Aerobic carbon cycling and cerium oxidation: significance for Archean oxygen levels and banded iron-formation deposition

Abstract

The amounts of organic carbon and iron generally estimated to have been preserved in early Precambrian sediments suggest by implication that only the rapid, oxidative recycling of organic carbon by aerobic heterotrophy could plausibly have kept pace with the oxygen produced annually by net photosynthesis, even with a low global primary productivity of less than 1% of modern global values (Towe, 1990). A global aerobic recycling process would require at least a moderately oxygenated atmosphere to operate. The presence of such an atmosphere as far back in Earth history as 3.8 Ga ago is consistent with, but not proven by, the abundance of organic carbon and the “anomalous” distribution of rare earths in the banded iron-formations of the Isua sediments (Dymek and Klein, 1988). The absence of positive cerium anomalies and the presence of positive europium anomalies in these and other banded iron-formations (Klein and Beukes, 1989; Derry and Jacobsen, 1990) imply that iron oxidation during BIF deposition took place in waters at some depth below the zone of mixing rather than at the surface. A modified Klein-Beukes model using shallow, seasonal downwelling in an otherwise deep-water stratified basin under a moderately oxygenated atmosphere may help explain the origin of some of these enigmatic deposits.