Carbonate precipitation associated with microbial mats in freshwater streams (Brianza Foothills, Lombardy, Northern Italy)

Calcareous tufa are carbonate deposits forming in ambient temperature freshwater streams in association with vegetation and microbial mats. Carbonate precipitation results from physico-chemical and biologically induced and... [ view full abstract ]

Calcareous tufa are carbonate deposits forming in ambient temperature freshwater streams in association with vegetation and microbial mats. Carbonate precipitation results from physico-chemical and biologically induced and influenced processes. The investigation of tufa can provide fundamental information for the understanding of the role played by microorganisms and organic substrates in carbonate precipitation.
Tufa deposits, actively forming metre-scale stepped barrage systems along streams in the Brianza Foothills (Northern Italy), were investigated in terms of facies, petrography and scanning electron microscopy (SEM) to identify the precipitated carbonate fabrics and the relationships with microbial biofilms, microphytes and macrophytes. Carbonate and associated microbial mat samples were fixed in the field with formaldehyde and glutaraldehyde solutions and dehydrated with increasing ethanol concentrations.
The stepped profile forming the tufa barrage system is nearly 20 m long and consists of pools (0.2-2 m wide, 0.1-1.1 m deep) separated by pool rims with sub-horizontal crests (0.2-1.5 m long downstream) fronted by sub-vertical walls, 0.1-1.5 m high. The concave-upward pool floors are draped by detrital clays to gravel deriving from Cretaceous-Palaeogene turbidite sandstone and pelagic calci-mudstone/wackestone of the substrate rocks, plant fragments, and carbonate coated oncoids. The rim crests and walls are draped by green microbial mats; on the rim walls Vaucheria algae are sparse whereas bryophytes and liverworts thrive mostly where the stream water is not temporally flowing. Along the 20 m distance of the barrage system, measured summer water temperatures are 18-20°C, pH increases downstream from 7.7 to 8.1, whereas electric conductivity and alkalinity decrease from 537 to 499 µS/cm and 251 to 188 mg/l, respectively.
Carbonate precipitates as low Mg calcite forming: a) laminated boundstone building pool rims and walls with cyanobacteria filaments encrusted in vertical upright orientation, b) Vaucheria boundstone consisting of micrite and microsparite coated algal filaments, c) dendritic clotted peloidal micrite and crystal fan centimetre-thick crusts coating detrital grains, d) phytoclast rudstone and boundstone with coated macrophytes and Charophytes algae, e) rare encrusted bryophytes boundstone. SEM analysis shows that carbonate tubes around filamentous cyanobacteria (Phormidium) and Vaucheria are made of nanometre-size micrite aggregated in 5-10 µm size crystals and internally are draped by Mg-Fe silicates. Cyanobacteria, such as possible Spirulina and Oscillatoria, are entombed within the carbonates together with pennate diatoms. In the analysed freshwater streams, carbonate precipitation exclusively occurs on organic substrates whether consisting of microbial filaments, biofilm extracellular polymeric substances, algal tubes and plant stems and leaves.