Microbial fabric formation in spring mounds ("microbialites") of alkaline salt lakes in the Badain Jaran Sand Sea, PR China

Abstract

The large sandy desert of Badnin 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 aragonitie pinnacles rising from the lake floor, Lake Nuoertu and Lake Huhejaran haue been investigated in detail. The porous limes tones of the tufa pinnacles ("spring mounds'') result {rom a hydrochemieally forced, exopolymer-mediated calcification of cyanobacteria- dominated mierobial mats at sublacustrine springs. The development oftheir enigmatie fabries is documented for the first time. Siekle-cell-like and bubbleshaped fabries reflect successive mineralization during degradation, shrinkage of the organie mucus, and gas formation by bacteria. Fossil analogues are known from the Pleistocene and Miocene (Ries-crater lake), but older non-marine, Phanerozoie examples are expected. 1b the extent that these textures are associated exclusively with alkaline environments, they can be used to test the hypothesis of an alkali ne ocean early in Earth history.