Biostabilization vs. bioturbation: the analysis of biogenic structures associated with modern microbial mats in the Bahia Blanca estuary.

CARMONA, NOELIA B.; CUADRADO, DIANA G.; PONCE, JUAN JOSE; BOURNOD, CONSTANZA N.

Mendoza

Congreso; 18th International Sedimentary Congress.; 2010

International Asociation of Sedimentology

Studies of recent environments with microbial mats increased considerably during the last years, not only due to their unique sedimentologic and ecologic characteristics, but also because they provide important implications for the understanding of fossil environments and paleocommunities. Microbial mats were widespread during the Proterozoic times, but with the increment in bioturbation during the Cambrian, they were largely restricted to extreme or stressed environments (Seilacher, 1999). Field studies in a mesotidal flat in the Bahia Blanca estuary, Argentina, reveal the presence of extensive areas with microbial mats, covering the upper-intertidal and lower-supratidal areas. Different microbially induced sedimentary structures (MISS) have been recognized in this estuarine setting (e.g. multidirected ripple marks, mat chips, polygonal oscillation cracks, and gas domes). The main purpose of this study is to characterize the biogenic structures associated to this biostabilized mat ground. In the lower supratidal area, grazing traces of the small gastropod (Heleobia australis) occur, mainly in areas with ripples and places with relatively thick microbial mats. Additionally, undermat mining of diptera larvae produces horizontal burrows few millimeters below the sediment surface, just along the oxic-anoxic boundary. Footprints of shorebirds are also common. The anoxic sediments preclude the presence of other biogenic structures within the substrate. So, in the upper zones of the tidal flat, biogenic activity is restricted to the uppermost tiers. However, this situation changes in the intertidal area, where abundant decapod burrows of Neohelice (=Chasmagnathus) granulata occur. These burrows were previously restricted to the lower parts of the intertidal zone, although in the last year there was a significant expansion of these organisms to the upper zones of the intertidal flat, and even to the lower supratidal area. These decapods produce reworking and overturning of the sediment, oxygenating the substrate in the deep, and affecting the whole sedimentation and geochemistry of the tidal flat. Growth and development of microbial mats depends mostly on the ability of the benthic cyanobacteria to cope with the background sediment deposition. As the presence of these large bioturbators is changing the sedimentary dynamics of the tidal flat, removing large amounts of sediment (up to 5.9 kg. m-2. day-1, Iribarne et al., 1997), their activity is having outstanding effects on the studied environment, and thus, the occurrence of microbial mats is largely dependent upon these deep-penetrating burrowers. This modern example helps us to understand the role of bioturbation in the decline of matgrounds and the onset of mixground in the early Phanerozoic.