Pumice from the ∼3460 Ma Apex Basalt, Western Australia: A natural laboratory for the early biosphere

Abstract

It has recently been hypothesised that pumice, a low-density vesicular volcanic rock, could have acted as a natural floating laboratory for the accumulation and concentration of chemical reactants needed for the origin of life. To test the plausibility of his hypothesis, we here turn to the earliest rock record for evidence of pumice deposits and their associated mineralogy and biogeochemistry. We report abundant clasts of pumice from within a volcaniclastic breccia bed immediately above the ∼3460Ma ‘Apex chert’ unit of the Apex Basalt, Pilbara region, Western Australia. Textural and geochemical analyses reveal that the body of these pumice clasts was deeply permeated by intimate associations of C, O, N, P and S. Pumice and scoria vesicles were also lined with carbon or with catalysts such as titanium oxide or potential biominerals such as iron sulfide, while many were infilled with aluminosilicate minerals. The latter may be the metamorphosed remains of potentially catalytic clay and zeolite minerals. It is not yet possible to distinguish between chemical signals left by prokaryote biology from those left by prebiology. That being so, then early prokaryotes may well have colonised and modified these Apex pumice clasts prior to burial. Nevertheless, our data provide the first geological evidence that the catalysts and molecules needed for the earliest stages of life may be found within pumice rafts from the earliest oceans on Earth.