C52A-01
Do Glacier Dams Retard River Incision Into Southeast Tibet?
The southeastern fringe of the Tibetan plateau around the Namche Barwa syntaxis is characterized by rapid uplift and fluvial incision, having created spectacular relief along the Yarlung Tsangpo river gorge. This region hosts numerous monsoonal temperate glaciers, some of which have been historically dynamic, and many of which have advanced from steep tributaries to repeatedly block or divert trunk rivers during the Quaternary. We propose that repeated glacial damming has resulted in knickpoint formation, as well as valley-floor aggradation and increases in active channel widths that serve to retard fluvial bedrock incision into the flanks of the Tibetan Plateau. The coupled reduction of fluvial incision upstream of glacier dams and enhanced incision downstream of the lowermost dams would act to maintain an edge to the Tibetan Plateau by defeating propagation of knickpoints into the Plateau in a region of high uplift and precipitation. The positions of several large moraine dams in an area where local relief and erosion are highest require depressions of the regional equilibrium line altitude (ELA) of ~ 450 m, which is within the range of ELA depression for Holocene glacial advances. Such an ELA depression would on average impound ~ 40% of the river network of major rivers upstream of glacier dams; estimated, though debated, Last Glacial Maximum ELA depression of 1000 m would result in damming of about 75% of these river networks. Whether driven by increased monsoon precipitation, temperature fluctuations, or changes to supraglacial sediment flux, Quaternary glacier advances indirectly contribute to effectively retarding river incision into southeast Tibet.
C52A-02 INVITED
Testing the bed-deformation hypothesis using magnetic fabrics of tills sheared experimentally and subglacially
The bed-deformation hypothesis asserts that glaciers can move and transport sediments primarily by shearing their soft beds. Numerous models of both glacier flow and development of subglacial landforms (e.g., till plains, drumlins, flutes, megalineations, tunnel valleys, Rogen moraines, and boulder pavements) incorporate this process. It requires that basal sediments be sheared to very high strains (> 100) and over depth ranges sufficient to account for most basal motion and sediment transport. Thus, the key issue in testing this hypothesis is not whether basal sediment has been sheared but how much and over what thickness of the bed. We have studied basal tills of southern lobes of the Laurentide ice sheet in ring-shear experiments to determine effects of strain magnitude and direction on anisotropy of magnetic susceptibility (AMS). These experiments indicate that fabrics defined by directions of the maximum principal susceptibility (k1) strengthen progressively with shear strain but at exponentially decreasing rates, becoming steady at strains of 10-20. Steady k1 fabrics are strong (S1 eigenvalue = 0.83-0.94), with k1 orientations clustered in the direction of shear and plunging gently "up-glacier." Ancillary experiments indicate that anisotropy results from alignment of silt-sized and smaller magnetite grains present in most tills. AMS fabrics integrate effects of many such particles and involve little subjectivity, so AMS fabrics are more accurate strain indicators than traditional particle fabrics. AMS fabrics measured along profiles through till units of the Superior (Douglas till, Wisconsin) and Lake Michigan lobes (Batestown till, Illinois) indicate that deformation to strains greater than ~ 10 can be ruled out in many parts of the bed. Elsewhere, fabrics are sufficiently strong to indicate that such strains were equaled or exceeded. However, spatial variability of fabric strength and direction indicates that only minor fractions of the thicknesses of these till units sheared at one time and that deformation was heterogeneous over the bed area. These field data are the first to benefit from laboratory fabric-strain calibrations and provide little support for deep, pervasive deformation of the bed to high strains.
C52A-03
Detailed Seafloor Evidence of Palaeo-ice Streaming Offshore N. Wales, UK: Implications for Decline of the Irish Sea Glacier
Recently acquired high resolution swath bathymetry data, 20 km offshore North Wales, UK reveal part of a glaciated terrain, unique in its kind on the European Shelf and provides the sole evidence for a grounded part of the Irish Sea Glacier. Exceptionally well-preserved ribbed moraines, drumlins, flutes, eskers, De Geer moraines and iceberg ploughmarks are all associated on a submarine landscape created in a subglacial and proglacial environment during times of final deglaciation of the Irish Sea. The shallow character of the till-bedrock interface offshore North Wales is enhanced by the presence of a major anticline in Carboniferous strata. In this setting, the spatial distribution and morphology of the observed ribbed moraines, drumlins, flutes and eskers can be explained using the time-space domain model for glacial bedform formation (Kleman and Hätterstrand, 1999). Spatial analysis of all the observed glacial features suggests a deglaciation mechanism for the grounded Irish Sea Glacier in the central and northern Irish Sea. It illustrates how a thawing front migrated up-glacier, drumlinising the subglacial bed, and halted at the edge of the ribbed moraine field close to where the anticline crops out. A quick and stepwise retreat of a grounded glacier followed, actively calving into a shallow proglacial water body. This water body then rapidly uplifted the glacier and almost simultaneously the subglacial terrain became ice free. This fast flooding prevented any terrestrial or coastal sediment from burying the glacial topography, allowing the glacial terrain to be beautifully preserved. Kleman, J., and Hätterstrand, C., 1999, Frozen-bed Fennoscandian and Laurentide ice sheets during the Last Glacial Maximum: Nature, v. 402, p. 63-66.
C52A-04
Influence of a frozen glacier margin on sediment transport: measurements and modeling
Broad belts of high-relief hummocky topography near the margins of former glaciers and ice sheets are commonly considered to be paleoclimatic indicators. These landscapes are thought by some to indicate a frozen bed at the glacier margin, thus requiring cold atmospheric temperatures at the time of formation. Such interpretations are usually justified by analogy with some modern polythermal glaciers where debris of apparent subglacial origin accumulates at the glacier surface in ridges and subsequently melts out over stagnating ice at the glacier terminus. In these modern glaciers, basal debris may be uplifted to the surface along shear zones or faults originating from the transition between warm-based ice upglacier and cold-based ice at the terminus. However, the relationship between supraglacial debris accumulation and the mechanics of ice flow at a frozen margin is not well established, so caution is needed in drawing paleoclimatic conclusions from these landscapes. Through a combination of field measurements and numerical modeling at Storglaciaren, Sweden, we attempt to better constrain this relationship. Storglaciaren is a small polythermal glacier with a cold surface layer that varies in thickness from 20 to 35 m. Borehole thermistor measurements and radar indicate that the zero-degree isotherm dips into the bed as the ice thins toward the terminus. Ice downglacier of this basal thermal transition (BTT) is frozen to the substrate, inhibiting basal slip whereas a short distance upglacier, borehole measurements (slidometers) indicate that the ice is slipping over the bed. Resulting longitudinal compression may result in uplift of basal ice near the BTT. Indeed, bands of sediment-rich ice, dipping steeply upglacier and apparently derived from the glacier bed, outcrop at the ice surface just downglacier from the BTT. Surface velocity measurements indicate that emergence velocity peaks just upglacier from the debris bands, but evidence for discrete shear across the debris bands is lacking. On the contrary, a finite element model of ice flow under similar conditions suggests that a slip/no-slip transition at the bed is sufficient to locally enhance emergence velocities without strongly localized shear and thereby convey basal ice to the glacier surface.
C52A-05
A Sediment Wedge and an Instantaneous End-Moraine: a Twofold Ice-marginal Product of the 1890 Glacier Surge of Bruarjokull, Iceland
Contemporary understanding of the behaviour of surging glaciers and ice streams is hampered by the lack of data on landsystem evolution and sedimentary environments. This study concerns the ice-marginal environment of the surge-type Brúarjökull in Iceland. The sediment distribution in the glacier forefield as well as the morphology, sedimentology and tectonic architecture of the 1890 end moraine is investigated for highlighting the interaction between very dynamic ice and sediment/landform associations. As a result of substrate/bedrock decoupling during the 1890 surge, subglacial sediment was dislocated across the bedrock surface and deformed compressively, leading to gradual substrate thickening and the formation of a sediment wedge in the marginal zone. A drop in subglacial porewater pressure at the very end of the surge led to substrate/bedrock coupling and a stress transfer up into the sediment sequence causing brittle deformation of the substrate. Simultaneously, the glacier toe ploughed into the topmost part of the marginal sediment wedge initiating the moraine-ridge construction. Fine-grained and incompetent sediment deformed in ductile manner, resulting in a narrow rooted-fold-dominated moraine while coarse-grained and competent sediment deformed in brittle fashion, resulting in wide imbricated moraine. A new sequential model of subglacial and ice marginal processes operating during a glacier surge is proposed, illustrating the stepwise formation of a surging-glacier marginal sediment wedge and an instantaneous end moraine - a twofold, inseparable marginal end-product of the 1890 surge. As a result of high ice-flow velocities (100-120 m/day) the sediment wedge is thought to have formed in approximately five days and the end moraine in about one day.
C52A-06
Subglacial Landforms and Processes: new Information From 3D Seismic Technology
Three-dimensional (3D) seismic interpretation and imaging techniques provide a unique means of investigating submarine geomorphic features produced by former ice sheets. An extensive two-dimensional (2D) and 3D seismic data base is here used to image the imprints left behind by glaciers that flowed out a major cross-shelf trough (Bjornoyrenna) of the north-Norwegian continental shelf during repeated glacial episodes. Mega-scale glacial lineations characterize the seafloor geomorphology of Bjornoyrenna and smaller, contributing cross-shelf troughs, where they are inferred to represent flow-lines of former ice streams that where active during the most recent (Weichselian) glacial period. Similar features are commonly observed on buried horizons. Large- scale seafloor imprints from an early readvance after the last glacial maximum are especially well preserved. Streamlined landforms and associated lobe-shaped ridges indicate that this major cross-shelf trough hosted six separate ice stream lobes that diverged fan-like at their margins, but were not all active simultaneously. A 300 km wide grounding-zone wedge results from high sediment flux within sub-ice stream deformable beds. A 2 to 3 km thick Pleistocene record is preserved at the mouth of Bjornoyrenna, in the Bjornoya Trough Mouth Fan. The preservation of up to several hundred meters of glacigenic sediments between the buried, glacially eroded surfaces, provides here the opportunity to study the internal structure of till units. 3D seismic attribute maps reveal that megablocks and rafts commonly occur within the till units. The sediments blocks are often aligned in chains that may be up to 2 km wide and over 50 km long. The largest individual megablocks have an areal extent of over 2 km2. The sediment chains are interpreted to have been eroded, transported and deposited by grounded ice, most probably fast-flowing ice streams. This is based on the relationship between the sediment chains and the horizons revealing mega-scale glacial lineations. A borehole documents that one of the sediment blocks consists of a 15 to 25 m thick sequence of consolidated mid Cretaceous sedimentary rock, buried in a muddy till. Both mega-scale glacial lineations and chains of sediment blocks and rafts are inferred to be products of fast- flowing ice streams, and hypothesized to represent different modes of ice-stream sediment deformation. The mega-scale glacial lineations indicate deformation of subglacial sediments close to the ice-sediment interface. The megablocks and rafts suggest deformation deeper in the sediments, probably related to freezing-on to the base of the ice.
C52A-07
Slow Erosion by a Fast Glacier
The dramatic imprints left on landscapes by Quaternary glaciations provide abundant evidence that glaciers can be efficient erosive agents. Understanding their erosive capability is key to unravelling the complex coupling of climate and the dynamics of the solid earth. Recent studies have focused on quantifying rates of glacial erosion to provide key data for understanding the physics that drives glacial erosion. While these studies concur that glacial erosion can outpace fluvial erosion, the magnitude of the measured erosion rates varies from ≫10 to <1.0 mm/yr. This discrepancy is, in part, because these studies estimate erosion rates over vastly different spatial and time scales. One excellent region for addressing the long-term, orogen-scale impact of glaciers on the landscape is the tectonically quiescent Transantarctic Mountains (TAM) that have been glaciated since ~34 Ma. Recent geomorphic investigations in the region of Byrd Glacier (the largest glacier in the TAM) indicate that the current geomorphology reflects ancient fluvial and tectonic processes active in the Cretaceous through Eocene, suggesting minimal reshaping of the landscape by glacial processing during the past ~34 Ma. These geomorphic conclusions are supported by new apatite fission track thermochronology from the valley wall of the Byrd Glacier that show that average erosion rates have been minimal since onset of glaciation (≪ 1 mm/yr). In light of the high ice velocities of the Byrd Glacier, these surprisingly low erosion rates challenge the current paradigm that erosion rate simply scales with ice flux. These results indicate that critical physics is missing from our glacial erosion laws.
C52A-08
Origin, Evolution, and Preservation of Cold Based Debris Covered Glaciers: Quantifying Sublimation Rates of Ancient Buried Ice in Antarctica
Growing interest in our planet's climate history has placed a premium on acquiring detailed records of past climate change. Of considerable interest are archives of ancient atmosphere trapped within the debris-covered alpine glaciers of the western Dry Valleys region of Antarctica. The Mullins Valley debris-covered glacier (~8 km in length) is sourced from local snowfall at the steep headwall of the valley. The first 1.2 km of this glacier is generally free of overlying debris except for isolated cobbles and boulders. Thereafter, the ice surface is covered with a thin, continuous sheet of dolerite-rich rubble. Factors that influence the origin and modification of this ice include atmospheric temperature and relative humidity, precipitation, incoming solar radiance, surface albedo, till texture, winds, surface roughness, salts, and secondary ice lenses. We applied a diffusion model to track vapor flux within a sublimation till overlying the Mullins Valley debris-covered glacier, purportedly the world's oldest debris-covered alpine glacier. As input, we used meteorological data from HOBO data loggers that captured climate change and till temperatures. Results show that vapor flows into and out of the sublimation till at rates dependent on the non-linear variation of soil temperature with depth. Sublimation rates along the Mullins Glacier varied as a function of till thickness, local climate (using a calculated regional lapse rate of 0.88°C per 100 m), and till texture. Ice loss during the study interval (November 27, 2006 to December 24, 2006) ranged from as high as 2.12 mm for exposed glacier ice in the upper ablation zone, to as low as 0.01 mm for buried ice beneath till >50 cm in thickness. Averaged over the entire ablation zone (6.7 km2), this yields a net ice-surface lowering of 0.32 mm during the study interval. Numerical modeling suggests that a modest ice accumulation rate at the headwall of ~1 cm a-1 appears sufficient to maintain current ice volumes. Overall, our model results are consistent with an inferred Miocene age for distal portions of the Mullins Valley debris-covered glacier and suggests that ancient atmosphere may be preserved in buried glaciers in the western Dry Valleys region.