Global and local drivers of the Ediacaran Shuram carbon isotope excursion

The Ediacaran Shuram carbon isotope excursion (CIE) follows the regional Gaskiers glaciation and occurs before the appearance of macroscopic animal fossils. Previous interpretations for the Shuram CIE have proposed global perturbations to Earth's carbon cycle accompanied by significant climatic and environmental change. These studies assume that carbonate carbon isotopes through the Shuram CIE record the composition of dissolved inorganic carbon in seawater. Through a compilation of new geochemical, sedimentological, and stratigraphic data from five localities on four separate paleocontinents, we find that all of the analyzed Shuram CIE successions share general similarities including a common transgressive-regressive sequence that is coincident with similar shifts in a host of geochemical proxies (δ¹³C, δ¹⁸O, δ^44/40Ca, Sr/Ca, Mn/Sr, U/Ca). Despite these broad similarities between sections, there are important differences in the proxies between water depths that may be linked to changes in carbonate diagenesis. Specifically, we find the excursion's magnitude is largest in slope environments where diagenesis is sediment-buffered and smaller in shallow-water settings dominated by fluid-buffered diagenesis. The transgression that accompanied the Shuram CIE is consistent in amplitude (∼10-100s m) and duration (>1 My) with eustatic fluctuations driven by plate reorganization. These coupled geochemical and stratigraphic observations argue against meteoric and burial diagenesis as drivers for the excursion; instead, they are best explained by global changes to the locus and intensity of photosynthetic primary productivity in shallow-water environments and do not necessarily require large changes in global climate or marine redox.