Englacial and subglacial water flow at Skálafellsjökull,Iceland derived from ground penetrating radar,in situ Glacsweb probe and borehole water level measurements

We reconstruct englacial and subglacial drainage at Skálafellsjökull, Iceland, using ground penetrating radar (GPR) common offset surveys, borehole studies and Glacsweb probe data. We find that englacial water is not stored within the glacier (water content ~0–0.3%). Instead, the glacier is mostly impermeable and meltwater is able to pass quickly through the main body of the glacier via crevasses and moulins. Once at the glacier bed, water is stored within a thin (1 m) layer of debris‐rich basal ice (2% water content) and the till. The hydraulic potential mapped across the survey area indicates that when water pressures are high (most of the year), water flows parallel to the margin, and emerges 3 km down glacier at an outlet tongue. GPR data indicates that these flow pathways may have formed a series of braided channels. We show that this glacier has a very low water‐storage capacity, but an efficient englacial drainage network for transferring water to the glacier bed and, therefore, it has the potential to respond rapidly to changes in melt‐water inputs. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Hydrology and dynamics of a polythermal (mostly cold) High Arctic glacier

To improve our understanding of the interactions between hydrology and dynamics in mostly cold glaciers (in which water flow is limited by thermal regime), we analyse short‐term (every two days) variations in glacier flow in the ablation zone of polythermal John Evans Glacier, High Arctic Canada. We monitor the spatial and temporal propagation of high‐velocity events, and examine their impacts upon supraglacial drainage processes and evolving subglacial drainage system structure. Each year, in response to the rapid establishment of supraglacial–subglacial drainage connections in the mid‐ablation zone, a ‘spring event’ of high horizontal surface velocities and high residual vertical motion propagates downglacier over two to four days from the mid‐ablation zone to the terminus. Subsequently, horizontal velocities fall relative to the spring event but remain higher than over winter, reflecting channelization of subglacial drainage but continued supraglacial meltwater forcing. Further transient high‐velocity events occur later in each melt season in response to melt‐induced rising supraglacial meltwater inputs to the glacier bed, but the dynamic response of the glacier contrasts with that recorded during the spring event, with the degree of spatial propagation a function of the degree to which the subglacial drainage system has become channelized. Copyright © 2006 John Wiley & Sons, Ltd.     

Initial results from a distributed,physically based model of glacier hydrology

This paper describes the development and testing of a distributed, physically based model of glacier hydrology. The model is used to investigate the behaviour of the hydrological system of Haut Glacier d'Arolla, Valais, Switzerland. The model has an hourly time-step and three main components: a surface energy balance submodel, a surface flow routing submodel and a subglacial hydrology submodel. The energy balance submodel is used to calculate meltwater production over the entire glacier surface. The surface routing submodel routes meltwater over the glacier surface from where it is produced to where it either enters the subglacial hydrological system via moulins or runs off the glacier surface. The subglacial hydrology submodel calculates water flow in a network of conduits, which can evolve over the course of a melt season simulation in response to changing meltwater inputs. The main model inputs are a digital elevation model of the glacier surface and its surrounding topography, start-of-season snow depth distribution data and meteorological data. Model performance is evaluated by comparing predictions with field measurements of proglacial stream discharge, subglacial water pressure (measured in a borehole drilled to the glacier bed) and water velocities inferred from dye tracer tests. The model performs best in comparison with the measured proglacial stream discharges, but some of the substantial features of the other two records are also reproduced. In particular, the model results show the high amplitude water pressure cycles observed in the borehole in the mid-melt season and the complex velocity/discharge hysteresis cycles observed in dye tracer tests. The results show that to model outflow hydrographs from glacierized catchments effectively, it is necessary to simulate spatial and temporal variations in surface melt rates, the delaying effect of the surface snowpack and the configuration of the subglacial drainage system itself. The model's ability to predict detailed spatial and temporal patterns of subglacial water pressures and velocities should make it a valuable tool for aiding the understanding of glacier dynamics and hydrochemistry. © 1998 John Wiley & Sons, Ltd.     

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

Bulk 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.     

Seasonal changes in the morphology of the subglacial drainage system,Haut Glacier d'Arolla,Switzerland

Dye tracing techniques were used to investigate the glacier-wide pattern of change in the englacial/subglacial drainage system of Haut Glacier d'Arolla during the ablation seasons of 1990 and 1991. Analysis of breakthrough curve characteristics indicate that over the course of a melt season, a system of major channels developed by headward growth at the expense of a hydraulically inefficient distributed system. By the end of the melt season, this channel system extended at least 3·3 km from the snout of the 4 km long glacier and drained the bulk of supraglacially derived meltwater passing through the glacier. The upper limit of the channel system closely followed the retreating snowline up-glacier. Rates of headward channel growth reached c. 65 m d⁻¹, although these rates decreased in the upper 1 km of the glacier where snowline retreat exposed a patchy firn aquifer. It appears that the removal of snow (with its high albedo and significant water storage capacity) from the glacier surface resulted in a dramatic increase in the volume of runoff into moulins, and in the peakedness of daily runoff cycles. This induced transient high water pressures within the distributed drainage system, which caused it to evolve rapidly into a channelised system. It is therefore likely that, at a local scale, channel growth occurred down-glacier from moulins, and that the overall up-glacier-directed pattern of channel formation was caused by the retreating snowline exposing new moulins and crevasses to inputs of ice-derived meltwater. Damping of diurnal melt inputs by storage in the firm aquifer accounts for the slowing of channel growth in the upper glacier. © 1998 John Wiley & Sons, Ltd.     

Meltwater discharge through the subglacial bed and its land‐forming consequences from numerical experiments in the Polish lowland during the last glaciation

Numerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in NW Poland: primarily its flow pattern, velocities and fluxes. We have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady‐state and transient conditions. The results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin. All model runs show water discharge at the ice forefield driven by ice‐sheet‐thickness‐modulated, down‐ice‐decreasing hydraulic heads. In relation to non‐glacial times, the transient 3D model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca. 90 m below the ice–bed interface and up to 40 km in front of the glacier. Comparison with empirical data suggests that, depending on the model run, only between 5 and 24% of the meltwater formed at the ice sole drained through the bed as groundwater. This is consistent with field observations documenting abundant occurrence of tunnel valleys, indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system. Groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely. In these areas the relief shows distinct palaeo‐ice lobes, indicating fast ice flow, possibly triggered by the undrained water at the ice–bed interface. Owing to the abundance of low‐permeability strata in the bed, the simulated groundwater flow depth is less than ca. 200 m. Copyright © 2009 John Wiley & Sons, Ltd.     

Sub-annual moraine formation at an active temperate Icelandic glacier

This paper presents detailed geomorphological and sedimentological investigations of small recessional moraines at Fjallsjökull, an active temperate outlet of Öræfajökull, southeast Iceland. The moraines are characterized by striking sawtooth or hairpin planforms, which are locally superimposed, giving rise to a complex spatial pattern. We recognize two distinct populations of moraines, namely a group of relatively prominent moraine ridges (mean height ~1.2 m) and a group of comparatively low-relief moraines (mean height ~0.4 m). These two groups often occur in sets/systems, comprising one pronounced outer ridge and several inset smaller moraines. Using a representative subsample of the moraines, we establish that they form by either (i) submarginal deformation and squeezing of subglacial till or (ii) pushing of extruded tills. Locally, proglacial (glaciofluvial) sediments are also incorporated within the moraines during pushing. For the first time, to our knowledge, we demonstrate categorically that these moraines formed sub-annually using repeat uncrewed aerial vehicle (UAV) imagery. We present a conceptual model for sub-annual moraine formation at Fjallsjökull that proposes the sawtooth moraine sequence comprises (i) sets of small squeeze moraines formed during melt-driven squeeze events and (ii) larger push moraines formed during winter re-advances. We suggest the development of this process-form regime is linked to a combination of elevated temperatures, high surface meltwater fluxes to the bed and emerging basal topography (a depositional overdeepening). These factors result in highly saturated subglacial sediments and high porewater pressures, which induces submarginal deformation and ice-marginal squeezing during the melt season. Strong glacier recession during the summer, driven by elevated temperatures, allows several squeeze moraines to be emplaced. This process-form regime may be characteristic of active temperate glaciers receding into overdeepenings during phases of elevated temperatures, especially where their englacial drainage systems allow efficient transfer of surface meltwater to the glacier bed near the snout margin. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Seasonal reorganization of subglacial drainage inferred from measurements in boreholes

The effect of the formation of a major subglacial drainage channel on the behaviour of the subglacial drainage system of Haut Glacier d'Arolla, Switzerland, was investigated using measurements of borehole water level and the electrical conductivity and turbidity of basal meltwaters. Electrical conductivity profiles were also measured within borehole water columns to identify the water sources driving water level changes, and to determine patterns of water circulation in boreholes. Prior to channel formation, boreholes showed idiosyncratic and poorly coordinated behaviour. Diurnal water level fluctuations were small and driven by supraglacial/englacial water inputs, even when boreholes were connected to a subglacial drainage system. This system appeared to consist of hydraulically impermeable patches interspersed with storage spaces, and transmitted a very low water flux. Drainage reorganization, which occurred around 31 July, 1993, in response to rapidly rising meltwater and rainfall inputs, seems to have involved the creation of a connection between an incipient channel and a well-established channelized system located further down-glacier. Once a major channel existed within the area of the borehole array, borehole water level fluctuations were forced by discharge-related changes in channel water pressure, although a diversity of responses was observed. These included (i) synchronous, (ii) damped and lagged, (iii) inverse, and (iv) alternating inverse/lagged responses. Synchronous responses occurred in boreholes connected directly to the channel, while damped and lagged responses occurred in boreholes connected to it by a more resistive drainage system. Pressure variations within the channel resulted in diurnal transfer of mechanical support for the ice overburden between connected and unconnected areas of the bed, producing inverse and alternating patterns of water level response. © 1998 John Wiley & Sons, Ltd.     

Excavation of subglacial bedrock channels by seasonal meltwater flow

Subglacial water flow drives the excavation of a variety of bedrock channels including tunnel valleys and inner gorges. Subglacial floods of various magnitudes – events occurring once per year or less frequently with discharges larger than a few hundred cubic metres per second – are often invoked to explain the erosive power of subglacial water flow. In this study we examine whether subglacial floods are necessary to carve bedrock channels, or if more frequent melt season events (e.g. daily production of meltwater) can explain the formation of substantial bedrock channels over a glacial cycle. We use a one‐dimensional numerical model of bedrock erosion by subglacial meltwater, where water flows through interacting distributed and channelized drainage systems. The shear stresses produced drive bedrock erosion by bed‐ and suspended‐load abrasion. We show that seasonal meltwater discharge can incise an incipient bedrock channel a few tens of centimetres deep and several metres wide, assuming abrasion is the only mechanism of erosion, a particle size of D=256 mm and a prescribed sediment supply per unit width. Using the same sediment characteristics, flood flows yield wider but significantly shallower bedrock channels than seasonal meltwater flows. Furthermore, the smaller the shear stresses produced by a flood, the deeper the bedrock channel. Shear stresses produced by seasonal meltwater are sufficient to readily transport boulders as bedload. Larger flows produce greater shear stresses and the sediment is carried in suspension, which produces fewer contacts with the bed and less erosion. We demonstrate that seasonal meltwater discharge can excavate bedrock volumes commensurate with channels several tens of metres to a few hundred metres wide and several tens of metres deep over several thousand years. Such simulated channels are commensurate with published observations of tunnel valleys and inner gorges. Copyright © 2018 John Wiley & Sons, Ltd.     

Basal sediment evacuation by subglacial meltwater: suspended sediment transport from Haut Glacier d'Arolla,Switzerland

Proglacial suspended sediment transport was monitored at Haut Glacier d'Arolla, Switzerland, during the 1998 melt season to investigate the mechanisms of basal sediment evacuation by subglacial meltwater. Sub‐seasonal changes in relationships between suspended sediment transport and discharge demonstrate that the structure and hydraulics of the subglacial drainage system critically influenced how basal sediment was accessed and entrained. Under hydraulically inefficient subglacial drainage at the start of the melt season, sediment availability was generally high but sediment transport increased relatively slowly with discharge. Later in the melt season, sediment transport increased more rapidly with discharge as subglacial meltwater became confined to a spatially limited network of channels following removal of the seasonal snowpack from the ablation area. Flow capacity is inferred to have increased more rapidly with discharge within subglacial channels because rapid changes in discharge during highly peaked diurnal runoff cycles are likely to have been accommodated largely by changes in flow velocity. Basal sediment availability declined during channelization but increased throughout the remainder of the monitored period, resulting in very efficient basal sediment evacuation over the peak of the melt season. Increased basal sediment availability during the summer appears to have been linked to high diurnal water pressure variation within subglacial channels inferred from the strong increase in flow velocity with discharge. Basal sediment availability therefore appears likely to have been increased by (1) enhanced local ice‐bed separation leading to extra‐channel flow excursions and[sol ]or (2) the deformation of basal sediment towards low‐pressure channels due to a strong diurnally reversing hydraulic gradient between channels and areas of hydraulically less‐efficient drainage. Copyright © 2005 John Wiley & Sons, Ltd.     

Till deposition by glacier submarginal,incremental thickening

Macro‐ and micro‐scale sedimentological analyses of recently deposited tills and complex push/squeeze moraines on the forelands of Icelandic glaciers and in a stacked till sequence at the former Younger Dryas margin of the Loch Lomond glacier lobe in Scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment. Where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris‐rich basal ice by cumulative freeze‐on processes, glacier submarginal processes are dictated by seasonal cycles of refreezing and melt‐out of tills advected from up‐ice by a combination of lodgement, deformation and ice keel and clast ploughing. Although individual till layers may display typical A and B horizon deformation characteristics, the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record. At the macro‐scale, Icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings; very strong fabrics typify unequivocally lodged clasts. Despite previous interpretations of these tills as subglacial deforming layers, micro‐morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes, and water escape and sediment flowage features are widespread. A model of submarginal incremental thickening is presented as an explanation of these data, involving till slab emplacement over several seasonal cycles. Each cycle involves: (1) late summer subglacial lodgement, bedrock and sediment plucking, subglacial deformation and ice keel ploughing; (2) early winter freeze‐on of subglacial sediment to the thin outer snout; (3) late winter readvance and failure along a decollement plane within the till, resulting in the carriage of till onto the proximal side of the previous year's push moraine; (4) early summer melt‐out of the till slab, initiating porewater migration, water escape and sediment flow and extrusion. Repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements. Copyright © 2005 John Wiley & Sons, Ltd.     

Hydrochemistry of meltwaters draining a polythermal‐based,high‐Arctic glacier,south Svalbard: II. Winter and early Spring

The hydrochemistry of naled and upwelling water sampled from the forefields of Finsterwalderbreen, Svalbard, during spring are used for the first time to infer the hydrology of overwinter meltwaters at a polythermal‐based glacier. Hydrochemical variations in naled are explained in terms of different water sources and their chemical alteration during freezing. Two water sources to naled are identified: surficially routed snowmelt and subglacial water. Naled that results from the freezing of the former is enriched in atmospherically derived ions such as Na⁺ and Cl⁻, and is believed to be formed during winter warm periods. Naled of subglacial origin contains relatively high proportions of crustally derived solute. It reflects the freezing of subglacial meltwaters that continue to issue from a subterranean upwellling during winter. An increasing dominance of SO²⁻₄ Mg²⁺, Na⁺ and Cl⁻ in subglacial naled with increasing distance from the upwelling reflects the progressive freezing of this water body and the associated removal of Ca²⁺ and HCO by calcite precipitation. These spatial trends are accentuated by the leaching of soluble ions from the naled close to its source by subsequent upwelling waters. The chemistry of spring upwelling waters, also of subglacial origin, strongly reflects this process. Meltwater produced by geothermal heating of glacier basal ice is believed to be the principal source of water to the subglacial drainage system during winter. Solute acquisition by this meltwater is limited by a scarcity of proton suppliers. Evolution of this dilute meltwater carries an imprint of ion exchange processes. Some stored subglacial water from the end of the previous ablation season may supplement the basal meltwater component in early winter. Copyright © 2000 John Wiley & Sons, Ltd.     

Subglacial drainage system structure and morphology of Brewster Glacier,New Zealand

A global positioning system and ground penetrating radar surveys is used to produce digital elevation models of the surface and bed of Brewster Glacier. These are used to derive maps of subglacial hydraulic potential and drainage system structure using three different assumptions about the subglacial water pressure (P_w): (i) P_w = ice overburden; (ii) P_w = half ice overburden; (iii) P_w = atmospheric. Additionally, 16 dye‐tracing experiments at 12 locations were performed through a summer melt season. Dye return curve shape, together with calculations of transit velocity, dispersivity and storage, are used to infer the likely morphology of the subglacial drainage system. Taken together, the data indicate that the glacier is underlain by a channelised but hydraulically inefficient drainage system in the early summer in which water pressures are close to ice overburden. By mid‐summer, water pressures are closer to half‐ice overburden and the channelised drainage system is more hydraulically efficient. Surface streams that enter the glacier close to the location of major subglacial drainage pathways are routed quickly to the channels and then to the glacier snout. Streams that enter the glacier further away from the drainage pathways are routed slowly to the channels and then to the snout because they first flow through a distributed drainage system. Copyright © 2008 John Wiley & Sons, Ltd.     

The hydrochemistry of Lake Vostok and the potential for life in Antarctic subglacial lakes

Our understanding of Lake Vostok, the huge subglacial lake beneath the East Antarctic Ice Sheet, has improved recently through the identification of key physical and chemical interactions between the ice sheet and the lake. The north of the lake, where the overlying ice sheet is thickest, is characterized by subglacial melting, whereas freezing of lake water occurs in the south, resulting in ～210 m of ice accretion to the underside of the ice sheet. The accreted ice contains lower concentrations of the impurities normally found in glacier ice, suggesting a net transfer of material from meltwater into the lake. The small numbers of microbes found so far within the accreted ice have DNA profiles similar to those of contemporary surface microbes. Microbiologists expect, however, that Lake Vostok, and other subglacial lakes, will harbour unique species, particularly within the deeper waters and associated sediments. The extreme environments of subglacial lakes are characterized by high pressures, low temperatures, permanent darkness, limited nutrient availability, and oxygen concentrations that are derived from the ice that provides the meltwater. Copyright © 2002 John Wiley & Sons, Ltd.     

Subglacial drumlins and englacial fractures at the surge-type glacier,Múlajökull,Iceland

The interaction between drumlins and overriding glacier ice is not well studied, largely due to the difficulty of identifying and accessing suitable active subglacial environments. The surge-type glacier Múlajökull, in central Iceland, overlies a known field of actively forming drumlins and therefore provides a rare opportunity to investigate the englacial structures that have developed in association with ice flow over the subglacial drumlins. In this study detailed ground-penetrating radar surveys are combined with field observations to identify clear sets of up-glacier and down-glacier dipping fractures at Múlajökull's margin. These are interpreted as conjugate shear planes or P- and R-type Reidel shears that developed and filled with saturated sediment derived from the glacier bed, during a previous surge. The fracture sets exhibit focused spatial distributions that are influenced by the subglacial topography. In particular, down-glacier dipping fractures are strongly focused over drumlin stoss slopes. These fractures, although well developed at depth, were mostly unable to transmit basal water and sediment up to the glacier surface during the surge cycle. In contrast, up-glacier dipping fractures formed over drumlin lee sides and in more gently sloping swales, and more frequently connected to the glacier surface, providing a pathway for the evacuation of basal water and water-saturated sediment. The study suggests that the subglacial drumlins under Múlajökull's margin have influenced the nature and distribution of englacial fractures, which could potentially contribute to spatial variations in basal water pressure during a surge. BGS © UKRI 2018     

Subglacial sediment production and snout marginal ice uplift during the late ablation season of a temperate valley glacier

Sediment export from glaciated basins involves complex interactions between ice flow, basal erosion and sediment transfer in subglacial and proglacial streams. In particular, we know very little about the processes associated with sediment transfer by subglacial streams. The Haut Glacier d'Arolla (VS, Switzerland) was investigated during the summer melt season of 2015. LiDAR survey revealed positive surface changes in the ablation zone, indicating glacier uplift, at the end of the morning during the period of peak ablation. Instream measures of sediment transport showed that suspended load and bedload responded differently to diurnal flow variability. Suspended load depended on the availability of fine material whereas bedload depended mainly on the competence of the flow. Interpretation of these results allowed development of a conceptual model of subglacial sediment transport dynamics. It is based upon the mechanisms of clogging (deposition) and flushing (transport/erosion) in sub-glacial channels as forced by diurnal flow variability. Through the melt season, the glacier hydrological response evolves from being buffered by glacier snow cover with a poorly developed subglacial drainage system to being dominated by more rapid ice melt with a more hydraulically efficient subglacial channel system. The resultant changes in the shape of diurnal discharge hydrographs, and notably higher peak flows and lower base flows, causes sediment transport to become discontinuous, with overnight clogging and late morning flushing of subglacial channels. Overnight clogging may be sufficient to reduce subglacial channel size, creating temporarily pressurized flow and lateral transfer of water away from the subglacial channels, leading to the late morning glacier surface uplift. However, without further data, we cannot exclude other hypotheses for the uplift. © 2018 John Wiley & Sons, Ltd.     

ISOTOPIC AND IONIC VARIATIONS IN GLACIER RIVER WATER DURING THREE CONTRASTING ABLATION SEASONS

The significance of the baseflow component of glacier river discharge in summer varies with geographical location, altitude, glacier geometry and glacier size. Baseflow is maintained by meltwater generated above the transient equilibrium line and by water released from temporary storage on, in or beneath the glacier. At the Norwegian glacier Austre Okstindbreen, where precipitation is generally high throughout the year and the summers are cool and wet, observations in three successive, but contrasting, years have shown that Na⁺ ion concentrations in the glacier river water are influenced strongly by the amount of snowmelt. This itself depends on the preceding winter conditions, which determine the amount of accumulation, and on the current summer's weather. The efficiency of the glacier's drainage systems depends on the general progress of summer ablation. The speed with which the systems develop influences ion provision from subglacial sources. Ca²⁺ ion concentrations are largely determined by subglacial conditions. Oxygen isotope variations in glacier river water reflect the relative contributions made to total discharge by snow meltwater and other sources; the composition of the snow cover, which is a function of winter temperatures, has a strong influence. Ice meltwater has low isotopic variability, but the isotopic composition of rainfall varies markedly. A simple model of mixing of englacial and subglacial waters, each of a constant composition, cannot be applied to a high-latitude glacier of the size and altitudinal range of Austre Okstindbreen.     

The hydrochemistry of runoff from a ‘cold-based’ glacier in the High Arctic (Scott Turnerbreen,Svalbard)

There are still relatively few hydrochemical studies of glacial runoff and meltwater routing from the high latitudes, where non-temperate glacier ice is frequently encountered. Representative samples of glacier meltwater were obtained from Scott Turnerbreen, a ‘cold-based’ glacier at 78° N in the Norwegian high Arctic archipelago of Svalbard, during the 1993 melt season and analysed for major ion chemistry. Laboratory dissolution experiments were also conducted, using suspended sediment from the runoff. Significant concentrations of crustal weathering derived SO²⁻₄ are present in the runoff, which is characterized by high ratios of SO²⁻₄: (SO²⁻₄+HCO⁻₃) and high p(CO₂). Meltwater is not routed subglacially, but flows to the glacier terminus through subaerial, ice marginal channels, and partly flows through a proglacial icing, containing highly concentrated interstitial waters, immediately afront the terminus. The hydrochemistry of the runoff is controlled by: (1) seasonal variations in the input of solutes from snow- and icemelt; (2) proglacial solute acquisition from the icing; and (3) subaerial chemical weathering within saturated, ice-cored lateral moraine adjoining drainage channels at the glacier margins, sediment and concentrated pore water from which is entrained by flowing meltwater. Diurnal variations in solute concentration arise from the net effects of variable sediment pore water entrainment and dilution in the ice marginal streams. Explanation of the hydrochemistry of Scott Turnerbreen requires only one major subaerial flow path, the ice marginal channel system, in which seasonally varying inputs of concentrated snowmelt and dilute icemelt are modified by seepage or entrainment of concentrated pore waters from sediment in lateral moraine, and by concentrated interstitial waters from the proglacial icing, supplied by leaching, slow drainage at grain intersections or simple melting of the icing itself. The ice marginal channels are analogous neither to dilute supra/englacial nor to concentrated subglacial flow components. © 1998 John Wiley & Sons, Ltd.     

Flow separation on Zongo Glacier,Cordillera Real,Bolivia

Meltwaters collected from the proglacial stream escaping from Zongo Glacier (2·1 km²), Bolivia (16°S), have been monitored in order to analyse the internal drainage system of an Andean glacier. Electrical conductivity has been measured sporadically between February 1995 and March 1996, during 16 one-day field surveys, under various meteorological conditions in summer and winter. The mixing-model technique based on the electrical conductivity is used for a quantitative separation of discharge which is derived from continuous water level registration. Tracer experiments (mainly uranine dye and NaCl salt) have been carried out from March to June 1997 to obtain information about the internal drainage system. In the tropical Andes, accumulation only occurs in austral summer, whereas ablation occurs throughout the year and is higher during the accumulation season, between November and March. The assumptions involved in the use of mixing models for analysis of glacial drainage structure are applicable for tropical glaciers because glacial conduits do not suffer complete closure, and are permanently supplied by meltwaters, even in wintertime. Two components of discharge are separated: an englacial flow originating from surface meltwater which is routed without chemical enrichment, and offering low electrical conductivity; and a subglacial one routed in contact with bedrock or sediments showing high ionic concentrations. Electrical conductivity of meltwater varies diurnally, inversely to discharge fluctuations. According to this behaviour, total discharge is mainly formed by the englacial component. The drainage structures for englacial and subglacial flow have to be widely interconnected, as indicated by diurnal variations of the subglacial discharge. Comparison of hydrograph separation based on conductivity and on ¹⁸O isotope confirms that the subglacial flow is influenced by surface melting. A hydrograph separation of the subglacial flow is proposed, between a diurnal variable component, composed of water coming from the englacial network, and a base flow, which may vary seasonally. The dye tracing experiments confirm the drainage complexity of Zongo Glacier and demonstrate the interest of identifying three main drainage components. © 1998 John Wiley & Sons, Ltd.