Characteristics and Significance of Carbon and Oxygen Isotopic Compositions of the PTB Boundary in Haidai Section, Xuanwei Area of China

The End-Permian mass extinction (EPME), Earth's most severe biocrisis, occurred proximal to the Permian-Triassic Boundary (PTB), with marine ecosystems experiencing catastrophic collapse. This study employs stable carbon (δ¹³C) and oxygen isotopes from marine carbonates in the Haidai Section (Xuanwei, northeastern Yunnan) to decipher paleoenvironmental drivers. The well-preserved stratigraphic sequence encompasses the Upper Permian (Yangxin and Xuanwei Formations) transitioning into the Lower Triassic (Feixianguan and Jialingjiang Formations), providing a continuous marine sedimentary archive. A marked negative δ¹³C excursion (-9.66‰ V-PDB) occurs at the PTB, initiating from +0.82‰ with subsequent gradual recovery. This geochemical signature correlates with: 90% reduction in primary productivity Biodiversity collapse exhibiting cluster extinction patterns Prolonged suppression of ecological recovery Concurrently, reconstructed seawater temperatures reveal extreme thermal fluctuations, surging from 23℃ to 32℃ at the PTB before precipitously declining to 16℃. These perturbations demonstrate coupled biogeochemical dynamics wherein: • Carbon cycle destabilization disrupted nutrient fluxes • Temperature oscillations exceeded marine taxa thermal tolerances • Synergistic environmental stresses amplified extinction selectivity The δ¹³C-temperature covariance (r²=0.085) establishes mechanistic linkages between physicochemical perturbations and biotic responses. Our findings demonstrate that the EPME was driven by positive feedback loops in which: Volcanic CO₂ emissions triggered carbonate saturation decline Thermal stratification exacerbated anoxia Biogeochemical cycling perturbations suppressed primary producers This integrated geochemical record from the Haidai Section provides critical insights into environment-organism coevolution during Phanerozoic Earth's most profound mass extinction.