Tufa-forming biofilms of German karstwater streams: microorganisms, exopolymers, hydrochemistry and calcification - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 81

mirrors that of the bulk water body only reflecting climate fluctuations. Tufa stromatolites with
their cyanobacterial - photosynthesis-related calcification fabrics form an analogue to porostromate
cyanobacterial stromatolites in fossil settings high in CaCO 3 mineral supersaturation but com-
paratively low in dissolved inorganic carbon. Here, the sum-effect of heterotrophic exopolymer-
degradation and secondary Ca 2þ -release rather decreases calcite saturation, contrary to settings
high in dissolved inorganic carbon such as soda lakes.

Geobiological significance of tufa
stromatolites

Muschelkalk Group: Usdowski et al. 1979; Salzwe-
del 1992; Malm Group in Weser and Leine Hills:
Menzel 1909; Weißjura Group of the Franconian-
Swabian Alb: Burger 1911; Stirn 1964; Gr¨ninger
1965; Merz-Preiß & Riding 1999; Arp et al.
2001b), and Upper Cretaceous chalk deposits (e.g.
M ¨nsterland: Hartkopf-Fr ¨der et al. 1989) are
favourable for present-day fluvial tufa deposition
(Fig. 1). In addition, a number of small streams
and rivers of the Northern Alpine Molasse Basin
show tufa stromatolites and oncoids due to Ca -
HCO 3 -dominated groundwaters discharging from
Miocene-Pliocene

Tufa-forming phototrophic biofilms are widespread
in freshwater streams in European regions where
Palaeozoic, Mesozoic and Cenozoic limestone for-
mations are subject to present-day karstification
(Ford & Pedley 1996). With respect to Germany,
areas of outcropping Devonian reef carbonates
(Herzynian Fold Belt with Rhenish Slate Mountains
and Harz Mountains; e.g. Sommermeier 1913),
Middle Triassic and Upper Jurassic epicontinen-
tal limestones (escarpments and plateaus of the gravel and Quarternary till

Fig. 1. Location of the studied tufa-forming karstwater streams in Germany.