Suboxic diagenesis in banded iron formations.

Anomalous isotopic composition has been reported for the carbon in carbonate minerals of banded iron formations. Well studied examples show and enrichment in the light isotope of carbon, 12C. This enrichment presumably reflects unusual circumstances in the deposition of these sedimentary rocks. It is suggested here that the isotopically-light carbonate results from early diagenetic oxidation by bacteria of substantial amounts of isotopically light organic carbon. The electron acceptor that permits oxidation in the absence of free oxygen is presumed to the iron(III) which may have been significantly more abundant in the initial chemical precipitate than in the post-diagenetic sedimentary rock.