Organic C-N Contents and Isotopes Based Marine Productivity Responses to the Deglaciation Following the Late Paleozoic Ice Age in the Tethyan Lhasa Block

As a geological analogue of future climatic warming, the transition from the Late Paleozoic Ice Age (LPIA) to its post-greenhouse climate in the early–middle Permian profoundly impacted marine ecosystems and productivity through carbon-nitrogen (C-N) cycles. Here, we present high-resolution profiles of organic carbon and nitrogen contents and isotopes from a Lower–Middle Permian glacio-to-carbonate section at Shenza of the Lhasa Block to reconstruct productivity and palaeo-oceanic circulation during deglaciation. During the latest glacial phase (uppermost Lagar Formation; late Artinskian), low organic nitrogen isotope (δ15Norg) and negative organic carbon isotope (δ13Corg) values reflect intensified mixing that suppressed denitrification and reduced biological fractionation in a cold, well-ventilated oligotrophic ocean. During the climate-transition interval (Member 1, Angie Formation; Artinskian–Kungurian transition), increased total organic carbon (TOC), δ13Corg, and δ15Norg suggest a brief rise in productivity and intensified nitrogen cycling, marking a transitional stage of progressively enhanced nutrient supply. Subsequently, recurring high TOC coupled with low δ13Corg and persistently high δ15Norg (Member 2, Angie Formation; middle–late Kungurian) indicate an upwelling-driven regime of elevated productivity and organic-carbon burial associated with cold-seep activity. In contrast, the early Roadian greenhouse (basal Xiala Formation) records an oligotrophic, well-oxygenated carbonate-platform setting with extremely low TOC, relatively heavy δ13Corg, and reduced burial efficiency. These results suggest that mid–low-latitude marginal basins of the Paleo-Tethys shifted from a high-productivity upwelling system to an oligotrophic, ventilated system during the climate reorganization from the Artinskian–Roadian, highlighting the perturbed carbon–nitrogen cycle and fluctuating productivity responses to post-LPIA climatic warming.