Constructive and destructive processes affecting a tufa system in semi-arid Sierras Pampeanas, Argentina

This study focuses on a modern tufa system developing under a semi-arid climate at mid-latitude in the Sierras Pampeanas of central Argentina, currently undergoing degradation. The research integrates a geomorphological, sedimentological, mineralogical, and hydrochemical approach to analyze constructive and destructive processes in tufa systems and their preservation potential in the stratigraphic record. The tufa features a 12.2-m-high, semi-conical structure, perched in a waterfall at the confluence of the Los Quebrachitos stream and Cabana River, overlying crystalline basement rocks rich in marble belts. The main lithofacies identified are: biohermal tufa, laminated tufa crusts, sandy tufa. Additionally, rubble tufa deposits occur at the base of the waterfall. Biohermal tufas are dominated by phytohermal components and associated invertebrate remains, with irregular clotted-micrite and peloidal fabrics. At the microscale, microbial activity becomes more significant, where cyanobacteria, diatoms, and exopolymeric substances contribute to the development of irregular laminated fabrics. Laminated tufa crusts consist of alternating micritic, microsparitic, and sparitic low-Mg calcite layers. The Los Quebrachitos stream, classified as fresh (total dissolved solids between 81.57 and 79.5 mg/L), slightly alkaline (pH between 7.9 and 8.3) waters, has a calcium-bicarbonate composition (Ca²⁺ between 23.10 and 26.16 mg/L, carbonate alkalinity between 95.95 and 112.18 mg/L). Flow rate variations, due to rainfall, appear to control the carbonate saturation state of waters in the Los Quebrachitos system. During dry season, the combined effects of aeration, low pressure, and jet flow enhance CO₂ degassing, enabling slightly saturated conditions with respect to calcite (Ω_cal = 1.1), promoting tiny water droplets evaporation, and thereby, inducing calcite precipitation. Conversely, increased rainfall dilutes water, reducing total dissolved solids and calcite saturation. Currently, the tufa system is in a destructive phase, showing gravitational collapse, bio-fracturing from tree root colonization, and surface erosion-dissolution features. The ongoing destructive processes seem to exceed the precipitation rates in this depleted carbonate system, mainly due to changes in climate conditions.