Abstract
Cryoconite is a dark-coloured surface dust deposited on glaciers that consists of wind-blown mineral particles, as well as organic matter derived from microbes living on glaciers. In this paper, we analyse the Sr and Nd isotopic ratios of four mineral fractions (i.e., the saline, carbonate, phosphate, and silicate mineral fractions), as well as the organic fraction, of cryoconite samples obtained from six Asian glaciers (the Altai, Pamir, Tien Shan, Qillian Shan, and Himalayan regions), and discuss their geographical variations in terms of the geological origins of the mineral particles and the biogeochemical processes on the glaciers. The silicate mineral fraction showed lower Sr and higher Nd ratios for the glaciers located to the north (Altai, 87Sr/86Sr: 0.713 490–0.715 284, εNd(0): −6.4 to −5.6), while higher Sr and lower Nd ratios for the glaciers located to the south (Himalayas, 87Sr/86Sr: 0.740 121–0.742 088, εNd(0): −16.4 to −15.7); the ratios were similar to those of desert sand, loess, and river sediments in the respective regions of the glaciers. This result suggests that the silicate minerals within the cryoconites were derived from different sources depending on the geographical locations of the glaciers. The isotopic ratios of the saline, carbonate, and phosphate mineral fractions were distinct from those of the silicate fraction, and were similar to those of evaporites and apatite deposits from the Asian deserts, but also varied geographically, indicating that they are likely to reflect their geological origin. The Sr isotopic ratios of the organic fraction were similar to those of the saline and carbonate fractions from glaciers in the central area (Tien Shan and Qillian Shan), but were higher than those of the saline and carbonate fractions, and lower than the phosphate mineral fraction, in the northern and southern areas. The ratios of organic fraction may be determined from the mixing ratio of calcium sources incorporated by microbes on the glaciers.
Highlights
We aim to describe the geographical variations in origins of minerals in cryoconite and nutrient sources incorporated into microbes on glaciers in Asia
Microscopic observations showed that the cryoconite deposits on the glaciers analysed here consisted mainly of mineral particles and organic matter, including dark-coloured amorphous particles, snow algae, and cyanobacteria cells
The mineral particles in the cryoconites were larger in the Altai and Himalayas than in the other glaciers
Summary
Cryoconite is a dark-coloured surface dust deposited on glacial ice, and is commonly found on glaciers worldwide. It usually consists of mineral particles, which originate from basal till. The dark colouration of cryoconite acts to reduce the surface albedo of glaciers and so accelerates their melting in many parts of the world. Oerlemans et al (2009) reported that the accumulation of mineral and biogenic dust (cryoconite) has substantially darkened a glacier in Switzerland since 2003, which is likely to cause surface albedo reduction and removal of ice from the glacier. It is important to study the formation process of cryoconite to understand the variations in the recent change in glacial mass balance
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