Shallow-marine ironstones formed by microaerophilic iron-oxidizing bacteria in terminal Paleoproterozoic

Abstract

Abstract The microbial origin of Precambrian iron formations is debated due to the lack of direct fossil evidence. In order to reveal the genesis of ironstones under low-oxygen levels, integrative studies of sedimentology, petrography, mineralogy, and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal Paleoproterozoic Chuanlinggou Formation (ca. 1.65–1.64\u202fGa) of North China, using microscopy, SEM, EDS, ICP-OES, ICP-MS and MC-ICP-MS techniques. Mineralogical study shows that the Fe-rich mineral is predominantly hematite that resulted from dehydration of amorphous Fe-oxyhydroxide during diagenesis. Petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus. Basinward increases of the ironstone abundance, Eu anomalies (from 1.39 to 1.56) and δ56Fe values of the ironstones (from\xa0+0.5‰ to\xa0+1.0‰) suggest that the iron was mainly sourced from seafloor hydrothermal fluids, and partially oxidized and precipitated in shallow subtidal to intertidal environments. The common existence of Fe-oxide coated sheaths, spiral stalks, residual extracellular polymer substances (EPS) and other biogenic fabrics indicates that microaerophilic iron-oxidizing bacteria (FeOB) may have played an important role in precipitating the Chuanlinggou ironstones. The extremely low oxygen concentrations implied by the proliferation of microaerophilic FeOB in the shallow waters, the weak positive Ce anomalies (0.94–1.12) and low Mn concentrations in the ironstones are broadly consistent with the previous result of a Cr isotope study. Thus the establishment of a microaerophilic FeOB genetic model for the widespread Chuanlinggou ironstones in North China provides new insight into the origin of Precambrian iron formations and the redox evolution of ocean-atmosphere systems during the “Boring Billion”.