BIOMINERALIZATION OF AN EDIACARAN SPONGE IN SHURAM EXCURSION STRATA OF THE PATOM UPLIFT IN SIBERIA

The discovery of cloudinomorphs in an unconformity-bounded sequence between the Mara and Kliphoek members of the middle Ediacaran lower Nama Group in southern Namibia juxtaposes the earliest evidence for animal seashells with the end of the Shuram Excursion, one of the most profound carbon cycle anomalies in Earth history. This coincidence suggests that environmental factors, most importantly high concentrations of Ca and CO₃^2- alkalinity in the world oceans, drove the onset of animal biomineralization. To test this hypothesis, we explored middle Ediacaran strata of the 550 m thick carbonate-dominated Nikol’skoe and Chencha formations from the Patom Uplift of central Siberia in search of sedimentary evidence for high seawater alkalinity and for shelly fossils. Limestones in these formations preserve C, Sr, and U isotope trends known to be associated with the Shuram Excursion worldwide. Alkalinity indicators in the subtidal ramp setting include widespread evidence of seafloor cementation resulting in abundant intraclastic plate breccias, as well as aragonite seafloor cements, microbial mud mounds, ooids, and grapestones. A reef-to-lagoonal carbonate facies was observed near the top of the Chencha Fm., wherein carbonate bioclasts of animal affinity were discovered. The bioclasts stand out by their red coloration – the result of the oxidation of pyrite that formed inside of thin carbonate shells, most likely due to the remineralization of animal tissue through microbial sulfate reduction. Petrographic observations suggest that carbonate mineralization was template-directed, and that the bioclasts were porous, attached to diaphanous sheets, and had a complex interior architecture of canals. Thin section, electron probe, and X-ray µCT analyses of the pyrite interior to the leaf-like bioclasts reveal a syconid wall geometry, which in concert with other observations suggest sponge-grade organization for the early animal. The discovery of the earliest known biomineralized animals on two continents in similar temporally-correlated depositional settings supports the view that the Shuram Excursion was the result of a worldwide oceanographic alkalinity event, initially driven by middle Ediacaran mountain building and weathering, rather than a diagenetic artefact.