Insights into subglacial processes inferred from the micromorphological analyses of complex diamicton stratigraphy near Illmensee‐Lichtenegg,Höchsten,Germany

Menzies, J. & Ellwanger, D. 2010: Insights into subglacial processes inferred from the micromorphological analyses of complex diamicton stratigraphy near Illmensee‐Lichtenegg, Höchsten, Germany. Boreas, 10.1111/j.1502‐3885.2010.00194.x. ISSN 0300‐9483. Investigations of a 30‐m‐high section of Pleistocene sediments at Illmensee‐Lichtenegg, Höchsten in Baden‐Württemberg provide detailed information on subglacial conditions beneath the Rhine Glacier outlet of the Alpine ice sheet in southern Germany. The sediment exposure extends from an upper cemented sand and gravel (Deckenschotter) into diamictic units that extend down to weathered Molasse bedrock. The exposure reveals sediments symptomatic of active syndepositional stress/strain processes ongoing beneath the ice sheet. Macrosedimentology reveals diamicton subfacies units and a strong uni‐direction of ice motion based on clast fabric analyses. At the microscale level, thin‐section analyses provide a substantially clearer picture of the dynamics of subglacial sediment deformation and till emplacement. Evidence based on detailed micromorphological analyses reveals microstructural strain and depositional markers that indicate a subglacial environment of ongoing soft bed deformation in which the diamictons can be readily identified as subglacial tills. Within this subglacial environment, distinct changes in pore‐water pressure and sediment rheology can be detected. These changes reveal fluctuating conditions of progressive, non‐pervasive deformation associated with rapid changes in effective stress and shear strain leading to till emplacement. This site, through the application of micromorphology, increases our understanding of localized subglacial conditions and till formation.     

Explosive rhyolite tuya formation: classic examples from Kerlingarfjöll,Iceland

Rhyolite eruptions in Iceland mostly take place at long-lived central volcanoes, examples of which are found associated with each of the present-day rift-zone ice caps. Subglacial eruptions at Kerlingarfjöll central volcano produced rhyolite tuyas that are notable for their exposures of early-erupted pyroclastic material. Observations from a number of these edifices are synthesised into a general model for explosive rhyolite tuya formation. Eruptions begin with violent phreatomagmatic explosions that generate massive tuff (mT), but the influence of water quickly declines, leading to the formation of massive lapilli-tuffs (mLT) containing magmatically-fragmented vesicular pumice and ash. These are deposited rapidly near the vent, probably by moist pyroclastic density currents, confined by ice but not within a meltwater lake. The explosive-effusive transition is controlled by the ascent rate and gas content of the magma. An unusual obsidian-rich massive lapilli-tuff lithofacies (omLT) is identified and interpreted as pyroclastic material that was intruded into gas-fluidised deposits at the explosive-effusive transition. The effusive phase of eruption involves the emplacement of intrusions and lava caps. Intrusions of lava into the early-erupted phreatomagmatic deposits are characterised by peperitic margins and the formation of hyaloclastite. Intrusions into stratigraphically higher levels of the pyroclastic material show more limited interaction with the host tephra and have microcrystalline cores. Large lava bodies with columnar-jointed margins cap the tuyas and have intrusive basal contacts with the tephras. The main influence of the ice is to confine the rhyolite eruptive products to immediately above the vent region. This is in contrast to subglacial basaltic tuya-forming eruptions, which are characterised by the formation of meltwater lakes, phreatomagmatic fragmentation and subaqueous deposition. The lack of meltwater storage may reduce the potential for large jökulhlaups.     

A new model for the evolution of table mountains: volcanological and petrological evidence from herdubreid and Herdubreidartögl volcanoes (Iceland)

We present a new model for the evolution of volcanic table mountains, based on volcanological and petrological studies of Herdubreid/Herdubreidartögl, an upper Pleistocene volcanic complex within the active Icelandic rift zone. The evolution of these table mountains is highly complex and influenced substantially by different eruptive environments as well as changing magma sources and volcanic and magmatic processes. Whereas current models imply entirely subglacial conditions and continuous compositional (“monogenetic”) evolution for these volcanoes, we subdivide their evolution into four eruptive periods characterized by different environments: (a) Subaerial lava flows erupted during an interglacial period. (b) Lavas and voluminous hydroclastic deposits formed in a lake during a waning period of the last glaciation. (c) Subglacial eruptions during thickening of the ice sheet as a result of a climatic deterioration built pillow lava piles overlain by steep-sided complexes of hydroclastic deposits. These deposits differ from those of the second eruptive period in structure, texture, and degree of alteration. Subaerial lava flows and agglutinates covered these deposits after buildup above the ice sheet. (d) Subaerial lava flows and fallout deposits during a postglacial period. The detailed analysis of volcanic table mountains appears to be a very sensitive indicator of climatic fluctuations. Although most deposits of the studied volcanoes were formed during the waning period of the last glaciation, the subglacial deposits in the upper part of the volcanic complex reflect a temporary, but major, thickening of the ice sheet.     

Trace element degassing and enrichment in the eruptive plume of the 2000 eruption of Hekla volcano, Iceland

During its last eruption in February 2000, Hekla volcano (Iceland) emitted a sub-Plinian plume that was condensed and scavenged down to the ground by heavy snowstorms, offering the unique opportunity to study the chemistry of the gaseous plume released during highly explosive eruptions. In this paper, we present results on trace element and minor volatile species (sulfates, chlorides, and fluorides) concentrations in snow samples collected shortly after the beginning of the eruption. The goal of this study is to better constrain the degassing and mobility of trace elements in gaseous emissions. Trace element volatility at Hekla is quantified by means of enrichment factors (EF) relative to Be. Well-known volatile trace elements (e.g., transition metals, heavy metals, and metalloids) are considerably enriched in the volcanic plume of Hekla. Their abundances are governed by the primary magmatic degassing of sulfate and/or halide compounds, which are gaseous at magmatic temperature. Their volatility is, however, slightly lower than in basaltic systems, most likely because of the lower magma temperature and higher magma viscosity at Hekla. More surprisingly, refractory elements (e.g., REE, Th, Ba, and Y) are also significantly enriched in the eruptive plume of Hekla where their apparent volatility is two orders of magnitude higher than in mafic systems. In addition, REE patterns normalized to the Hekla 2000 lava composition show a significant enrichment of HREE over LREE, suggesting the presence of REE fluorides in the volcanic plume. Such enrichments in the most refractory elements and REE fractionation are difficult to reconcile with primary degassing processes, since REE fluorides are not gaseous at magma temperature. REE enrichments at Hekla could be attributed to incongruent dissolution of tephra grains at low temperature by F-rich volcanic gases and aerosols within the eruptive plume. This view is supported by both leaching experiments performed on Hekla tephra and thermodynamic considerations on REE mobility in hydrothermal fluids and modeling of glass dissolution in F-rich aqueous solutions. Tephra dissolution may also explain the observed enrichments in other refractory elements (e.g., Th, Y, and Ba) and could contribute to the degassing mass balance of some volatile trace elements, provided they are mobile in F-rich fluids. It thus appears that both primary magmatic degassing and secondary tephra dissolution processes govern the chemistry of eruptive plumes released during explosive eruptions.     

火山喷发形式与挥发分含量

火山喷发是由地球深部物质发生部分熔融产生的岩浆上涌至地表所形成的一种地质现象,是地球内部能量释放的主要途径之一。有些火山喷发极为猛烈,产生的大量火山灰能够在长达几个月的时间内影响当地气候环境,甚至可以在一瞬间掩埋整座城市;而有些火山喷发时只有大量的熔岩从火山口中静静地溢出,人们甚至可以在不远处进行观赏。火山喷发具有何种程度的破坏力取决于其喷发形式,而挥发分含量是影响喷发形式的重要因素之一。本文简述了几种常见的喷发形式及其相互之间可能存在的转化关系,着重论述了挥发分含量在其中所起到的作用,同时介绍了几种可能的去气模型及常见的测量岩浆挥发分含量的方法。其中使用单斜辉石斑晶来反演大陆玄武岩原始岩浆水含量的方法预计会在未来的研究中得到普及。     

Subglacial erosion forms in northwest Ireland

Subglacial erosional forms are commonly found on bedrock substrates inside the Late Weichselian ice margin in County Donegal, northwest Ireland, and can be used to provide detailed information on subglacial processes and environments. The erosional forms occur on spatial scales from whalebacks (tens of metres in scale), to asymmetric and channelized bedrock-cut scours (tens of cm in scale) and striations (mm scale). Processes responsible for development of subglacial erosional forms occur along a continuum, from free meltwater existing as a laterally extensive sheet at the ice-bed interface, to abrasion by basal ice. Channelized bedrock-cut scours are particularly common in County Donegal, and show asymmetric and meandering thalwegs, U-shaped cross-profiles and steep lateral margins. Innermost parts of the scours are highly polished and have striations that follow thalweg direction. In places, bedrock surfaces are overlain by a delicate polish and thin calcite cement, and are buried beneath glacial till. Based on their morphology, the bedrock scours are interpreted as s-forms caused by high-pressure subglacial meltwater erosion. Striations within the scoured channels reflect periods of ice-bed coupling and subglacial abrasion. The range of features observed here was used to consider relationships between subglacial topography, hydraulic processes and ice-bed coupling. Precipitation of calcite cement took place in depressions on the bedrock surface by CO₂ degassing. Infilling of depressions by glacial till formed a new type of 'sticky spot' related to spatial variations in subglacial water pressure. The temporal evolution of sticky spots reflects interactions within the subglacial environment between subglacial relief, hydraulic regime and ice-bed coupling.     

Seafloor glacial features reveal the extent and decay of the last British Ice Sheet,east of Scotland

Three‐dimensional (3D) seismic datasets, 2D seismic reflection profiles and shallow cores provide insights into the geometry and composition of glacial features on the continental shelf, offshore eastern Scotland (58° N, 1–2° W). The relic features are related to the activity of the last British Ice Sheet (BIS) in the Outer Moray Firth. A landsystem assemblage consisting of four types of subglacial and ice marginal morphology is mapped at the seafloor. The assemblage comprises: (i) large seabed banks (interpreted as end moraines), coeval with the Bosies Bank moraine; (ii) morainic ridges (hummocky, push and end moraine) formed beneath, and at the margins of the ice sheet; (iii) an incised valley (a subglacial meltwater channel), recording meltwater drainage beneath former ice sheets; and (iv) elongate ridges and grooves (subglacial bedforms) overprinted by transverse ridges (grounding line moraines). The bedforms suggest that fast‐flowing grounded ice advanced eastward of the previously proposed terminus of the offshore Late Weichselian BIS, increasing the size and extent of the ice sheet beyond traditional limits. Complex moraine formation at the margins of less active ice characterised subsequent retreat, with periodic stillstands and readvances. Observations are consistent with interpretations of a dynamic and oscillating ice margin during BIS deglaciation, and with an extensive ice sheet in the North Sea basin at the Last Glacial Maximum. Final ice margin retreat was rapid, manifested in stagnant ice topography, which aided preservation of the landsystem record. Copyright © 2008 John Wiley & Sons, Ltd.     

Subglacial processes and drumlin formation in a confined bedrock valley,northwest Ireland

Knight, J. 2010: Subglacial processes and drumlin formation in a confined bedrock valley, northwest Ireland. Boreas, 10.1111/j.1502‐3885.2010.00182.x. ISSN 0300‐9483. Subglacial processes beneath the Late Weichselian ice sheet in northwest Ireland are deduced from sediments and structures within drumlins in a bedrock valley at Loughros Beg, County Donegal. Here, a glacially smoothed bedrock surface underlies the drumlins, which are composed on their up‐ice side of stacked, angular rafts of local bedrock. Overlying and down‐ice from these rafts are down‐ice‐dipping beds of massive to bedded diamicton that contain sand and gravel interbeds. In a down‐ice direction the diamicton matrix coarsens and the beds become laterally transitional to water‐sorted gravels. The down‐ice end of one drumlin shows a concentrically bedded stratified gravel core aligned parallel to ice flow and resembling the internal structure of an esker. With distance away from this core, the gravels become more poorly sorted with an increase in matrix content, and are transitional to massive to stratified diamicton. A four‐stage model describes the formation of drumlins in this sediment‐poor setting. The sediments that are located directly above the bedrock represent deposition in a semi‐enclosed subglacial cavity. A trigger for this process was the formation of subglacial relief by the thrusting up of bedrock rafts, which created the leeside cavity. Subsequent sediment deposition into this cavity represents a form of feedback (self‐regulation), which may be a typical characteristic of subglacial processes in sediment‐poor settings.     

Basaltic subglacial sheet-like sequences: Evidence for two types with different implications for the inferred thickness of associated ice

Subglacially-erupted volcanic sequences provide proxies for a unique range of palaeo-ice parameters and they are potentially highly useful archives of palaeoenvironmental information, particularly for pre-Quaternary periods. They can thus be incorporated by climate and ice sheet modellers in the same way as other environmental proxies, yet they remain largely under-utilised. Basaltic volcanic sequences erupted subglacially consist empirically of two major types, corresponding to eruptions under “thick” and “thin” ice, respectively. The latter are called subglacial sheet-like sequences and only one generic type of sequence has been described so far. However, there is now evidence that there are at least two generic types, with significantly different implications for interpretations of associated palaeo-ice sheet thicknesses. One type, which is relatively well described, is believed to be a diagnostic product of eruptions associated with a relatively thin glacial cover (< c. 150–200 m), probably corresponding most commonly to mountain glaciers but also conceivably thin ice caps or sheets, of any thermal regime (temperate, sub-polar, polar). It is here called the Mount Pinafore type. By contrast, a second subglacial sheet-like sequence, described in this paper for the first time and called the Dalsheidi-type, represents products of eruptions under much thicker ice (probably > 1000 m). Eruptions that form the Dalsheidi-type of sequence commence with the injection and inflation of a sill along the ice:bedrock interface. Such “interface sills” were predicted theoretically but had no known geological example, until now. Subsequent evolution commonly involves floating of the ice cover, catastrophic meltwater drainage and emplacement of widespread sheets of hyaloclastite, as cohesionless mass flows and hyperconcentrated flows. The water-saturated hyaloclastite is characteristically intruded by apophyses sourced in the underlying “interface sill”. Eruptions are commonly not explosive until their later stages. Dalsheidi-type deposits are outflow sequences probably linked to subglacial pillow volcanoes, which in Iceland were erupted along fissures. They only provide an indication of minimum thicknesses of the associated overlying ice, although theoretical considerations suggest substantial ice thicknesses in excess of 1000 m. However, they are likely to be characteristic products of eruptions under the thick West Antarctic Ice Sheet, but are currently inaccessible. Such eruptions may be capable of destabilising that ice sheet.     

The relative importance of supraglacial versus subglacial meltwater escape in basaltic subglacial tuya eruptions: An important unresolved conundrum

The hydraulic behaviour of meltwater during subglacial basaltic eruptions in temperate ice is of paramount importance in understanding the eruptive processes, lithofacies and architecture of the edifices formed. Hydraulics also determines the timing, location and volume of meltwater discharge, which may be sudden and catastrophic and via subglacial and/or supraglacial routes. Increasing our knowledge of meltwater hydraulics is therefore important for understanding, predicting and mitigating the impact of meltwater release on vulnerable human communities. New observations about eruption-related meltwater hydraulics are presented for well-exposed glaciovolcanic lava-fed deltas on James Ross Island, Antarctica, and accounts of historical eruptions are also re-examined to identify the major meltwater discharge routes. The study provides the first conceptual model for how meltwater escapes supraglacially. In the absence of a crevassed layer (which will dominate any meltwater flow), overflowing may be initiated by enhanced rates of seepage, despite the intrinsically low hydraulic conductivities of snow and firn. Once overflowing is established, the rate of spillway incision is a likely overriding control on the evolution of the system and whether the discharge is unstable (fast) or stable (slower). The James Ross Island sequences demonstrate that meltwater discharge is highly dynamic and may have involved both subglacial and supraglacial escape. Subglacial discharge probably occurs throughout basaltic tuya eruptions but some periods may be dominated by concurrent overflowing. It is still unclear if overflowing systems are sufficiently stable to enable the growth of laterally extensive glaciovolcanic lava-fed deltas.     

The geography of basal ice and its relationship to glaciohydraulic supercooling: Svínafellsjökull,southeast Iceland

Glaciohydraulic supercooling is potentially an important mechanism of basal ice formation with significant implications for Quaternary Science, but remains controversial. Some studies cite similarities between basal ice and the ice forming at vents of upwelling supercooled subglacial water as evidence that where supercooling occurs it dominates basal ice formation. Other studies find no evidence linking supercooling to basal ice even at glaciers where supercooling occurs, questioning the supposed genetic link between basal ice and vent ice. At Svínafellsjökull, southeast Iceland, we compare the physical characteristics and geographical distribution of stratified basal ice with the characteristics and distribution of phenomena such as upwellings and anchor ice terraces associated with supercooling. Two physically and geographically distinct basal ice populations emerge: one occurs only where there is evidence for glaciohydraulic supercooling and has physical characteristics consistent with an origin by supercooling; the other is ubiquitous, even at locations where there is no evidence for supercooling, and has characteristics consistent with non-supercooling modes of origin. We suggest that glaciohydraulic supercooling does not account for all of the basal ice at Svínafellsjökull, and that other mechanisms of basal ice formation are also significant.     

Crustal volatile release at Merapi volcano; the 2006 earthquake and eruption events

High‐temperature gas in volcanic island arcs is widely considered to originate predominantly from the mantle wedge and from subducted sediments of the down‐going slab. Over the decade (1994–2005) prior to the 2006 eruption of Merapi volcano, summit fumarole CO₂ gas δ¹³C ratios are relatively constant at ?4.1 ± 0.3‰. In contrast, CO₂ samples taken during the 2006 eruption and after the May 26th 2006 Yogyakarta earthquake (M6.4) show a dramatic increase in carbon isotope ratios to ?2.4 ± 0.2‰. Directly following the earthquake (hypocentre depth 10–15 km), a 3–5‐fold increase in eruptive intensity was observed. The elevated carbon isotope gas data and the mid‐crustal depth of the earthquake source are consistent with crustal volatile components having been added during the 2006 events, most probably by the thick local limestone basement beneath Merapi. This ‘extra’ crustal gas likely played an important role in modifying the 2006 eruptive behaviour at Merapi and it appears that crustal volatiles are able to intensify and maintain eruptions independently of traditional magmatic recharge and fractionation processes.     

Basaltic explosive volcanism: Constraints from deposits and models

Basaltic pyroclastic volcanism takes place over a range of scales and styles, from weak discrete Strombolian explosions (～10²–10³ kg s^?1) to Plinian eruptions of moderate intensity (10⁷–10⁸ kg s^?1). Recent well-documented historical eruptions from Etna, Kīlauea and Stromboli typify this diversity. Etna is Europe's largest and most voluminously productive volcano with an extraordinary level and diversity of Strombolian to subplinian activity since 1990. Kīlauea, the reference volcano for Hawaiian fountaining, has four recent eruptions with high fountaining (>400 m) activity in 1959, 1960, 1969 (–1974) and 1983–1986 (–2008); other summit (1971, 1974, 1982) and flank eruptions have been characterized by low fountaining activity. Stromboli is the type location for mildly explosive Strombolian eruptions, and from 1999 to 2008 these persisted at a rate of ca. 9 per hour, briefly interrupted in 2003 and 2007 by vigorous paroxysmal eruptions. Several properties of basaltic pyroclastic deposits described here, such as bed geometry, grain size, clast morphology and vesicularity, and crystal content are keys to understand the dynamics of the parent eruptions.The lack of clear correlations between eruption rate and style, as well as observed rapid fluctuations in eruptive behavior, point to the likelihood of eruption style being moderated by differences in the fluid dynamics of magma and gas ascent and the mechanism by which the erupting magma fragments. In all cases, the erupting magma consists of a mixture of melt and gaseous bubbles. The depth and rate of degassing, melt rheology, bubble rise and coalescence rates, and extent of syn-eruptive microlite growth define complex feedbacks that permit reversible shifts between fragmentation mechanisms and in eruption style and intensity. However, many basaltic explosive eruptions end after an irreversible shift to open-system outgassing and microlite crystallization in melt within the conduit.Clearer understanding of the factors promoting this diversity of basaltic pyroclastic eruptions is of fundamental importance in order to improve understanding of the range of behaviors of these volcanoes and assess hazards of future explosive events at basaltic volcanoes. The three volcanoes used for this review are the sites of large and growing volcano-tourism operations and there is a public need both for better knowledge of the volcanoes’ behavior and improved forecasting of the likely course of future eruptions.     

Pre- and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano,Indonesia

The 2010 eruption of Merapi (VEI 4) was the volcano’s largest since 1872. In contrast to the prolonged and effusive dome-forming eruptions typical of Merapi’s recent activity, the 2010 eruption began explosively, before a new dome was rapidly emplaced. This new dome was subsequently destroyed by explosions, generating pyroclastic density currents (PDCs), predominantly consisting of dark coloured, dense blocks of basaltic andesite dome lava. A shift towards open-vent conditions in the later stages of the eruption culminated in multiple explosions and the generation of PDCs with conspicuous grey scoria and white pumice clasts resulting from sub-plinian convective column collapse. This paper presents geochemical data for melt inclusions and their clinopyroxene hosts extracted from dense dome lava, grey scoria and white pumice generated during the peak of the 2010 eruption. These are compared with clinopyroxene-hosted melt inclusions from scoriaceous dome fragments from the prolonged dome-forming 2006 eruption, to elucidate any relationship between pre-eruptive degassing and crystallisation processes and eruptive style. Secondary ion mass spectrometry analysis of volatiles (H₂O, CO₂) and light lithophile elements (Li, B, Be) is augmented by electron microprobe analysis of major elements and volatiles (Cl, S, F) in melt inclusions and groundmass glass. Geobarometric analysis shows that the clinopyroxene phenocrysts crystallised at depths of up to 20 km, with the greatest calculated depths associated with phenocrysts from the white pumice. Based on their volatile contents, melt inclusions have re-equilibrated during shallower storage and/or ascent, at depths of ~0.6–9.7 km, where the Merapi magma system is interpreted to be highly interconnected and not formed of discrete magma reservoirs. Melt inclusions enriched in Li show uniform “buffered” Cl concentrations, indicating the presence of an exsolved brine phase. Boron-enriched inclusions also support the presence of a brine phase, which helped to stabilise B in the melt. Calculations based on S concentrations in melt inclusions and groundmass glass require a degassing melt volume of 0.36 km³ in order to produce the mass of SO₂ emitted during the 2010 eruption. This volume is approximately an order of magnitude higher than the erupted magma (DRE) volume. The transition between the contrasting eruptive styles in 2010 and 2006 is linked to changes in magmatic flux and changes in degassing style, with the explosive activity in 2010 driven by an influx of deep magma, which overwhelmed the shallower magma system and ascended rapidly, accompanied by closed-system degassing.     

Boulder-gravel hummocks and wavy basal till contacts: products of subglacial meltwater flow beneath the Saginaw Lobe, south-central Michigan, USA

Hummocky terrain composed of boulder gravel and a wavy contact between stratified till and sand are described and explained as products of subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet in south-central Michigan. Exposures and geophysical investigations of hummocky terrain in a tunnel channel reveal that hummocks (˜100m diameter) are glaciofluvial bedforms with a supraglacial melt-out till or till flow veneer. The hummocky terrain is interpreted as a subglacial glaciofluvial landscape rather than one of stagnant ice processes commonly assumed for hummocky landscapes. Sandy bedforms at another site are in-phase with a wavy contact at the base of a stratified till exposed for 50m along the margin of a tunnel channel. The 0.4m thick stratified till is overlain by up to 5m of compact, pebble-rich, sandy subglacial melt-out till. The contact between the till and sand has a wave form with a 0.5m amplitude and 3-5m wavelength. Bedding within the stratified till, sandy bedforms and melt-out till are mostly in-phase with each other. Clasts from the overlying stratified till penetrate and deform the underlying sand recording recoupling of the ice to its bed. Ice ripples cut into the base of river ice have a similar morphology and are considered analogs for cavities cut into the base of the glacier and subsequently filled with sand. Subglacial meltwater activity was not coeval at each study site, indicating that subglacial meltwater played important roles in the evolution of the subglacial environment beneath the Saginaw Lobe at different times.     

Thermo-mechanical facies representative of fast and slow flowing ice sheets: the Weichselian ice sheet,a central west Poland case study

This study deals with an issue of thermo-mechanical facies, reflecting specific thermal and mechanical properties of the subglacial environment. The main objective of this study was to develop a model of glacitectionic deformation and its sedimentary record beneath fast and slow flowing ice sheets, based on investigations conducted in Wielkopolska (west central Poland). Sedimentary structures, mainly at the contact between subglacial tills and glacifluvial sediments, were recognized to delineate typical facies associations in a Weichselian glacigenic succession. Each association was interpreted as a record of the different depositional environments related to different subglacial conditions. Those investigations suggest the substratum was composed of frozen and dry, and wet and mobile spots, and four thermo-mechanical facies were distinguished: A – is representative of slower ice flow, dry and cold subglacial conditions, where driving stresses and normal effective pressure were high; B – is also related to slow ice flow and occurrence of cold subglacial permafrost, but with little amount of unfrozen water (however, higher than in facies A), with similar physical characteristics of the ice sheet as facies A; thermo-mechanical facies C and D represent wet and warm ice sole, with low normal effective pressure and driving stresses, thus lowering sediments’ shear strength and enabling high ice-flow velocities. We suggest that these facies have specific and non-random location, thereby revealing the relationship between subglacial thermo-mechanical conditions and ice sheet dynamics. Slow moving, cold-based ice occurred along ice sheet margins and inter-stream areas, whereas fast-moving, warm-based, well-lubricated ice, was typical of the axial parts of ice streams.     

Glacial erosion beneath ice streams and ice-stream tributaries: constraints on temporal and spatial distribution of erosion from numerical simulations of a West Antarctic ice stream

Geologic evidence such as subglacial troughs and grounding zone wedges indicate that soft-bedded, West Antarctic ice streams are capable of eroding, transporting and depositing large volumes of debris at high rates (˜100 m³ yr^-1 per meter width). In order to understand the dynamics of ice streams and the geologic effects of their activity, it is important to understand the physical mechanisms that control these high rates of sub-ice-stream sediment generation and transport. Here, we use a numerical model of Ice Stream C run over c. 8500 model years to quantify the effects of a recently proposed, till-ploughing mechanism of till formation and redistribution beneath ice streams (Tulaczyk et al. 2001; Clark et al. in press). Our results show that this 'transport-limited' mechanism, in which till transport rates scale with ice velocity and erosion rates with spatial gradients of velocity, is consistent with existing constraints. For instance, our model results predict significantly higher (˜0.6 mm yr^-1) average erosion rates beneath ice-stream tributaries, which are underlain by deep subglacial troughs, than beneath ice-stream trunks (˜0.2mm yr^-1), whose subglacial troughs have a significantly smaller relief. We would not obtain this satisfactory result if subglacial erosion was parametrized in the model using the more traditional approach of scaling erosion rates with ice velocity (what we call the 'production-limited' parametrization). Because of the requirement of ice continuity, the magnitude of ice velocity generally increases downstream in polar ice streams, and so do the production-limited erosion rates. Pending further investigations, we propose that geologic and geomorphic effects of soft-bedded ice streams should be quantified using some form of a 'transport-limited' parametrization of subglacial erosion rates, e.g. the till-ploughing mechanism.     

Water,ice and mud: lahars and lahar hazards at ice‐ and snow‐clad volcanoes

Large‐volume lahars are significant hazards at ice and snow covered volcanoes. Hot eruptive products produced during explosive eruptions can generate a substantial volume of melt water that quickly evolves into highly mobile flows of ice, sediment and water. At present it is difficult to predict the size of lahars that can form at ice and snow covered volcanoes due to their complex flow character and behaviour. However, advances in experiments and numerical approaches are producing new conceptual models and new methods for hazard assessment. Eruption triggered lahars that are ice‐dominated leave behind thin, almost unrecognizable sedimentary deposits, making them likely to be under‐represented in the geological record.     

Sediment‐rich meltwater plumes and ice‐proximal fans at the margins of modern and ancient tidewater glaciers: Observations and modelling

Turbid meltwater plumes and ice‐proximal fans occur where subglacial streams reach the grounded marine margins of modern and ancient tidewater glaciers. However, the spacing and temporal stability of these subglacial channels is poorly understood. This has significant implications for understanding the geometry and distribution of Quaternary and ancient ice‐proximal fans that can form important aquifers and hydrocarbon reservoirs. Remote‐sensing and numerical‐modelling techniques are applied to the 200 km long marine margin of a Svalbard ice cap, Austfonna, to quantify turbid meltwater‐plume distribution and predict its temporal stability. Results are combined with observations from geophysical data close to the modern ice front to refine existing depositional models for ice‐proximal fans. Plumes are spaced ca 3 km apart and their distribution along the ice front is stable over decades. Numerical modelling also predicts the drainage pattern and meltwater discharge beneath the ice cap; modelled water‐routing patterns are in reasonable agreement with satellite‐mapped plume locations. However, glacial retreat of several kilometres over the past 40 years has limited build‐up of significant ice‐proximal fans. A single fan and moraine ridge is noted from marine‐geophysical surveys. Closer to the ice front there are smaller recessional moraines and polygonal sediment lobes but no identifiable fans. Schematic models of ice‐proximal deposits represent varying glacier‐terminus stability: (i) stable terminus where meltwater sedimentation produces an ice‐proximal fan; (ii) quasi‐stable terminus, where glacier readvance pushes or thrusts up ice‐proximal deposits into a morainal bank; and (iii) retreating terminus, with short still‐stands, allowing only small sediment lobes to build up at melt‐stream portals. These modern investigations are complemented with outcrop and subsurface observations and numerical modelling of an ancient, Ordovician glacial system. Thick turbidite successions and large fans in the Late Ordovician suggest either high‐magnitude events or sustained high discharge, consistent with a relatively mild palaeo‐glacial setting for the former North African ice sheet.     

Soft-bedded subglacial meltwater channel from the Welzow-Süd opencast lignite mine, Lower Lusatia, eastern Germany

An excellent section in the Welzow-Süd open-cast lignite mine in Lower Lusatia, eastern Germany, provided a rare opportunity to study a small (5 m deep), buried subglacial meltwater channel of Saalian age. The channel is steep-sided and distinctly U-shaped. It is separated from undeformed outwash deposits in which it is incised by a sharp erosional contact and it is filled with meltwater sand and till. The till was possibly squeezed into the channel from the adjacent ice/bed interface. Directly beneath the channel, there is a partly truncated diapir of clayey silt, evidencing sediment intrusion into the channel from below. During channel formation, the pressure gradient was oriented from the surrounding sediments into the channel, so that the channel served as a drainage conduit for groundwater from the adjacent subglacial aquifer. The substratum consists largely of sandy aquifers with a total thickness of about 100 m, separated by two aquitards. Channel formation was initiated when hydraulic transmissivity of the bed did not suffice to evacuate all the subglacial meltwater as groundwater flow. As the Welzow-Süd channel belongs to a dense network of subglacial channels in eastern Germany, temporary ice-sheet instability in this region prior to channel formation seems possible.