Similar sources but distinct δ13C signatures in adjacent low-temperature travertines from Laguna Amarga (Southern Patagonian Andes)

Abstract

This study examined the waters and carbonates from two cold spring travertines (ca. 13 °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°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 87Sr/86Sr composition (0.70720–0.70740) and isotopic signatures of the spring waters, including δ2H (ca. -110 ‰ VSMOW), δ18Owater (ca. -14 ‰ VSMOW) and δ13C-DIC (ca. -5 ‰ 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 (δ13C ca. -1 ‰ VPDB) and Upper Cretaceous (δ13C ca. -10 ‰ VPDB) during shallow subsurface circulation of meteoric waters through the bedrock. The carbon isotopic composition of the deposits resembles those of endogenic travertines (δ13Ctrav − 1.2 to 5.3 ‰ VPDB), with the highest δ13Ctrav values associated with carbonates from the vents. However, the involvement of deep CO2 sources is unclear and epigenic processes capable of producing the observed 13C enrichments are discussed.Despite their common sources, similar sedimentological features and δ18Otrav compositional range (−12.4 to −10.1 ‰ VPDB), the δ13Ctrav values are lower in the south-facing travertine (−1.2 to 1.9 ‰ VPDB) compared to the north-facing travertine (1.8 to 5.3 ‰ 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 δ13Ctrav signatures to lower and higher values, respectively. These relationships highlight the sensitivity of low-temperature spring carbonates to subtle environmental changes at basin scales.