Optical characteristics of cryoconite (surface dust) on glaciers: the relationship between light absorbency and the property of organic matter contained in the cryoconite

Optical characteristics of the cryoconite collected from nine glaciers in the Himalaya, Tibet and the Arctic (Canada and Svalbard) were analyzed. The spectral light reflectance (visible region) of the cryoconite on the six glaciers in the Arctic and the Himalayawas generally low, indicating high light absorbency (dark coloration) of the cryoconite. In contrast, the spectral reflectances of the cryoconite on the three glaciers in Tibet were significantly higher than on the other glaciers. There was no significant difference in the spectral reflectance of mineral particles contained in the cryoconite between the Tibetan and the other glaciers, indicating that the difference in the albedo of the cryoconites is not due to the mineral particles, but due to organic matter contained in the cryoconite. Chemical analysis of the organic matter in the cryoconites revealed that the light absorbency of cryoconites is due to the amount of humic substances, which are dark-colored organic substances, the residue of bacterial decomposition of organic matter. The cryoconite of the three glaciers in Tibet contained significantly smaller amounts of humic substances than that of the other glaciers, probably due to different biological or chemical conditions. Results show that the formation of the humic substances in the cryoconite affects its optical characteristics, and possibly affects the surface albedo of the glaciers.