Barite in the Ediacaran Doushantuo Formation and its implications for marine carbon cycling during the largest negative carbon isotope excursion in Earth’s history

•Doushantuo excess Ba shows a rapid increase with a lateral gradient over the Shuram Excursion.•Barites at Jiulongwan and Siduping were respectively formed in the SMTZ and upwelling water column.•Higher δ34S in barite than in CAS indicate limited influence of methane oxidation on Shuram at JLW.•Reverse spatial pattern of Ce/Ce* versus Baex indicates productivity control on local oceanic oxygenation during the Shuram.•Oxidation of DOC upwelled to the ocean-surface layer contributes to the Shuram Excursion. The Ediacaran Period (∼635–539 Ma) witnessed the largest negative inorganic carbon isotope (δ13Ccarb) excursion in Earth’s history (i.e., the Shuram Excursion), which is characterized by decoupling from the organic carbon isotope (δ13Corg) record. The cause(s) of this event remains highly debated. Here, we report a major (∼8–9-Myr-long) episode of strong barium (Ba) accumulation during the Shuram Excursion in the form of barite, as recorded in the Ediacaran Doushantuo Formation (∼635–551 Ma) of South China. The inner-shelf Zhangcunping section exhibits minimal Ba enrichment, while the slope Siduping section shows maximal Ba enrichment with the intrashelf basinal Jiulongwan section in the middle. The Siduping section contains ∼5 μm-diameter, ellipsoidal barite crystals of marine origin; and the Jiulongwan section contains large (>50 μm), euhedral barite crystals and cements that are partly replaced by pyrite, pointing to a diagenetic origin with barite formation within the sulfate-methane transition zone. The barite δ34S is ∼10‰ higher than δ34S of carbonate-associated sulfate at Jiulongwan in contrast to similar values of these two components at Siduping, suggesting a limited influence of methane oxidation, if any, on the formation of Shuram Excursion at Jiulongwan. The Ce/Ce* exhibits a lateral gradient among study sections which is reverse to the Ba enrichment, supporting the hypothesis that local surface-water productivity controlled dissolved oxygen levels in the Ediacaran surface ocean. Based on these findings, we attribute the highest Ba enrichment at Siduping to oceanic upwelling which enhanced local marine productivity. Furthermore, we propose that episodic oceanic upwelling in the Ediacaran shelf regions likely transported phosphorus and dissolved organic carbon (DOC) to shallow waters, increasing their productivity and facilitating the oxidation of DOC, contributing to the largest negative carbon isotope excursion in Earth’s history.