EARLY ARCHAEAN BACTERIAL MATS/BIOFILMS AND HYDROTHERMAL ACTIVITY

The oldest morphological evidence for life on Earth documents a sometimes close association between hydrothermal activity and microbial mats and biofilms. Silicified bacterial biofilms and mats occur in chert horizons in the 3.3-3.5 b.y.-old Early Archaean Barberton and Pilbara greenstone belts, some of which are associated with hydrothermal activity (Byerly et al., 1986; Walsh, 1992; Nijman et al., 1999; Westall et al., 2001). Sometimes visible macroscopically as wavy, tabular to hummocky, finely stratified structures, in thin section the microbial mats are often intimately interlayered with and/or replaced by synsedimentary, hydrothermal chert, often also associated with barite, carbonate or iron oxides). The mats were principally constructed by long, thin, silicified filamentous bacteria (1-2 µm wide, up to 100 µm long), possibly representing anaerobic phototrophs. SEM imaging of organic mats on bedding plane surfaces documents thick, permineralised polymeric sheets containing fossilised gas bubbles, desiccation cracks, and embedded evaporite minerals, such as halite hoppers, calcite (and gypsum?), as well as hydrothermal tourmaline. Some of these features indicate intermittent, subaerial exposure of the microbial mats. High resolution SEM studies also document thinner bacterial biofilms on top of water-lain volcaniclastic sediments which contain embedded smooth-surfaced, rod shaped bacteria (2-3.8 µm long, 1 µm wide) and smooth to wrinkled-surfaced coccoidal bacteria (1 µm diameter). They could represent anaerobic heterotrophs.

All the sedimentological indications point to formation of the mats and biofilms on shallow water volcanic and volcaniclastic sediment surfaces which were occassionally subaerially exposed. The mats and biofilms often occur immediately adjacent to hydrothermal vents, as well as on the sides of the chimneys.

Byerly, G.R. et al., 1986, Nature, 319, 489. Nijman, W. et al., 1998, Precambrian Res., 95, 247. Walsh, M.M. 1992, Precambrian Res., 54, 271. Westall, F. et al., 2001, Precambrian Res., 106, 93.