Structure and formation process of cryoconite granules on Urumqi glacier No. 1, Tien Shan, China

Cryoconite granules are dark-colored spherical aggregates of organic and inorganic material on glacier ice, and are commonly observed on glaciers the world over. The structure of cryoconite granules on Urumqi glacier No. 1, Tien Shan, China, was analyzed. Granules were distributed over the entire ice surface of the ablation area, and ranged in size from 0.26 to 3.5 mm (mean 1.1 mm). The granule surface was densely covered with filamentous cyanobacteria. Microscopy of a thin section revealed various inner structures. Most granules had concentric layers of dense organic matter, which are probably derived from annual growth of the granules by the activity of cyanobacteria. The number of layers averaged 3.5 and ranged up to 7, which is likely to indicate their mean and maximum growth ages, respectively. Some granules contained two or more subgranules, showing that small granules had combined and enlarged. Such structures suggest that granule formation was mainly due to the activity of filamentous cyanobacteria, and that the granules repeatedly grew and disintegrated over a cycle of several years on the glacier.