Physico-chemical and biological controls in a travertine system in the high Andes of northwestern Argentina

Abstract

Unraveling the processes involved in travertines and tufa formation, and the textural and geochemical signatures they preserve, are relevant given their potential as world-class oil reservoirs and analogues for ancient microbial carbonates. This study yields insights into the processes controlling hydrochemistry, isotopic fractionation and carbonate precipitation within a travertine system located in the southern border of the Puna plateau (Argentina), at an active high-altitude volcano-tectonic setting. Geochemical and isotopic composition of hot spring waters, together with its physico-chemical parameters, are typical of deep-cycled meteoric waters with most of the CO2 provided by a magmatic source. Downstream CO2 degassing rises the pH and shifts carbonate chemical equilibrium providing the bulk conditions for CaCO3 precipitation. Precipitates occur mainly as aragonite near hot springs and calcite throughout the rest of the system (~1 km extent) in close association with cyanobacteria, diatoms and exopolymeric substances (EPS). Constructive morphologies and crystalline crusts develop where strong CO2 degassing occurs, driven by contrasting CO2 partial pressures between hot spring waters and the atmosphere, and also associated to incremental downstream turbulence (e.g., dams and waterfalls). Biological influence is preponderant within downstream calm pools by providing an EPS substrate suitable for calcite nucleation, contributing to clotted micrite textures. Downstream trend towards heavier isotopic compositions of waters and related carbonates is given by the preferential degassing and evaporation of lighter isotopes (12C and 16O) and enrichment of heavy isotopes (13C and 18O) in the carbonate phase. Nonequilibrium isotopic fractionation seems to relate with kinetically-controlled processes and pH changes along the system. This study highlights that, while at the macroscale physico-chemical processes control bulk water and carbonate chemistry, providing favorable conditions for carbonate precipitation, at the microscale microbial activity has some control, particularly on the development of carbonate microtextures.