GEOCHEMICAL EVIDENCE FOR OXYGENATED ARCHEAN OCEANS

The currently popular model for the chemical and biological evolution of Precambrian oceans (e.g., Canfield, 1999) suggests the following: (1) the deep (> ~400 m) oceans remained anoxic until ~ 600 Ma; (2) methanogenic bacteria were the (only) primary producers prior to ~2.7 Ga; (3) the first major divergence of organisms, resulting in the appearance of cyanobacteria, sulfate-reducing bacteria (SRB), methanotrophs and eukaryotes, occurred ~ 2.7 Ga; (4) the second major divergence of organisms, resulting in the appearance of sulfide-oxidizing bacteria, animals and plants, occurred ~ 600 Ma.

We have carried out systematic investigations on the chemical and isotopic characteristics (S, C, N, P, Mo, U, V, REE; Fe³⁺/Fe ²⁺; d¹³C and d³⁴S, etc.) of minerals, organics and rocks in more than 300 samples of marine sedimentary rocks ~3.5 Ga to ~1.9 Ga in age. The average contents and ranges of redox-sensitive elements and bio-essential elements (S, C, N, P, Mo, U, V, REE; Fe³⁺, Fe ²⁺) of the Archean and early Proterozoic shales are found to be essentially the same as those of Phanerozoic shales. In several stratigraphic columns studied in detail, we have recognized well-defined trends, both in time and space, in the C and S contents and the d¹³C and d³⁴S values. These data are inconsistent with the conventional model of chemical and biological evolution, but they are consistent with a model postulating the development of globally oxygenated and sulfate-rich oceans and of the emergence of SRB, cyanobacteria and methanotrophs prior to 3.5 Ga. The development of regional anoxic basins hosting submarine ore deposits (VMS and BIF) was common in the Archean oceans, as in the Proterozoic and Phanerozoic oceans.