Microscopic and field characterization of microbial mats in the Bahía Blanca estuary.

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

Mendoza

Congreso; 18th International Sedimentary Congress.; 2010

Microbial mats and their associated sedimentary structures (Microbially Induced Sedimentary Structures - MISS, Noffke, 1997) have been recently identified in the Bahía Blanca Estuary, being this the first mention of modern microbial mats in Argentina. Since then, a number of studies have been conducted to characterize morphologically these structures, and to determine their implications for the fossil record. Results from field observations, and sedimentologic and petrographic analyses are presented herein. The study tidal flat is located in Puerto Rosales, in the central area of the estuary, where a wide variety of MISS occurs. These sedimentary structures show a distributional pattern related to tidal inundation, which is determined by the micro-scale topography of the tidal flat that controls the moisture content of the substrate. Therefore, in the highest zone of lower supratidal areas, frequently exposed, desiccation cracks are well developed. Toward lower zones, mat cracks with curled edges (mat curls), and polygonal oscillation cracks, usually associated with mat chips and flipped-over mats, occur. Frozen ripples are widespread over the flat, being often subtle due to ripple leveling. Gas domes has also been observed, showing different sizes, and being sometimes cracked at the top and collapsed due to gas escape. Erosional remnants and pockets occur in the lower intertidal zone, most likely in relation to higher hydrodynamic conditions, especially during winter months. Besides, multidirected ripple marks are also common in this intertidal zone. Undisturbed samples were taken with tube corers and sectioned in millimeter-thick intervals in order to characterize them as a function of depth. Granulometric distributions, moisture, and organic matter contents have been measured. Grain size analyses show predominance of silt-sized sediments (20-40 µm) in the upper 0.5 cm of the profile, which corresponds to the layer with microbial mat. This layer usually has more than 6% of organic matter and elevated values of water content, exceeding the 30%. These parameters tend to decrease with depth. Also, below this upper layer, there is a distinctive change in colour, from light brownish to dark, indicating the presence of an anoxic horizon. Petrographic analyses of thin sections revealed the presence of fabrics resulting from trapping/binding and post-burial processes. The first includes matrix-supported grains (silt- fine sand sized) incorporated in the fibrillar meshwork of the mat, arranged with their long axes parallel to the sediment surface. Orientation of grains in association with this biolaminite type is a common signature in the analyzed sections. Also, framboidal pyrite (or greigite) was observed, mostly in the reduced, lower layers of the sections. Geochemical changes resulting from the activity of the microorganisms are essential for precipitation of early diagenetic minerals, which in turn may enhance the preservation of the modern sedimentary structures in the studied tidal-flat. These actualistic analyses will help to recognize and characterize microbially induced sedimentary structures occurring in fossil tidal-flats.