Daniel E. Lawson

How glaciers entrain and transport basal sediment: Physical constraints

Abstract Simple insights from the physics of ice, water and sediment place constraints on the possible sediment-transport behavior of glaciers and ice sheets. Because glaciers concentrate runoff, streams generated by glaciers transport much sediment and may erode bedrock rapidly. Deforming glacier beds also can transport much sediment, particularly in marginal regions. Rapid sediment entrainment producing thick debris-rich basal zones may occur by regelation into subglacial materials, and by freeze-on from rising supercooled waters. Numerous other mechanisms may be important but primarily near ice margins, especially those of advancing or fluctuating glaciers. Several sediment-entrainment mechanisms may be active beneath a single glacier, but one process is likely to be dominant at any place and time.

Glaciohydraulic supercooling : a freeze-on mechanism to create stratified, debris-rich basal ice : I. Field evidence

Debris-laden ice accretes to the base of Matanuska Glacier, Alaska, U.S.A., from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening. Frazil ice grows in the water column and forms aggregates, while other ice grows on the glacier sole or on substrate materials. Sediment is trapped by this growing ice, forming stratified debris-laden basal ice. Growth rates of >0.1 m a -1 of debris-rich basal ice are possible. The large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings, including Heinrich events and the till sheets south of the Laurentian Great Lakes.

Stabilizing feedbacks in glacier-bed erosion.

Glaciers often erode, transport and deposit sediment much more rapidly than nonglacial environments, with implications for the evolution of glaciated mountain belts and their associated sedimentary basins. But modelling such glacial processes is difficult, partly because stabilizing feedbacks similar to those operating in rivers have not been identified for glacial landscapes. Here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks. We find that the long profiles of beds of highly erosive glaciers tend towards steady-state angles opposed to and slightly more than 50 per cent steeper than the overlying ice–air surface slopes, and that additional subglacial deepening must be enabled by non-glacial processes. Climatic or glaciological perturbations of the ice–air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate.

Ground-penetrating radar reflection profiling of groundwater and bedrock in an area of discontinuous permafrost

We have used ground‐penetrating radar to profile the depth of permafrost, to groundwater beneath permafrost, and to bedrock within permafrost in alluvial sediments of interior Alaska. We used well log data to aid the interpretations and to calculate dielectric permittivities for frozen and unfrozen materials. Interfaces between unfrozen and frozen sediments above permafrost were best resolved with wavelet bandwidths centered at and above 100 MHz. The resolution also required consideration of antenna configuration, season, and surface conditions. Depths to subpermafrost groundwater were profiled where it was in continuous contact with the bottom of the permafrost, except near transitions to unfrozen zones, where the contact appeared to dip steeply. The complexity of the responses to intrapermafrost bedrock, detected at a maximum depth of 47 m, appears to distinguish these events from those of subpermafrost saturated sediments. The relative dielectric permittivity ranged between 4.4 and 8.3 for the permafro...

Glaciohydraulic supercooling : a freeze-on mechanism to create stratified, debris-rich basal ice : II. Theory

Simple theory supports field observations (Lawson and others, 1998) that subglacial water flow out of overdeepenings can cause accretion of layered, debris-bearing ice to the bases of glaciers. The large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths. If that flow ascends a sufficiently steep slope out of an overdeepening, the water will supercool as the pressure-melting point rises, and basal-ice accretion will occur. Diurnal, occasional or annual fluctuations in water discharge will cause variations in accretion rate, debris content of accreted ice or subsequent diagenesis, producing layers. Under appropriate conditions, net accretion of debris-bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget.

Flux of debris transported by ice at three Alaskan tidewater glaciers

The stability of tidewater terminus is controlled by glacial dynamics. Calving processes and sedimentary processes at the grounding line. An investigation of grounding-line sediment dynamics and morainal-bank sediment budgets in Glacier Bay, Alaska, U.S.A., has yielded data that enable us to determine the debris fluxes of Grand Pacific, Margerie and Muir Glaciers, Debris flux ranges from 10 5 to 10 6 m 3 a −1 , one to two orders of magnitude lower than the glacifluvial sediment fluxes (10 6 -10 7 m 3 a −1 ). Combined these fluxes represent the highest yields known for glacierized basins. Large debris fluxes reflect the combined effecrs of rapid glacier flow, driven by the maritime climate of southeast Alaska, and highly erodible bedrock, Englacial-debris distribution is affected by valley width and relief both of which control the availability of sediment. The number of tributaries controls the distribution and volume of debris in englacial and supraglacial moraines. At the terminus, iceberg-rafting removes up to two orders of magnitude more sediment from the ice-proximal environment than is deposited by melt-out or is dumped during calving events. Rough estimates of the sediment flux by deforming beds suggests that soft-bed deformation may deliver up to an order of magnitude more sediment to the terminus than is released from within the glacier ice.

Glaciohydraulic supercooling in Iceland

We present evidence of glaciohydraulic supercooling under jokulhlaup and ablation- dominated conditions from two temperate Icelandic glaciers. Observations show that freezing of sediment-laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes. Intraglacial frazil ice propagation under normal ablation-dominated conditions can trap copious volumes of sediment, which forms anomalously thick sections of debris-rich ice. Glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development. Extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling, which may explain the concentration of englacial sediments deposited in Heinrich layers in the North Atlantic during the last glaciation.

Basal-crevasse-fill origin of laminated debris bands at Matanuska Glacier, Alaska, U.S.A.

The numerous debris bands in the terminus region of Matanuska Glacier, Alaska, U.S.A., were formed by injection of turbid meltwaters into basal crevasses. The debris bands are millimeter(s)-thick layers of silt-rich ice cross-cutting older, debris-poor englacial ice. The sediment grain-size distribution of the debris bands closely resembles the suspended load of basal waters, and of basal and proglacial ice grown from basal waters, but does not resemble supraglacial debris, till or the bedload of subglacial streams. Most debris bands contain anthropogenic tritium (3H) in concentrations similar to those of basal meltwater and ice formed from that meltwater, but cross-cut englacial ice lacking tritium. Stable-isotopic ratios (δ 18 O and δD) of debris-band ice are consistent with freezing from basal waters, but are distinct from those in englacial ice. Ice petrofabric data along one debris band lack evidence of active shearing. High basal water pressures and locally extensional ice flow associated with overdeepened subglacial basins favor basal crevasse formation.

Microstructures of glacigenic sediment-flow deposits, Matanuska Glacier, Alaska

Microstructures of glacigenic sediment gravity-flow deposits formed at the terminus of the Matanuska Glacier, Alaska, were analyzed to characterize flow type. These sediment flows have been classified into four types based primarily on water content and sedimentological characteristics (Lawson, 1979a, 1982). Thin sections of flow deposits show a variety of microand mesoscale characteristics that vary according to water content of the source flow. Wet-type flow deposits are characterized in thin section by a well-defined parallel and imbricated microclast fabric and thin laminations resulting from laminar to plastic flow regimes. Dry-type flow deposits are characterized in thin section by bior polymodal or random microclast fabrics, greater textural heterogeneity, and deformational microstructures associated with plastic to brittle flow regimes. Thin laminations and a “laminar flow fabric” in wet-type flow deposits may be characteristic of sediment gravity flow in a glacial environment. Characterization of these microstructures supports the contention that micromorphological analyses can be used to elucidate sediment flow genesis and the conditions of the flow just prior to deposition. Thus, micromorphology may also be useful for differentiating sediment-flow type in Pleistocene diamictons in other locations. Lachniet, M. S., Larson, G. J., Strasser, J. C., Lawson, D. E., Evenson, E. B., and Alley, R. B.,1999, Microstructures of glacigenic sediment-flow deposits, Matanuska Glacier, Alaska, in Mickelson, D. M., and Attig, J. W., eds., Glacial Processes Past and Present: Boulder, Colorado, Geological Society of America Special Paper 337. 45 *Current address: Department of Earth Sciences, Syracuse University, Syracuse, New York 13244 type sediment-flow deposits correspond approximately to Lawson type III and IV flow deposits (high water content; Lawson, 1979a, 1982; see below for sediment flow type characteristics). Here we evaluate the use of micromorphological analysis to differentiate contemporary dry-type from wet-type sediment-flow deposits formed at the terminus of the Matanuska Glacier. This study deals exclusively with the micromorphology of sediment-flow deposits; the study of the micromorphology of tills is beyond the scope of this study and has not been undertaken at the Matanuska Glacier. Future investigation on the micromorphology of known glacial sediments will allow the further distinction between sediment-flow deposits and true tills.

Orogenic and glacial research in pristine southern Alaska

Southern Alaska is an exceptional natural laboratory for studying a range of geologic problems, including the links between orogenic processes, landscape modification by glacial processes, and continental margin sedimentation. Geologic processes operate at rapid rates along the southern Alaskan margin, which allows scientists to concurrently collect data on tectonic deformation, uplift, erosion, and sedimentation, and develop comprehensive models that connect these diverse processes. Significant advancements have been made in studying fundamental geologic processes in this region. However, efforts to link these into comprehensive models are in their infancy and fundamental research questions remain. The National Science Foundations MARGINS Source-to-Sink program (see http://www.ldeo.columbia.edu/margins/SciencePlan.15Novpdf) will provide an interdisciplinary group of geoscientists the opportunity to merge these studies and significantly advance our understanding of continental margins.