Redox stratification and ecological implications of microbial dolomites in the late Ediacaran Dengying Formation, Yangtze platform

Abstract

The late Ediacaran ocean witnessed a critical phase in the evolution of early life, during which redox conditions were closely linked to biological innovation. Nevertheless, the nature of these redox conditions and their interactions with evolving ecosystems remain debated. This study provides a systematic investigation of the petrographic and geochemical characteristics of microbial dolostones from the Dengying Formation on the Yangtze Platform, South China. The results reveal that microbial dolomites in this unit comprise a diverse suite of lithofacies—bioclastic, stromatolitic, oncoidal, foam-spongy, and thrombolitic dolomites—indicative of intertidal to subtidal depositional settings. Microbialite components are classified into microbial matrix, fibrous dolomite cements, and clear equant dolomite cements. Among these, Fibrous dolomite is particularly well-developed, and its cathodoluminescence zoning and length-slow optical properties suggest syn-depositional precipitation during early diagenesis. Combined δ¹³C and ⁸⁷Sr/⁸⁶Sr data suggest a warm and humid climate with high primary productivity and intense continental weathering during the Z₂dn² interval, followed by a decline in sedimentation rates during Z₂dn⁴, albeit with continued robust carbonate deposition. All dolomite components exhibit distinct negative cerium anomalies (Ce/Ce* < 1), heavy rare earth elements (HREEs) enrichment, and elevated Y/Ho ratios, collectively indicating deposition under well‑oxygenated shallow marine conditions. Notably, the fibrous dolomite component displays the most pronounced oxidative signature, with oxygen levels at the sediment–water interface potentially approaching atmospheric values. In contrast, the clear equant dolomite cements components reflect suboxic to weakly oxic conditions, suggesting a stratified redox water column during the late Ediacaran. These observations imply that localized oxygen-rich zones on the seafloor may have developed through microbial oxygen production. When compared across the entire Ediacaran, the widespread microbial dolomites of the Dengying Formation and their associated oxic conditions likely provided key ecological niches for early benthic metazoans, such as bilaterians. The oxygenation process recorded in this formation may have played an important role in setting the stage for the Cambrian Explosion. The Dengying Formation thus captures a significant shallow-marine oxygenation event closely associated with algal proliferation and the emergence of multicellular life, underscoring the role of biologically mediated environmental feedbacks during the terminal Neoproterozoic. This study enhances our understanding of the interplay between seawater redox structure and microbial activity at the close of the Ediacaran, and supports the interpretation that microbial metabolism may have been a key regulatory mechanism in the development of oceanic redox stratification.