PROTEROZOIC DIAGENETIC SPHEROIDS AND CONNECTIONS TO CHEMICALLY OSCILLATING REACTIONS

This work presents the first ever timeline of occurrences of diagenetic spheroids in Proterozoic strata between 0.541 to 1.8 billion years ago (Ga). Diagenetic spheroids are enigmatic structures in sedimentary rocks that span several orders of size dimensions: from tens of micrometres in rosettes, to millimetre size granules and ooids, and onto centimetre to metre size botryoids, nodules and concretions. Connections between these objects with chemically oscillating reactions (COR) are elucidated from chemical and pattern similarities. For instance, equidistant circular waves propagate in the radial direction in COR and are akin to circularly concentric laminations and radially aligned crystals observed within diagenetic spheroids. Notably, elongated cavities and ‘open-book’ structures are commonly observed patterns within diagenetic spheroids (e.g. formed by twinned crystal in botryoids) and COR experiments due to destructive interference of the circularly concentric chemical waves. A significant increase of diagenetic spheroid occurrence is observed after the Marinoan “Snowball Earth” glaciation event, and this is attributable to thawing ice causing increased continental oxidative weathering. As a result, seawater became more enriched in sulphate and iodate, which created ideal conditions for the diagenetic oxidation of biomass and possibly also for the associated large fluctuations of biogeochemical cycles. Sulphate reducing microorganisms prospered and the decarboxylation of organic matter enabled an increased availability of carboxylic acids, which are central to COR reactions. An influx of ions is delivered into post-glacial waters, facilitating COR, since a greater concentration of reactants is induced. The trends observed during the Proterozoic may be more broadly applicable to trends throughout Earth history and possibly on other planets. For instance, there has been evidence of ancient glaciers on Mars, so analysis of Martian rock samples may yield similar circular concentric patterns and could display biosignatures.