LARGEST KNOWN MICROBIALITES DISCOVERED IN LAKE VAN, TURKEY

MICROBIALITES are organosedimentary deposits produced by benthic microbial communities interacting with detrital or chemical sediments 1. Calcareous cyanobacterial microbialites defined as stromatolites and thrombolites were common in ancient shallow marine environments 2. Today, they are restricted to a few lacustrine and perimarine settings. This restriction may result from changes in seawater chemistry through time 3-6, particularly from alteration in supersaturation with respect to carbonate minerals 7. The largest known calcareous microbialites (several metres high) were formed in the late Precambrian 8. Here we report the discovery of enormous (approximately 40 m high) tower-like microbialites from alkaline (pH > 9.7) Lake Van, eastern Anatolia. Growth is by mats of coccoid cyanobacteria (Pleurocapsa group) permineralizing in situ with aragonite and by inorganically precipitated calcite. Certain aspects of these microbialites resemble Proterozoic marine stromatolites 9