Genesis of the Fulu Cryogenian iron formation in South China: Synglacial or interglacial?

•Sedimentological and geochemical variability indicate a range in depositional environments from distal to proximal facies for two sections.•Fe–Nd isotope variability is indicative of mixing between detrital and hydrothermal components for the iron formation.•The Cryogenian Fulu iron formation formed during an interglacial rather than synglacial period. Iron formations reappeared globally in the Cryogenian geological record associated with the Sturtian glaciation (ca. 720–660 Ma), and are interpreted to have been deposited during a critical period of extreme climate change that preceded the Ediacaran appearance of metazoans in the fossil record. The origin of Cryogenian iron formations has been hotly debated, as they have been variously interpreted as the products of submarine hydrothermalism related to tectonic activity or the oxidation of ferruginous seawater related to widespread ice cover during the “Snowball Earth” event. These end-member hypotheses have different implications for our understanding of seawater chemistry in the Cryogenian. The Cryogenian Fulu iron formation from South China is central to this debate as it can provide a test of the different genetic models for Cryogenian iron formations. In this study, we combine major and trace element data and Fe–Nd isotope systematics to constrain the genesis of the Fulu iron formation. We document geochemical variability within the Fulu iron formation and argue that this is caused by mixing between detrital and hydrothermal components. Further, we suggest that the sedimentology of the Fulu Formation is incompatible with a synglacial origin for the iron formation. A model of interglacial Fe2+ oxidation is proposed to account for the sedimentological and geochemical characteristics of the Fulu iron formation. In this model, the Fe2+-rich Cryogenian seawater was partially oxidized upon deglaciation, leading to the deposition of hematite-rich sedimentary layers with heavy Fe isotope compositions. Meanwhile, a significant amount of continental debris was delivered into the ocean via meltwater from deglaciation. This interglacial model is consistent with earlier models for Cryogenian iron formations but stands in contrast to more recent synglacial models.