Redox evolution and marine anoxia across the Permian–Triassic boundary (PTB): Evidence from Neo-Tethyan successions of the NW Himalaya

In the present study, we reconstruct depositional conditions and provenance changes across the PTB from Guryul Ravine (Kashmir) and Attargoo (Spiti) sections of the Tethys Himalaya. The Guryul section consists of shale, limestone, and calcareous sandstone from the Zewan and Khunamuh Formations. The Spiti section has organic-rich black shales from the Gungri Formation that are overlain by Tamba-Kukur (Mikin) Formation, separated by 8–10 cm ferruginous layer. Upper Permian sediments of the Zewan Formation (E1 unit) show stable euxinic conditions, with δ 34 S values of −22‰ to −9‰ and abundant syngenetic framboids (<10 μm; ∼60%). In contrast, Lower Triassic strata of the Khunamuh Formation (E3 unit) record more oxygenated conditions, characterized by larger framboids (30–35 μm) and less negative δ 34 S values (−5.39‰ to +0.60‰). This redox variability coincides with pronounced negative shifts in δ 13 Corg (−27‰ at Guryul Ravine and −26.8‰ at Attargoo) and elevated TOC values, reaching ∼2% in the E1 unit of the Zewan Formation at Guryul and ∼2–3% in the ferruginous layer at Attargoo section. Together, these changes show that the marine carbon and sulfur cycles were severely disrupted during the extinction period and was driven by multiple distinct pulses of environmental disturbance. Chemical weathering indices (CIA, α Al Ca, α Al Na, α Al Mg) show an increase upward from Upper Permain to Lower Triassic sediments, suggesting increasing chemical weathering across the boundary. Upper Permian sediments with high Th/Sc (∼2), (La/Yb) n (∼14), Th/Yb n (∼10), negative Eu anomalies, and enriched LREE patterns, point at felsic source rocks. In contrast, Lower Triassic strata display a sharp decrease in Th/Sc (∼0.39) and La N /Yb N (∼9), suggesting a change toward more mafic contributions. This transition is supported by isotopic data from the Guryul Ravine, where Upper Permian sediments of the Zewan Formation yield non-radiogenic εNd(0) values (−14.9 to −15.6), high 87 Sr/ 86 Sr(0) ratios (0.718–0.724), and older TDM ages (1.71–1.76 Ga), whereas Lower Triassic sediments of Khunamuh Formation show more radiogenic εNd(0) values (−9.0 to −9.7), lower 87 Sr/ 86 Sr(0) ratios (0.718–0.721), and younger TDM ages (1.35–1.56 Ga). Geochemical proxies, δ 34 S values and framboid size distribution, carbon isotopes (δ 13 Corg) and total organic carbon, indicate persistent anoxia near the PTB. Overall, the our data reveal a noticeable provenance shift from felsic to mafic sources across the PTB and provide new constraints on marine redox evolution helping to improve global models of the end-Permian mass extinction.