Geochemical Insights Into an Active Calcareous Tufa Depositing System in Southern Germany

Abstract Calcareous tufa deposition in bicarbonate-rich karstic spring waters is essentially linked to complex microbiological communities that alter the geochemistry of the deposited tufa and hydrochemistry of the creek water. A monthly monitoring of the creek water chemistry of a natural tufa depositing site in the Franconian Alb in southern Germany reflects the tufa formation well, which is expressed in a decrease of calcium and bicarbonate concentrations and SI calcite oversaturation. The accompanying decrease of barium ions is most likely due to chemoselective chelation by Extracellular Polymeric Substances (EPS) of the tufa biofilms that favour divalent ions with low charge densities in the order of Ba>Sr>Mg. Tufa geochemistry is altered with respect to increasing Mg/Ca ratios downstream reflecting incresing Mg/Ca ratios downstream of the creek water due to continouing low-Mg-Calcite precipitation. Especially the decrease of barium holds the potential to monitor seasonal variabilities of the relative portion of biofilm influence on tufa formation, which is important regarding the use of calcareous tufa as an archive of paleoclimate information.