MISS generated in a temperate hypersaline relic estuarine saltpan with current continental influence

YORLANO, MARÍA FLORENCIA; MAISANO, LUCIA; CUADRADO, DIANA G.; PAN, JERÓNIMO

Congreso; 36th International Meeting of Sedimentology; 2023

International Association of Sedimentologists

The Salitral de La Vidriera (38°44´S, 62°34´W), once part of the Bahía Blanca estuary (~3000 years BP), is currently a continental saltpan, that acts as a hypersaline evaporitic environment. This study provides a first-hand characterization of the epibenthic microbial mats that biostabilize its surface sediments, and the environmental dynamics and physical deformation agents behind the formation of modern Microbially Induced Sedimentary Structures (MISS).MISS and sediment cores were sampled on five field trips from Dec 2020 to Nov 2021, comprising all consecutive seasons in a year. The organic matter (OM) content was quantified by the mass LOI method, and granulometric (Malvern Mastersizer particle analyzer) and petrographic (Nikon Eclipse POL 50 transmitted light microscope) analyses of sediment were performed. Environmental parameters in water and sediment were recorded in situ. The community of photoautotrophic organisms in mats was characterized by light microscopy and the chlorophyll a content (a proxy for biomass) was estimated spectrophotometrically. The MISS identified were reticulate surfaces, pinnacles, gas domes, desiccation cracks, folds and wrinkles, mat chips, flipped-over mats, and roll-up mats. Gypsum crystals were found in surface and subsurface sediments and carbonate precipitation was corroborated through petrographic analyses. The microbial community in the hypersaline mats responded seasonally and was dominated by filamentous (Oscillatoriales) and coccoid cyanobacteria (Chroococcales and Pleurocapsales). The processes behind the MISS genesis involve an interplay between hydration, the flexibility and cohesion conferred by microbial colonization, and physical deformation exerted by hydrodynamic wind-generated currents. The sediment cores had a homogeneous texture; while it was not possible to identify a record of microbial mats, average OM content was > 10% in the upper 10 mm and remained ≥ 5% throughout the 80 mm of cores. Gypsum crystals are produced by rainwater evaporation, and preserved by mat recolonization. The micritic carbonate precipitation is characterized by peloids morphology rather than laminae. The high content in autotroph pigments, OM and carbonate precipitation highlight the potential role of this environment as a sink for atmospheric carbon.