Chemo-Biostratigraphy and Evolution of Paleoenvironment in Late Ediacaran-Early Cambrian from SW Mongolia

The late Ediacaran to early Cambrian period (ca.550-514Ma) is recognized as a globally significant juncture, with sedimentary records found across numerous cratons worldwide. This period witnessed intensified plate tectonic activity (e.g., the amalgamation of Gondwana), a substantial increase in atmospheric oxygen levels (the “Neoproterozoic Oxidation Event”), and major faunal turnovers, including the “Ediacaran Mass extinction” and the “Cambrian Bioradiation”. While extensive research has explored the links between oxygenation and the early evolution of metazoans, far fewer studies have examined the combined influence of palaeotemperature, palaeosalinity, palaeoproductivity, palaeoclimate, volcanism, and redox conditions, particularly at high temporal resolution. Moreover, the definition of the global Ediacaran-Cambrian (Ed-C) boundary remains unresolved, and the validity of the currently established GSSP continues to be a matter of debate. Here we report a sedimentary sequence from the Dzabkhan micro-continent in SW Mongolia, spanning the late Ediacaran-Cambrian Stage 3 interval, revealing several major positive and negative C isotope excursions and interpret the first excursion as the onset of the basal Cambrian carbon isotope excursion (BACE). Our study comprehensively discusses synchronous stratigraphies from Mongolia, Kazakhstan, South China, Tarim, Siberia, Morocco, and Newfoundland. Most records consistently exhibit a distinctive pattern that includes the first appearance datum (FAD) of Small Skeletal Fossils, negative carbon isotope excursions, and strontium isotope uptrends. Since it has been previously agreed upon that the Ed-C boundary is defined by the FAD of ichnospecies Treptichnus pedum, but it´s appearance is limited to clastic environments and partly is diachronous, other bio- or chemostratigraphical tools have been discussed to define the onset of Cambrian Period. Therefore, the small skeletal fossils Protohertzina anabarica, P. unguliformis and Anabarites trisulcatus, strontium and carbon isotope excursions are here utilized as a potential supplementary diagnostic tool. Furthermore, our palaeoenvironmental reconstructions elucidate the variations in carbon and strontium isotope trends spanning this period, which may reveal carbon cycling and tectonic processes during sedimentation on the Dzabkhan Craton. These environmental changes may further support redefining the boundary. Studies of carbonate Ce anomalies, Fe speciation, and redox sensitive elements from the Orolgain Gol section suggest an anoxic setting in the late Ediacaran, with gradually rising oxygen levels persisting into the Fortunian, coinciding with metazoan radiation. This trend is accompanied by a shift from restricted basinal to open marine environments. In Cambrian Stage 2-3, redox conditions fluctuated more frequently, with episodic anoxia potentially driving evolutionary innovation. Additionally, multi-proxy reconstructions of temperature, salinity, nutrients, and climate indicate several intervals of favourable environmental conditions, likely promoting the early diversification of life. Volcanic activity peaked during the middle to late Stage 3, which may have contributed to faunal turnover. Overall, our integrated palaeoenvironmental reconstruction is consistent with the episodic model of the Cambrian explosion, offering new constraints on its tempo and drivers.