Organic composition and diagenetic mineralization of microfossils in the Ediacaran Doushantuo chert nodule by Raman and petrographic analyses

Abstract

We studied microfossils and their embedding matrix minerals from chert nodules in the Ediacaran Doushantuo Formation of the Yangtze Gorges area in South China using Raman spectroscopy as well as transmitted- and polarized light microscopy. The microfossils are composed of organic carbonaceous material with weak structural organization/ordering, which indicates a low degree of thermal maturation and alteration. Raman spectral disparity of carbonaceous material among different portions (vesicle/inclusion) of microfossils reveals some differences in organic composition and structure of their precursor biological matters. The mineral phases of silica matrix that entombs the microfossils are recognized as opal-CT, cryptocrystalline quartz, and their transitional phases. Overall, Raman spectral analyses and petrographic observations document a complete and continuous sequence of silica phase transformation from opal-CT to quartz. When reconstructing the diagenetic history of sediments and mineralization process of microfossils in a localized microenvironment, the silica in pore waters precipitated preferentially at the periphery of microorganisms in the form of opal, gradually decreasing in abundance as it transformed into more stable cryptocrystalline quartz. The recrystallization took place in cryptocrystalline quartz to form grains. Authigenic fluorapatite readily precipitated in the vicinity of some microfossils and within their degraded organic fragments. While apatite was crystallizing around organisms and their biostructures, silica precipitated in the surrounding voids, inhibiting the space of potential apatite crystallization. The silicification and phosphatization of microfossils occurred in the microbial sulfate reduction (MSR) zone of the sediment column, and both were driven by the localized pH change resulting from MSR and pyrite precipitation in the microenvironment, thus facilitating the exceptional fossil preservation in the Doushantuo chert nodules.