Englacial Sediment Research Articles

Blue-ice moraines formation in the Heritage Range, West Antarctica: Implications for ice sheet history and climate reconstruction

Blue ice is found in areas of Antarctica where katabatic winds, focussed by steep surface slopes or by topography around nunataks, cause enhanced surface ablation. This process draws up deeper, older ice to the ice sheet surface, often bringing with it englacial sediment. Prevailing theories for dynamically stable moraine surfaces in East Antarctica suggest that: (i) it is this material, once concentrated, that forms blue-ice moraines (BIM), (ii) that the moraine formation can be dated using cosmogenic isotope approaches, and that, (iii) since we expect an increase in exposure age moving away from the ice margin towards bedrock, dating across the moraine can be used to constrain ice-sheet history. To test this lateral accretion model for BIM formation we visited Patriot, Marble and Independence Hills in the southern Heritage Range, West Antarctica. Detailed field surveys of surface form, sediment and moraine dynamics were combined with geophysical surveys of the englacial structure of the moraines and cosmogenic nuclide analysis of surface clasts. Results suggest sediment is supplied mainly by basal entrainment, supplemented by debris-covered valley glaciers transferring material onto the ice sheet surface, direct deposition from rock-fall and slope processes from nunataks. We find that once sediment coalesces in BIM, significant reworking occurs through differential ablation, slope and periglacial processes. We bring these processes together in a conceptual model, concluding that many BIM in West Antarctica are dynamic and, whilst they persist through glacial cycles, they do not always neatly record ice sheet retreat patterns since linear distance from the ice margin does not always relate to increased clast exposure age. Understanding the dynamic processes involved in moraine formation is critical to the effective interpretation of the typically large scatter of cosmogenic nuclide exposure ages, opening a deep window into the million-year history of the West Antarctic Ice Sheet.

Ice stream subglacial access for ice-sheet history and fast ice flow: the BEAMISH Project on Rutford Ice Stream, West Antarctica and initial results on basal conditions

AbstractThree holes were drilled to the bed of Rutford Ice Stream, through ice up to 2154 m thick, to investigate the basal processes and conditions associated with fast ice flow and the glacial history of the West Antarctic Ice Sheet. A narrative of the drilling, measuring and sampling activities, as well as some preliminary results and initial interpretations of subglacial conditions, is given. These were the deepest subglacial access holes ever drilled using the hot-water drilling method. Samples of bed and englacial sediments were recovered, and a number of instruments were installed in the ice column and the bed. The ice–bed interface was found to be unfrozen, with an existing, well-developed subglacial hydrological system at high pressure, within ~1% of the ice overburden. The bed itself comprises soft, water-saturated sediments, consistent with previous geophysical interpretations. Englacial sediment quantity varies significantly between two locations ~2 km apart, and possibly over even shorter (~20 m) distances. Difficulties and unusual observations encountered while connecting to the subglacial hydrological system in one hole possibly resulted from the presence of a large clast embedded in the bottom of the ice.

Radar‐Detected Englacial Debris in the West Antarctic Ice Sheet

AbstractStructural glaci‐geological processes can entrain basal sediment into ice, leading to its transportation and deposition downstream. Sediments potentially rich in essential nutrients, like silica and iron, can thus be transferred from continental sources to the ocean, where deposition could enhance marine primary productivity. However, a lack of data has limited our knowledge of sediment entrainment, transfer, and distribution in Antarctica, until now. We use ice‐penetrating radar to examine englacial sediments in the Weddell Sea sector of the West Antarctic Ice Sheet. Radargrams reveal englacial reflectors on the leeside of nunataks and subglacial highlands, where Mie scattering analysis of the reflectors suggests particle sizes consistent with surface moraine sediments. We hypothesize that these sediments are entrained at the thermal boundary between cold and warm‐based ice. Conservative estimates of >130 × 109 kg of englacial sediment in Horseshoe Valley alone suggest that the ice sheet has significant entrainment potential unappreciated previously.

Probing supraglacial debris on Mars 1: Sources, thickness, and stratigraphy

Geomorphic and geophysical evidence supports a debris-covered glacier origin for a suite of landforms at the mid-latitudes of Mars, including lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF). These large reservoirs of ice and their near-surface structure provide a rich record for understanding the planet's climate and history of global volatile exchange over the past billion years. LDA, LVF, and CCF are also potential sites for future robotic and human missions but the accessibility of glacial ice for direct sampling and in situ resource utilization depends largely on the geotechnical properties of the surface debris (“supraglacial debris”), including its thickness, grain sizes, and density structure. The physical properties of this supraglacial debris layer have been poorly constrained. We use images of morphology, digital elevation models, thermal inertia data, and radar sounding data to probe the near surface of LDA, LVF, and CCF in Deuteronilus Mensae in order to place constraints on the sources, grain sizes, thickness, and stratigraphy of supraglacial debris. We find evidence for at least a two-layer stratigraphy. Layered mantle consisting of atmospherically emplaced dust and ice superposes boulder-rich sediment sourced by rockfalls glacially transported downslope. High thermal inertia, boulder-rich termini and debris bands reminiscent of medial moraines are found throughout the study region, supporting a rockfall origin for at least a fraction of the debris exposed at the surface. This supraglacial debris layer would have thickened with time from sublimation of glacial ice and liberation of englacial sediment and dust. At present, the entire supraglacial debris package is a minimum of a few meters in thickness and is likely tens of meters in thickness in many locations, possibly thinning regionally at lower latitudes and locally thinning toward the headwalls. The lack of terracing or interior structures in craters formed within LDA, LVF, and CCF and the absence of near-surface reflectors in SHARAD radar data further suggest that no strong contrasts in permittivity or strength occur at the interface of the layers.

Numerical modeling of glacial sediment production and transport during deglaciation

We aim to improve the understanding of sediment fluxes originating from ice sheet outlet glaciers in order to explain the evolution of sedimentary fjord fills. A numerical glacial erosion and sediment flux process model has been developed to quantify sediment generation and subglacial transport. Dynamic ice sheet cover is generated from topography and climate parameters, affecting the glacial net mass balance. Pressure, temperature and velocity characteristics of the ice sheet force erosion at the glacier bed. Bed erodability depends on bedrock hardness and potential shielding by sediment. After erosion, sediment is either stored at the bed, or transported either by meltwater or englacially (within basal ice). Model experiments show ice retreat is driven by valley topography; rapid retreat occurs in the flatter valley stretches and slower retreat in the steeper parts of a glacial valley. Distinct depocenters of exposed sediment develop during retreat. Predicted erosion rates in the glaciated valley range from 37 to 56mm/yr for different climate scenarios, which is well within the range of reported field reconstructions. Erosion is significantly accelerated beneath warm-based ice and the effect of an enlarged warm-based footprint during warming compensates for decreasing ice mass. Topographic shielding of bedrock by previously eroded material in large areas constrains erosion by a factor 4. Simulated glacier retreat and melt water flux initially occur at a high rate and subsequently decelerate, while sediment delivery exhibits a distinct lagging of several thousands of years behind the meltwater flux. Upon the transition from glaciation to deglaciation, the subglacial system is in a sediment-starved state. Late-deglacial erosion and sub- and englacial sediment storage cause a lagging sediment delivery to downstream periglacial, fluvial and lacustrine domains.

Application of borehole optical televiewing to investigating the 3-D structure of glaciers: implications for the formation of longitudinal debris ridges, midre Lovénbreen, Svalbard

AbstractDigital optical televiewing (OPTV) of hot-water-drilled boreholes is evaluated as a technique for the investigation of englacial ice and debris structures on the basis of six boreholes drilled in the terminus region of midre Lovénbreen, Svalbard. The resulting OPTV logs successfully reveal several visually distinctive englacial ice properties and deformation structures (e.g. oblique englacial fractures imaged here for the first time). Combining these OPTV logs with surface mapping has resulted in the identification of eight separate structural elements, several of which can be interpolated onto 3-D grids at a node spacing of 1 m vertically and 10 m horizontally. Basally derived englacial sediment layers are also found to be intercalated with primary stratification, elevated into near-vertical planes around a central fold axis by large-scale lateral folding. The analysis also allows supraglacial longitudinal debris ridges to be subclassified into two types: a previously described (type-I) form, which are the exposed fold axes of large-scale lateral folds, and a new (type-II) form experiencing secondary deformation by small-scale horizontal folding in association with vertical displacements across arcuate shear planes in response to longitudinally compressive stresses near the glacier terminus. Although using boreholes to investigate glacier structure is limited (e.g. by parallelism with vertical planes), applying OPTV to multiple boreholes at midre Lovénbreen has successfully revealed a range of 3-D structural elements at high spatial resolution. As such, interpolating between multiple OPTV logs overcomes many of the problems associated with interpretations made solely on the basis of surface-based structural mapping, and combining the two techniques represents a powerful glaciological tool.

Subglacial erosion and englacial sediment transport modelled for North American ice sheets

Reconstructions of North American glacial history have either been based on interpretation of the geologic record or on numerical models of continental ice dynamics. Because such models make no explicit reference to the geological products of glaciation, the full potential of a combined approach has not been realized. We present the results of a process-based model of sediment production, entrainment, transport and deposition that represents a first step toward uniting these two lines of investigation. In this model basal-ice processes are dependent on ice sheet geometry, hydrology and dynamics. A three-dimensional thermomechanical ice sheet model yields the ice geometry, flow and thermal conditions that are used by a model of subglacial hydrology; together these drive the basal process model. Comparisons of simulation results with mapped debris distributions of the Hudson Bay Paleozoic carbonates and of the distinctive Dubawnt Group are used to test the validity of the transport model. Dispersion of debris radially out of Hudson Bay is greatly overestimated by the model, and dispersion toward the bay equally underestimated. This is because the modelled ice sheet has a single dome over Hudson Bay at glacial maxima, including Last Glacial Maximum, rather than a saddle separating ice domes over Quebec-Labrador and Keewatin. Estimated rates of glacial erosion for the Cordilleran and Laurentide ice sheets are compared against those predicted by the model. The model underestimates these rates because it underestimates the vigour of the quarrying process.

Controls on englacial sediment deposition during the November 1996 jökulhlaup, Skeiđarárjökull, Iceland

This paper presents sedimentary evidence for rapid englacial debris entrainment during jökulhlaups. Previous studies of jökulhlaup sedimentology have focused predominantly on proglacial impact, rather than depositional processes within glaciers. However, observations of supraglacial floodwater outbursts suggest that englacial sediment emplacement is possible during jökulhlaups. The November 1996 jökulhlaup from Skeiđarárjökull, Iceland presented one of the first opportunities to examine englacial flood deposits in relation to former supraglacial outlets. Using observations from Skeiđarárjökull, this paper identifies and explains controls on the deposition of englacial flood sediments and presents a qualitative model for englacial jökulhlaup deposition. Englacial jökulhlaup deposits were contained within complex networks of upglacier-dipping fractures. Simultaneous englacial deposition of fines and boulder-sized sediment demonstrates that englacial fracture discharge had a high transport capacity. Fracture geometry was an important control on the architecture of englacial jökulhlaup deposits. The occurrence of pervasively frozen flood deposits within Skeiđarárjökull is attributed to freeze-on by glaciohydraulic supercooling. Floodwater, flowing subglacially or through upglacier-dipping fractures, would have supercooled as it was raised to the surface faster than its pressure-melting point could increase as glaciostatic pressure decreased. Evidence for floodwater contact with the glacier bed is supported by the ubiquitous occurrence of sheared diamict rip-ups and intra-clasts of basal ice within jökulhlaup fractures, deposited englacially some 200–350 m above the bed of Skeiđarárjökull. Evidence for fluidal supercooled sediment accretion is apparent within stratified sands, deposited englacially at exceptionally high angles of rest in the absence of post-depositional disturbance. Such primary sediment structures cannot be explained unless sediment is progressively accreted to opposing fracture walls. Ice retreat from areas of former supraglacial outbursts revealed distinct ridges characterized by localized upwellings of sediment-rich floodwater. These deposits are an important addition to current models of englacial sedimentation and demonstrate the potential for post-jökulhlaup landform development. Copyright © 2001 John Wiley & Sons, Ltd.

Controls on englacial sediment deposition during the November 1996 jökulhlaup, Skeiđarárjökull, Iceland

AbstractThis paper presents sedimentary evidence for rapid englacial debris entrainment during jökulhlaups. Previous studies of jökulhlaup sedimentology have focused predominantly on proglacial impact, rather than depositional processes within glaciers. However, observations of supraglacial floodwater outbursts suggest that englacial sediment emplacement is possible during jökulhlaups. The November 1996 jökulhlaup from Skeiđarárjökull, Iceland presented one of the first opportunities to examine englacial flood deposits in relation to former supraglacial outlets. Using observations from Skeiđarárjökull, this paper identifies and explains controls on the deposition of englacial flood sediments and presents a qualitative model for englacial jökulhlaup deposition. Englacial jökulhlaup deposits were contained within complex networks of upglacier‐dipping fractures. Simultaneous englacial deposition of fines and boulder‐sized sediment demonstrates that englacial fracture discharge had a high transport capacity. Fracture geometry was an important control on the architecture of englacial jökulhlaup deposits. The occurrence of pervasively frozen flood deposits within Skeiđarárjökull is attributed to freeze‐on by glaciohydraulic supercooling. Floodwater, flowing subglacially or through upglacier‐dipping fractures, would have supercooled as it was raised to the surface faster than its pressure‐melting point could increase as glaciostatic pressure decreased. Evidence for floodwater contact with the glacier bed is supported by the ubiquitous occurrence of sheared diamict rip‐ups and intra‐clasts of basal ice within jökulhlaup fractures, deposited englacially some 200–350 m above the bed of Skeiđarárjökull. Evidence for fluidal supercooled sediment accretion is apparent within stratified sands, deposited englacially at exceptionally high angles of rest in the absence of post‐depositional disturbance. Such primary sediment structures cannot be explained unless sediment is progressively accreted to opposing fracture walls. Ice retreat from areas of former supraglacial outbursts revealed distinct ridges characterized by localized upwellings of sediment‐rich floodwater. These deposits are an important addition to current models of englacial sedimentation and demonstrate the potential for post‐jökulhlaup landform development. Copyright © 2001 John Wiley & Sons, Ltd.

Sediment deposition in an iceberg-dominated glacimarine environment, East Greenland: basin fill implications

Kangerdlugssuaq Fjord, east Greenland (68°N), is an iceberg dominated fjord, receiving in excess of 15 km 3/a in iceberg volume, representing about a thousand fairly-large icebergs each year. In contrast, the estimated combined flux of subglacial and supraglacially derived meltwaters is of the order of 4 km 3/a. Iceberg melt within the fjord accounts for half of the freshwater delivered to the fjord from all sources. Iceberg appear to lose their subglacial debris during their lenghty residence time (2 year) within the sikussaks pinned to the ice margins. Once icebergs escape the confines of the sikussaks, they transit the fjord in about 68 days, melting at rates between 0.05 and 0.1 m/d and releasing englacial sediment to a mid-water, iceberg melt zone. Sedimentation beneath this plume is predicted to range from 2.4 cm/a, near the head of the fjord, to 0.6 cm/a at the most distal (50 km) part of the fjord basin. Much of the sediment that initially accumulates on the steep sidewall margins becomes remobilized and redeposited on the distal basin floor via turbidity currents. Predicted accumulation rates agree with long-term (1400 year) rates obtained from dated cores collected in Kangerdlugssuaq Fjord.

˝Stagnant-Ice˝ Topography and its Relation to Drumlin Genesis, with Reference to South-Central Ulster (Abstract Only)

Previous work on drumlin genesis considers the problem at the ice-rock interface and ignores the environmental significance of inter-drumlin sediments and superimposed landform associations. Present work by the author examines two drumlin fields in south-central Ulster, Northern Ireland, and proposes a mechanism of drumlin formation which is time-dependent and results from changes of thermal regime through time.Drumlin exposures in south-central Ulster reveal a particular facies association in drumlin tills showing considerable lithological variation; striae, roches moutonnées, and basal lodgement till beneath drumlins indicate an event early in the last glaciation characterized by a temperate thermal regime. This association is overlain by a basal melt-out till, characterized by striated clasts and containing numerous pockets of stratified sediment. The melt-out till is related to changes in provenance of erratics in the till, associated with a shift in the centre of ice accumulation during the course of glaciation.The stratigraphy suggests that a considerable amount of drift was formed prior to drumlin formation and that a change of thermal state may have characterized the shift in the ice accumulation zone. This change is also suggested by evidence of high-level melt-water transport in the Sperrin Mountains, indicating a sub-polar or sub-temperate thermal regime existing in the early part of the late Weichselian deglaciation.A model based on thermal convection theory of polar ice sheets (Hughes 1976) is proposed in an attempt to relate the juxtaposition of drumlins and glaciofluvial assemblages in southcentral Ulster. In an ice accumulation zone, as characterized this area during the late Weichselian glacial maximum, advection is generally absent and convection is the dominant flow in the ice sheet. In this environment motive ice may develop in the basal zone, resulting from vertical buoyancy stresses acting downward on a column in the ice sheet. This flow environment may coexist within an ice sheet which is "stagnant" and perhaps subject to supraglacial insolation melting, depending on the overall climatic regime. If convection is dominant, it may create areas of basal compression and tension, acting to establish co-existing zones of "hot" and "cold" ice; hot-ice zones are subject to basal erosion, leading to migration of subglacial sediments to points of englacial storage above the density inversion layer. Cold-ice zones act to protect underlying sediments and result in isolation of drift nuclei, protection being afforded by the yield strength of frozen drift being greater than either debris-laden or clean ice.If the model is tenable, it seems likely that drift nucleation in drumlin fields may have been accomplished before the drumlin formation or streamlining event. Vertical migration of subglacial materials also explains the similarity of sediment sources of glacigenic and glaciofluvial assemblages which occur in juxtaposition, and, without reduction in drumlin density. The sequence of events in south-central Ulster may be summarized as follows: (1)Build-up of ice, associated with sub-drumlin erosion forms and basal lodgement till. Change of thermal regime from temperate to sub-temperate.(2)Shift in the ice-accumulation zone from highland to lowland, indicated by erratic dispersals of glaciogenic sediments. Change of thermal regime from sub-polar to polar at the late Weichselian glacial maximum.(3)High-level insolation melting with concomitant basal freezing. Initiation of polygonal array in basal zone. Change of thermal regime from polar to sub-polar or sub-temperate.(4)Down-wastage of ice and nunatak formation. Formation of high-level melt-water phenomena. Deposition of basal melt-out till. Further development of basal polygonal array. Till nuclei established and englacial sediment isolated. Change of thermal regime from sub-polar to sub-temperate.(5)Change of thermal state to temperate. Uncoupling of ice from bedrock. Streamlining of till nuclei and reworking of englacial sediment by melt water.(6)Final disintegration of ice.