Rock glaciers in the Central Eastern Alps – How permafrost degradation can cause acid rock drainage, mobilization of toxic elements and formation of basaluminite

• Published in: Global and Planetary Change
• Main Authors: Wanner, Christoph, Moradi, Hoda, Ingold, Philipp, Cardenas Bocanegra, Miguel A., Mercurio, Romano, Furrer, Gerhard
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier Science 2023
• Subjects: 550 Earth sciences & geology; Ice; permafrost
• Online Access: https://boris.unibe.ch/184604/1/WannerEtAlGloplacha_23.pdf; https://boris.unibe.ch/184604/2/WannerEtAlGloplacha_23SI.pdf; https://boris.unibe.ch/184604/

Naturally occurring acid rock drainage (ARD) appears to be promoted in permafrost environments with pyritebearing host rocks. However, it is poorly understood how the interaction between solid ice, meltwater and pyrite causes the formation of sulfuric acid and the subsequent mobilization of toxic elements. To elucidate the governing processes and to assess the general hazard of ARD in permafrost areas in the context of global warming, we present chemical water analyses for six high-alpine surface waters downstream of intact rock glaciers in the Central Eastern Alps. In addition, we provide a detailed chemical and structural characterization of nanocrystalline Al-hydroxysulfate precipitates forming along the streams and serving as a visual manifestation of ARD. Finally, we show results from column experiments, experimentally simulating the interaction between water and pyrite-bearing paragneisses. Studying ARD on both, the field and laboratory scales, demonstrated that under field-site conditions intact rock glaciers may act as highly efficient chemical reactors, resulting in high concentrations of toxic elements such as aluminum, nickel, manganese, and fluorine in the sampled high-alpine streams. The most likely reason for the strong mobilization of these elements is their temporal storage and enrichment in the frozen rock glacier core, leading to a quick and focused export in summer when ice melt production rates are high. The analyses of the Al-hydroxysulfate precipitates confirmed the presence of basaluminite (Al4(OH)10(SO4) x 3 H2O), controlling the solubility of aluminum. Geochemical modeling allowed to quantify the inverse behavior of the basaluminite solubility with temperature, resulting in a field-derived standard reaction enthalpy (ΔrH◦) of - 580 kJ mol -1. This value can now be used to determine the solubility of aluminum also at temperatures relevant for permafrost settings (0–5 ◦C). Aerial photographs of the study sites suggest a strong intensification of ARD in the Central Eastern Alps over the ...