Carbon isotope fluctuations linked to the Norian-Rhaetian boundary and the End-Triassic mass extinction in the Tethys Himalaya, Southern Tibet (Xizang)

Abstract

The Late Triassic was a time of complex and rapid carbon cycle perturbations, which can be better understood through global carbon-isotope correlations. This study presents an integrated biostratigraphic, geochronological, and carbon isotope chemostratigraphic analysis across the Norian-Rhaetian boundary (NRB) and End-Triassic Mass Extinction (ETME) intervals from the Tela section in Southern Tibet. Four conodont biozones are identified, Mockina bidentata Zone, Parvigondolella andrusovi-Misikella hernsteini Zone, Misikella posthernsteini Zone, and Misikella ultima Zone, indicating a continuous succession from the Sevatian (upper Norian) to the upper Rhaetian. Globolaxtorum tozeri, one of the most indicative upper Rhaetian radiolarian taxa, was found within the M. ultima Zone. Zircon UPb dating of three volcanic detritus-bearing limestone samples from the lower part of the section yields ages of 217.0 ± 1.4 Ma, 214.5 ± 1.9 Ma, and 214.4 ± 1.4 Ma, aligning with the Mo. bidentata Zone further refining the biostratigraphy. Carbon and oxygen isotope data (δ13Ccarb and δ18Ocarb) are subdivided into a lower (0–54 m) and an upper (70–120 m) interval. Four carbon isotope excursions (CIEs) are here documented. Three negative excursions (CIE1, CIE2, CIE3) occur in the Sevatian, each visible in both δ13Ccarb and δ13Corg curves. The fourth carbon excursion of δ13Corg, the Initial-CIE (−5.24 ‰), coinciding with the appearance of radiolarians G. tozeri and spans the M. posthernsteini and M. ultima zones, linking it to the ETME. While the cause of the Initial-CIE is generally attributed to Central Atlantic Magmatic Province, the drivers behind the three Sevatian CIEs and around the NRB remain uncertain. Volcanogenic materials from the Tela section suggest that prolonged volcanic activity—likely associated with seafloor spreading in the Neo-Tethys Ocean and the rifting of the Lhasa Block from eastern Gondwana may have contributed to global carbon cycle disruptions from the Sevatian through the NRB interval.