Taphonomic study of Ediacaran organic-walled fossils confirms the importance of clay minerals and pyrite in Burgess Shale-type preservation

摘要

Burgess Shale-type (BST) macrofossils and organic-walled microfossils are preserved as carbonaceous compressions and may share similar taphonomic processes. Previous taphonomic investigations of carbonaceous compressions have primarily focused on the microchemistry of Cambrian BST fossils, but comparative data from organic-walled microfossils were not available. To address these issues, we analyzed two organic-walled taxa from the Yangtze Gorges area of the South China block, Chuaria (an acritarch) and Vendotaenia (a ribbon-shaped fossil). Their abundance offers the opportunity for destructive microanalysis, including petrography, electron microscopy, and elemental mapping on both bedding planes and in cross sections. Our data suggest that Chuaria preservation is remarkably similar to BST fossils in that its vesicle walls are closely associated with clay minerals. In addition, like many BST macrofossils, Chuaria and Vendotaenia are also closely associated with pyrite; Chuaria vesicles are often filled with framboidal pyrite, and Vendotaenia fossils are associated with sulfate, partly derived from pyrite oxidation. The comparative taphonomy of Chuaria and Vendotaenia and BST macrofossils indicates that the preservation of organic-walled acritarchs may be aided by clay and pyrite mineralization, and that these abundant microfossils may serve as proxies for uncovering in more detail the taphonomic histories of BST preservation.