Regional Weathering Feedbacks Vs. Global Volcanic Forcing: Multi-Isotope Evidence for a Diachronous Terrestrial Collapse Across the Permian-Triassic Transition

The end-Permian mass extinction (EPME, ~252 Ma) is widely attributed to the Siberian Traps Large Igneous Province (STLIP) volcanism. While marine records often suggest a globally synchronous event, the timing and driving mechanisms of terrestrial ecosystem collapse remain highly debated. Here, we present high-resolution organic carbon (δ13Corg), mercury (Hg) concentration and its isotope (Δ199Hg), and multiple sulfur isotope (δ34S, Δ33S, Δ36S) records from two adjacent drill cores representing terrestrial upland (TK-1) and paleotropical coastal peatland (HK-1) environments in Southwest China. Our results reveal a striking spatial decoupling in geochemical signatures. The coastal lowland core exhibits massive Hg enrichments characterized by negative Δ199Hg values, indicating that the Hg spike was primarily driven by enhanced terrestrial runoff rather than direct volcanic atmospheric deposition. Concurrently, δ34S values in the coastal section record a dramatic negative shift from an average of +17.0‰ to -12.7‰ across the Permian-Triassic transition. It likely marks a shift from a semi-closed sulfate system to an open system, driven by a massive influx of terrestrial sulfate following the catastrophic collapse of the Cathaysian rainforest and subsequent intense continental weathering. Crucially, the multiple sulfur isotope record shows near-zero Δ33S values (+0.01‰ to +0.12‰) throughout the crisis, entirely lacking the mass-independent fractionation signature expected from stratospheric volcanogenic sulfate aerosols. Geochronological constraints demonstrate that this paleotropical terrestrial collapse postdates the ecosystem collapse in higher latitudes (e.g., the Sydney Basin) by hundreds of thousands of years, highlighting a clear latitudinal diachroneity. Collectively, our multi-isotope evidence challenges the paradigm of a single, globally synchronous volcanic kill mechanism on land. Instead, we propose that protracted global climate pressures crossed local ecological thresholds at different times, triggering regional post-deforestation weathering feedbacks that drastically altered local biogeochemical cycles and overprinted global volcanic signals.