A new high-resolution δ¹³C record for the Early Triassic: Insights from the Arabian Platform

Current understanding of biodiversity changes in the Permian is presented, especially the consensus and disagreement on the tempo, duration, and pattern of end-Guadalupian and end-Permian mass extinctions. The end-Guadalupian mass extinction (EGME; i.e., pre-Lopingian crisis) is not as severe as previously thought. Moreover, the turnovers of major fossil groups occurred at different temporal levels, therefore the total duration of the end-Guadalupian mass extinction is relatively extended. By comparison, fossil records constrained with high-precision geochronology indicate that the end-Permian mass extinction (EPME) was a single-pulse event and happened geologically instantaneous. Variation of geochemical proxies preserved in the sedimentary records is important evidence in examining potential links between volcanisms and biodiversity changes. Some conventional and non-traditional geochemical proxy records in the Permian show abrupt changes across the Permian-Triassic boundary, reflecting climate change, ocean acidification and anoxia, carbon cycle perturbation, gaseous metal loading, and enhanced continental weathering. These, together with the stratigraphic coincidence between volcanic ashes and the end-Permian mass extinction horizon, point to large-scale volcanism as a potential trigger mechanism.

To further define the nature of volcanism which was responsible for global change in biodiversity, main characteristics of four Permian large igneous provinces (LIPs; i.e., Tarim, Panjal, Emeishan, and Siberian) are compared, in terms of timing and tempo, spatial distribution and volume, and magma-wall rock interactions. The comparison indicates that volcanic fluxes (i.e., eruption rates) and gas productions are the key features distinguishing the Siberian Traps from other LIPs, which also are the primary factors in determining the LIP’s potential of affecting Earth’s surface system. We find that the Siberian Traps volcanism, especially the switch from dominantly extrusive eruptions to widespread sill intrusions, has the strongest potential for destructive impacts, and most likely is the ultimate trigger for profound environmental and biological changes in the latest Permian-earliest Triassic. The role of Palaeotethys subduction-related arc magmatism cannot be fully ruled out, given its temporal coincidence with the end-Permian mass extinction. As for the Emeishan LIP, medium volcanic flux and gas emission probably limited its killing potential, as evident from weak changes in geochemical proxies and biodiversity. Because of its long-lasting but episodic nature, the Early Permian magmatism (e.g., Tarim, and Panjal) may have played a positive role in affecting the contemporaneous environment, as implicated by coeval progressive climate warming, termination of the Late Palaeozoic Ice Age (LPIA), and flourishing of ecosystems.

The transition from the Smithian substage to the Spathian substage of the Olenekian stage of the late Early Triassic was a critical time marked by a series of biological and environmental changes during the multimillion-year recovery interval following the end-Permian mass extinction. However, the Smithian/Spathian boundary (SSB) does not yet have an agreed definition, a shortcoming that complicates high-resolution analysis of events during the Smithian-Spathian transition. Here, we review key biostratigraphic (i.e., ammonoid and conodont) studies of the Smithian and Spathian substages in historically important regions (e.g., the Canadian Arctic for the Boreal realm, western North America for the eastern Panthalassic Ocean) and more recently re-studied locations (e.g., Pakistan and India in the southern Tethys, South China in the eastern Tethys) as well as the carbon isotope chemostratigraphy of 29 major Smithian-Spathian sections globally. Key ammonoid genera (e.g., Wasatchites, Anasibirites, Glyptophiceras and Xenoceltites of the late Smithian, and Bajarunia, Tirolites and Columbites of the early Spathian), conodont species (e.g., Scythogondolella milleri, Novispathodus waageni, and Borinella buurensis of the late Smithian, and ‘Triassospathodus’ hungaricus, Neogondolella aff. sweeti, and Icriospathodus spp. of the early Spathian), and carbonate carbon isotope excursions provide appropriate markers for constraining the SSB. Use of the first occurrence of the conodont Novispathodus pingdingshanensis as a potential marker of the SSB is also discussed. Based on correlations of biostratigraphic and carbon isotope data globally, we propose to revise previous placements of the SSB transition in some sections. Finally, we show that the Smithian Thermal Maximum (STM; herein named) was middle Smithian in age and not correlative with the SSB, as inferred in some earlier studies, and that the SSB coincided with a subsequent major global cooling event.

The Permian–Triassic mass extinction (PTME) is not only a dramatic loss in biodiversity and major change in ecosystem structures, but also coincided with the formation of abundant unusual sedimentary structures. Of these, ooids were widespread in shallow marine carbonate settings during the Permian–Triassic (P–Tr) transition, and giant ooids occurred more frequently at this critical period than any time during the Phanerozoic. Global review of 43 oolite-bearing Permian–Triassic boundary (PTB) sections with reliable biostratigraphic controls indicates that ooids occurred mostly in coincidence with the latest Permian extinction (LPE) and its immediate aftermath. Ooids became widespread over extensive regions just after the LPE during the interval corresponding to the conodont Hindeodus changxingensis Zone. They persisted into the earliest Triassic until the conodont Isarcicella isarcica Zone. In addition, oolites are often found in association with microbialites in low-latitude shallow-marine settings. Proliferation of ooids over the P–Tr transition indicates an extensive range of warm waters with high level of carbonate saturation state that prevailed in the oceans during that time. The latest Permian ooids were usually small (0.3 to 0.7 mm in diameter), aragonitic, poorly preserved and recrystallized, while moderately to well-preserved morphology, bimineralic, and oversized forms usually occurred in the I. isarcica Zone of the earliest Triassic and afterwards. Widespread aragonitic ooids in the end of the Permian reinforce the scenario that an “aragonite sea” period may have resulted in the dramatic losses of skeletal organisms that precipitated low-Mg calcite and hampered their recovery in the aftermath. The anomalous primary mineralogy of Lower Triassic ooids implies that previously assumed stable seawater composition during the Early Triassic needs to be revaluated.

The Permian–Triassic mass extinction was marked by a massive release of carbon into the ocean-atmosphere system, evidenced by a sharp negative carbon isotope excursion. Large carbon emissions would have increased atmospheric pCO₂ and caused global warming. However, the magnitude of pCO₂ changes during the PTME has not yet been estimated. Here, we present a continuous pCO₂ record across the PTME reconstructed from high-resolution δ¹³C of C₃ plants from southwestern China. We show that pCO₂ increased from 426 +133/−96 ppmv in the latest Permian to 2507 +4764/−1193 ppmv at the PTME within about 75 kyr, and that the reconstructed pCO₂ significantly correlates with sea surface temperatures. Mass balance modelling suggests that volcanic CO₂ is probably not the only trigger of the carbon cycle perturbation, and that large quantities of ¹³C-depleted carbon emission from organic matter and methane were likely required during complex interactions with the Siberian Traps volcanism.