Laminated microbial mats, laguna Guerrero Negro, Mexico

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Abstract
Laguna Guerrero Negro and Ojo de Liebre are large, restricted embayments characterized by an arid climate and by a salinity ranging from that of normal seawater to the hypersalinity of sabkha environments. Intertidal microbial mats develop in some of the slightly hypersaline marshes and in many of the moderately hypersaline flats. Each major mat type corresponds to the lower, middle or upper intertidal. In the lower intertidal mat, the photosynthetic horizon is a bilayer composed of a surface, blue-green algal layer (about 3 mm thick) dominated by Microcoleus chtonoplastes, and an underlying purple layer (about 2 mm thick) of photosynthetic bacteria, predominantly Chromatium sp. The mat may accrete to a thickness of 10 cm or more. It is characterized by fine laminations of alternating layers of blue-green algae and of photosynthetic bacteria. The middle intertidal mat, dominated by Lyngbya aestuarii, is relatively thin (usually 1 or 2 cm). Only a very thin, lower horizon of purple photosynthetic bacteria develops in this mat. The 1-cm tall pinnacles that resemble the Precambrian stromatolite, Conophyton, occur locally in the Lyngbya mats. The upper intertidal is characterized by a crusty, wrinkled mat dominated by Calothrix crustacea; the total thickness of these mats is about 0.5 cm. None of the mats is lithified. Aragonite precipitates as uncemented grains in the mats. Non-algal mat sediments contained only 2% CaCO3. Absolute CaCO3 content was positively correlated both with total mat thickness and with organic C concentration in the mat. CaCO3 content decreased with depth in the mats, which demonstrates that in a predominantly silicoclastic and slightly hypersaline environment, the CaCO3 produced in the mats is not stable. Analogies between this modern microbial environment and ancient, marine stromatolite environments are probably limited.