Fluvial tufa formation in a hard-water creek (Deinschwanger Bach, Franconian Alb, Germany)

Cyanobacteria-dominated biofilms involved in tufa deposition in the hardwater creek Deinschwanger Each, Bavaria, were investigated with regard to their effect on the carbonate equilibrium and fabric formation. Current tufa deposition is evident by up to 1.5 mm thick crusts that have formed on substrate plates placed in the creek for 10 months. Hydrochemistry data indicate that carbonate precipitation along the creek is physicochemically driven by CO2 degassing, whereas photosynthetic carbon assimilation is without detectable effect on the macroscale carbonate equilibrium. However, stable isotope data indicate a minor photosynthetic effect, but only for the lower creek section where the pCO(2) already drops to the two-fold of the atmospheric level. Though the initial process of external nucleation on cyanobacterial sheaths in the lower creek section might be promoted of by a photosynthetically-induced microscale pH gradient, the effect is not strong enough to cause a CaCO3 impregnation of the sheaths. The fabric of the laminated tufa crusts in the creek reflects the temporal alternation of porous microsparitic Phormidium incrustatum-Phormidium foveolarum - diatom biofilms in spring, micrite-impregnated Phormidium incrustatum - Phormidium foveolarum - diatom biofilms in summer-autumn, and detritus-rich non-calcified diatom-biofilms in winter. By contrast, exopolymer-poor surfaces of cascade tufa mosses show large, euhedral spar crystals. Non-phototrophic bacteria, which occur in large numbers in Phormidium incrustatum - Phormidium foveolarum - diatom - communities, thrive on extracellular polymeric substances (EPS) and dead cells of the cyanobacteria and are unlikely to promote CaCO3 precipitation.