Geomorphic signatures of large-scale glacier detachments

The catastrophic detachment of Kolka Glacier in Russia was long thought to be a unique occurrence (e.g., Haeberli et al., 2004), but recent events in Tibet, Alaska, Argentina and China have increased the urgency to understand these processes and the risk they pose to mountain communities and infrastructure. Most notably, the tongues of two neighboring glaciers in Tibet detached only a few weeks apart in 2016, the first killing nine herders and hundreds of their livestock. In 2013 and 2015 Flat Creek Glacier in Alaska’s Saint Elias Mountains lost half of its total area in two large detachments. The resulting destructive mass flows left a clear scar in the landscape, piling debris up to 30 m thick and spreading it over 8 km². Recent investigations by Kääb et al. (2018), Gilbert et al. (2018) and Jacquemart et al. (in review) suggest that the failures in Tibet and Alaska share three main drivers: temperate ice restricted by a frozen glacier tongue, a clay-rich bed, and increased meltwater input to the base of the glacier, driven by increasing summer temperatures.

Here we ask whether these glacier detachments are indeed a new, emerging hazard or whether we simply have not previously recognized the signs they leave in the landscape. Only a long-term record of observations stretching beyond the modern satellite era, can reliably answer the question about possibly increasing frequencies. In order to start building some understanding of the nature of such deposits, we investigated the internal structure and landscape setting of the 2013 and 2015 detachment deposits at Flat Creek. We performed electrical resistivity tomography surveys to estimate their ice content and ice distribution. In addition we analyzed grain size distributions and orientations in the deposits to see if they can be clearly distinguished from other glacio-fluvial deposits. To understand if glacier detachments have happened in this region before, we performed the same analysis on large debris deposits found downstream of a neighboring glacier. We combine this field evidence with remote sensing analysis of the temporal evolution of the glaciers and detachment deposits in Alaska, Tibet and Russia to understand the signatures of these catastrophic events in the landscape. Our preliminary results for Alaska show that the glacier itself is a bad indicator of past events, as the ice response quickly masks the detachment. Additionally, we found ice in the deposits to be highly broken up and ground, though never the less able to endure multiple years. Unlike a traditional debris-flow deposits, the glacier-detachment deposits exhibit a lack of grain-size sorting, and the grain orientations appear highly chaotic, with a tendency toward vertical orientations. As such, the deposits appear clearly distinct from the surrounding hillslope, and further analysis will show to what extent they can be distinguished from other glacio-fluvial deposits.