CO2 emission at a hypersaline shallow lake at sediment-atmosphere interface. The significance of the organomineral upper crust as an active barrier.

Saline endorheic shallow playa-lakes are ecosystems susceptible to extreme geochemical changes because of severe hydro-climatic fluctuations. Under dry conditions, a rigid salt crust can separate the underlying sediments from the atmosphere. This interface is an organic-mineral assemblage of benthic biofilm encapsulated by evaporitic salts. It is well known that its structure, composition and consistency control water evaporation from underlying sediments, but its role in CO2 fluxes is unknown. We therefore measured the CO2 exchanges from sediments and the atmosphere in a hypersaline playa-lake characterized by a thin organic-mineral benthic crust upon drying. Results show that the largest CO2 release to the atmosphere occurs when ambient temperature and sediment humidity are high and low respectively. Nonetheless fluxes were lower than those reported for typical dry freshwater sediments and other hypersaline lagoons. The dry crust contains sedimentary structures that likely reflect the gaseous pressure from the underlying sediments, and its removal provokes a significant increase in net CO2 fluxes. Thus, this interface exerts physical control over both water evaporation and gases exchange such as CO2. Nevertheless, prolonged and severe droughts threaten the integrity of the crust: cracks together with bio-induced burrows and tunnels, likely create preferential pathways for CO2 leakage and enhance oxygen diffusion within the sediments and likely promote aerobic heterotrophic activity, explaining the CO2 leakage observed just below the organic-mineral crust.