Subglacial Drainage Network Research Articles

Topographically mediated ice stream subglacial drainage networks

AbstractSatellite laser altimetry reveals short timescale changes in Antarctic ice sheet surface elevation that are suggested to be driven by subglacial water transport and storage. Here details of the interaction between the dynamics of ice stream flow, subglacial water system, and bed elevation relief are examined in the context of idealized, heterogeneous bed geometries. Using a two‐way coupled model of ice and subglacial water flow, we show that basal topography controls the temporal and spatial variability of the sub–ice stream hydraulic system. The orientation and characteristic dimensions of the topographic undulations determine the morphology (connected subglacial ponds or channel‐like subglacial water features) and timescales of the sub–ice stream drainage system. The short‐term (several years to decades) variability of the simulated coupled ice stream/subglacial water system suggests that the short‐term surface variations detected in remote sensing observations may be indicative of a rapidly evolving subglacial water system. Our simulations also show that interaction between ice flow and the highly dynamic subglacial water system has a strong effect on effective stress in the ice. Large effective stress magnitudes arise over areas where the basal traction is characterized by strong spatial gradients, that is, transitions from high to low basal traction or vise versa. These transitions migrate on multiyear timescales and thus cause large effective stress variability on the same temporal scales.

An extensive subglacial lake and canyon system in Princess Elizabeth Land, East Antarctica

The subglacial landscape of Princess Elizabeth Land (PEL) in East Antarctica is poorly known due to a paucity of ice thickness measurements. This is problematic given its importance for understanding ice sheet dynamics and landscape and climate evolution. To address this issue, we describe the topography beneath the ice sheet by assuming that ice surface expressions in satellite imagery relate to large-scale subglacial features. We find evidence that a large, previously undiscovered subglacial drainage network is hidden beneath the ice sheet in PEL. We interpret a discrete feature that is 140 × 20 km in plan form, and multiple narrow sinuous features that extend over a distance of ∼1100 km. We hypothesize that these are tectonically controlled and relate to a large subglacial basin containing a deep-water lake in the interior of PEL linked to a series of long, deep canyons. The presence of 1-km-deep canyons is confirmed at a few localities by radio-echo sounding data, and drainage analysis suggests that these canyons will direct subglacial meltwater to the coast between the Vestfold Hills and the West Ice Shelf.

Antarctic subglacial hydrology: current knowledge and future challenges

AbstractFlood-carved landforms across the deglaciated terrain of Victoria Land, East Antarctica, provide convincing geomorphological evidence for the existence of subglacial drainage networks beneath the Antarctic ice sheet, and motivate research into the inaccessible environment beneath the contemporary ice sheet. Through this research, our understanding of Antarctic subglacial hydrology is steadily building, and this paper presents an overview of the current state of knowledge. The conceptualization of subglacial hydrological behaviour was developed at temperate and Arctic glaciers, and is thus less mature in the Antarctic. Geophysical and remote sensing observations have demonstrated that many subglacial lakes form part of a highly dynamic network of subglacial drainage beneath the Antarctic ice sheet. Recent research into subglacial water flows, other than those directly concerned with lakes, has discovered potentially significant impacts on ice stream dynamics, ice sheet mass balance, and supplies of water to the ocean potentially affecting circulation and nutrient productivity. Despite considerable advances in understanding there remain a number of grand challenges that must be overcome in order to improve our knowledge of these subglacial hydrological processes.

Identifying weathering sources and processes in an outlet glacier of the Greenland Ice Sheet using Ca and Sr isotope ratios

Chemical and isotope data (ε40Ca, δ44/42Ca, 87Sr/86Sr, δ18O) of river water samples were collected twice daily for 28 days in 2009 from the outlet river of Leverett Glacier, West Greenland. The water chemistry data was combined with detailed geochemical analysis and petrography of bulk rock, mineral separates and sediment samples in order to constrain the mineral weathering sources to the river. The average isotopic compositions measured in the river, with 2SD of all the values measured, were ε40Ca=+4.0±1.4, δ44/42Ca=+0.60±0.10‰ and 87Sr/86Sr=0.74243±0.00327. Based on changes in bulk meltwater discharge, the hydrochemical data was divided into three hydrological periods. The first period was marked by the tail-end of an outburst event and was characterised by water with decreasing suspended sediment concentrations (SSC), ion concentrations and pH. During the second hydrological period, discharge increased whilst 87Sr/86Sr decreased from 0.74550 to 0.74164. Based on binary mixing diagrams using 87Sr/86Sr with Na/Sr, Ca/Sr and ε40Ca, this is interpreted to reflect an increase in reactive mineral weathering, in particular epidote, as the water residence time decreases. The decrease in water residence time is a result of the evolution from a distributed (long water residence time) to a channelised (short water residence time) subglacial drainage network. The third hydrological period was defined as the period when overall discharge was decreasing. This hydrological period was marked by prominent diurnal cycles in discharge. During this period, significant correlations between δ44/42Ca and SSC and δ18O were observed which are suggestive of fractionation during adsorption. This study demonstrates the potential of radiogenic Ca to both identify temporally changing mineral sources in conjunction with 87Sr/86Sr values and to separate source and fractionation effects in δ44/42Ca values.

Dendritic subglacial drainage systems in cold glaciers formed by cut‐and‐closure processes

The routing and storage of meltwater and the configuration of drainage systems in glaciers exert a profound influence on glacier behaviour. However, little is known about the hydrological systems of cold glaciers, which form a significant proportion of the total glacier population in the climate sensitive region of the igh rctic. Using glacio‐speleological techniques, we obtained direct access to explore and survey three conduit systems and one moulin within the tongue area of Tellbreen, a small cold‐based valley glacier in central pitsbergen. More than 600 m of conduits were surveyed and mapped in plan, profile and cross‐section view to analyse the configuration of the drainage system. The investigations revealed that cold‐based glaciers can exhibit a dendritic drainage network with supra‐, en‐ and subglacial components formed most likely by cut‐and‐closure processes as well as surface‐to‐bed drainage via moulins. Furthermore, we observed that water is stored within the glacier and released gradually via subglacial conduits during the winter months, forming a large and active icing in the proglacial area. The presence of supra‐, en‐ and subglacial components, the surface‐to‐bed moulin and the dendritic subglacial drainage network suggest that existing models and understanding of the hydrology of cold glaciers needs to be re‐evaluated, mostly concerning the different possible pathways and processes that form the hydrological system.

A microbial ecosystem beneath the West Antarctic ice sheet.

Liquid water has been known to occur beneath the Antarctic ice sheet for more than 40 years, but only recently have these subglacial aqueous environments been recognized as microbial ecosystems that may influence biogeochemical transformations on a global scale. Here we present the first geomicrobiological description of water and surficial sediments obtained from direct sampling of a subglacial Antarctic lake. Subglacial Lake Whillans (SLW) lies beneath approximately 800m of ice on the lower portion of the Whillans Ice Stream (WIS) in West Antarctica and is part of an extensive and evolving subglacial drainage network. The water column of SLW contained metabolically active microorganisms and was derived primarily from glacial ice melt with solute sources from lithogenic weathering and a minor seawater component. Heterotrophic and autotrophic production data together with small subunit ribosomal RNA gene sequencing and biogeochemical data indicate that SLW is a chemosynthetically driven ecosystem inhabited by a diverse assemblage of bacteria and archaea. Our results confirm that aquatic environments beneath the Antarctic ice sheet support viable microbial ecosystems, corroborating previous reports suggesting that they contain globally relevant pools of carbon and microbes that can mobilize elements from the lithosphere and influence Southern Ocean geochemical and biological systems.

Biogeochemistry: Microbes eat rock under ice.

The first description of the microorganisms inhabiting a subglacial lake deep below the Antarctic ice sheet reveals some of the complex interactive metabolic processes that sustain these microbial communities. See Letter p.310 Whether there is microbial life in subglacial lakes in the Antarctic has been a matter of controversy, as early results were compromised when it was discovered that contamination may have occurred during drilling. Discovered less than a decade ago using satellite data, Lake Whillans lies beneath some 800 metres of ice on the lower portion of the Whillans Ice Stream (WIS) in West Antarctica and is part of an extensive and evolving subglacial drainage network. In the first study to sample Antarctic subglacial waters directly, analysis of sediments obtained by the WISSARD drilling program shows that Lake Whillans' water contains more than 3,900 different types of bacteria and archaea, including one closely related to the nitrite oxidizing betaproteobacterium 'Candidatus Nitrotoga arctica', which comprised 13% of the sequence data. The lake waters contain a diverse range of metabolically active microorganisms, many of which seem to gain nutrients from the melting ice and from the rock and sediment beneath the ice.

Increased channelization of subglacial drainage during deglaciation of the Laurentide Ice Sheet

The configuration of subglacial meltwater is a critical control on ice sheet dynamics, and the presence of pressurized water distributed across the bed can induce dynamic instabilities. However, this process can be offset by efficient evacuation of water within large subglacial channels, and drainage systems beneath alpine glaciers have been shown to become increasingly channelized throughout the melt season in response to the increased production of meltwater. This seasonal evolution has recently been inferred beneath outlet glaciers of the Greenland Ice Sheet, but the extent to which this process occurs across much larger spatial and temporal scales is largely unknown, introducing considerable uncertainty about the evolution of subglacial drainage networks at the ice sheet scale and associated ice sheet dynamics. This paper uses an unprecedented data set of over 20,000 eskers to reconstruct the evolution of channelized meltwater systems during the final deglaciation of the Laurentide Ice Sheet (13–7 kyr B.P.). We demonstrate that eskers become more frequent during deglaciation and that this coincides with periods of increased rates of ice margin recession and climatic warming. Such behavior is reminiscent of the seasonal evolution of drainage systems observed in smaller glaciers and implies that channelized drainage became increasingly important during deglaciation. An important corollary is that the area of the bed subjected to a less efficient pressurized drainage system decreased, which may have precluded dynamic instabilities, such as surging or ice streaming.

Glacial conditions that contribute to the regeneration of Fountain Glacier proglacial icing, Bylot Island, Canada

Proglacial icings are one of the most common forms of extrusive ice found in the Canadian Arctic. However, the icing adjacent to Fountain Glacier, Bylot Island, is unique due to its annual cycle of growth and decay, and perennial existence without involving freezing point depression of water due to chemical characteristics. Its regeneration depends on the availability of subglacial water and on the balance between ice accretion and hydro-thermal erosion. The storage and conduction of the glacial meltwater involved in the accretion of the icing were analyzed by conducting topographic and ground penetrating radar surveys in addition to the modelling of the subglacial drainage network and the thermal characteristics of the glacier base. The reflection power analysis of the geophysical data shows that some areas of the lower ablation zone have a high accumulation of liquid water, particularly beneath the centre part of the glacier along the main supraglacial stream. A dielectric permittivity model of the glacier – sediment interface suggests that a considerable portion of the glacier is warm based; allowing water to flow through unfrozen subglacial sediments towards the proglacial outwash plain. All these glacier-related characteristics contribute to the annual regeneration of the proglacial icing and allow for portions of the icing to be perennial. Copyright © 2013 John Wiley & Sons, Ltd.

Evolution of subglacial overdeepenings in response to sediment redistribution and glaciohydraulic supercooling

AbstractGlaciers erode bedrock rapidly, but evacuation of sediments requires efficient subglacial drainage networks. If glaciers erode more rapidly than evacuation proceeds, a protective subglacial till layer can form to armor the bed. Where glaciers cross overdeepenings, local closed depressions, the bed slope opposes the ice surface and lowers the hydraulic potential gradient that drives water flow. Here, we present results of a dynamic, distributed model of coupled basal water flow and sediment transport to show how overdeepenings evolve over the course of a melt season. We use steady‐state calculations as well as numerical simulations to understand how alluvial bed erosion alters overdeepenings. Numerical results from a modified form of the Spring‐Hutter equations show behaviors that cannot be inferred from either local or steady‐state calculations. In general, opposition of surface and bed slopes lessens sediment transport regardless of ice accretion from glaciohydraulic supercooling. Drainage efficiency strongly affects erosion and deposition rates. Results show characteristic behaviors of flow through overdeepenings such as overpressured water systems and accretion rates compatible with field measurements. Simulations that start with overdeepened glacier configurations progress out of a freezing regime where glaciohydraulic supercooling occurs. This progression indicates that glacier hydrology is more strongly affected by erosion and deposition than by freezing from glaciohydraulic supercooling. We discuss how this outcome affects glacier erosion and sediment transport under modern and past ice sheets.

Paleo ice flow and subglacial meltwater dynamics in Pine Island Bay, West Antarctica

Abstract. Increasing evidence for an elaborate subglacial drainage network underneath modern Antarctic ice sheets suggests that basal meltwater has an important influence on ice stream flow. Swath bathymetry surveys from previously glaciated continental margins display morphological features indicative of subglacial meltwater flow in inner shelf areas of some paleo ice stream troughs. Over the last few years several expeditions to the eastern Amundsen Sea embayment (West Antarctica) have investigated the paleo ice streams that extended from the Pine Island and Thwaites glaciers. A compilation of high-resolution swath bathymetry data from inner Pine Island Bay reveals details of a rough seabed topography including several deep channels that connect a series of basins. This complex basin and channel network is indicative of meltwater flow beneath the paleo-Pine Island and Thwaites ice streams, along with substantial subglacial water inflow from the east. This meltwater could have enhanced ice flow over the rough bedrock topography. Meltwater features diminish with the onset of linear features north of the basins. Similar features have previously been observed in several other areas, including the Dotson-Getz Trough (western Amundsen Sea embayment) and Marguerite Bay (SW Antarctic Peninsula), suggesting that these features may be widespread around the Antarctic margin and that subglacial meltwater drainage played a major role in past ice-sheet dynamics.

Instabilities on Alpine temperate glaciers: new insights arising from the numerical modelling of Allalingletscher (Valais, Switzerland)

Abstract. The processes leading to a glacier instability depend on the thermal properties of the contact between the glacier and its bedrock. Assessing the stability of temperate glacier (i.e. the glacier can slide on its bedrock) remains problematic. In order to scrutinize in more detail the processes governing such "sliding" instabilities, a numerical model designed to investigate gravitational instabilities in heterogeneous media was further developed to account for the presence of water at the interface between the bedrock and the glacier for Allalingletscher. This model made it possible to account for various geometric configurations, interaction between sliding and tension cracking and water flow at the bedrock. We could show that both a critical geometrical configuration of the glacier tongue and the existence of a distributed subglacial drainage network were the main causes of the Allalingletscher catastrophic break-off. Moreover, the analysis of the modelling results diagnosed the phenomenon of recoupling of the glacier to its bed followed by a pulse of subglacial water flow as a potential new precursory sign of the final break-off in 1965. This model casts a gleam of hope for a better understanding of the ultimate rupture process resulting from such glacier sliding instabilities.

Hydrological control of stream water chemistry in a glacial catchment (Damma Glacier, Switzerland)

The temporal and spatial controls of stream water chemistry in a small, granitic, glacial catchment were investigated in conjunction with high-resolution hydrological and meteorological measurements. Significant systematic seasonal and diurnal variations were observed in the stream water chemistry, which were not caused by the mixing of water draining different lithologies. A hydrological model (ALPINE3D, Lehning et al., 2006) was used to calculate the relative contributions of the principal water sources, snow melt and ice melt, throughout the period of this study. Pronounced seasonal minima in δ 18O and 87Sr/ 86Sr were attributed to spring snow melt. Clear changes in X/Si ratios (X = Ca, Mg, K, Na, Sr) between summer and winter were observed. These changes are interpreted to reflect seasonal changes in the average residence time of water in the sub-glacial drainage network with short residence times in summer, when the discharge was greatest, leading to high X/Si ratios, and long residence times in winter, when the discharge was lowest, resulting in low X/Si ratios. This study shows that the time dependent stoichiometry of cation to Si ratios in glacial stream water (and likely all catchments) strongly depends on the hydrological state of the catchment at the time of sampling. Annual fluxes based on spot samples varied by a factor of six depending on the time of year in which the sample was collected, highlighting the importance of long-term catchment monitoring. An improved understanding of the spatial and temporal controls acting on stream water chemistry will allow silicate weathering processes to be more precisely quantified.

Assessing the relative efficiency of fluvial and glacial erosion through simulation of fluvial landscapes

The relative rates of erosion by rivers and glaciers, and the topographic effects of these two different styles of erosion, remain outstanding problems in geomorphology. We use a quantitative description of local fluvial landscapes to estimate how glaciated landscapes might look now had glaciers not developed. This indicates the landscape modification attributable to glacial erosion. We present examples from the Sierra Nevada, California and the Sangre de Cristo Range, Colorado. In smaller drainage basins, glacial modification is focussed above the mean Quaternary equilibrium line altitude (ELA), where both ridgelines and valley floors have been lowered as a consequence of glaciation. At lower elevations, small glaciers have apparently widened valleys without incising the valley floor beyond what a river would have. This may reflect the short residence time of the glaciers at their full extent, or differences in the subglacial drainage network between the glacier margins and the thalweg. In larger drainage basins, the pattern of glacial erosion is dramatically different. Here, the glaciers have modified longitudinal profiles, as well as valley cross sections, far below the mean Quaternary ELA. Possible causes of this difference in the larger basins include the larger accumulation area, greater shading of the valley floor, longer residence times for ice at its full extent, and the influence of the shallower valley slope prior to glaciation on the subsequent glacier and subglacial drainage conditions.

The drainage pattern of Hansbreen and Werenskioldbreen, two polythermal glaciers in Svalbard

To improve our understanding of Svalbard-type polythermal glacier drainage, hydraulic geometry models of the subglacial hydrology of two contrasting glaciers in Svalbard have been constructed. The models are tested against a uniquely long and rich set of field observations spanning 45 years. Digital elevation models (DEMs) were constructed from bedrock data measured with ground penetrating radar and surface data of two medium-sized polythermal glaciers, Hansbreen and Werenskioldbreen, in south-west Spitsbergen. Hansbreen has a low angle bed with over-deepenings and a calving front, while Werenskioldbreen has steeper bed and terminates on land. Together they are representative of many Svalbard glaciers. The DEMs were used to derive maps of hydraulic potential and subglacial drainage networks. Validation of the models was done using field observations including location mapping and speleological exploration of active moulins, positions of main river outflows, dyetracing and water chemistry studies, and observations of water pressure inside moulins. Results suggest that the water pressure is generally close to ice overburden pressure but varies greatly depending on local conditions such as bed location, the thickness of cold ice layer, the thickness of the glacier and seasonal changes in meltwater input.

Evidence for seasonal subglacial outburst events at a polythermal glacier, Finsterwalderbreen, Svalbard

AbstractBulk runoff and meteorological data suggest the occurrence of two meltwater outburst events at Finsterwalderbreen, Svalbard, during the 1995 and 1999 melt seasons. Increased bulk meltwater concentrations of Cl− during the outbursts indicate the release of snowmelt from storage. Bulk meltwater hydrochemical data and suspended sediment concentrations suggest that this snowmelt accessed a chemical weathering environment characterized by high rock:water ratios and long rock–water contact times. This is consistent with a subglacial origin. The trigger for both the 1995 and 1999 outbursts is believed to be high rates of surface meltwater production and the oversupply of meltwater to areas of the glacier bed that were at the pressure melting point, but which were unconnected to the main subglacial drainage network. An increase in subglacial water pressure to above the overburden pressure lead to the forcing of a hydrological connection between the expanding subglacial reservoir and the ice‐marginal channelized system. The purging of ice blocks from the glacier during the outbursts may indicate the breach of an ice dam during connection. Although subglacial meltwater issued continually from the glacier terminus via a subglacial upwelling during both melt seasons, field observations showed outburst meltwaters were released solely via an ice‐marginal channel. It is possible that outburst events are a seasonal phenomenon at this glacier and reflect the periodic drainage of meltwaters from the same subglacial reservoir from year to year. However, the location of this reservoir is uncertain. A 100 m high bedrock ridge traverses the glacier 6·5 km from its terminus. The overdeepened area up‐glacier from this is the most probable site for subglacial meltwater accumulation. Copyright © 2001 John Wiley & Sons, Ltd.

Inversion of borehole-response test data for estimation of subglacial hydraulic properties

AbstractResponse tests are widely used in ground-water studies to assess the hydraulic properties of sub-surface water-flow systems. The simplicity of such tests also makes them attractive for investigation of subglacial hydraulic conditions. This paper describes a systematic, quantitative approach to the analysis of borehole-response test data. The approach uses the theoretical model of Stone and Clarke (1993), which describes water motion in a coupled borehole—subglacial flow system; this framework provides the basis for an inversion scheme that is focused on quantifying physical properties of the basal-flow system, as it is characterized in the theoretical model. The inversion procedure was applied to response-test data from Trapridge Glacier, Yukon Territory, Canada. Results of the inversions suggest that the subglacial drainage network can be described as a confined layer comprising coarse-sand-to fine-gravel-sized sediments, having a thickness of 0.1 – 0.3 m, and a hydraulic conductivity of about 5 × 10−4ms−1. Based on the water-drainage rates from boreholes, as they connect with the subglacial water-flow system, specific storage of the sediment layer was estimated to be approximately 1 × 10−4m−1. Further consideration of subglacial water-flow conditions suggests that connection drainage test results may tend to underestimate specific storage of the overall glacier substrate.

Inversion of borehole-response test data for estimation of subglacial hydraulic properties

AbstractResponse tests are widely used in ground-water studies to assess the hydraulic properties of sub-surface water-flow systems. The simplicity of such tests also makes them attractive for investigation of subglacial hydraulic conditions. This paper describes a systematic, quantitative approach to the analysis of borehole-response test data. The approach uses the theoretical model of Stone and Clarke (1993), which describes water motion in a coupled borehole—subglacial flow system; this framework provides the basis for an inversion scheme that is focused on quantifying physical properties of the basal-flow system, as it is characterized in the theoretical model. The inversion procedure was applied to response-test data from Trapridge Glacier, Yukon Territory, Canada. Results of the inversions suggest that the subglacial drainage network can be described as a confined layer comprising coarse-sand-to fine-gravel-sized sediments, having a thickness of 0.1 – 0.3 m, and a hydraulic conductivity of about 5 × 10−4ms−1. Based on the water-drainage rates from boreholes, as they connect with the subglacial water-flow system, specific storage of the sediment layer was estimated to be approximately 1 × 10−4m−1. Further consideration of subglacial water-flow conditions suggests that connection drainage test results may tend to underestimate specific storage of the overall glacier substrate.

Glacial and glaciofluvial deposits in the interlobate areas of the Scandinavian ice sheet

Rapid flow within ice streams took place in a broad marginal zone of the Scandinavian Ice Sheet during deglaciation. Ice streams and interlobate zones have been characterised by analysis of glacial flow patterns, and examination of sedimentary features of interlobate glacial and glaciofluvial deposits. Reconstructed glaciodynamic systems are compared with ice streams of modem ice sheets. Wide interstream areas of inactive ice were left between some adjacent ice streams where they separated during the retreat of the margin. These areas are characterised by morainic hummocks composed of both till and glaciofluvial sediment, and sections show indications of subglacial deformation. Observations from Fennoscandia suggest that these interstream areas were sites of net accumulation of glacial and glaciofluvial sediment during deglaciation. Glaciofluvial sediment was deposited in subglacial tunnels or cavities simultaneously with till deformation and deposition resulting from subglacial melting. Substantial glaciofluvial complexes were formed in narrow interlobate joints where adjacent ice lobes coalesced. A case study made in southern Finland shows converging flow patterns of two ice lobes and intervening glaciofluvial deposits. Concentration of meltwater in interlobate zones was a consequence of ice-sheet configuration. Thinner ice in interlobate zones caused convergence of supraglacial and subglacial drainage. The crevassed and strain-softened ice may also have allowed surface meltwater to penetrate down to the ice/bed interface close to the ice margin. Steep ice-velocity gradients in interlobate zones would have increased subglacial melting rates, and this environment was favourable for establishment of a subglacial drainage network and deposition of glaciofluvial sediment.

IMPACT OF SUBGLACIAL GEOTHERMAL ACTIVITY ON MELTWATER QUALITY IN THE JÖKULSÁ Á SÓLHEIMASANDI SYSTEM, SOUTHERN ICELAND

The influence of subglacial geothermal activity on the hydrochemistry of the Jokulsa a Solheimasandi glacial meltwater river, south Iceland, is discussed. A radio echosounding and Global Positioning System survey of south-west Myrdalsjokull, the parent ice-cap of the valley glacier Solheimajokull, establishes the geometry and position of a subglacial caldera. A cauldron in the ice-cap surface at the basin head is also defined, signifying one location of geothermally driven ablation processes. Background H 2 S concentrations for the Jokulsa meltwaters in summer 1989 show that leakage of geothermal fluids into the glacial drainage network takes place throughout the melt season. Chemical geothermometry (Na + /K + ratio) applied to the bulk meltwaters tentatively suggests that the subglacial geothermal area is a high-temperature field with a reservoir temperature of 289-304°C. A major event of enhanced geothermal fluid injection was also detected. Against a background of an apparently warming geothermal reservoir, the event began on Julian day 205 (24 July) with a burst of subglacial seismic activity. Meltwater hydrochemical perturbations followed on day 209 and peaked on day 213, finally leading to a sudden and significant increase in flow on day 214. The hydrochemical excursions were characterized by strong peaks in meltwater H 2 S, SO 2- 4 and total carbonate concentrations, transient decreases in pH, small increases in Ca 2+ and Mg 2+ and sustained increases in electrical conductivity. The event may relate to temporary invigoration of the subglacial convective hydrothermal circulation, seismic disturbance of patterns of groundwater flow and geothermal fluid recruitment to the subglacial drainage network, or a cyclic 'sweeping out' of the geothermal zone by the annual wave of descending groundwater. Time lags between seismic events and meltwater electrical conductivity responses suggest mean and maximum intraglacial throughflow velocities of 0.032-0.132 ms -1 , respectively, consistent with a distributed drainage system beneath Solheimajokull. Because increases in flow follow hydrochemical perturbations, the potential exists to use meltwater hydrochemistry to forecast geothermally driven flood events in such environments.