FIG. 7.

Concentrations of interpreted chemotrophic colonies on volcanic clasts in a strongly hydrothermally influenced location, Josefsdal Chert. (a) Field view of black, carbon-rich, silicified volcanic sediments from which the details are shown in micrographs (b–i). (b) Thin section micrographs of this rock showing a dark-colored (carbon-rich, ∼0.5% C), clotted biofilm consisting of thickly carbon-coated volcanic clasts (c–i) overlain by a layer of relatively carbon-poor, fine-grained sediments. In the latter, the bedding plains are outlined by very fine-grained detrital carbon as shown in the inset (small arrow). The large arrows in the inset document larger detrital flocks of carbon. (c) Optical micrograph showing a detail of the clotted biofilm in (b) and, in particular in the red box, a carbon-coated volcanic particle. (d–f) Raman mapping of individual mineral and carbon phases—quartz (d), carbon (e), and anatase (f)—to show the distribution of carbon as a thick layer on the volcanic particle, as well as in the matrix, and the presence of the volcanic clast just visible as an anatase-coated alteration surface beneath the carbon coat in (f). (g–i) Optical views and Raman compositional map of a large, sand-sized, carbon-coated volcanic particle observed in cross section. The silica-replaced volcanic particle exhibits an up to 30 μm thick carbon coat. The Raman map in (i) shows silica (quartz in yellow-orange) permeating the whole sample, carbon (green) coating the grain, and a trace of anatase (blue) indicating surface alteration of the volcanic particle.