Geochemistry and molybdenum isotopes of the basal Datangpo Formation: Implications for ocean-redox conditions and organic matter accumulation during the Cryogenian interglaciation

Abstract

The basal Datangpo Formation in South China deposited in the Cryogenian interglacial period has gained increasing interest with the well development of organic-rich black shale and its hosted manganese (Mn) deposits (Mn carbonate), which contain important clues to the paleoclimate and ocean redox of this critical interval in Earth's history. The depositional environment of Mn deposits is still uncertain, as suggested by different suites of redox-indicating parameters. In addition, the Mn deposits are notable for their enrichment in organic matter (OM), despite the fact that the diagenetic origin for the Mn deposits would result in large consumption of OM. To gain insight into these issues, we present the geochemical and molybdenum (Mo) isotopic data of black shale and Mn carbonate samples from a new section. The Datangpo Mn carbonates are characterized by low Mo content (mostly <3.2 ppm) and slightly light Mo isotopic composition (mostly from +0.07‰ to +0.52‰), suggesting that Mo, once adsorbed in Mn-oxide particles, was released in suboxic or anoxic (non-sulfidic) pore water due to Mn-oxide reduction and then diffused back into the water column. Hence the extremely light Mo isotopic signatures might be rarely preserved in sediments. The Mn carbonate formation accompanied by OM re-mineralization within sediments may have caused lower total organic carbon (TOC) content relative to that in black shales. However, elevated nutrient input related with hydrothermal fluid activity probably enhanced primary productivity during the Mn deposition period. It is evidenced by: (1) the Mn-TOC-total inorganic carbon covariation displayed by the Datangpo section and (2) a positive correlation between TOC and Mn content when Mn content is higher than 10% in a large compiled dataset. A compilation of Mo concentration and Mo isotope data suggests that widespread anoxia prevailed during the Cryogenian interglaciation. Instead, Neoproterozoic ocean oxygenation was postponed to the Ediacaran.