CO2 fluxes at a hypersaline shallow playa. The organomineral crust makes the difference.

Shallow hypersaline playas are flat endorheic basins that form a salt crust at the top sediment layer. They are episodically flooded after intense rainfall events. After flooding, if precipitation does not persist, evaporation causes the water to recede, and salinity can increase up to 35–45%. Ultimately, the system dries up and evaporites like eugsterite (Na₄Ca(SO₄)₃·2H₂O), blödite (Na₂Mg(SO₄)₂·4H₂O), halite (NaCl) and gypsum (CaSO₄·2H₂O) can precipitate forming a rigid crust layer.

The present study focuses on measuring the exchange of CO₂ between the upper sediments and the atmosphere at “La Muerte” playa-lake in the Monegros region (Aragon, Northeast Spain). Its main characteristic is the presence of a benthic organicmineral film dominated by cyanobacteria which uniformly covers the basin after rain events. As the water body evaporates, the biofilm contracts, while precipitated minerals replace organics and partially cement pore spaces, forming a rigid, salt crust–biofilm assemblage up to 1 cm thick. The two main objectives of the present study are:

• To estimate the net CO₂ exchanges under wet and dry conditions and therefore to verify the impact of water availability in modulating the magnitude and sign of CO₂ fluxes
• To investigate the significance of the upper organomineral crust on modulating the CO₂

 

To accomplish these objectives this study relies on field in-situ short-term incubations complemented by additional ex-situ laboratory estimates. Main preliminary outputs are:

• Net CO₂ emission typically predominated over net consumption.
• Net CO₂ emission increased in summer, under dry conditions. Net CO₂ consumption is detected under water-saturated conditions only.
• The CO₂ emissions decreased in incubations where the surface biofilm remained undisturbed, as opposed to those where it was removed.
• Most subsurface CO₂ emission originates from just below the crust (ca. 1 cm depth) to 8 cm depth.
• Under dry conditions, primary productivity of phototrophs at the top biofilm crust is insufficient to account for the observed decrease in CO₂

Overall, these results suggest that although the upper layer is not an impermeable barrier to gas flows, such as CO₂, it transiently mitigates its seepage into the atmosphere.