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The first animals (metazoans) with skeletons belong to the tubular cloudinid morphogroup, the lowest occurrence of which marks the base of the Nama biotic assemblage (ca. 551550 Million years ago, Ma). This evolutionary first appearance coincided with, or immediately post-dated, a major faunal turnover event associated with the loss of many soft-bodied White Sea assemblage taxa that dominated the preceding ca. 610 Myrs. At present, there is no evidence that the majority of cloudinid skeletons were biomineralized under strong biological control. Instead, these early biomineralizing metazoans may have acquired their skeletons with relative ease in response to ambient seawater chemistry in carbonate settings. The trigger for the origin of metazoan biomineralization remains unknown, but may have been linked to changes in seawater Mg/Ca and/or environmental oxygen concentration. Weathering-derived nutrient input can fuel marine productivity and regional deoxygenation on short-medium timescales, leading to organic carbon and pyrite burial and atmospheric oxygenation on longer timescales. Changes to the intensity and style of weathering on the global scale can also alter the flux of dissolved cations (e.g., Ca and Mg) and alkalinity to the oceans. Despite their importance, global weathering dynamics at the dawn of animal biomineralization remain poorly understood. Carbonate-hosted Sr and Li isotopes have the potential to track the degree and style of weathering, and temporal trends in both datasets may therefore provide meaningful insights into the dynamics of associated elemental fluxes to regional palaeoenvironments. Late Ediacaran sedimentary rocks of the Nama Group (ca. 551538 Ma) host a rich fossil assemblage that includes impressions of both soft-bodied organisms and the lowest known occurrence of the skeletal cloudinid, Cloudina. Here we present new Sr and Li isotope data from carbonates in four outcrop sections, and new data of carbonate carbon isotopes, major and trace element concentrations, and Fe speciation from two cores drilled as part of the ICDP GRIND-ECT project, which together span the entire Ediacaran portion of the Nama Group succession. The combination of these data, when considered within a sequence stratigraphic framework, clearly reveals the influence of changes in regional weathering intensity/style on marine palaeoredox dynamics. Furthermore, calibration of these new data within a global chronostratigraphic age model reveals cyclicity in weathering proxies from multiple cratons that respond directly to changes in eustatic sea level. The implications of these new time-calibrated geochemical and stratigraphic data are considered relative to the timing of the earliest metazoan biomineralization, and major faunal turnover events that preceded and coincided with deposition of the Nama Group succession.
Bowyer et al. (Mon,) studied this question.