Iron isotopes constrain biogeochemical redox cycling of iron and manganese in a Palaeoproterozoic stratified basin

The Hotazel Formation in the uppermost stratigraphic portion of the Neoarchaean–Palaeoproterozoic Transvaal Supergroup of southern Africa is an unusual sedimentary sequence of banded iron-formation (BIF) intercalated with three manganese-rich layers. As such, it is a succession that holds great potential to offer a unique view of one of the most dramatic transitions in early Earth history — the switch to a full oxidative cycle in shallow oceans at ca. 2.3 Ga. We present iron isotope results from BIF and Mn-rich samples collected across the entire Hotazel sequence, with a view to constraining processes of biogeochemical redox cycling for both metals close to the transition from a reducing to an oxidizing ocean–atmosphere system. The recorded de-coupling of Fe- and Mn reduction during anaerobic organic carbon cycling in the Hotazel strata, suggests that manganese became an important electron acceptor in stratified marine environments of the Palaeoproterozoic during periods of increased primary manganese precipitation relative to iron. Very low δ57Fe values registered across the entire Hotazel sequence and especially in manganese-rich samples (−2.4 to −3.5‰) signify deposition of iron and manganese in a terminal, stratified aqueous reservoir that was depleted in the heavy iron isotopes. These isotopic signatures, in conjunction with the unusual endowment of the Hotazel sequence in manganese, are interpreted to have evolved by Rayleigh distillation processes during protracted deposition of Mn-poor BIFs as preserved in the lower stratigraphic portion of the Transvaal Supergroup (Kuruman and Griquatown BIFs). The unique end-member geochemical and isotopic characteristics of the Hotazel rocks may therefore constitute a potential link between the widespread deposition of BIF during the Neoarchaean and Palaeoproterozoic, and the postulated rise in atmospheric oxygen levels around 2.3 Ga ago.