| Summary: | 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 <i>Spirogyra</i> during spring and show lighter isotopic signatures (δ<sup>13</sup>C aver. −4.10‰ and δ<sup>18</sup>O aver. −0.75‰, VPDB) than a second association of carbonates, such as hydromagnesite, northupite and traces of magnesite among other sodium-bearing carbonates (δ<sup>13</sup>C aver., −1.34‰ and δ<sup>18</sup>O aver. 4.52‰, VPDB). The latter precipitate in association with degraded microbial mats as the lake desiccates during summer. Covariant trends between carbonate δ<sup>13</sup>C and δ<sup>18</sup>O 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 δ<sup>13</sup>C within a range of −12.40‰ to −0.43‰. The more depleted <sup>13</sup>C 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 δ<sup>13</sup>C and δ<sup>18</sup>O 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 <sup>13</sup>C by 5‰ with respect to dissolved inorganic carbon (δ<sup>13</sup>C 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.
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