Carbonate precipitation in microbial mats developed in a siliciclastic coastal environment of N Patagonia (Argentina)

QUIJADA, I. EMMA; CUADRADO, DIANA G.; MAISANO, LUCÍA; PAN, JERÓNIMO

Toulouse

Congreso; International Meeting of Sedimentology 2017; 2017

ASF-IAS

Microbial mats can alter the microenvironment where they live, changing the geochemical conditions and favoring the precipitation of autigenic minerals such as calcium carbonate. The metabolic processes of the microbial community, together with the production of EPS and extrinsic factors, such as environmental conditions, are decisive for mineral precipitation. Although carbonate precipitation induced/influenced by microbial communites has been extensively studied in carbonate environments and laboratory cultures, it has been scarcely reported in siliciclastic environments. This study documents the precipitation of continuous, well-defined carbonate laminae within microbial mats from a siliciclastic environment. Paso Seco, located in northern Patagonia (Argentina), is a supratidal coastal plain (~2.5 x 0.3 km) developed in a lowlying area among eolian dunes, which constitutes the remnant of an old tidal channel that was disconnected from daily connection with the sea by the formation of a sand spit at its mouth. After the spit formation, the exposed bottom of the channel was colonized by extensive thick microbial mats. Nowadays the study area is flooded during storm events and presents an annual variation of the water table of ~1 m (up to 70 cm above the sediment surface during storms and up to 30 cm below the surface during summer) and high solar radiation in summer. 20 cm-deep sedimentary cores extracted from the plain show that the sediments consist of an alternation of up to 4 cm-thick sand layers and biolaminites. The petrographic study of the biolaminites allowed the identification of characteristic, repetitive, up to 2 mm-thick sequences composed of four different types of laminae. Each sequence begins with a silt and fine to medium-grained sand lamina that shows a sharp lower contact. The basal lamina is overlain by an up to 400 μm-thick lamina of clay to fine-grained silt, amorphous organic matter, diatoms, and cyanobacteria sheaths. This layer displays abundant, generally subvertical, cyanobacteria filament molds that continue into the overlying lamina, which consists in a 100-200 μm-thick layer of dense micritic calcite. Finally, the uppermost 75-100 μm-thick lamina is composed of amorphous organic matter, diatoms and cyanobacteria sheaths, showing a horizontal laminated texture. These sequences might record the changing conditions achieved in the study area. The basal silty-sandy lamina is probably the result of a sudden sediment input during an episode of seawater income. During winter, the plain remains flooded for most of the time and phototrophic organisms may generate the abundant organic matter that overlies the silty-sandy lamina. The cyanobacteria filaments are subvertically oriented in the lower part of the mat and lie horizontal on the surface. During summer, water evaporation is intense and carbonate saturation is achieved, allowing the precipitation of calcite in the boundary between the laminae with vertically and horizontally oriented filaments. Therefore, the carbonate precipitation in the studied microbial mats developed in a siliciclastic environment is the result of the combination of microbial processes plus the environmental factors that allow carbonate saturation to be reached.