Abstract

The Laguna Amarga is an E-W oriented endorheic lake located in a mountainous area on the eastern slope of the Patagonian Andes. On the NE and SE margin of the lake are vegetated limemud ponds fed by cold groundwaters of meteoric origin leading to precipitation of travertines. The travertines are flat-lying bedded deposits mostly made of crystalline calcite, being the southern one with larger lateral extension and thickness (130 m and 40 cm, respectively). They are mostly composed by boundstones and feather-like crystal crusts in the proximal zone, and fan/ray crystals crust in the intermediate zone. Also, (micro)stromatolite, oncoidal packstones and lime-mudstones are marginally present. Aragonite contents (5- 55%) are restricted to the southern travertine where it occurs as: 1) layers of spherulites (200 – 400 μm) having porous nuclei with nanocrystals in the feather-like crystal crust; 2) in the stromatolite; 3) as botryoids in the inner portions of oncoids; and 4) spherulitic fabrics (20 – 30 μm) in the fan/ray crystals crusts. Close relationships between the aragonite crystals and Extracellular Polymeric Substance (EPS) suggest bio-mediation in the aragonite precipitation, with the microbiological matrix serving as substrate for heterogeneous nucleation or through mineral supersaturation in EPS-related physicochemical microdomains. The δ13Ccarb and δ18Ocarb composition of the southern travertine is about 2‰ and 1‰ heavier, respectively, than the north travertine, and in both cases the larger C-O values correspond to carbonates from the proximal zone. This might result from the combined effect of microbial photosynthesis in the nearly stagnant waters of the proximal zone and disequilibrium isotopic effects during precipitation from turbulent waters in the intermediate/distal zones. Bulk-rock XRF analyses show that the southern travertine is relatively depleted in Fe and Mn, but enriched in Sr, the last being positively correlated with the aragonite contents. The sun exposure of the southern travertine is greater than that of the northern travertine due to geographical effects, resulting in different microenvironments between the two. This induces to a higher vegetal biomass, moisture, and soil capacity for CO2 retention in the northern travertine, and possibly larger photosynthetic productivity in the southern travertine. It’s interpreted that higher δ18Ocarb in the southern travertine is related to larger evaporation/evapotranspiration while the lower δ13Ccarb and higher Fe an Mn of the northern travertine to larger contributions from soil. It’s suggested that special care should be taken when inferring paleoenvironmental conditions from isotope data of cold travertine formed at subpolar latitudes.