GAS PRODUCTION WITHIN ARCHEAN STROMATOLITES: EVIDENCE FOR ANCIENT MICROBIAL METABOLISMS

Determining which microbial metabolisms existed in the Archean is a fundamental question of deep-time geobiology. Specific fenestral textures within Archean stromatolites provide evidence for the presence of gas within mats, and thus gas-releasing metabolisms within ancient microbial mats. Paleoenvironmental analysis indicates many of the stromatolites formed in shallow, agitated aqueous environments, with relatively rapid gas production and lithification of fenestrae. Proposed gases include oxygen, carbon dioxide, methane, hydrogen sulfide, and various nitrogen species, produced by appropriate metabolisms. This study examines the potential for various gas-releasing metabolisms to produce fenestral textures seen in Archean stromatolites.

First, the volume of gas within Archean microbial mats was estimated by measuring fenestrae in ancient stromatolites and bubbles within modern microbial mats. The time needed for metabolisms to produce appropriate gas volumes was calculated using modern rates obtained from the literature. Given the paleoenvironmental conditions, the longer a metabolism takes to make large amounts of gas, the less likely large bubbles will remain long enough to become preserved. Additionally, limiting reactants were estimated for each metabolism using previous Archean geochemical models. Metabolisms with limited reactants are less likely to produce large amounts of gas. For example, in modern natural environments, sulfate reduction produces amounts of gas necessary for large fenestrae within hours; however, Archean environments were low in sulfate, severely limiting potential sulfate reduction rates. Methanogenesis can produce enough gas for large fenestrae in several hours, and unlike sulfate, the hydrogen required for methanogenesis was potentially less limited in the Archean. Oxygenic photosynthesis, however, can produce large amounts of gas within minutes, and the necessary reactants (carbon dioxide and water) were readily available in Archean environments. In the absence of clear sedimentary or geochemical evidence for abundant hydrogen or oxidized sulfur and nitrogen species during stromatolite morphogenesis, oxygenic photosynthesis is the metabolism with the highest potential for producing fenestrae before the GOE.