A SHEET OF AUFEIS IN THE KHARKHIRAA MOUNTAINS, MONGOLIAN
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A SHEET OF AUFEIS IN THE KHARKHIRAA MOUNTAINS, MONGOLIAN ALTAI Nicholas Swanson-Hysell, Carleton College Bob Carson and Mary Savina, advisors Introduction Aufeis, also known as icings or by the Russian term naled, is a sheet-like mass of Abstract At an elevation of ~2900 m in the glaciated Ugwi Yamaa Valley (N49º34.63’, E091º 27.23’), aufeis a layered ice that forms from successive flows of ground water during freezing temperatures (Harden et al., 1977; Hu and Pollard, 1997). In the river channels of covered an area of ~60,500 m2 in mid-July, 2004. This laminated ice sheet developed on a braided permafrost regions aufeis is evidence for perennial groundwater discharge (Clark and stream channel that is constricted by a small canyon below the sheet. The aufeis had a maximum Lauriol, 1997). Aufeis typically melts out during a summer and will form in the same thickness of 4.3 m. One 2.15 m section of the ice had 90 laminations while another section of 2.35 Ulaangom b place year after year (Hu and Pollard, 1997). m had 64. There are three different types of ice: equidimensional crystals, candle ice, and plate ice. Sheets of aufeis have been observed in Alaska (Harden et al., 1977; Kane, 1981), Arctic Canada (Veillette and Thomas, 1979; Reedyk et al., 1995; Clark and Lauriol, 1997; Equidimensional crystals, ranging from 3-12 mm, dominate the aufeis. Candle ice, vertically Uvs Aimag Priesnitz and Schunke, 2002), Russia (Sokolov, 1978) and Mongolia (Froelich 1982). oriented cylindrical ice, occurred both in beds (1-5 cm thick) and in lenses. The candle ice lenses have flat bottoms and convex tops, and can be as thick as 35 cm resulting in candle ice of that length. The ice plates are amalgamations of crystals that clip downstream at angles between 40º and 65º. Between July 21 and 29, 2004, the stream was blocked by the ice and eroded a cut bank 3.90 m Kharkhiraa Mountains high. Evidence of similar past erosional events indicates that aufeis plays an important role in widening the channel. Alluvial clasts under aufeis have responded to the load by preferentially aligning so that their flat surfaces create an alpine sub-aufeis cobble pavement. c large scale site of strat actively eroding column evidence for past evidence for past cutbank large scale cutbank large scale cutbank Figure 1: The aufeis sheet on the braided Figure 2: The weight of the ice on the channel stream channel in the glaciated Ugwi Yamaa gravel aligns the cobbles to form a pavement valley. A series of glacial advances and termed, in this study, as an alpine sub-aufeis retreats has left lateral and terminal moraines cobble pavement. N throughout the valley. Aufeis stratigraphy equidimensional (diameter in cm) plates candles The laminations of the aufeis each represent an Figure 5: LANSAT images of field area. In image “a” the mountains in 4 8 12 overflow event (Williams and Smith, 1989). Two sections the west running north-south are part of the Mongolian Altai. “B” is a centimeters estimated max extent of ice extent of ice 7/23/2004 closer view of the Kharkiraa mountain group that is part of the of ice were measured in detail. One 2.15 m section of the Mongolian Altai. “C” is the Ugwi Yamaa valley that flows north out of ice had 90 laminations (Fig. 3) while another section of downstream the Kharkiraa group. Outlined in red is the aufeis sheet that was 200 N river extent of ice 8/04/2004 2.35 m had 64. The 2.15 m section was a fresher surface observed for this study. The aufeis covers a larger area of the stream and it likely represents a more complete record of 130 65 0 130 Meters channel in the LANSAT than at the time of study. deposition. As the aufeis reached a thickness of 4.3 m it is 180 possible that 180 laminations are present in the entire sheet. The stream made these sections inaccessible so the Figure 4: Map of aufeis sheet made with GPS measurements taken at the beginning of study (7/23/2004) and the number of laminations in the sections of such thickness end (8/04/2004). Features such as the stream, major cutbanks and the location of the stratigraphic column (see a) 160 could not be confirmed. figure 7), were also mapped with a GPS. Equidimensional ice makes up the most layers of the section. Since these crystals form in an overflow slush 140 their vertical growth is limited and they do not progress to Aufeis effect on channel morphology the columnar growth face, unlike candle ice (Schohl and Ettema, 1986). During July of 2004 aufeis covered most of the channel on a 0.8 km section of the stream . From 7/21-7/29 the stream was 120 Candle ice occurred both in lenses and in distinct beds. blocked from usual down gradient flow by the ice, causing it to turn and for its flow to be directed straight towards the right bank Lenses of candle ice have a characteristic convex up shape. of the channel. The resulting cut bank was 3.9 m high. The bank had a 1.5 m thick layer of pure ice 1.5 m down from the surface. These lenses are recognized in the literature, and Hu and While ice in the soil profile of the bank could be due to segregated permafrost, Kane (1981) proposes that hydrostatic pressures Pollard (1997) attribute their formation to hydrostatic can cause separation in the soil that lead to the development of laterally extending ice wedges in the soil adjacent to stream banks. 100 pressure forcing water through fractures and forming an This ice supported the bank and allowed it to be undercut ~4 m by the stream. On the inside bank of the active meander, 5.5 m “icing blister.” This hydrostatic pressure is aided by the away from the current bank, there was grass that was still green under the ice suggesting that the inside bank has been the outside upward forces of c-axis growth create the convex lense. bank the summer before. This, combined with evidence of very active bank failure (e.g. chunks of soil in the stream), suggests 80 The ice described as plates were amalgamations of that the bank had retreated the 5.5 m during that melt season. This bank erosion was contributing significantly to the bedload of b) equidimensional crystals. Often the plates of ice align so the stream, as the water going into the meander was clear while the water coming out was brown. they dip downstream. Late in the day on 7/29/04, or early in the day on 7/30/04, the stream cut through the ice, bypassing the meander and cut bank 60 (Fig. 6). After the stream left this part of the channel the undercut bank continued to collapse. This erosional event caused Ice crystal classification key significant widening of the river channel. Evidence of similar past erosional events on river right above and below the ice sheet suggest that aufeis has played an important role in widening the channel. Because aufeis is known to form preferentially in wider plate ice 40 downstream apparent dip depicted parts of channels (Hu and Pollard, 1997), there is a positive feedback cycle associated with its development: aufeis forms in a candle ice wider part of the channel, aufeis diverts stream flow creating cut banks, channel widens, and the next winter a wider sheet of equidimensional ice aufeis forms. length of box correlates with crystal diameter 20 equidimensional ice lower limit of crystal diameters shown by small Acknowledgments References box, upper limit by larger box I would like to thank Enkhbayar Dandar, Molor Clark, I.D., and Laurial, B., 1997, Aufeis of the Firth River basin, northern Figure 6: The down gradient flow of the stream in the Ugwi Yamaa Yukon, Canada; insights into permafrost hydrogeology and karst: Arctic 0 channel gravel Eredenebat and Richard Hazlett for their assistance in the valley was blocked by the ice sheet causing the stream to turn 90º and Alpin Research, v. 29, no. 2, p. 240-252. field, Bob Carson for his work as my advisor and for his Harden, D., Barnes, P., and Reimnitz E., 1977, Distribution and character of and flow directly into the bank (Fig. 4). This created a 3.9 m tall cut- Figure 3: A 2.15 meter section of the aufeis sheet comprised of 90 laminations. This section amazing work organizing such logistically complicated naleds in northeastern Alaska: Arctic, v. 30, no.1, p. 28-40. bank and undercut the permafrost supported bank by ~4 m. Picture was recently exposed as it was next to the stream which had undercut the sheet and caused fieldwork and Mary Savina for taking me on as a Hu X., and Pollard W.H., 1997, The hydrologic analysis and modeling of a, taken on 7/23/2004, shows this water flow. Picture b shows the blocks to break off exposing fresh faces of ice. Often the plates of ice align so they dip Independent Research Advisee. The Keck Geology river icing growth, North Fork Pass, Yukon Territory, Canada: Permafrost same channel on 7/30/2004 after the stream had cut a more direct downstream. Where this was measured the dip angle of the plates is shown accurately above. Consortium and the NSF REU program funded the and Periglacial Processes, v. 8, p. 279-294. coarse through the ice bypassing the cut-bank. The bank collapse, Beds that are indicated as plates above, but for which no orientation is shown had the Schohl G.A., and Ettema R., 1990, Two-dimensional spreading and evident in picture b, is causing the channel to widen and will expedition. dimensions of plates but had no, or no recognized, preferential orientation. thickening of aufeis: Journal of Glaciology, v. 36, no. 123, 169-178. continue to do so as the exposed permafrost melts.