Progressive oxidation of anoxic and ferruginous deep-water during deposition of the terminal Ediacaran Laobao Formation in South China

摘要

The Laobao Formation, a black chert sequence, was deposited in deep-water in the terminal Ediacaran Nanhua Basin. The chert samples have finely laminated fabric, and have no distinct recrystallization or metasomatic relict texture. There is an extremely limited detrital component and no obvious hydrothermal influence. Therefore, the chert appears to be an authigenic deep-water precipitate. The trace elements in the chert were mainly derived from seawater and thus could be a relatively unique proxy for late Precambrian seawater conditions. Redox-sensitive trace elements and relevant ratios were used to reconstruct the depositional environment of the chert sequence. Combined with iron species results of the Laobao Formation, enrichments in redox-sensitive trace elements (U, V, Mo), depleted Mn, very low Th/U ratios (< 2), high V/Sc (> 7.9) and V/(V + Ni) (> 0.6) ratios suggest an overall anoxic and ferruginous conditions over the deposition of the chert. The pyrite sulfur isotope compositions are between - 15.4% and 11.0%, and show a decreasing trend toward the top of the section, possibly implying sulfate concentration gradually increased in the terminal Ediacaran Nanhua Basin, which is consistent with implication of their pyrite framboid diameter characteristics. Our data confirm that deep-water in the Nanhua Basin was still anoxic and ferruginous, and it was progressively oxidized during the terminal Ediacaran period, which could create necessary conditions for the emergence and rapid evolution of multicellular metazoans.