Origin of the Cryogenian iron formations: Climatic fluctuation coupling with local hydrothermal iron input

Abstract

The brief resurgence of iron formation deposition during the Neoproterozoic Era appears to have been closely associated with the glaciation deposits. However, the genesis of these Cryogenian iron formations (CIFs) remains unclear. Here, we present Fe isotopic compositions and rare earth element (REE) contents for four Fulu iron formation (IF) lithostratigraphic sequences in the Sanjiang area, South China. The Fulu IF strata are in conformable contact with the underlying Chang’an glacial succession. However, owing to lack of glacial deposits evidence, they are interpreted to have formed in non-glacial conditions, probably during a warm period Sturtian glaciation. Because of the shrinking ice cover over the ocean surface, more oxidants have been transferred from the atmosphere to the ocean during this Sturtian warm period, leading to reconstruction of a redox-stratified ocean. Local hydrothermal vent systems provided dissolved Fe(II) to form the IFs, as inferred from the δ56Fe values and Eu anomalies of the Fulu IFs. Our results allow us to propose a new genetic model for the origin of CIFs; whereby partial Fe(II) oxidation caused the precipitation of Fe(III) oxide and enabled the deposition of CIFs. This occurred under redox-stratified conditions over an extended warm climatic period in combination with iron sourced from local hydrothermal activities during Cryogenian glaciations.