Iron and carbon isotope evidence for microbial iron respiration throughout the Archean

Abstract

Banded Iron-Formations (BIFs) are voluminous chemical sediments that are rich in iron-oxide, carbonate and silica and whose occurrence is unique to the Precambrian. Their preservation in the geological record offers insights to the surface chemical and biological cycling of iron and carbon on early Earth. However, many details regarding the role of microbial activity in BIF deposition and diagenesis are unresolved. Laboratory studies have shown that reaction between carbon and iron through microbial iron respiration [2Fe 2O 3∙ nH 2O + CH 2O + 7H + → 4Fe 2+ + HCO 3 − + (2n + 4)H 2O + chemical energy] can impart fractionation to the isotopic compositions of these elements. Here, we report iron (δ 56Fe, vs. IRMM-014) and carbon isotopic (δ 13C, vs. V-PDB) compositions of magnetite and of iron-rich and iron-poor carbonates in BIFs from the late Archean (~ 2.5 Ga) Hamersley Basin, Australia and the early Archean (~ 3.8 Ga) Isua Supracrustal Belt (ISB), Greenland. The range of δ 56Fe values measured in the Hamersley Basin, including light values in magnetite and heavy values in iron-rich carbonates (up to + 1.2‰), are incompatible with their precipitation in equilibrium with seawater. Rather, the data together with previously reported light δ 13C values in iron-rich carbonates record evidence for diagenetic reduction of ferric oxide precursors to magnetite and carbonate through microbial iron respiration (i.e., dissimilatory iron reduction, DIR). Iron and carbon isotope data of iron-rich metacarbonates from the ISB are similar to those of late Archean BIFs. The isotopic signatures of these metacarbonates are supportive of an early diagenetic origin despite metasomatic overprint, and preserve evidence of microbial iron respiration within the oldest recognized sedimentary rocks on Earth.