Uncovering the spatial heterogeneity of Ediacaran carbon cycling.

Records of the Ediacaran carbon cycle (635-541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to -12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ13 C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ13 Ccarb and δ13 Corg ) records from the Ediacaran Doushantuo Formation along a proximal-to-distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ13 Corg are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ34 S of carbonate-associated sulfate (CAS) and increased 87 Sr/86 Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ13 Corg records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O2 and sulfate) limited to marginal near-surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ13 C presented in this study.

Volcanic perturbations of the marine environment in South China preceding the latest Permian mass extinction and their biotic effects.

The Dongpan section in southern Guangxi Province records the influence of local volcanic activity on marine sedimentation at intermediate water depths (~200-500 m) in the Nanpanjiang Basin (South China) during the late Permian crisis. We analyzed ~100 samples over a 12-m-thick interval, generating palynological, paleobiological, and geochemical datasets to investigate the nature and causes of environmental changes. The section records at least two major volcanic episodes that culminated in deposition of approximately 25- to 35-cm-thick ash layers (bentonites) and that had profound effects on conditions in both the Dongpan marine environment and adjacent land areas. Intensification of eruptive activity during each volcanic cycle resulted in a shift toward conifer forests, increased wildfire intensity, and elevated subaerial weathering fluxes. The resulting increase in nutrient fluxes stimulated marine productivity in the short term but led to a negative feedback on productivity in the longer term as the OMZ of the Nanpanjiang Basin expanded, putting both phytoplankton and zooplankton communities under severe stress. Radiolarians exhibit large declines in diversity and abundance well before the global mass extinction horizon, demonstrating the diachroneity of the marine biotic crisis. The latest Permian crisis, which was probably triggered by the Siberian Traps flood basalts, intensified the destructive effects of the earlier local eruptions on terrestrial and marine ecosystems of the South China craton.