Sediment sources and transport by the Kahiltna Glacier and other catchments along the south side of the Alaska Range, Alaska A. Matmon, P. J. Haeussler, M. Arnold, D.L. Bourlès, Georges Aumaitre, Karim Keddadouche To cite this version: A. Matmon, P. J. Haeussler, M. Arnold, D.L. Bourlès, Georges Aumaitre, et al.. Sediment sources and transport by the Kahiltna Glacier and other catchments along the south side of the Alaska Range, Alaska. Geosphere, Geological Society of America, 2020, 16 (3), pp.787-805. 10.1130/GES02190.1. hal-03152420 HAL Id: hal-03152420 https://hal.archives-ouvertes.fr/hal-03152420 Submitted on 25 Feb 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Research Paper THEMED ISSUE: Geologic Evolution of the Alaska Range and Environs GEOSPHERE Sediment sources and transport by the Kahiltna Glacier and other catchments along the south side of the Alaska Range, Alaska GEOSPHERE, v. 16, no. 3 A. Matmon1, P.J. Haeussler2, and ASTER Team3,* 1Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel https://doi.org/10.1130/GES02190.1 2U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508, USA 3Centre de Recherche et d’Enseignement de Géosciences de l’Environnement (CEREGE), UMR 6635 Centre National de la Recherche Scientifique (CNRS), Aix-Marseille University, BP 80, 13 545 10 figures; 3 tables Aix en Provence, Cedex 4, France CORRESPONDENCE: [email protected] ABSTRACT drainage system are not in erosional steady state with respect to cosmogenic isotope accumulation. CITATION: Matmon, A., Haeussler, P.J., and ASTER Team, 2020, Sediment sources and transport by the Erosion related to glacial activity produces enormous amounts of sediment. Kahiltna Glacier and other catchments along the south However, sediment mobilization in glacial systems is extremely complex. side of the Alaska Range, Alaska: Geosphere, v. 16, Sediment is derived from headwalls, slopes along the margins of glaciers, ■ INTRODUCTION no. 3, p. 787–805, https://doi.org/10.1130/GES02190.1. and basal erosion; however, the rates and relative contributions of each are unknown. To test and quantify conceptual models for sediment generation Geomorphologists have long recognized the important role of glaciers in Science Editor: Andrea Hampel 10 Guest Associate Editor: Jeffrey A. Benowitz and transport in a simple valley glacier system, we collected samples for Be shaping some of the most spectacular landscapes on Earth (e.g., Esmark, 1824), analysis from the Kahiltna Glacier, which flows off Denali, the tallest mountain including deep fjords, rugged mountains, and broad valleys that drain vast polar Received 11 August 2019 in North America. We collected angular quartz clasts on bedrock ledges from ice sheets. Glaciers flowing down alpine cirques and into deep valleys create Revision received 19 January 2020 a high mountainside above the equilibrium line altitude (ELA), amalgamated a distinctive landscape by their vigorous erosion. They produce prodigious Accepted 10 February 2020 clast samples from medial moraines, and sand samples from the river below quantities of sediment ranging from house-size boulders to fine silt. Such val- the glacier. We also collected sand from nine other rivers along the south ley glaciers derive their sediment loads from weathering and mass wasting on Published online 10 March 2020 flank of the Alaska Range. In the upper catchment of the Kahiltna drainage adjacent slopes and from the abrasion of rock beneath (e.g., Church and Ryder, system, toppling, rockfall, and slab collapse are significant erosional processes. 1972; Boulton, 1996; Alley et al., 2003; MacGregor et al., 2009). One problem of Erosion rates of hundreds of millimeters per thousand years were calculated glacial geomorphology is that many of the processes related to glacial erosion from 10Be concentrations. The 10Be concentrations in amalgamated samples work beneath ice and are exceptionally difficult to observe or infer. from medial moraines showed concentrations much lower than those mea- The specific processes of sediment mobilization in glacial systems are sured from the high mountainside, a result of the incorporation of thick, and complex. Conceptual models describing the source for glacial sediment, the effectively unexposed, blocks into the moraine, as well as the incorporation ways in which glaciers mobilize underlying sediments, and the rapidity with of material from lower-elevation nearby slopes above the moraines. The 10Be which they do so have been described in several studies (e.g., Bloom, 1998, sediment samples from downstream of the Kahiltna Glacier terminus showed figure 16-2, p. 356; Ward and Anderson, 2011; Fig. 1). For example, Bloom (1998) decreasing concentrations with increasing distance from the moraine, indi- stated that most of the sediment transported by valley glaciers was derived cating the incorporation of material that was less exposed to cosmic rays, from slopes of the upper glacial valley. He also described the route taken by most likely from the glacier base as well as from slopes downstream of the sediment. In that model, sediment was delivered to the glacier surface in the glacier. Taken together, 10Be concentrations in various samples from the the upper reaches of the glacier, and then gradually buried by the addition of Kahiltna drainage system indicated erosion rates of hundreds of millimeters snow, which became ice as it was buried and transported down valley. In the per thousand years, which is typical of tectonically active terrains. We also lower part of the glacier, where melting and ablation are dominant processes, measured 10Be concentrations from river sediment samples collected from sediment was gradually exposed to form medial and lateral moraines (Fig. 1). across the south flank of the Alaska Range. Calculation of basinwide weighted Ward and Anderson (2011), based on Anderson (2000) and Bozhinskiy et al. erosion rates that incorporated hypsometric curves produced unrealistically (1986), described in more detail the route taken by glacial sediment. According high erosion rates, which indicates that the major source of sediment was to their description, sediment shed from cliffs along the sides of valley glaciers not exposed to cosmic rays and was primarily derived from the base of gla- was progressively buried in the accumulation zone of the glacier and became ciers. Moreover, the apparently high erosion rates suggest that parts of each entrained in the ice. It was transported down the glacier, embedded in the ice This paper is published under the terms of the near the margin of the glacier, until the glacier converged with another. The CC-BY-NC license. *ASTER Team: M. Arnold, G. Aumaître, D. Bourlès, and K. Keddadouche converging edges merged into a band of debris in the center of the combined © 2020 The Authors GEOSPHERE | Volume 16 | Number 3 Matmon et al. | Sediment sources and transport along the south side of the Alaska Range Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/3/787/5018380/787.pdf 787 by guest on 25 February 2021 Research Paper glacier. This continued to move down the glacier until it crossed the equilibrium ELA line altitude (ELA) and began to be exhumed in the ablation zone. Zone of accumulation Zone of ablation Upon exhumation, debris accumulates at the surface, to form the medial moraine. Once this layer of debris becomes more than a few centimeters thick, it insulates the ice beneath it from ablation (Bozhinskiy et al., 1986; Lundstrom et al., 1993). At that point, the debris-free ice nearby ablates faster than the debris-covered ice, and topography develops on top of the glacier’s surface. This creates a lateral slope between debris-covered and debris-free areas, and debris migrates down this topographic slope. The exhumation rate of debris from within the ice comes into balance with the rate of removal of debris downslope, and the moraine attains a consistent thickness of debris. During englacial transport of the debris, it is effectively entombed in ice and does not mix much from the way it fell on the surface. Certainly, some mixing Figure 1. Conceptual description of sediment transport within a glacier, may occur (Hambreky et al., 1999), but it is unlikely to be mixed with sediment modified from Bloom (1998) and Ward and Anderson (2011). All descrip- other than the other debris surrounding it. Along the margins of a glacier, new tions are two-dimensional and follow a longitudinal cross section down the glacier. The major difference between previous descriptions and the debris is constantly added to the ice surface as the englacial debris from farther one presented here is expressed by the blue thick arrows. While previous up the glacier translates below it, but these sources do not mix because debris descriptions suggested that the majority of sediment is derived from the that fell on the glacier farther upstream takes a deeper path through the glacier upper headwalls of the glacial system, our description indicates that the than does debris that falls farther downstream (Ward and Anderson, 2011). Based major role of sediment input is from slopes immediately above the gla- cier along the entire length of the glacier. This enables the incorporation on the processes described above, Ward and Anderson (2011) argued that the of sediment with low 10Be concentrations, as measured in the medial concentration of cosmogenic 10Be measured in any medial moraine material moraines. ELA—equilibrium line altitude. represents the rate at which the cliffs at the upper part of the glacier wear back.