68 D21. Laying the Foundation for the Modern Earth System at the Neoproterozoic-Paleozoic Transition

Monday, 10 October 2022
8:00 AM - 12:00 PM
Mile High Ballroom 4C

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

Paper number: 68-11
Program: Technical Sessions
Time: 10:20 AM – 10:35 AM

Authors

Abstract

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 CO32- 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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-379736

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.