CARBON ISOTOPIC BIOGEOCHEMICAL CLUES ABOUT THE EARLY BIOSPHERE

The two major crustal carbon reservoirs are carbonates (5.8x10exp21 moles) and kerogen (1.2x10exp21 moles). Changes over time in 13C/12C values of these reservoirs indicate changes both in isotopic discrimination during biosynthesis and processing of organics, and in relative rates of burial of organics vs carbonates. The range in 13C/12C values of kerogens older than 2.1 Ga exceeds that of younger kerogens. Anaerobic microbes assimilate CO2 by a variety of pathways that differ substantially in isotopic discrimination. The decrease over time in the range of kerogen 13C/12C perhaps reflects the transition from a world where global primary production (GPP) was dominated by diverse anaerobes to a world where cyanobacteria and algae dominated. The onset after 2.3 Ga of major isotopic excursions for carbonates indicates large episodic burial events of organic carbon and shows that oxygenic photosynthesis was globally significant. Increases in the range of 34S/35S of sulfides largely coincide with these organic burial events and are consistent with rising seawater sulfate levels. The rise of O2 photosynthesis increased GPP by 100-fold or more. Fermentation of those abundant photosynthetic organics produced hydrogen that, in an anoxic world, increased the rate of H2 escape to space and hastened the oxidation of the global environment. Evidence for the existence of O2 photosynthesis at 3.5 Ga is NOT compelling. We are challenged to discover the oldest evidence of its existence, when it ascended to global dominance, and how the ascent affected reservoirs of Fe3+, sulfate, nitrate and O2. The products of O2 photosynthesis can be actively and almost completely recycled within modern microbial mats, with little or no net effect upon the external environment. Ecological studies of such mats can offer clues about an ancient anoxic world having "oases" of O2 photosynthetic communities. Geologists and geochemists must determine when, and for how long, such a world actually existed.