Supraglacial landslides: improved detection, and their deposits as sources of bioavailable Fe

Abstract

Large landslides (rock avalanches [RA]) onto glaciers are thought to be increasing in frequency and magnitude due to feedbacks with both climate warming and permafrost degradation. However, there are no rapid, standardised detection methods to quantify these events. Understanding their frequencies and magnitudes is key to determining their glaciological and geochemical impacts from source to sink. As point sources of highly reactive, comminuted sediment they may be important delivery mechanisms for lithogenic nutrients such as iron (Fe). In this thesis, a new rapid, free-to-use tool; the Google Earth Engine supRaglAciaL Debris INput dEtector (GERALDINE), is presented, which can semi-automatically detect supraglacial RAs with 92% accuracy. This tool is applied to Glacier Bay National Park and Preserve, Alaska (~5000 km2 ); a known slopefailure hotspot, to assess an existing inventory of 24 supraglacial rock avalanches that were identified between 1984 and 2016. GERALDINE analysed an order of magnitude more Landsat imagery to aid in the creation of a new inventory of 69 RAs between 1984 and 2020. The original inventory underestimated RA frequency by 53%, missing predominantly small events, resulting in an area/volume bias in current detections. RAs increased in magnitude and frequency in the last decade, their triggering clustered temporally between May-September, and they clustered spatially at high altitudes and around the modelled permafrost boundary, with one ridge producing 29% of all events. Additionally, 58% were deposited in accumulation zones resulting in sequestering of debris into englacial and possible subglacial transport pathways. The largest of these RAs; the Lamplugh RA, was sampled to analyse its contribution to glacial iron (Fe) cycling. Its bioavailable Fe content was lower than other glacial sources, due to crystallisation on the surface and a lack of pyrite available for oxidation within the source rock. However, a singular RA is a large, rapid, point-source delivery of Fe into the glacial domain, and over wider areas their spatial and temporal clustering results in seasonal pulses of Fe along key glacial pathways, making them previously overlooked components of glacial Fe cycles