Isotopic Signatures of Microbial Mg-Carbonates Deposited in an Ephemeral Hyperalkaline Lake (Central Spain): Paleoenvironmental Implications

Interpretation of stable isotope (C and O) composition of lacustrine carbonates requires in-depth knowledge about the interplay between the abiotic and biotic processes in sedimentary environments. The present study, focused on Mg-carbonates from a well-characterized alkaline and ephemeral lake, gives new insight into the behavior of the stable isotopes during the seasonal precipitation of a variety of carbonates. Dolomite and Mg-calcite precipitate intracellularly within Spirogyra during spring and show lighter isotopic signatures (δ¹³C aver. −4.10‰ and δ¹⁸O aver. −0.75‰, VPDB) than a second association of carbonates, such as hydromagnesite, northupite and traces of magnesite among other sodium-bearing carbonates (δ¹³C aver., −1.34‰ and δ¹⁸O aver. 4.52‰, VPDB). The latter precipitate in association with degraded microbial mats as the lake desiccates during summer. Covariant trends between carbonate δ¹³C and δ¹⁸O reflect isotope enrichment related to evapoconcentration. The seasonal cycling of inorganic carbon among carbonate minerals, microbial biomass, lake water and pore water was also analyzed, revealing variations of δ¹³C within a range of −12.40‰ to −0.43‰. The more depleted ¹³C derives from the decay of the microbial mats. The less negative values are distinctive of the bulk carbonates forming crusts in summer. Intracellular calcite and dolomite have δ¹³C and δ¹⁸O values (VPDB) ranging, from −5.45‰ to −3.07‰ and −2.48‰ to 1.58‰, respectively, that are intermediate between those two endmembers. These intracellular carbonates are enriched in ¹³C by 5‰ with respect to dissolved inorganic carbon (δ¹³C in the range of −11.79‰ to −6.87‰, VPDB) due to the vital effect of photosynthesis. The crust of carbonates deposited as the lake desiccates dissolve interannually. Alternatively, dolomite and Mg-calcite as well as their isotopic compositions persist during synsedimentary diagenesis, confirming that carbonate biominerals provide isotopic signatures related to the environmental conditions of formation with potential of preservation in the rock record.