Glaciers, erosion and climate change in the Himalaya and St. Elias Range, SE Alaska
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The important roles of glaciers in topographic evolution, relief development, and sediment production are well recognized but understanding remains rather limited largely because of the inherent complexity of glacial erosion in diverse terrains, the lack of validated glacial erosion models, and the sparse nature of the data available on erosion rates. The primary focus of this research is defining and understanding rates of glacial erosion and their spatio-temporal variation at the scales of single glacier basins and entire orogens. I study glacial erosion in two tectonically active mountain ranges, the Himalaya and the St. Elias Range (Alaska), ideally suited for this study because of the wealth of pertinent data already available, and because they represent a broad range of climates and glacier types. In the Himalaya study, I also examine the impact of the debris produced by glacial erosion that accumulates on the glacier surface on glacier mass balance and the response of the glacier to climate change. In the SE Alaska study, I model the spatial pattern of erosion rates over an entire glacial cycle, and compare the temporally averaged rates to published rates of exhumation to validate and calibrate an erosion model. These model results illuminate the source region and temporal aspects of the offshore sediment record that have received considerable attention in the context of climate-driven modulation of erosion and sediment production. Large data sets on exhumation rates spanning the entire Himalayan arc have documented spatial and temporal variations in erosion rates; however, data on glacial erosion rates at the heavily glaciated crest of the Himalaya are very sparse. In light of this weakness in the knowledge base, I integrate several types of field research to investigate rates of erosion for a single glaciated basin at the base of Mt. Everest. I found that erosion rates for two timescales, contemporary (101 yr) and over the Holocene (104 yr), are similar to published long-term (O ~ 107 yr) exhumation rates (~1 mm/yr) derived from thermochronometric data in the region. The apparent uniformity of erosion and exhumation rates over a large range of time implies a surprising insensitivity to likely variations in climate, structural development, and relief evolution; it also contrasts with recent studies emphasizing the variation of rates over different timescales. Moreover, measurements of the suspended sediment flux out of the proglacial stream suggest that the fluvial evacuation rate of suspended sediments is ~50x less than the contemporary sediment production rate. This, together with the known time over which sediment has accumulated in the basin, the downglacier decrease in sediment flux, and evidence that the contemporary glacier is perched on a 20–100 m thick debris edifice, implies that most of the eroded debris remains within the basin. This result provides new insights into the geomorphic development of the high relief in the Himalaya, and the episodic nature of the downstream transfer of sediment from high glaciers. To investigate links between basin erosion, debris transfer, and the evolution of debris-covered glaciers during periods of climate change, I numerically model the coupled evolution of ice and debris for Khumbu Glacier. For the first time, I define a relationship between ice-melt rate and debris thickness, representative of the thick surface debris characteristic of the Khumbu region, and implement it in the model to quantitatively explore the “debris-covered glacier anomaly”. The model simulates the response of a debris-covered glacier to changes in climate forced by variations in the net mass balance represented by vertical shifts in the equilibrium line. Model results indicate that despite the thick debris cover, Khumbu Glacier has thinned at rates similar to current rates measured by remote sensing, averaging 0.4 m w.e./yr, for over a century since the Little Ice Age (LIA). Even under a constant climate, it will continue to thin into the future, by about 6–8% by AD2100, largely in the middle part of the glacier with minor changes in the terminus ice thickness and extent. In SE Alaska, I expand the study region from a single catchment to an entire orogen where I model the spatial distribution of erosion rates on two timescales, the present-day and the longer-term. The latter represents the past ~100-kyr when much larger ice masses covered the study area and underwent large oscillations; by inference, it represents the Quaternary during which these large oscillations prevailed and much of the orogen was exhumed. I hypothesize that the rate of erosion increases with the glacier power, the amount of energy available for erosion per unit time and per unit area of the glacier bed, which has the advantage of representing the strength of the ice-bed coupling, the basal shear stress, as well as the sliding rate. When averaged over an entire major cycle, glacier power accounts for nearly 70% of the variation in the published exhumation rates inferred from thermochronology data from the entire orogen despite the large range of substrate characteristics expected in the region. The strong correlation between exhumation rates and glacier power validates the hypothesis that the rate of erosion scales with power and the numerical erosion model. Model results define the zones of rapid exhumation as the zones of steep and rapid glaciers. Moreover, the results dispel the notion that rapid erosion is spatially coincident with the long-term position of the equilibrium line; averaged over the major Quaternary glaciations, the position of the equilibrium line is well south of zones of rapid exhumation, close to the continental shelf break in the Gulf of Alaska.