Termination of Cryogenian ironstone deposition by deep ocean euxinia

Abstract

Widespread deposition of iron-rich sedimentary rocks (ironstones) occurred during the Sturtian ice age, the earlier of two Cryogenian 'Snowball Earth' glaciations. However, the reasons for the termination of Cryogenian Iron Formation (CIF) deposition remain poorly understood. Here we report a multi-isotope (Fe-C-S) study of the Xinyu CIF in South China that can directly address this question. The isotopic compositions of these multivalent elements exhibit remarkable covariance at the top of the Xinyu CIF that reflects progressive redox stratification during iron oxide deposition. Iron oxide deposition ended abruptly due to the titration of ferrous iron as pyrite following a large influx of riverine sulfate. Although termination of CIF deposition is commonly attributed to oxygenation, our data show that at least in some Cryogenian ocean basins, CIF deposition ended with the spread of euxinic rather than oxygenated waters.

The deposition and significance of an Ediacaran non‐glacial iron formation

“…The reappearance of IFs in the Neoproterozoic roughly coincided with a transition to higher surface oxygen levels, termed the Neoproterozoic Oxygenation Event (NOE) by some authors (e.g., Shields‐Zhou & Och, 2011). However, it is typically assumed that essentially all significant Neoproterozoic IFs (NIFs) are associated with glacial deposits, specifically, the Sturtian glaciation (e.g., Cox et al, 2013; Halverson et al, 2011; Hoffman et al, 2017; Klein, 2005; Lechte et al, 2018; Wu et al, 2020). This facies association has been used to argue for global or regional ice cover during the Cryogenian “Snowball Earth” events, which would restrict atmosphere–ocean mixing and gas exchange, thereby allowing the development of ferruginous conditions (i.e., Fe(II)‐enriched, H 2 S‐depleted, and anoxic) in large parts of the global ocean (Hoffman et al, 1998; Kirschvink, 1992).…”

Mesoarchean banded iron-formation from the northern Yangtze Craton, South China and its geological and paleoenvironmental implications

“…In China, most Neoarchean-Paleoproterozoic BIFs are deposited in the North China Craton (Li et al, 2012b;Wang et al, 2014;Wang et al, 2017;Wu et al, 2020), while the presence of BIFs in the older Mesoarchean South China Block, host to the Yangtze Craton, remains poorly explored.…”