THE ARCHEAN-PROTEROZOIC TRANSITION: CARBON ISOTOPIC INDICATORS OF CHANGE
DES MARAIS, David J., NASA Astrobiology Institute, Mail Stop 239-4, Ames Research Center, Moffett Field, CA 94035-1000, ddesmarais@mail.arc.nasa.gov.

Archean and early Proterozoic sedimentary rocks have recorded the effects of both biological evolution and environmental change upon carbon isotopic discrimination as well as upon the relative rates of burial of carbonates and organics. Thermally-driven postdepositional increases in kerogen d13Cred were usually minor. Archean d13Cred ranges from >-10 to <-60 and exhibits multiple isotopic modes, consistent with a diversity of pathways for carbon assimilation among globally-prominent anaerobic microbial ecosystems. Archean carbonate d13Ccarb values cluster near 0, consistent with total dominance of the redox balance of the Archean carbon cycle by the mantle, which even today is a major redox buffer. The rise of oxygenic photosynthesis enabled life to obtain additional reducing power by splitting H2O. Thus, during the Proterozoic, photosynthetic production rose above the level determined by mantle-derived fluxes of reduced species. Today, chemoautotrophs harvesting energy from hydrothermal emanations synthesize globally at most between 0.2x1012 and 2x1012 mol organic C yr-1. In contrast, global photosynthetic productivity is estimated as 9000x1012 mol C yr-1. During the early Proterozoic, major excursions in d13Ccarb, and thus in relative sedimentation rates of organics versus carbonates, confirm the global importance of oxygenic photosynthesis by that time. Secular changes in the Proterozoic sedimentary d13Cred record are consistent with an increase in the oxidation state of the environment and a transition to global dominance by producers and utilizers of O2 and SO4=. This progressive oxidation was episodic. For example, between 2.2 and 1.9 Ga, crustal and atmospheric reservoirs of O2, Fe3+ and SO4= increase was relatively rapid and too large to be attributed solely to the declining rate of oxidant consumption by volcanic and hydrothermal reduced species. Additional changes seem required, e.g., in the style of organic sedimentation and preservation. Episodic changes in the carbon cycle and in O2 levels likely influenced the evolution of Eukarya.

Earth System Processes - Global Meeting (June 24-28, 2001)
Session No. T1
Archean Earth and Contemporary Life: The Transition from an Anaerobic to an Aerobic Marine Ecosystem (Sponsored by NASA Astrobiology Institute)
Edinburgh International Conference Centre: Sidlaw
10:00 AM-4:30 PM, Tuesday, June 26, 2001