to differentiate subglacial from supraglacial inputs
Optical properties in July highlight microbial sources of dissolved organic matter, with implications for downstream bioavailability
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Subglacial chemical weathering plays a key role in global silicate weathering budgets, contributing to the cycling of silicon (Si) in terrestrial and marine systems and the potential drawdown of ...carbon dioxide from the atmosphere. Here, we use data from two Greenland Ice Sheet (GrIS) catchments to demonstrate how Si isotopes from dissolved and amorphous particulate fractions (δ30DSi and δ30ASi respectively) can be used together with major ion data to assess the degree of secondary silicate weathering product formation and redissolution in subglacial environments. We compare a time-series of summer melt seasons from the two study sites, which differ in catchment size (∼600 km2 for Leverett Glacier (LG) and ∼36 km2 for Kiattuut Sermiat (KS)). Subglacial waters from LG have elevated Na+ and K+ ions in relation to Ca2+ and Mg2+ ions, indicating a predominance of silicate weathering, whilst meltwaters from KS are characterised by carbonate weathering (hydrolysis and carbonation) throughout the melt season. Both catchments have mean δ30DSi values substantially lower than average riverine values (KS 0.41‰, LG −0.25‰, versus a global riverine mean of 1.25‰) and display a seasonal decline, which is more pronounced at LG. The δ30ASi values (discharge weighted mean values KS −0.44‰, LG −0.22‰) are lighter than the bedrock (mean values KS −0.18 ± 0.12‰, LG 0.00 ± 0.07‰) in both catchments, indicating a secondary weathering product origin or leaching of lighter isotopes during initial weathering of crushed rock. When used in combination, the major ion and silicon isotope data reveal that the extent of silicate weathering and secondary phase redissolution are more pronounced at LG compared to KS. Contrasting weathering regimes and subglacial hydrology between catchments need to be considered when estimating the δ30Si composition of silica exported into polar oceans from the GrIS, with larger catchments likely to produce fluxes of lighter δ30Si. As larger catchments dominate freshwater export to the ocean, GrIS meltwater is likely to be very light in isotopic composition, and the flux is likely to increase with ice melt as the climate warms.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The effect of temperature, light and nutrient composition on morphological traits was determined in seven nostocacean cyanobacteria
(Anabaena planctonica, A. sphaerica
var.
conoidea, A. spiroides, ...Aphanizomenon gracile, Nostoc
sp.,
Scytonema
sp., and
Tolypothrix
sp.). Their morphological variability was high but only some of the features showed changes reflecting varied growth conditions. The frequency of heterocyst occurrence decreased with increasing nitrogen concentration. Within the range studied, the effect of temperature on heterocyst frequency of
Tolypothrix
sp. and planktonic
Anabaena
strains could be fitted by a normal curve with a clear optimum while linear correlation was found in
Aphanizomenon gracile
. T-and S-type branching was observed in both
Scytonema
sp. and
Tolypothrix
sp. strains. T-type branching was found to be markedly dependent on nitrogen concentration. The abundance of necridic cells of
Tolypothrix
sp. increased linearly with temperature and light intensity. Regularity of trichome coiling of
A. spiroides
depended on culture medium, suggesting that nutrient composition may be the main controlling factor. In contrast, the effect of the experimental conditions on the dimensions of vegetative cells and heterocysts was weak. Their variability was markedly higher within each experimental treatment than between treatments.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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