Chert precipitation in the aftermath of the Cryogenian Sturtian glaciation

The cap carbonates are markers indicating the end of the Sturtian (717–660 Ma) and Marinoan (∼650–635 Ma) global glaciations. These carbonates precipitated from oversaturated seawater, which was caused by intense continental chemical weathering under extremely high atmospheric CO2 levels. Unlike the widespread Marinoan cap carbonate, the Sturtian cap carbonate is restricted, questioning the uniformity of the two post-glacial weathering processes. This study presents a sedimentological analysis of the basinal Sturtian deglacial succession in the Cryogenian Fulu 4th member, South China. The analysis identifies four facies associations: the proglacial diamictite facies association, the glaciomarine shale facies association, the distributary gravelly sandstone facies association, and the turbidite facies association. Notably, the glacial diamictite is directly overlain by chert, revealing an abrupt end of the Sturtian glaciation. The chert is composed of equidimensional granular microquartzs, and is characterized by high total Fe2O3 contents (0.29–0.97 %, mean = 0.50 %), low Ge/Si ratios (0.50–0.68 μmol/mol, mean = 0.61 μmol/mol), left-leaning PAAS-normalized REE patterns, coupled Eu/Eu* and Y/Ho ratios. These results suggest that the Si source is from post-glacial weathering, with a minor hydrothermal influence. This study supports an enhanced weathering response in the termination of the Sturtian glaciation, providing insights from a Si perspective into the deglacial geochemical processes. The restricted occurrence of chert implies that silicate weathering and its signature may have been spatially variable, which is an important consideration for future research.