Chemical–to–reverse weathering triggered a pronounced positive carbon isotope excursion in a forced regressive to transgressive dolostone succession during the terminal Ediacaran glaciation
Journal:
Global and Planetary Change
Key Words:
Terminal Ediacaran； glaciationPositive ；carbon isotope excursion (CIE)；C–Sr–Li isotopic system；Silicate chemical weatheringReverse weathering
Abstract:
The terminal Ediacaran (∼551–541 Ma) was a critical period in Earth's history witnessing the coupled evolution of tectonism, climate, marine environment, and biodiversity, during which a global pronounced positive carbon isotope excursion (i.e., EP3) was recorded in marine carbonates. However, there is still a lack of systematic research on its triggering mechanism. Here, we present high-resolution C–Sr–Li isotopic records from a forced regressive to transgressive microbialite-dominated dolostone succession in the Xigou section of Aksu, northwestern Tarim Basin, aiming to reflect coeval hydrogenous (i.e., seawater-sourced) signals. The forced regressive succession shows distinct positive δ13Ccarb, δ13Corg, and 87Sr/86Srcarb excursions, and negative δ7Licarb excursions, reflecting increased terrestrial flux into the ocean and enhanced chemical weathering, which caused an increase in primary productivity and 12C-enriched organic carbon burial, as well as incremental inorganic carbon isotope of shallow seawater. The intense continental weathering in this succession is mainly attributed to sea-level fall under the combined control of the Gondwana assemblage and the Luoquan glaciation. The overlying transgressive succession presents relatively stable high δ13Ccarb, low 87Sr/86Srcarb, and positive δ7Licarb excursions, implying the weakened continental weathering, which is probably due to sea-level rise as Luoquan glaciers began to partially melt, and the dominance of reverse weathering. During the early stage of transgression, upwelling transported deep marine nutrients to the photic zone, allowing the high primary productivity of shallow water to sustain for a long time, resulting in the continuous burial of organic carbon with the mass formation of clay minerals and stable high δ13Ccarb values. Subsequently, as reverse weathering continues, primary productivity decreases, shallow marine oxygen levels decline, and inorganic carbon isotopes gradually fall. This study demonstrates the great potential of Precambrian dolostones as archives of coeval seawater Li isotope, and provides an insight into the co-evolution of carbon cycle, marine redox, and Ediacaran biota from Shuram excursion to EP3 event.
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