Supraglacial dust and debris characterization via in situ and optical remote sensing methods

Abstract

Supraglacial dust and debris affects many glaciologic variables, including radiative absorption, ablation, generation of supraglacial melt as well as mass flux. Earth observing satellite technology has advanced greatly in recent decades and allows for unprecedented spatial, temporal and spectral imaging of Earth’s glaciers. While remote sensing of ‘clean’ glacier ice can be done quite successfully, strategies for satellite mapping of supraglacial debris remain in development. This work provides the first visible to thermal infrared full optical spectrum satellite data analysis of supraglacial dust and debris characterization and differentiation. Dust and debris covered glaciers in the following six contrasting study regions were targeted: Iceland, Nepal, New Zealand, southern Norway, Svalbard and Switzerland. A combination of field spectrometry and surface samples of snow, ice and debris were utilized to investigate supraglacial dust and debris diversity. This in situ data served as ground truth for evaluating spaceborne supraglacial debris mapping capabilities.

Glacier snow, ice and debris samples were analyzed for mineral composition and inorganic elemental abundances via the following analytical geochemical techniques: X-ray diffraction, X-ray fluorescence spectroscopy and inductively coupled plasma mass spectrometry. A synoptic data set from four contrasting alpine glacier regions – Svalbard, southern Norway, Nepal and New Zealand – and 70 surface snow, ice and debris samples was presented, comparing supraglacial composition variability. Distinct supraglacial geochemical abundances were found in major, trace and rare earth elemental concentrations between the four study regions. Elemental variations were attributed to both natural and anthropogenic processes.

Over 8800 glacier surface spectra were collected in Nepal, Svalbard and Switzerland, as well as from Nepal, New Zealand and Switzerland debris samples. Surface glacier debris mineralogy and moisture content were assessed from field spectra. Spaceborne supraglacial dust and debris mineral mapping techniques using visible to shortwave reflective and thermal emissive data were evaluated. Successful methods for mineral identification allowed mapping of volcanic vs. continental supraglacial debris, as well as different mineral classes within one glacier’s supraglacial debris. Granite- vs. schist-dominant debris was mapped on Khumbu glacier in Nepal. Iron-rich vs. iron-poor serpentine debris was mapped on Zmutt glacier in the Swiss Alps. Satellite emissivity derived silica mapping suggested potential use of silica thresholds for delineation of debris covered glacier extent or sediment transport and weathering processes. Satellite derived surface temperatures were compared in Iceland, Nepal, Switzerland and New Zealand glacier study regions, with results demonstrating variations in supraglacial temperatures coincident with changing mineral abundances. Consistently higher surface temperatures with increasing dust and debris cover were mapped at all four glacier study regions. Repeat supraglacial debris imagery was used to estimate ablation area velocities and particulate transport times at debris covered glaciers. Velocity derivations used in conjunction with supraglacial composition variation analysis from shortwave and thermal infrared false color composites, allowed for estimation of glacial mass flux in the Khumbu Himalayas.

In short, the visible to thermal infrared satellite spectral analysis, combined with in situ spectral and geochemical ground truth data, proved that glacier dust and debris characterization is possible via satellite spectral data. Furthermore, this supraglacial dust and debris satellite characterization can be applied to a range of glaciologic studies, including thermal, mass balance and surface process interpretations on large spatial and temporal scales.