13. Archaean Soils, Lakes and Springs: Looking for Signs of Life

Microbial life in Archaean non-marine settings like soils, lakes and springs would have faced several challenges. These would have included exposure to UV light; aridity, salinity and temperature changes; and nutrient availability. Current understanding is that none of these challenges would have been insurmountable. Microbial organisms of Archaean marine environments are likely to have been similar in their lifestyles and habits to those of the Archaean terrestrial world. Non-marine stromatolites, microbial filaments, microbial borings and microbially-induced sedimentary structures might therefore have been preserved. But Archaean subaerial surfaces would have been very prone to erosion by wind and rain, so the oldest fossil ‘soils’ of subaerially weathered surfaces (up to 3.47 Ga) are mostly identified using geochemistry. However, some ancient duricrusts like calcretes have been reported. Archaean lacustrine microbial life may have included stromatolites of the Tumbiana Formation of Western Australia. The case that these were lacustrine rather than marine is critically assessed, with the conclusion that the stratigraphy provides the strongest supporting evidence here. Archaean terrestrial hot springs, though often mentioned in origin of life studies, are not yet known from the rock record. In the Palaeoproterozoic to present these silica and carbonate-precipitating environments are commonly found in proximity to volcanic sediments and faults, where the deposits form terraced mounds, fissure ridges and hydrothermal lakes. It remains plausible that life could have existed and even evolved in these hypothesised Archaean hot-spring settings, and there is cause for optimism that the evidence for this might one day be found.