Papineau, D; She, Z; Jiao, L; Liu, S; Liu, D; Luo, G; Li, C; (2025) Widespread chemically oscillating reactions during oxidative organic diagenesis recorded during the Ediacaran. Chemical Geology , 683 , Article 122753. 10.1016/j.chemgeo.2025.122753.

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Abstract

The Neoproterozoic oxygenation event co-occurred with widespread phosphate deposition in sediments, the formation of concretionary structures, perturbations in stable carbon and sulphur isotope compositions, and possibly with the emergence of animals, all well-recorded in South China. However, the significance of concretionary structures in this context remains enigmatic. Occurrences of fossils inside Ediacaran apatite granules are also poorly explained by rounding through sediment reworking, which is the model usually invoke the origin of granules. The main problem with the reworking model is that it does not predict that granules should contain specific minerals, geometric patterns, or fossils. Here we show that concretionary diagenetic spheroids such as botryoids, rosettes, granules, nodules, and concretions are widespread in the Ediacaran Doushantuo Fm and we compare their patterns and composition to those involved in chemically oscillating reactions (COR). We report that diagenetic spheroids in the Doushantuo Fm have distinct radial, concentric, twinned, and cavity-like patterns and geometries that span several orders of dimension sizes. Their mineral assemblage is documented to be variably dominated by a small range of minerals including apatite, carbonate, chert, and sulphide. Their ubiquitous content of redox sensitive trace elements and halogens shows that they contain some residual reactants from their original sedimentary environments. The organic matter (OM) therein includes more than six different types of crystallinities and disorder, which is interpreted to suggest variable abundance of molecular functional groups from biomass decomposition or low-grade metamorphism. Hence, Doushantuo diagenetic spheroids have self-similar geometric patterns and, combined with their mineralogy, trace elements, and diverse OM, they are comparable to compounds involved in pattern formation during new COR experiments reported here. The COR model therefore points to diagenetic spheroids as a continuum of fractal objects and patterns, which is argued to have formed during the diagenetic decarboxylation of carboxylic acids from biomass, although carboxylic acids on other planetary bodies could also be abiotic in origin. The abiotic COR model is not inconsistent with previous models for the origin of diagenetic spheroids, but it is more specific in explaining their patterns, mineralogy, and content of OM and fossils. Hence, COR should be considered as a major geochemical process involved in the formation of diagenetic spheroids in the Doushantuo Fm, which represents a new model to understand abiotic carbon cycling during diagenesis, and which is also applicable to extraterrestrial settings.

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