Volcanic glass shards as a substrate for early life

Explosive volcanoes eject large amounts of ashes in the form of fine-grained glass fragments (shards) during eruption. Depending on their size, degree of vesicularity and composition, pyroclastic glass shards have chemically reactive catalytic surfaces with high surface-to-volume ratios. They are able to adsorb organics, metals, and phosphates, as well as create microenvironments attractive for microbial growth. Pyroclastic material – deposited in both aquatic and terrestrial environments – was abundant on early Earth and some of the first habitats for life may have been glass-rich. Our new sedimentological, geomicrobiological and geochemical-petrological comparative studies (LAICPMS, EMPA, TEM, Raman) aim at evaluating the significance of volcanic glass shards as a substrate and source of nutrients for microbes and as a medium for preservation of biosignatures in the geological record.

Here we show that modern (Holocene) and Paleoarchean volcanic glass shards deposited in aqueous settings (hyaloclasts) preserve evidence of alteration by microbial activity. For example, sub-recent (ca. 0.37 ka; Kaźmierczak & Kempe 2006) shards of island arc basalt composition (containing phenocrysts of the early crystallization process i.e., forsterite olivine, spinel, plagioclase-bytownite, pyroxene) are documented from the alkaline caldera lake Vai Lahi on Niuafo’ou Island, Tonga (Kempe & Kaźmierczak 2012). Analyses by 3D Raman spectroscopy (depth profiling) reveal aragonite and calcite in the entire shard volume with associated carbonaceous matter, as well as spectra of anorthite and olivine.

Most Niuafoʻou shards are coated with a laminated envelope of alternating aragonitic and silicate layers resembling oncoids cortex. Open vesicles and external faces of the shards host an organic matter and mineral assemblage texturally identical to that of the laminated envelope. Two types of alterations are identified in the Niuafo’ou shards: i) pit-like etchings; and, ii) alveolar-spongy textures. Transmission electron microscopy reveals etch-like alterations (weathering or microbial activity?) on shard surfaces to a depth of ca. 2 µm. Elemental compositions of the altered layer point to a mixture of glass and the carbonate-silicate envelope.

Niuafo’ou shards were deposited in water of increased alkalinity that favored silica dissolution and carbonate precipitation. In turn, this leads to the growth of aragonite coatings as well as sizeable stromatolites in the lake. Such habitat is ideal for alkalophilic cyanobacteria that form biofilms and participate in the precipitation of mineral envelopes. Coated by carbonate-silicate, such glass shards can effectively preserve biosignatures even as far back as the Paleoarchean (<3.5 Ga) geologic record.

Kazmierczak, J. & Kempe, S. (2006)  Naturwissenschaften 93, 119- 126.

Kempe, S. & Kazmierczak, J. (2012) Life on Earth and Other Planetary Bodies, Springer, 197-234.