Interaction between physical and microbial processes in a coastal modern environment: the stabilization of flaser and wavy bedding

CUADRADO, DIANA GRACIELA; I. EMMA QUIJADA; LUCIA MAISANO

Encuentro; 36th Meeting of Sedimentology; 2023

Benthic microbiota in clastic deposits can greatly contribute to the stabilisation of sedimentary structures such as ripples. While microbial activity is typically favoured in low-energy environments, flaser and wavy bedding are formed in environments recurrently affected by high-energy typically-tidal currents. However, this study documents the participation of microbial activity in the stabilization of flaser and wavy bedding in a coastal flat. The study area, Paso Seco (Argentina), is an elongated supratidal area (3.5 x 0.4 km) perpendicular to the coast with a narrow mouth. The zone is inundated intermittently by seawater only during storm surges, creating a high-energy environment at the mouth where fast water currents erode slightly the bottom sediment, creatinga semi-permanent pond. During floodings, sand ripples are formed, and when the energy diminishes, finegrained sediments fall out of suspension, draping the bedforms with mud. During calm conditions after thestorm, field observations reveal a cyanobacteria biofilm that covers the bottom of the pond, coating sand ripplesand mud, and forming reticulate structures with Oxygen-bubbles.Microbial activity is recognized in the surroundings: the subaerially exposed areas adjacent to the pond exhibitstabilized ripples or planar surfaces colonized by microbial mats. Moreover, microbial mats remain hangingfrom the eroded border of the pond, and thread-like structures enriched in organic matter are piled up againstthe pond margin. Sedimentary cores reveal flaser and wavy bedding in the sediments. In addition, opticalmicroscope and SEM studies show the presence of filaments of cyanobacteria and EPS in mud sediments,and filaments of different sizes entangling sand grains. The hydrodynamic of the zone is characterised by afew days of inundation due to storms (high-energy period) when the sand ripples are formed, followed by thereceding of seawater, which can last up to several months during a calm state. The quiescence period providesoptimal conditions for bacterial colonization of sediments underwater, allowing the formation of a coherentcyanobacteria biofilm. This microbial layer protects the flaser/wavy bedding from erosion. Thus, the interaction of microbial and physical processes creates the flaser and wavy bedding.