MICROFOSSILS FROM THE DOUSHANTUO FORMATION OF SOUTH CHINA: PETROGRAPHIC AND RAMAN HYPERSPECTRAL STUDIES

The Ediacaran Doushantuo Formation (630~550Ma) in South China is a carbonate-black shale-phosphorite sequence that was deposited in the aftermath of the Marinoan glaciation. It is notable for containing well-preserved fossils interpreted to be earliest animal embryos and bilaterians. Although other primitive micro-organisms might have been predominant in the Doushantuo environment, they have received relatively little attention. Characterization of the microfossil diversity in this context is important to trace how these microbial communities adapted to increased atmospheric oxygen in the aftermath of the Marinoan glaciation. Here we present results of petrographic and laser Raman hyperspectral analyses of the phosphorites from the lower part of the Doushantuo Formation from Yichang, South China. The phosphorites are characterized by sand-sized collophane grains cemented by sparitic apatite. Coccoidal and filamentous carbonaceous structures are abundant in these phosphatic grains. The coccoids are commonly less than 10 microns in diameter and occur alone or in clusters of a few dozens. The filaments range in thickness from 0.5 to 12 microns and in length from 10 to 115 microns. In addition, our survey revealed coiled and segmented filaments, tiny coccoids (<1 micron in diameter), and possible multi-cellular microfossils. Raman hyperspectral images document that these microfossils are phosphatized and have organic walls. The carbonaceous material (CM) associated with microfossils show sharp and nearly symmetric “D-band” and “G-band” that have similar intensities, suggesting a high degree of disorder of CM and an insignificant effect of metamorphism on isotope compositions. Current work is aimed at measuring in situ carbon and nitrogen isotope compositions to better constrain the paleo-ecological significance of these ancient micro-organisms. The abundance and diversity of primitive organisms in the lower Doushantuo Formation imply rapid ecological revitalization in the aftermath of major glaciations and adaptation to higher atmospheric oxygen levels that led to the emergence of metazoans.