Andy Hodson

Contemporary rates of chemical denudation and atmospheric CO2 sequestration in glacier basins: an Arctic perspective.

This paper presents new estimates of solute fluxes from five high Arctic glacier basins in Svalbard. These estimates are combined with data from two other glacier basins to assess the effectiveness of chemical denudation on Svalbard and to estimate rates of temporary (or transient) CO2 drawdown. We use a solute provenance model to partition solutes into marine, aerosol, atmospheric and crustal components and to estimate their annual fluxes. Crustally derived solute fluxes are equivalent to a mean chemical denudation rate of 350 Σmeq+ m−2 a−1 for Svalbard (range: 160–560 Σmeq+ m−2 a−1), which lies within the global range of 94–4200 Σmeq+ m−2 a−1 for 21 glacier basins in the northern hemisphere, and is close to the continental average of 390 Σmeq+ m−2 a−1. Specific annual discharge is the most significant control upon chemical denudation in the glacierized basins, and basin lithology is an important secondary control, with carbonate-rich and basaltic lithologies currently showing the greatest chemical denudation rates. Estimates of transient CO2 drawdown are also directly associated with specific annual discharge and rock type. On Svalbard transient CO2 drawdown lies in the range 110–3000 kg C km−2 a−1, whilst the range is 110–13000 kg C km−2 a−1 for the northern hemisphere glacial data set. Transient CO2 drawdown is therefore usually low in the Svalbard basins unless carbonate or basalt rocks are abundant. The analysis shows that a large area of uncertainty in the transient CO2 drawdown estimates exists due to the non-stoichiometric release of solute during silicate hydrolysis. Silicate hydrolysis is particularly non-stoichiometric in basins where the extent of glacierization is high, which is most probably an artefact of high flushing rates through ice-marginal and subglacial environments where K-feldspars are undergoing mechanical comminution. Copyright

Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet

Darkening of parts of the Greenland ice sheet surface during the summer months leads to reduced albedo and increased melting. Here we show that heavily pigmented, actively photosynthesising microalgae and cyanobacteria are present on the bare ice. We demonstrate the widespread abundance of green algae in the Zygnematophyceae on the ice sheet surface in Southwest Greenland. Photophysiological measurements (variable chlorophyll fluorescence) indicate that the ice algae likely use screening mechanisms to downregulate photosynthesis when exposed to high intensities of visible and ultraviolet radiation, rather than non-photochemical quenching or cell movement. Using imaging microspectrophotometry, we demonstrate that intact cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the phototrophic community growing directly on the bare ice, through their photophysiology, most likely have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet.

Biogeochemical weathering under ice: Size matters

[1] The basal regions of continental ice sheets are gaps in our current understanding of the Earth’s biosphere and biogeochemical cycles. We draw on existing and new chemical data sets for subglacial meltwaters to provide the first comprehensive assessment of sub‐ice sheet biogeochemical weathering. We show that size of the ice mass is a critical control on the balance of chemical weathering processes and that microbial activity is ubiquitous in driving dissolution. Carbonate dissolution fueled by sulfide oxidation and microbial CO2 dominate beneath small valley glaciers. Prolonged meltwater residence times and greater isolation characteristic of ice sheets lead to the development of anoxia and enhanced silicate dissolution due to calcite saturation. We show that sub‐ice sheet environments are highly geochemically reactive and should be considered in regional and global solute budgets. For example, calculated solute fluxes from Antarctica (72–130 t yr −1 ) are the same order of magnitude as those from some of the world’s largest rivers and rates of chemical weathering (10–17 t km −2 yr −1 ) are high for the annual specific discharge (2.3–4.1 × 10 −3 m). Our model of chemical weathering dynamics provides important information on subglacial biodiversity and global biogeochemical cycles and may be used to design strategies for the first sampling of Antarctic Subglacial Lakes and other sub‐ice sheet environments for the next decade.

A glacier respires: Quantifying the distribution and respiration CO2 flux of cryoconite across an entire Arctic supraglacial ecosystem

Hodson, A., Anesio, A. M., Ng, F., Watson, R., Quirk, J., Irvine-fynn, T., Dye, A., Clark, C., McCloy, P., Kohler, J., Sattler, B. (2007). A glacier respires: Quantifying the distribution and respiration Co2 flux of cryoconite across an entire Arctic supraglacial ecosystem. Journal of Geophysical Research, 112 (G4).

Unmanned aerial vehicle measurements of volcanic carbon dioxide fluxes

[i] We report the first measurements of volcanic gases with an unmanned aerial vehicle (UAV). The data were collected at La Fossa crater, Vulcano, Italy, during April 2007, with a helicopter UAV of 3 kg payload, carrying an ultraviolet spectrometer for remotely sensing the SO 2 flux (8.5 Mg d- 1 ), and an infrared spectrometer, and electrochemical sensor assembly for measuring the plume CO 2 /SO 2 ratio; by multiplying these data we compute a CO 2 flux of 170 Mg d -1 . Given the deeper exsolution of carbon dioxide from magma, and its lower solubility in hydro-thermal systems, relative to SO 2 , the ability to remotely measure CO 2 fluxes is significant, with promise to provide more profound geochemical insights, and earlier eruption forecasts, than possible with SO 2 fluxes alone: the most ubiquitous current source of remotely sensed volcanic gas data.

The hydrochemistry of meltwaters draining a polythermal-based, high Arctic glacier, south Svalbard : I. The ablation season

Solute and runoff time-series at Finsterwalderbreen, Svalbard, provide evidence for considerable basal routing of water and the existence of at least two contrasting subglacial chemical weathering environments. The hydrochemistry of a subglacial upwelling provides evidence for a snowmelt-fed subglacial reservoir that dominates bulk runoff during recession flow. High concentrations of Cl− and crustal ions, high pCO2 and ratios of [*SO2−4/(*SO2−4+HCO−3)] close to 0·5 indicate the passage of snowmelt through a subglacial weathering environment characterized by high rock:water ratios, prolonged residence times and restricted access to the atmosphere. At higher discharges, bulk runoff becomes dominated by icemelt from the lower part of the glacier that is conveyed through a chemical weathering environment characterized by low rock:water ratios, short residence times and free contact with atmospheric gases. These observations suggest that icemelt is routed via a hydrological system composed of basal/ice-marginal, englacial and supraglacial components and is directed to the glacier margins by the ice surface slope. Upwelling water flows relatively independently of icemelt to the terminus via a subglacial drainage system, possibly constituting flow through a sediment layer. Cold basal ice at the terminus forces it to take a subterranean routing in its latter stages. The existence of spatially discrete flow paths conveying icemelt and subglacial snowmelt to the terminus may be the norm for polythermal-based glaciers on Svalbard. Proglacial mixing of these components to form the bulk meltwaters gives rise to hydrochemical trends that resemble those of warm-based glaciers. These hydrochemical characteristics of bulk runoff have not been documented on any other glacier on Svalbard to date and have significance for understanding interactions between thermal regime and glacier hydrology.

Fluvial suspended sediment transport from cold and warm-based glaciers in Svalbard

An analysis of temporal variability in proglacial suspended sediment concentration is undertaken using time series data collected from three Svalbard basins which include one largely cold-based glacier (Austre Broggerbreen), one largely warm-based glacier (Finsterwalderbreen) and one intermediate polythermal glacier (Erdmannbreen). The temporal variability in proglacial suspended sediment concentration is analysed using multiple regression techniques in which discharge is supplemented by other predictors acting as surrogates for variability in sediment supply at diurnal, medium-term and seasonal timescales. These multiple regression models improve upon the statistical explanation of suspended sediment concentration produced by simple sediment rating curves but need to account for additional stochastic elements within the time series before they may be considered successful. An interpretation of the physical processes which are responsible for the regression model characteristics is offered as a basis for comparing the different arctic glaciofluvial suspended sediment transport systems with that of their better known temperate glaciofluvial counterparts. It is inferred that the largely warm-based glacier is dominated by sediment supply from subglacial reservoirs which evolve in a similar manner to temperate glaciers and which cause a pronounced seasonal exhaustion of suspended sediment supply. The largely cold-based glacier, however, is dominated by sediment supply from marginal sources which generate a responsive system at short time scales but no significant seasonal pattern. The intermediate polythermal glacier basin, which was anticipated to be similar to the warm-based glacier, instead shows a highly significant seasonal increase in suspended sediment supply from an unusual subglacial reservoir emerging under pressure in the glacier foreland. The temperate model of glaciofluvial suspended sediment transport is therefore found to be of limited use in an arctic context. Copyright

Suspended sediment yield and transfer processes in a small High-Arctic glacier basin, Svalbard

Observations of suspended sediment concentration and discharge at two sites on the proglacial river network draining from a predominantly cold-based, High-Arctic glacier (Austre Broggerbreen) are described. Analysis of these observations illustrates: (i) the relatively low suspended sediment yield from this basin in comparison with many other glacier basins reported in the open literature; (ii) sustained and possibly increasing availability of suspended sediment to the fluvial system as the ablation season progresses; and (iii) the role of the proglacial sandur as both a sediment source and sink. Field observations coupled with the results of the data analysis are used to make inferences concerning the changing nature and relative importance of sediment sources within the basin.

The hydrochemistry of Bayelva, a high Arctic proglacial stream in Svalbard

Major ion, silica and pH data from a high Arctic proglacial stream network are examined. Factor analysis shows that the major controls upon hydrochemistry in the basin include snowpack solute elution, rapid alteration of minerals via surface reactions and slow, incongruent silicate dissolution. The importance of chemical weathering is found to increase downstream at the expense of solute acquisition from the transient snowpack. Data from a series of snow pits show that snowpack solute elution is characterised by the preferential elution of SO42−. Evidence for the preferential elution of NO3− is equivocal. However, NO3− levels in proglacial runoff may be enhanced by the leaching of cryotic soils later in the ablation season. Chemical weathering is dominated by calcite dissolution at high elevations and a combination of calcite and dolomite dissolution in the downstream proglacial environment. Silicate weathering at high elevations is characterised by the enrichment of K+ relative to Si. Concentrated active layer soil waters show less enrichment of K+ and constitute a potentially important source of dissolved silica to the river system. Na+ is also produced by silicate weathering, but its mobility is believed to be largely governed by ion exchange with Ca2+ early in the ablation season. All chemical weathering environments are characterised by high p(CO2) due to either respiration within the soil or, at higher elevations within the catchment, H+ provision from acid aerosol and sulphide oxidation. However, rates of sulphide oxidation are limited because the cold-based thermal regime of the glaciers effectively prevents access of meltwaters to freshly comminuted sub-glacial sulphide minerals. The restricted sub-glacial weathering therefore means that ice marginal and proglacial environments are the most important zones of solute acquisition by meltwaters, causing significant enrichment of major ions, silica and dissolved CO2 within only a short distance of the contemporary ice margin.

The microstructure and biogeochemistry of Arctic cryoconite granules

Abstract A cryoconite granule is a biologically active aggregation of microorganisms, mineral particles and organic matter found on glacier surfaces, often within shallow pools or cryoconite holes. Observations of the microstructure of a range of cryoconite granules from locations in Svalbard and Greenland reveal their structure and composition. Whereas bulk analyses show that the mineralogy and geochemistry of these granules are broadly similar, analyses of their microstructure, using optical, epifluorescence and confocal microscopy, indicate differences in the location and quantity of photosynthetic microorganisms, heterotrophic bacteria and organic matter. Using these findings, a hypothesis on the aggregation of cryoconite is presented, centred upon multilevel aggregation by bioflocculation and filamentous binding.