Ediacaran NeoProterozoic Doushantuo Biota Fossilization

Fossil preservation through phosphatization and silicification in the Ediacaran Doushantuo Formation (South China): A comparative synthesis

Authors:

Muscente et al

Abstract:

Phosphatized and silicified microfossils—preserved through replication of organic templates by authigenic calcium phosphate and silica, respectively—form through mechanistically similar mineralization processes but under notably different conditions. In order to characterize the environments and taphonomic processes associated with these preservational pathways, we compare phosphatized and silicified fossil assemblages in the Ediacaran Doushantuo Formation (South China) in terms of stratigraphic and paleogeographic distribution, preservation, and biodiversity. Several differences are recognized: (1) phosphatization and silicification both occur in sub-tidal Doushantuo sediments, but phosphatized fossils generally occur in shallower sub-tidal facies than silicified fossils; (2) although both phosphatized and silicified fossils are preserved three dimensionally, the latter are typically more compressed; and (3) whereas phosphatized assemblages are numerically dominated by multicellular acritarchs (putative metazoan eggs and embryos), silicified assemblages are dominated by prokaryotes. These differences can be explained by environmental and taphonomic distinctions. Phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates, and were likely fossilized near the suboxic/anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to P remineralization, phosphate burial via iron-pumping, and calcium phosphate precipitation. Reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial. In contrast, silicified fossils typically occur in chert nodules, which formed deeper in the sediment column—in ferruginous pore waters and perhaps under ferruginous bottom waters—where bacterial sulfate reduction and pyrite formation created low-pH conditions conducive to calcite dissolution and silica precipitation. Once formed, chert nodules mostly remained in situ, but some were reworked with turbidites and olistostromes developed in slope facies. Despite these distinctions, phosphatized and silicified fossil assemblages in the Doushantuo Formation generally overlap in time and space, and representatives of all major groups (and many common genera) of Doushantuo organisms are present in both phosphorites and cherts. Consequently, although both preservational pathways have paleoenvironmental and taphonomic biases, their relative biases on taxonomic presence/absence data are minimal given sufficient sampling efforts, and paleobiologic and biostratigraphic information based on presence/absence (but not abundance) data from the Doushantuo Formation appears to be robust.