High authigenic Co enrichment in the non-euxinic buff-grey and black shale of the Chandarpur Group, Chhattisgarh Supergroup: Implication for the late Mesoproterozoic shallow marine redox condition

Persistently low enrichment of conventional redox-proxy trace metals, i.e., V, Cr, Mo and U in Mesoproterozoic black shale records, has generally been interpreted as an oxygen-depleted surface environment. However, new evidence in this study contradicts this concept. We performed an integrated trace metals, major oxides and total organic carbon investigation on the late Mesoproterozoic black/buff-grey shale of the Chandarpur Group. The results revealed no significant enrichment of V, Cr, Mo and U, however, intermittent high enrichment of non-detrital Co, Fe and Mn. As the studied section is non-euxinic, sulphidic Co scavenging seems to be an unlikely mechanism for the Co enrichment. A significantly positive correlation between Co and Mn, and Co-EF covariation with Fe/Al, strongly suggests the co-precipitation of Co and Fe/Mn in a well-oxygenated environment. This intermittent oxygenation of the sediment-water interface would have promoted the re-dissolution loss of V, Cr, Mo and U from the sediments deposited under preceding anoxic conditions.

Automated summary

Persistently low enrichment of conventional redox-proxy trace metals, i.e., V, Cr, Mo and U in Mesoproterozoic black shale records, has generally been interpreted as an oxygen-depleted surface environment. However, new evidence in this study contradicts this concept. The results revealed no significant enrichment of V, Cr, Mo and U, however, intermittent high enrichment of non-detrital Co, Fe and Mn.