C23B-01 INVITED
Frictional Slip Resistance at Glacier Beds due to Rock Debris
Slip at the bases of wet-based ice masses may share more in common with slip along crustal faults than previously suspected. The ice plain of Whillans ice stream moves by persistent slick-slip motion with associated basal seismicity (Wiens, et al., 2007, Nature) Additional evidence for stick-slip over shorter time and length scales comes from seismic studies elsewhere in Antarctica (Danesi et al., 2006, EPSL) and from Trapridge Glacier, where brief but large water pressure pulses in closed boreholes are interpreted to reflect stress transients associated with minute, episodic slip events (Kavanaugh, in press, JGR-ES). These observations indicate that slip resistance over some portions of glacier beds can be dominantly frictional, rather than viscous or viscoplastic as is commonly assumed for both hard and soft beds. Rock debris, either within basal ice or in a soft bed, is a source of such friction. Measurements at Engabreen, Norway, indicate that friction between debris in ice and a hard bed can locally be comparable to the bed shear stress, indicating that over some parts of the bed rock friction can exceed viscous drag on bedrock bumps. Laboratory studies of till deformation relevant to soft beds indicate that Coulomb models best approximate steady-state deformation but with subtle rate dependencies at small strains. Glacier slip across a till bed, triggered by high water pressure with associated shear relaxation of till, has been measured beneath several glaciers. Moreover, laboratory experiments indicate that resistance to this slip can decrease with increasing slip velocity, the converse of viscous rate strengthening. This velocity weakening is measured commonly in friction experiments with fault gouge and is a fundamental requirement for stick-slip motion and consequent seismicity. These subglacial and laboratory data, therefore, are broadly consistent with larger-scale observations of stick-slip and basal seismicity on some glaciers. The implication, in agreement with some models of glacier flow, is that sections of glacier beds with slip resistance dominated by debris friction may have little or no capacity for viscous rate strengthening. In such zones stable flow can be achieved only through interaction with adjacent ice.
C23B-02
The Relationship Between Till Rheology and Water Content Derived From In-situ Wireless Multi-sensor Subglacial Probes
Subglacial till is a granular material, whose behaviour is an important control on glacier response to climate
change. Little is known about the rheology of till, because of the logistical problems of subglacial
experiments. We report the results from an innovative wireless probe ('smart pebble') embedded in a sand-
rich till. At low water contents, the till is relatively strong (due to liquid capillary bridges), and undergoes
deformation, reflecting the stick-slip mechanism of glacier motion. With high water content, the till undergoes
greater deformation and flows as a viscous Bingham material with grain rearrangement by sliding, dilation
and rolling. These experiments provide new field data to test theories of granular materials and show the
links between macroscale stress/strain relationships and microscale grain behaviour.
http://envisense.org/glacsweb/
C23B-03
Multiple, discrete till sheets created in differently sourced ice streams are deposited in the Des Moines lobe, USA
During review of the deposits of the Des Moines lobe--linked to drawdown of the southern Laurentide ice sheet--seven (upstream) and five (downstream) tills were distinguished within what had been considered a more homogenous deposit. Using lithology of the 1-2 mm sand fraction and texture of over 12,000 samples, several discrete till sheets were correlated for over 1,000 kilometers parallel to the flow direction. Lateral boundaries of till sheets are marked by abrupt lithologic and textural discontinuities and subtle geomorphic features including a broad, flow-parallel ridge overlain by pockets of sorted sediment. Deeply scalloped portions of moraine segments correspond to individual till sheets; cross-cutting subglacial landforms appear in areas where tills of similar age overlap; a till sheet is compositionally and texturally uniform in vertical section except for the lowermost few centimeters. We suggest that unique till sheets are created by ice from different ice-stream source areas, each with unique bed lithology. Substrate is incorporated and till homogenized where ice streams are confined by the ice sheet, but till is just exported as ice moves beyond these confines. A reduction in basal shear stress decreases the thickness of the deforming horizon, deposits till from the bottom up, and eliminates incorporation of new material. Independent behavior of flow zones derives from the activity of the ice stream source areas as well as the ability of the ice receiving area to maintain inherited initial conditions, i.e. ice thickness, velocity, and water pressure. Interplay between ice streams while still in the ice shed (e.g., capture) affects till provenance over time, but these changes take time to be transmitted down-ice and will result in a more subtle admixture of material. Contrasting velocity and/or ice thickness of neighboring flow zones may be apparent. The broad, flow-parallel ridge is interpreted as a subglacial feature, evidenced by oriented, striated clasts, overlain by sorted sediment that must have concentrated in an ice-surface low between two ice flows of contrasting velocity or thickness. The transverse- to diagonal- orientation of flanking stagnation landforms is consistent with the crevasse pattern expected in the stress field of a shear zone. Evidence of flow zones stagnating at different times comes from widespread ponding of meltwater over a zone that was confined by a more active ice flow. Ice margins are marked by a thickening of the till sheet, with basal melt-out till capped by supraglacial debris. This is interpreted as a result of freezing-on of subglacial material as ice slows and then stagnates. Sudden loss of subglacial water could help produce an ice-marginal thrust block and lead to subsequent incorporation of underlying, older material if thrust blocks with a deep detachment surface are overridden.
C23B-04
'Glacial Curvilineations': new Glacial Landforms Produced by Longitudinal Vortices in Subglacial Meltwater Flows.
The glacial landscape of the Dobrzyn Moraine Plateau (DMP) in central Poland contains a complex suite of enigmatic bedforms consisting of elongate and continuous sediment ridges occurring within tunnel valleys. We propose the term 'glacial curvilineations' for these new forms. The DMP consists of low-relief regional plateau composed of interbedded glaciofluvial sediments and diamictons. An anabranched network of tunnel valleys, containing tributary channels, dissects the plateau surface. Fields of parallel, sinuous ridges/troughs occur within tunnel valleys on at least three distinct topographic 'levels' decreasing in elevation toward the tunnel valley floors. These ridge-trough combinations are parallel with tunnel valley margins for up to 8-10 km. Ridge spacing is 50-150 m and intervening troughs exhibit localized overdeepenings occupied by lakes that are sometimes crescent-shaped. Tunnel valleys and curvilineations are eroded in the sediments of the moraine plateau. The terminus of tunnel valleys is marked by extensive glaciofluvial fans. The indissociable nature of tunnel valleys and curvilineations suggest a common genesis. We propose that they are the products of subglacial meltwater erosion. Curvilineations are erosional remnants produced by longitudinal vortices within the tunnel valley-forming flows. Tunnel valley inception determines the lateral extent of individual ridge fields. Parallelism of curvilineations reflects spacing of turbulent structures within the subglacial flow. Tunnel valley margins also control the sinuosity of curvilineations by dictating both the path and curvature of longitudinal vortices. Arguments for meltwater erosion are strengthened by the presence of extensive glaciofluvial fans at tunnel valley termini: they are the depositional counterparts of the extensive dissection of the DMP. Understanding curvilineation genesis may also shed light on glacier hydrologic processes. Tunnel valley tributaries develop suddenly in the plateau surface suggesting localized point-sources of meltwater, such as drainage of subglacial and/or supraglacial water bodies. As well, the regional extent of tunnel valleys and multiple topographic 'levels' containing curvilineations suggest sustained flows requiring more voluminous regional water storage and release.
C23B-05
Drumlins in reverse gear: observations against the relationship between drumlin stoss and lee asymmetry and flow direction
A well-known characteristic of drumlins is their longitudinal asymmetry in morphology with steep upstream slope angles (stoss) and more gentle downstream tails (lee). This characteristic is the main cue for deducing flow direction from drumlin fields. However, and although commonly accepted, few studies have actually quantified this drumlin property. How many drumlins in a field show this asymmetric form? Does it match with flow direction? Here we analyse drumlins from our mapping program of Great Britain, based on a high resolution DTM (5 m horizontal resolution, 0.7-1 m vertical accuracy) and yielding a GIS database of over 25,000 drumlins recorded as polygons, defined by their perimeter break-of-slope. Using GIS techniques we analysed long-profile asymmetry and compared this with the flow direction as derived from other information such as moraines, eskers, and spatial context within the ice sheet. Because drumlins may lie on a non- horizontal surface (i.e. hillslope), the first stage of analysis was to 'terrain correct' them. With all their bases adjusted to a horizontal plane, the highest point now also approximates the thickest part of the drumlin. We then computed a simple measure of asymmetry (Harry and Trenhaile, 1987) as the ratio of distances AB/(AB+BC), where A and C are the most upstream and downstream points respectively and B is the summit. In this paper we present the statistics of this analysis applied to the large sample of drumlins. The surprising result is that British drumlins are far from being "classically" asymmetrical. This is discussed in comparison to previous work and for its implication on drumlin formation theories.
C23B-06
Testing a numerical model of ribbed moraine formation arising from an instability in the coupled flow of ice and subglacial sediment
Ribbed moraines are large ridges of sediment produced transverse to ice flow direction that formed widely
beneath palaeo-ice sheets. Since ice sheet stability is sensitive to conditions operating at the bed, an
understanding of ribbed moraine genesis will provide critical information on ice sheet dynamics. Currently,
there is no consensus on ribbed moraine formation and various competing hypotheses have been presented
to account for their genesis. Only one of these theories has been developed into a physically based
numerical model that quantitatively describes ribbed moraine formation. This theory, known as the Bed
Ribbing Instability Explanation (BRIE), argues that ribbed moraines are produced by a naturally arising
instability in the coupled flow of ice and till. BRIE demonstrates that transverse subglacial ridges (i.e., ribbed
moraine) spontaneously grow under certain parameter combinations, and it predicts their wavelength
(spacing between ridges). The model represents a significant advance because it is the first time a theory of
subglacial bedform generation has been developed to make quantitative predictions which can be formally
tested. This talk discusses the types of tests that are currently possible and reports the results from the first
testing of BRIE. This analysis centres on the ability of BRIE to predict the primary characteristics of ribbed
moraine, which are patterning and wavelength. Results show that BRIE successfully predicts the correct
ribbed moraine pattern and appropriate wavelengths. The tests fail to falsify the model, and it is concluded
that BRIE remains a viable explanation of ribbed moraine formation.
http://www.agu.org/pubs/crossref/2008/2007JF000954.shtml
C23B-07
Meltwater Behavior in Ice Streams Inferred from Deposits in Shelf-crossing Troughs on the Southeast Canadian Margin
Shelf-crossing troughs draining Laurentide and Newfoundland sheets of the southeast Canadian margin are investigated using high and ultrahigh seismic reflection profiles and cores. Paleo-ice streams formed diverse seismostratigraphic facies and geometries which provide evidence for subglacial, grounding zone and ice marginal meltwater behavior. Common to most troughs is a down-ice transition from an overdeepened and erosive inner to mid-shelf situation. This transitions to net deposition, either as trough-flanking tills or broad mid or outer shelf moraines. Distal to the moraines are debris flow and/or mass failure deposits interbedded in various geometries with stratified glacimarine plume-dominated deposition. Finally, late-stage channelization and/or sheet erosion is common, some with an outburst nature. The moraines are broad and thick enough (up to 80 m) that they were constructed in a near-horizontal aggradational nature. Seismic interpretation suggests deposition from a broad grounding zone with constrained (non-channelized) water evolving to a more communicable or channelized system. Successively stacked up-ice-dipping till sheets, with preserved depositional surfaces (stratified sediment over a fluted surface), mimic the proximal moraine flank. They were deposited with progressive retreat and sheet- aggradation from the ice stream sole. The latest sheet has ubiquitous pockets of water-deposited sediment embedded in an otherwise till-like regime. Occasional channels at its top demonstrate an evolution to local free flow. Till tongues and glacigenic debris flows at the distal moraine margin evolve directly from a dilatent ice stream grounding line effluent, into debris flow channels and aprons without appreciable interim deposition. The tongues are interbedded with thin water-lain (plume) deposits, but devoid of associated channels, indicating intermittent switching between dilatant slurry efflux and meltwater plumes. Outburst flooding evidence is best recognized by hyperpycnal slope and rise deposits, a giant moraine breaching, mid-axis marginal meltwater channeling in late glacial sediments, a ubiquitous late glacial unconformity and large sandwave fields. Chronology relating erosion and deposition events places them penecontemporaneously, but more precision is needed. All the trough settings possess a mid-shelf overdeepened basin which is the presumed site of sub-glacial lake from which the outbursts originate. We suggest that the diversity in deposits and erosional events reflects evolution of a continuum from a situation where meltwater is entirely constricted interstitially in a subglacial dilatant slurry to a spatially transitional or periodic alternation from slurry to water-lain plume and finally to a free-flowing, channelized and occasionally periodic outburst process.
C23B-08
A glacial hydrofracture origin for the Sula Ridges, Skeidararsandur, Iceland.
The western edge of Skeidarársandur outwash plain near the Súla River has undergone a complex
history of ice surging, jökulhlaups and glacial melt resulting in a landscape dominated by moraines,
hummocks, glacial outwash, and a curious suite of straight ridges, here referred to as the Súla Ridges.
The ridges are covered by pebbles, cobbles and boulders at their surfaces, and they are typically 20-200 m
long, 2-10 m wide and 0.5-5 m high. A few are up to 3 km long. Locally, the ridges have a rectilinear pattern,
that is, they are sub-parallel and are cross-cut by smaller ridges at right angles. Aerial photos show that the
terrain was under ice in 1945 but some of the most prominent ridges coincide with cracks on the 1945 ice
surface that are oriented parallel or at right-angles to the ice margin. It is therefore assumed that structural
control for the ridges was emplaced between the turn of the 20th century when Skeidarárjökul Glacier
was near its maximum extent, and 1945.
Four 100 MHz ground penetrating radar (GPR) lines were collected over three small adjacent ridges that are
cross-cut by at least 2 more ridges at right angles. The GPR stratigraphy is fairly simple with, an average
penetration of 4m through layered sediments that are interpreted as glacial outwash. An irregular surface at
depth prevents further penetration of the GPR signal. Based on exposure in a nearby cliff, this surface is
likely glacial till. There is deeper penetration, up to 8 m over, or immediately adjacent to, some of the ridge
crests, although no discernible bedding is seen in the GPR profiles. Subsequent trenching demonstrates
that the deeper penetration represents near vertical sheets of open-worked sub-rounded pebbles and
cobbles, and thus the ridges are the surface expression of a network of clastic dykes.
It is hypothesized that the ridges are relics of hydrofracture fills that formed during major jökulhlaups.
The hydrofractures formed near the ice front when rapidly increasing water-pressure during the rising stage
of jökulhlaups drove water from overpressured subglacial channels and/or aquifers up through the
overlying ice and possibly through till that was already deposited. These ridges represent the surface
expression of where water escaped from the ice surface and continued to flow. As flow waned, sediment was
deposited in the cracks. In-situ melting of the ice allowed gradual lowering of these sediments and
preservation of the dykes. The study area in known to have been affected by several major jökulhlaups
during the timeframe of probable formation. The draining of Graenalón ice-marginal lake resulted in major
floods in1935 and 1939, and several subglacial volcanic eruptions occurred in 1903, 1922, and 1938 and
also resulted in major jökulhlaups .
http://neogeo.kent.edu/Munro/Sularidges