Multi-scale study of the role of the biofilm in the formation of minerals and fabrics in calcareous tufa

Three sites of actively-forming tufa, two barrage systems and one terraced slope system, located in northern Calabria (Italy) and in north-east England, have been investigated with the purpose of studying the neoformed carbonate minerals at the interface with the organic components that compose the associated biofilms. Several depositional facies are distinguished, notably peloidal to aphanitic, laminar and dendrolitic fabrics composed of micrite and microsparite, and isolated botryoids and continuous crusts composed of sparite. All fabrics occurring in all depositional facies are organized into layers with a more or less well-developed seasonal cyclicity.

Low-Mg calcite precipitates more or less constantly during all seasons within the active depositional zone. This extends for a few hundred microns upon the external surface of the deposits, where the biofilm occurs. The latter is composed of a heterogeneous community of green algae, filamentous cyanobacteria and other types of prokaryotes, Actinobacteria and fungi, with a variable amount of extracellular polymeric substances (EPS). Porous micro-columns (50 to 150 mu m in size), separated by interstitial spaces, characterize the active depositional zone. Here precipitation always begins with organomineral nanospheres (10 to 20 nm diameter), both along the external surfaces and within internal cavities of the micro-columns, by replacing degraded organic matter, and at point-sites suspended within living cyanobacterial tufts along the external surface of their sheaths, indicating that the biological activities of the biofilm are crucial, with its living organisms and non-living organic matter.

Organomineral nanospheres successively agglutinate to form irregular to rod-shaped crystal aggregates, 100-200 nm in size, that with their further agglutination create two basic types of larger, more ordered, crystal structure: polyhedrons in the range of 1-2 mu m, and minute triads of calcite fibres varying in length from similar to 0.4 (short triads) to similar to 1 mu m (long triads). Basic crystal structures coalesce to form larger crystals (mainly tetrahedrons ;tens of microns in size) that constitute the micro-columns. (C) 2011 Elsevier B.V. All rights reserved.