A REVERSED WATER-SEDIMENT REDOX GRADIENT IN THE EVE OF CAMBRIAN EXPLOSION

It is widely accepted that early animal evolution and change of marine redox landscape were tightly coupled. Sedimentological records provide the first order constraint on the organismal and environmental coevolution in the Ediacaran-Cambrian transition (550-521 Ma). Here, we focus on carbonate concretions in the black shale of early Cambrian Shuijingtuo Formation in the Yangtze Block, South China. Such concretion layer has a platform-wide distribution and can be used as a marker in regional stratigraphic correlation. The Shuijingtuo carbonate concretions contain three types of greigite (Fe₃S₄), a metastable precursor of pyrite: laminar greigite, greigite aggregates, and disseminated greigite. The laminar greigite was deposited syndepositionally within sediment pore water, while the greigite aggregates were distributed in the peripheral of carbonate concretions, suggesting the concurrent origin with carbonate concretions. Nevertheless, three types of greigite have nearly identical sulfur isotopic compositions (d³⁴S), suggesting that all greigite might have derived from the same sulfur source. We suggest H₂S diffusion from sulfidic seawater might be ultimate source of greigite. Furthermore, carbonate concretion formation was triggered by the dissimilatory iron reduction (DIR) that elevated porewater alkalinity and pH and accordingly increased carbonate saturation state. On the other hand, DIR also provided Fe²⁺ for greigite aggregates formation in the peripheral of carbonate concretion. Therefore, a reversed vertical redox profile, which was characterized by seawater euxinic and sediment porewater ferruginous, might have developed during the carbonate concretion formation. We suggest that the reversed redox profile could be generated by low seawater sulfate concentration and abundant supply of particulate reactive Fe (detrital iron oxides). Such iron cycle differs from the canonical Fe shuttle model in Black Sea, where Fe is supplied by the remobilization of Fe oxides in suboxic sediments.

Key words: Shuijingtuo Formation; carbonate concretions; greigite; sulfidic; iron reduction