Abstract

This study examined the waters and carbonates from two cold spring travertines (ca. 13 degrees C) located on the sun- exposed (north-facing travertine) and sun-shaded (south-facing travertine) margins of Laguna Amarga, an alkaline lake from the semiarid region of the eastern Patagonian Andes (51 degrees S). The travertines are composed of calcite + low-Mg calcite +/- aragonite. Both exhibit similar sedimentological transitions along their longitudinal profiles. In the proximal zones, biologically-influenced carbonates form in wetland-like environments. Spherulitic calcite precipitates in association with extracellular polymeric substances in microbial biofilms containing cyanobacteria-like molds at the vent of the south-facing travertine, while aragonite spherulite formation at the north-facing travertine vent also involves sulfate-reducing bacteria, as indicated by their close association with framboidal pyrite. Downstream, in the intermediate and distal zones, crystalline dendrites predominantly precipitate due to increased turbulence-induced CO2 degassing. Both travertines share a similar range of carbonate 87 Sr/ 86 Sr composition (0.70720-0.70740) and isotopic signatures of the spring waters, including S2H (ca.-110 %o VSMOW), S 18 O water (ca.-14 %o VSMOW) and S13C- DIC (ca.-5 %o VPDB), suggesting common sources and processes influencing fluid composition. This points to the dissolution of carbonates from mudstone-rich marine units of the Lower Cretaceous (S13C ca.-1 %o VPDB) and Upper Cretaceous (S13C ca.-10 %o VPDB) during shallow subsurface circulation of meteoric waters through the bedrock. The carbon isotopic composition of the deposits resembles those of endogenic travertines (S 13 C trav-1.2 to 5.3 %o VPDB), with the highest S 13 C trav values associated with carbonates from the vents. However, the involvement of deep CO2 sources is unclear and epigenic processes capable of producing the observed 13 C enrichments are discussed. Despite their common sources, similar sedimentological features and S 18 O trav compositional range (-12.4 to-10.1 %o VPDB), the S 13 C trav values are lower in the south-facing travertine (-1.2 to 1.9 %o VPDB) compared to the north-facing travertine (1.8 to 5.3 %o VPDB). This disparity is inferred to result from variations in local environmental conditions due to different levels of insolation, which favored the incorporation of soil-derived CO2 in the south-facing travertine and likely increased photosynthetic productivity in the north-facing travertine, thereby shifting their S 13 C trav signatures to lower and higher values, respectively. These relationships highlight the sensitivity of low-temperature spring carbonates to subtle environmental changes at basin scales.