THE ARCHEAN-PROTEROZOIC TRANSITION: CARBON ISOTOPIC INDICATORS OF CHANGE

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 d¹³C_red were usually minor. Archean d¹³C_red 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 d¹³C_carb 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 d¹³C_carb, 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 d¹³C_red 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.