Bryn Hubbard

Possible interactions between bacterial diversity, microbial activity and supraglacial hydrology of cryoconite holes in Svalbard

The diversity of highly active bacterial communities in cryoconite holes on three Arctic glaciers in Svalbard was investigated using terminal restriction fragment length polymorphism (T-RFLP) of the 16S rRNA locus. Construction and sequencing of clone libraries allowed several members of these communities to be identified, with Proteobacteria being the dominant one, followed by Cyanobacteria and Bacteroidetes. T-RFLP data revealed significantly different communities in holes on the (cold) valley glacier Austre Brøggerbreen relative to two adjacent (polythermal) valley glaciers, Midtre Lovénbreen and Vestre Brøggerbreen. These population compositions correlate with differences in organic matter content, temperature and the metabolic activity of microbial communities concerned. No within-glacier spatial patterns were observed in the communities identified over the 2-year period and with the 1 km-spaced sampling. We infer that surface hydrology is an important factor in the development of cryoconite bacterial communities.

Water exchange between the subglacial Lake Vostok and the overlying ice sheet

It has now been known for several years that a 200-km-long lake, called Lake Vostok, lies beneath the ice sheet on which sits Vostok Station in Antarctica. The conditions at the base of the ice sheet above this subglacial lake can provide information about the environment within the lake, including the likelihood that it supports life. Here we present an analysis of the ice-sheet structure from airborne 60-MHz radar studies, which indicates that distinct zones of basal ice loss and accretion occur at the ice–water interface. Subglacial melting and net ice loss occur in the north of the lake and across its 200-km-long western margin, whereas about 150 m of ice is gained by subglacial freezing in the south. This indicates that significant quantities of water are exchanged between the base of the ice sheet and the lake waters, which will enrich the lake with gas hydrates, cause sediment deposition and encourage circulation of the lake water.

A review of the use of radio-echo sounding in glaciology

Radio-echo sounding (RES), or radar, is an established geophysical technique that has been, and continues to be, applied to investigate a variety of ice-mass properties. This review presents the physical theory and principles of radio-glaciology, and describes the various types of radar equipment commonly used, including modern, ground-penetrating radar (GPR) systems. The range of glaciological applications these systems have been used to investigate is summarized, along with promising avenues of current and future research.

Basal Ice Facies and Their Formation in the Western Alps

Research at 11 glaciers in the western European Alps has resulted in the definition of 7 basal ice facies. The formation of each is investigated on the basis of its debris distribution, sedimentology, and stable isotope composition. Isotopic data are interpreted within an analytical framework that allows for variations in the isotopic composition of source waters. Ice facies formed by metamorphism are distinguished from those formed by open-system refreezing within basal cavities and from those formed by closed-system regelation. Individual basal ice facies are thereby linked to the subglacial physical conditions required for their formation. These associations allow inferences relating to bed type, basal temperature, and subglacial hydrology to be made on the basis of the presence of specific, diagnostic basal ice facies at any glacier where they may be observed.

Hydrological controls on patterns of surface, internal and basal motion during three "spring events" : Haut Glacier d'Arolla, Switzerland

Three early-melt-season high-velocity events (or “spring events”) occurred on Haut Glacier d’Arolla, Switzerland, during the melt seasons of 1998 and 1999. The events involve enhanced glacier velocity during periods of rapidly increasing bulk discharge in the proglacial stream and high subglacial water pressures. However, differences in spatial patterns of surface velocity, internal ice deformation rates, the spatial extent of high subglacial water pressures and in rates of subglacial sediment deformation suggest different hydrological and mechanical controls. The data from two of the events suggest widespread ice–bed decoupling, particularly along a subglacial drainage axis creating the highest rates of basal motion and “plug flow” in the overlying ice. The other event showed evidence of less extensive ice–bed decoupling and sliding along the drainage axis with more mechanical support for ice overburden transferred to areas adjacent to decoupled areas. We suggest that: (1) plug flow may be a common feature on glaciers experiencing locally induced reductions in basal drag; (2) under certain circumstances, enhanced surface motion may be due in part to non-locally forced enhanced bed deformation; and (3) subglacial sediment deformation is confined to a depth of the order of centimetres to decimetres.

The character, structure and origin of the basal ice layer of a surge-type glacier

The basal ice layer of surge-type Variegated Glacier, Alaska, appears to have formed by a combination of (i) open-system freezing of subglacial meltwaters over both rigid and unconsolidated substrates; (ii) apron over-riding during surge-induced glacier advance; (iii) incorporation of glacier ice by recumbent folding, thrust-faulting and nappe over-riding during down-glacier propagation of a surge front; and (iv) post-formational metamorphism involving recrystallization, partial internal melting and squeezing out of meltwaters and dissolved gases. Structural evidence and the characteristics of debris entrained in ice facies formed by basal freezing suggest that the layer includes a lower element formed under surge conditions and an upper element formed during the quiescent phase of a surge cycle. The lower element is depleted in comminution products and enriched in medium gravel, while the upper element contains comminution products but virtually no medium gravel. This distinction is attributed to the efficiency of bedrock fracture and meltwater flushing of comminution products under surge conditions. The basal ice layer thickens from <I m to >13m down-glacier in a manner consistent with the magni tu de of horizon tal shortening ind uced by the 198283 surge. Thickening is largely tectonic in origin, and the style and intensity of folding and thrust faulting change down-glacier as the magnitude of horizontal shortening increases. Tectonic processes associated with the down-glacier propagation of surge fronts therefore appear to be capable of creating thick basal ice layers which allow extensive supraglacial sedimentation of subglacially derived debris.

Basal ice formation and deformation: a review

A distinction is often made between the ice which constitutes the major part of glaciers and ice sheets and a relatively thin layer of debris-rich ice which is present at their base. This ’basal ice’, which has a vertical extent of up to tens of metres, is produced at and interacts with the glacier bed, while the overlying ’glacier ice’ is a product of firnification processes occurring at or near the upper surface of the ice mass (Paterson, 1981). As a result of its different mode and environment of formation, basal ice may differ from glacier ice in terms of its overall extent and structure and in the properties of the ice, debris, solutes and gases of which it is composed. In general, basal ice sequences have a much higher debris content than glacier ice and an anisotropic structure, consisting of individual layers, lenses and pods of variable lateral extent and distinctive chemical and isotopic composition. Several workers have combined study of these characteristics with theoretical considerations to postulate mechanisms for the formation of basal ice (Weertman, 1961; 1964; Kamb and LaChapelle, 1964; Boulton, 1970; Shaw, 1977; Tison and Lorrain, 1987). Given this compositional variability, basal ice may behave in a very different way rheologically from glacier ice or polycrystalline ’laboratory’ ice. A number of recent studies have reported field observations which indicate that strain rates in debris-rich basal ice or frozen subglacial sediments may be well in excess of those expected from theoretical or laboratory-based studies. This is of signicance since, although much of the internal deformation of ice masses occurs in layers close to the bed, most models of glacier and ice-sheet behaviour are based on assumptions of the uniform deformation of isotropic, polycrystalline ice based on Glen’s (1955) power flow law (see Weertman and Birchfield, 1984). This is no less the case today than in 1967, when Theakstone called for a greater comprehension of the response to stress of ice ’such as actually exists at glacier beds’, without which he felt that ’an adequate theory of glacier sliding is unlikely to be developed’ (Theakstone, 1967). The aim of this paper is to identify some of the processes responsible for the formation of basal ice sequences and to demonstrate the links likely to exist between these processes and the characteristics of the resultant sequences. The postulated rheological consequences of these properties are also discussed in the

Persistent flow acceleration within the interior of the Greenland ice sheet

We present surface velocity measurements from a high-elevation site located 140 km from the western margin of the Greenland ice sheet, and ~ 50 km into its accumulation area. Annual velocity increased each year from 51.78 ± 0.01 m yr−1 in 2009 to 52.92 ± 0.01 m yr−1 in 2012—a net increase of 2.2%. These data also reveal a strong seasonal velocity cycle of up to 8.1% above the winter mean, driven by seasonal melt and supraglacial lake drainage. Sole et al. (2013) recently argued that ice motion in the ablation area is mediated by reduced winter flow following the development of efficient subglacial drainage during warmer, faster, summers. Our data extend this analysis and reveal a year-on-year increase in annual velocity above the equilibrium line altitude, where despite surface melt increasing, it is still sufficiently low to hinder the development of efficient drainage under thick ice.

Solute generation and transfer from a chemically reactive alpine glacial–proglacial system

The environs of the Glacier de Tsanfleuron, Switzerland, was used as a study site to investigate the controls on the relative efficiency of solute generation and removal from glacial and proglacial environments. Here, a 1500 m wide glacier forefield consists of a karstic limestone plateau flanked to the north by a till-floored valley. Bedrocks and glacial debris are composed of chemically reactive pure and impure Mesozoic and Tertiary limestones with accessory pyrite. Spot sampling of ice, snow and meltwaters in the late melt season was supplemented by systematic measurements of the main meltstream, including during periods of rainfall, and simple laboratory leaching and weathering experiments.Isotopic parameters were used to investigate water sources. Most meltwater and glacier ice samples lay close to a meteoric water line (δ D= 8·3 δ18O + 14) defined by waters from small tributary streams. Heavy isotopic excursions of bulk meltwater chemistry were caused by rainfall events, recovering within days to a δ18O baseline around −12 permil. No regular diurnal variations in δ18 O were apparent.The atmosphere is the source of Cl− and most Na+, but the bulk of other solutes are generated in the environment. Ion loads of up to 1 meq l−1 are rapidly attained by calcite dissolution. Over periods of weeks to months pyrite oxidation generates sulphate and acidity that drives further calcite dissolution. Low water–rock ratio weathering environments have characteristically high SO42−, Mg2+, Sr2+, and ratios of these species to calcium. The characteristic cation ratios are influenced by non-congruent calcite dissolution. The ratio of sulphate to other species is highest where water–rock contact times are highest, although this relationship is complicated by spatial variations in pyrite abundance.Meltstream time series illustrate that 90 per cent of daily ion yields in fine weather are concentrated in the 12 h time period of higher discharge. Ion yields increase downstream mainly by a combination of dissolution of calcareous suspended sediment and input from tributaries and seepage from the till banks. Rainstorms lead to increased solute concentrations and resulting hourly fluxes can match daily fine weather ion fluxes. Excess nitrate appears to be largely sourced from the proglacial surface. The capacity of the proglacial environment for yielding significant subsurface water as a result of the storm seems low, unlike non-glacial environments. This implies that most of the excess solutes mobilized by storms comes from subglacial sources. Increased efficiency of yield of solutes from low water/rock ratio subglacial weathering environments persists after the isotopic signature of the rainfall event has died away.A simple conceptual model of the sources of water and solutes agrees with conclusions from attempts at hydrograph separation, that mixing of water reservoirs of fixed solute composition cannot be used for quantitative descriptions of the system. Estimated annual solute yields (17 ton per km2 per m precipitation) are high, but cannot be readily expressed in purely areal terms because of likely significant losses to the underlying karstic system. A tentative conclusion is that the proglacial environment is overall less efficient at producing solutes than the glacial environment, but more information is required on processes in the early melt season to substantiate this statement. Copyright

Interactions of calcareous suspended sediment with glacial meltwater: a field test of dissolution behaviour

Abstract Dissolution of calcite and associated interactions of suspended sediment with aqueous solution were investigated in a tributary-free 600 m reach of the main meltstream draining the Tsanfleuron glacier, Switzerland, over a 24-h cycle during which solute concentrations varied inversely with discharge. Downflow, solute calcium, strontium, and alkalinity increased because of calcite dissolution. Using flow-through times from salt-dilution gauging, a consistent small sulphate excess at the downstream site was observed. Given the slowness of sulphate supply by pyrite oxidation, this excess sulphate can be attributed to mixing of around 1% of ion-rich water (seeping from till banks) with the main meltstream. Calcite dissolution is normally directly proportional to exposed surface area of the mineral, yet only a small increase in calcite dissolution was observed when suspended sediment increased by a factor of 25 to 1.3 g/l at peak flow. The suspended sediment displays little variation in size distribution with total suspended load, and contains 30–40% calcite with a minimum specific surface area (S) of 0.25 m2/g sediment. Application of the Plummer–Wigley–Parkhurst (PWP) model predicts dissolution rates broadly similar to those found at lower suspended sediment concentrations given this value of S. At higher suspended sediment loads predicted dissolution rates are too high. This discrepancy is reduced by use of the Buhmann–Dreybrodt (B–D) model which takes explicit account of the slowness of hydration of aqueous carbon dioxide, and the problem of mass transfer of H2CO3 given the surface area of calcite to volume of solutions considered. The remaining discrepancy implies less interaction than expected of suspended sediment particles with turbulent meltwater at high suspended sediment concentrations. The effects of proglacial modification of meltstream geochemistry in this case is a strong decrease in PCO2 accompanied by an increase in total ion load, but decreases in Mg/Ca and Sr/Ca, from the high values characteristic of low water–rock ratio interactions in subglacial environments and till. Nevertheless, the distinctive chemical imprint in meltstream chemistry of non-congruent mineral dissolution in low water–rock ratio glacial weathering environments remain. In contrast, in terrains where calcite is scarce, it will tend to dissolve congruently, contributing significantly to total solutes, and its dissolution will be less limited by CO2 reaction kinetics.