Stable isotopic geochemistry of organic carbon and pyrite sulfur from the Early Cambrian black shales in Northwestern Hunan, China

The early Cambrian black shale sequence in Northwestern Hunan Province has been investigated for its stable isotopic geochemistry of organic carbon and pyrite sulfur. The relatively low delta(13)C values of organic matter, - 29.7parts per thousand similar to - 34.2parts per thousand, are interpreted as caused by the high CO2 concentration in atmosphere/ocean and an anoxic depositional environment in early Cambrian. Variable, but significantly positive delta(34)S values, + 10.2parts per thousand similar to + 28.2parts per thousand, for sedimentary pyrite reveals bacterial sulfate reduction as the pyrite-forming process but under sulfate-limited conditions. A protected anoxic basin, which had very limited access to open ocean, and its sediments, where metabolizable organic matter was rich and bioturbation was absent, are supposed to be the sulfate-limited environments and might have existed in South China in the early Cambrian.