Abstract

The large sandy desert of Badain Jaran Shamo, Inner Mongolia, has several salt lakes located among megadunes with crests as high as 400 m. Most of the lakes are hypo- to hypersaline and alkaline, with pH values between 8.5 and 10. Because of their reef-like aragonitic pinnacles rising from the lake floor, Lake Nuoertu and Lake Huhejaran have been investigated in detail. The porous limestones of the tufa pinnacles ("spring mounds") result from a hydrochemically forced, exopolymer-mediated calcification of cyano-bacteria-dominated microbial mats at sublacustrine springs. The development of their enigmatic fabrics is documented for the first time. Sickle-cell-like and bubble-shaped fabrics reflect successive mineralization during degradation, shrinkage of the organic mucus, and gas formation by bacteria. Fossil analogues are known from the Pleistocene and Miocene (Riescrater lake), but older non-marine, Phanerozoic examples are expected. To the extent that these textures are associated exclusively with alkaline environments, they can be used to test the hypothesis of an alkaline ocean early in Earth history.

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