Microbial mats: physical and geological implications

CUADRADO, D.G.; PRATOLONGO, P.D.; PERILLO, G.M.E.; PIZANI, N.V.

Bahia Blanca

Simposio; ECSA 44 Symposium: Science and management of estuaries and coasts: A tale of two hemispheres; 2008

Estuarine and Coastal Sciences Association (ECSA)

Tidal sediments often support large populations of phototrophic benthic microorganisms. They commonly appear as recognizable microbial mats or biofilms dominated by cyanobacterias and epipelic diatoms. The extracellular polymeric substances (EPS) segregated by them form a mucilaginous matrix causing the binding of particles, increasing their cohesiveness through a variation in the physical properties of the unconsolidated sediments. An observable consequence of this process is the presence of tidal flats showing higher resistance to erosion by waves and currents. The activity of intertidal benthic microorganisms also causes biogeochemical changes in pH inducing mineral precipitation. The authigenic minerals hence formed can preserve the morphology of tidal flats by giving rigid and preservable structures. This is the first step to an early diagenesis, leading to the formation of rocks conserving the previous morphology with them. The present study was carried out in modern tidal flats where microbial mats and biofilms were present. A Cohesive Strength Meter (CSM) was employed to evaluate the stability of sediments, and results showed a strong effect of biofilms in shielding sediment particles from erosion. Critical shear for erosion was nearly 20 times higher in sediments showing conspicuous microbial mats compared to uncovered sand grains. On the other hand, authigenic grains could be observed by means of a Scanning Electron Microscopy (SEM). Energy Dispersive X-Ray (EDX) analysis demonstrated that crystals found were zeolites, a common mineral in eogenesis (the first stage of diagenesis). The zeolites could have formed by hydration of clayed minerals in association with volcanic glass present in the tidal flats.   Keywords: EPS, biofilms, Cohesive Strength Meter, Tidal flats, diagenesis.