Organic Carbon Isotope Geochemistry of the Neoproterozoic Doushantuo Formation, South China

摘要

The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered: the Weng'an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng'an section of a carbonate platform (shelf of the Yangtze Platform, Guizhou Province) from the Songtao section and Nanming section of a transition belt (slope of the Yangtze Platform, Guizhou Province) and from the Yanwutan section (basin area of the Yangtze Platform, Hunan Province). Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon (mean delta~(13)C_(org) = - 35.0 per thousand) from the uppermost Nantuo Formation are followed by an overall increase in delta~(13)C up-section. Carbon isotope values vary between -9.9 per thousand and 3.6 per thousand for carbonate and between -35.6 per thousand and -21.5 per thousand for organic carbon, respectively. Heavier delta~(13)C_(carb) values suggest an increase in organic carbon burial, possibly related to increasing productivity (such as the Weng'an biota). The delta~(13)C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.