Iron mineralization of cyanobacteria Girvanella: Insights from Cambrian ferruginous stromatolites of southern North China Block

Girvanella , a cyanobacterial fossil commonly found in Cambrian carbonates, plays a crucial role in the formation of many microbialites. Despite considerable advancements in elucidating calcifying Girvanella and its biomineralization mechanisms, the understanding of iron-mineralized Girvanella and its iron-mineralization mechanism is still relatively vague. This paper investigates the ferruginous stromatolites from the Mantou Formation Member Ⅱ (Cambrian Miaolingian Series) of southern North China Block. Through an integrated analysis of sedimentology, mineralogy and geochemistry, we explore the sources of iron minerals, the genesis of chamosite, and the role of Girvanella in iron mineralization. The results indicate that the ferruginous stromatolites consist of dark laminae with densely distributed Girvanella and light laminae with sparsely distributed Girvanella . The tube core of Girvanella is primarily composed of calcite, whereas the tube wall of Girvanella contains hematite and chamosite. The iron minerals within the stromatolites were mainly originated from continental weathering of the Ordos Land and transported into the relatively enclosed study area in a colloidal suspension. This process supplied essential Fe–Al–Si material for the formation of chamosite. The low-energy hydrodynamic condition, coupled with suboxic to weakly reducing settings as reflected by the presence of residual organic kerogen, can provide favorable hydrochemical conditions for the formation of chamosite in ferruginous stromatolites. The authigenic chamosite formed by natural crystallization exhibits a widespread and random distribution within the stromatolitic laminae. In contrast, cryptocrystalline chamosite and hematite are restricted to the Girvanella sheaths and their surroundings. This strongly indicates a close relationship between the formation of cryptocrystalline iron minerals and the iron mineralization of Girvanella . Through CO 2 -concentrating mechanism, Girvanella generates a large number of anions in its surroundings. During respiration and organic matter decomposition, Fe 3+ is partially reduced to Fe 2+ , and these anions adsorb Fe 2+ , Mg 2+ along with dissolved SiO 4 2- and Al 3+ , facilitating nucleation on Girvanella cell wall and/or EPS (extracellular polymeric substances), leading to the formation of cryptocrystalline chamosite. A subset of chamosite is subsequently oxidized into hematite under the influence of Girvanella photosynthesis. The coupling of oxidation and reduction processes induced by microbial respiration and Girvanella photosynthesis result in the coexistence of iron and ferrous minerals on the Girvanella sheaths.