Mapping Supraglacial Debris on Aerin Basehart1, Sam Altenberger1, Michelle Koutnik2, and Claire Todd1 1 - Department of Geosciences, Pacific Lutheran University, Tacoma, WA 98447 2 - Department of Earth and Space Sciences, University of , Box 351310, , WA 98195

Abstract Sedimentological Results Remote-Sensing Data Analyses Debris-covered glaciers from around the world offer distinct environmental, climatic, Transects had a debris surface elevation range of ~150 feet, ranging from 5266 to 5431 feet for Six sediment units were qualitatively identified based on visually-derived color differences in and historical conditions from which to study the effects of debris on glacier-surface Transect 1, and from 5125 to 5275 feet for Transect 2. GPS elevation accuracy was consistently ±10-20 the debris cover as well as boulder density that was assigned as a relative measure as low, evolution. In general, for the ablation zone of a glacier to be covered by more than ~50% ft. For both transects, the eastern side of the glacier was higher in elevation (Figure 3). We found more moderate, or high. The unit with the largest area (unit D) is dominated by weathered andesite debris, the debris flux needs to be relatively high and the ice-flow rate needs to be angular samples on the eastern side of our transects (Figure 4). Sieving of fine-grained matrix samples that may be the remnants of the 1963 rockfall. All units except for D are oriented in the relatively low; this is the case for many Mt. Rainier glacier termini. On Emmons Glacier suggests a decrease in the presence of extremely fine-grained samples (smaller than 125 μm) near the direction of ice flow. Low boulder densities occur closer to the glacier centerline; sediment in we know that the debris cover, and especially sourced from the 1963 rock avalanche, has midpoint of our transects (Figure 5). these units may derive from bedrock fins that emerge close to the center of the glacier at insulated the lower glacier. However, heterogeneities in debris-cover thickness and elevations upflow from these units. Moderate and high boulder densities found closer to the character have led to significant (up to multi-meter scale) heterogeneities in surface N margins of the glacier are presumably due to the influence of rockfall and moraine elevation. We are particularly interested in how different terrain sources and different degradation. clast sizes may influence the efficiency of ice insulation. In order to address how debris cover controls the evolution of the lower Emmons A Glacier, we are mapping supraglacial debris units to constrain the source, character, and N movement of the sediment. During summer of 2016 we assessed sediment cover thickness, composition, size, sorting, as well as site temperature, slope angle, and if there were any stagnant ice formations along two transects at two different elevations C across the width of the lower glacier. Some key results from 2016 field work include B D Sample Sites finding 1) more fine-grained sediment at lower elevation, 2) more angular grained (A) Grey / Moderate Boulder Density sediment on the eastern side of the glacier where there was a rockfall event in 1963, and A Emmons C (B) Red / Moderate Boulder Density 3) fewer ultra-fine grained sediment near the center of the glacier. During summer of Glacier E (C) Grey / Low Boulder 2017, using visually derived differences in color and boulder density, we defined six F F Density (D) Red / Low to Moderate Boulder Density surface-sediment units based on August 2016 satellite imagery and compared these units Emmons Glacier (E) Dark Grey/Black / Low to sedimentological data collected in summer 2016. Boulder Density (F) Grey / High Boulder Density Introduction 0 100 200 km Previous studies have demonstrated the sensitivity of ablation rates to debris cover thickness. A thin debris cover can increase ablation rates, while thicker debris cover can Figure 7 (above): Debris-cover units defined first by color, and second by relative boulder insulate glacier ice and slow ablation rates. density as seen in satellite imagery. Emmons Glacier is located in the northeastern quadrant of and has the Figure 8 (below): Same image as in Figure 7 but without shading so that actual color 6 2 largest surface area of the mountain’s glaciers (11.28 10 m ; Sisson et al., 2011). This Figure 2 (above): Mount Rainier and Emmons Glacier. variations of the debris-covered surface can be seen with respect to our mapped thick debris cover is dominated by a range of lithologies derived from the mostly Orange and green lines represent the approximate boundaries. Boulder density was evaluated to the 0.5-meter resolution of the image. andesitic flows that surround the glacier (Fiske et al., 1964). A 2013-2014 study of locations of transects 1 and 2, respectively (see Figure 3). Emmons Glacier demonstrated that heterogeneities in debris-cover thickness can produce significant changes in melt rates on small scales (Dits and Moore, 2014). N In December 1963, a rockfall event from Little Tahoma peak sent 14 million cubic Figure 3 (right): Elevation transects 1 and 2. Sites are offset from the transect in locations where it was unsafe to collect yards of debris down the Emmons Glacier valley (Figure 1; Crandell and Fahnestock, samples. White concentric circles show the spacing of clast 1965). The thickening of lower Emmons Glacier has been attributed to the persistence of selection around the centerpoint of each site. The blue push this debris (Sisson et al., 2011). pin indicates the lowest elevation NPS ablation stake.

Figure 4 (right): The number of angular; subangular, and rounded clasts collected at each sample site. Nine total clasts were A B collected at each site. Note Figure 5 (above): Lines show the surface that transect 1 Figure 1A. 1963 rockfall from Little Tahoma Peak (Crandell and Fahnestock, 1965; elevation profiles for transects 1 and 2; note is upglacier photo by Austin Post). B. Little Tahoma above Emmons Glacier (photo Scurlock, 2007). that the green and orange lines correspond to Interpretations from transect 2. the green and orange lines in Figure 3 above. Samples are Bars indicate and the number of fine-grained While weathering could factor into the color variation of the debris cover, multiple units Methods ordered glacier trace upstream to distinct sources and are delineated in packets that are oriented with the Sediment samples were collected along two transects of the lower Emmons Glacier matrix collections from each site. The total left to right. ice-flow direction. The next step will be to quantitatively assess these unit designations ablation zone (Figure 3). One clast was selected at the centerpoint of each site, and at number of sample collection points at each site four locations at 0.5 and 1 m radii from the centerpoint of the site (Figure 4); in total, nine is 9, as shown in the white circles in Figure 3. against field measurements and analyze glacier-wide imagery to trace these lower-glacier clasts were collected at each site. In locations with a fine-grained matrix, a scoop of units upstream. Figure 6 (right): sediment was collected for sieving. August 2016 satellite imagery was also analyzed. Site-specific angularity and grain-size analyses help to distinguish rockfall-event sourced Aggregated grain-size analysis of fine-grained debris (angular) from ice-flow transport sourced debris (rounded). Since the Little Tahoma rock avalanche contained mostly light grey and red/brown sediment (Crandell and References and Acknowledgements matrix samples collected at Crandell, D.R., and R.K. Fahnestock (1965), Rockfalls and Avalanches from Little Tahoma Peak PLU Division of Natural Sciences and Department of each site. Transect 1 is Fahnestock, 1965) we can most likely attribute Unit D as remnant avalanche debris. We will on Mount Rainier Washington, Geological Survey Bulletin 1221-A Geosciences; NASA Solar System Workings NNX15AH42G; upglacier from transect 2. Dits, T., and P. Moore (2014), Small-Scale Variations in Melt of a Debris-Covered Glacier: GRAPL research team 2017: Hannah Bortel, Logan Krehbiel, use these analyses together with a glacier surface elevation model to evaluate how modern Emmons Glacier, Mount Rainier National Park, National Park Service and Alex Yannello. Samples are ordered Fiske, R. S., C. A. Hopson, and A. C. Waters (1963), Geology of Mount Rainier National Park, glacier shape relates to debris character and thickness. Then, we will use our structural Washington, Geological Survey Professional Paper 444. Sedimentological data collected from this research project glacier left to right. Scurlock image courtesy of Portland State University. is available online at http://goo.gl/ajuxRg. analyses with an ice-flow model to evaluate the co-evolution of ice and debris over the past Sisson, T.W., J.E. Robinson, and D.D. Swinney (2011), Whole-edifice ice volume change A.D. 1970 to 2007/2008 at Mount Rainier Washington, based on LiDAR surveying. 50 years.