Geology Club Seminar

Ediacaran and early Cambrian siliciclastic sediments commonly preserve the fossils of soft-bodied organisms, as well as microbial fossils, textures and patterns. However, the environmental and biological factors that promoted the exceptional preservation of these organisms remain unconstrained. To experimentally identify mechanisms of preservation in siliciclastic sediments, soft tissues (scallop adductor muscles) and cyanobacteria were incubated in quartz sand or kaolinite for up to 45 days. Muscles were preserved under anaerobic conditions, and specimens exhumed from quartz sand were coated by discontinuous veneers of iron sulfide. In contrast, muscles exhumed from kaolinite after the same interval of time were coated by continuous veneers of iron(II)-rich clay minerals. Filamentous cyanobacteria were preserved in oxic environments and were coated by clay mineral veneers, regardless of substrate type. These clay veneers were likely formed by two main processes: 1) the adhesion of fine particles from the sediment to organic material and 2) the precipitation of authigenic clay minerals. Taken together, our results show that the microbial reduction of iron(III) present in sedimentary minerals and the formation of mineral veneers are critical for the preservation of organic material, but that modes of fossilization in siliciclastic environments are determined by sediment composition and organism type. Similar processes likely facilitated the formation of exceptionally preserved fossils and textures throughout the Ediacaran and early Cambrian.