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), δ¹⁸O_water (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 (δ¹³C_trav −1.2 to 5.3 ‰ VPDB), with the highest δ¹³C_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 ¹³C enrichments are discussed.

Despite their common sources, similar sedimentological features and δ¹⁸O_trav compositional range (−12.4 to −10.1 ‰ VPDB), the δ¹³C_trav 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 δ¹³C_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.