GEOCHEMICAL-SEDIMENTARY-PALEONTOLOGICAL CONSTRAINTS FOR PERMIAN-TRIASSIC TRANSITION IN THE NEO-TETHYS SEA: EVIDENCE FROM SPITI VALLEY, INDIA

The 252 million year old Permian-Triassic (P-T) mass extinction event accompanied the strongest global environmental change in Earth’s geological history. The Spiti Valley in the Indian Himalaya represents a continental shelf section on the southern margin of the land-locked Neo-Tethys Sea and preserves this event in its rock record. We present new sedimentary-paleontological observations and high-resolution carbon and oxygen isotope data across the P-T boundary from two Spiti outcrops, Atargu and Guling. In these rock sections, brachiopod and crinoid-bearing Gungri shale layers with the presence of framboidal pyrites indicate an anoxic-dysaerobic environment during the Late Permian. Evidence of rapid transition to oxidizing conditions is clear from the iron-rich ferruginous layer, before the deposition of carbonates in the Early Triassic. Changes of 2.4‰ and 3.1‰ in δ¹³C_org of the two sections in Atargu and Guling respectively identify the Late Permian Event Horizon (LPEH) across the ferruginous layer and confirms a compositional change in the marine carbon pool and bio-productivity at this paleolatitude.

Unlike other global P-T sections, marine anoxia seems to have been restricted to the deeper waters of the Neo-Tethys and did not significantly affect the productivity of shallow waters in the Late Permian. Extinction occurred rapidly under oxic conditions in a regressing sea. Sharp fluctuations and highly depleted values in δ¹³C_carb and δ¹⁸O_carb in the Early Triassic Mikin Formation suggest a warmer arid climate with strong weathering fronts and rapid sea-level changes. We propose that the synchronicity of the Siberian Trap volcanism and the Araguainha impact in Brazil was the chief trigger for this catastrophe. Isotopically-depleted CO₂ dominated the atmosphere-marine exchange, impacting the geochemical composition of the Neo-Tethys Sea. This study adds new perspectives to the possible mechanism of rapid environmental change at the P-T transition, especially in the Peri-Gondwanan realm. While the discovery of fossils in the ferruginous layer suggests the need for more biostratigraphic and chronological control, we believe a combination of factors including volcanism, meteorite impact, marine regression and anoxia were the drivers to this ecological bottleneck.