High temperature methane emissions from Large Igneous Provinces as contributors to late Permian mass extinctions

Large Igneous Provinces (LIPs) eruptions coincide with major mass extinctions over the past 500 million years, attributed to magmatic activity and greenhouse gas release. The Late Permian Mass Extinction (LPME), Earth's most severe, involved two events—the Guadalupian-Lopingian Extinction (GLE) and the Permian-Triassic Extinction (PTE)—potentially linked to the Emeishan LIP (ELIP, ~260 Ma) and Siberian Traps LIP (STLIP, ~252 Ma), respectively. A negative shift in carbon isotopes supports the association between global warming and LPME. While volcanism triggered greenhouse gas outbursts, the mechanisms and relative contributions of different gases remain debated. Though CO₂ from magma degassing is a primary suspect, the role of CH₄ from various sources, including oil cracking in LIP regions, needs further investigation. Pyrobitumen, a byproduct of oil pyrolysis into CH₄, is abundant in ELIP and STLIP regions, suggesting this source might have played a more significant role than previously considered. CH₄ is a potent greenhouse gas (28 times that of CO₂ over 100 years). Previous research, based on indirect evidence, hinted at the release of CH₄ from oil pyrolysis in Sichuan Basin and Tunguska Basin, but lacked quantitative assessment and robust links to LIPs. This study uses clumped isotopes in methane, noble gas isotopes, and basin modeling to quantify ELIP-induced CH₄ generation and evaluate its impact on the LPME, focusing on the Sichuan Basin, an ideal location due to its ELIP proximity, identified heating events, and widespread pyrobitumen and natural gas.

The connection between LIP eruptions and mass extinctions has been established, with greenhouse gas emissions (CO₂ and CH₄) causing rapid warming and ecosystem collapse. Research has focused on CO₂ release from magma degassing, thermal metamorphism, and combustion of organic matter. CH₄ release from volcanic intrusion into coal, clathrate destabilization, and microbial methanogenesis have been proposed but considered secondary. However, the presence of large quantities of pyrobitumen in ELIP and STLIP regions indicates the potential for substantial CH₄ production via oil cracking induced by LIP magmatism. Previous studies have investigated pyrolysis of paleo-oil and associated CH₄ emissions based on petrological observations of gas-venting pipes, suggesting carbon gas release in Sichuan and Tunguska basins. Yet, these lacked the quantitative assessment of pyrolysis extent and a strong correlation with LIP activity, prompting this study's focus on combining isotope tracers with basin modeling to investigate the volcanism-CH₄ link.

This study analyzed 20 natural gas samples (mostly CH₄) from the Sinian-Cambrian dolostones in the Anyue gas field within the Sichuan Basin's ELIP outer zone. Nine samples underwent methane-clumped isotope analysis to determine formation temperatures, while 11 samples were analyzed for noble gas isotopic composition to understand mantle influence. Basin evolution and hydrocarbon generation were numerically simulated to constrain methane formation temperature and genesis. Gas composition analysis included determination of dryness index (C₁/ΣC₂₋₅), CH₄, C₂₋₅, CO₂, H₂S, and N₂ content. Carbon and hydrogen isotopes (δ¹³C and δD) were measured to characterize gas sources. Methane clumped isotope analysis (Δ¹³CH₂D) yielded formation temperatures. Noble gas analysis (⁴He/³He, ³He/²⁰Ne, ⁴⁰Ar*/⁴He) assessed mantle contributions. A numerical basin modeling approach, using software PetroMod, reconstructed the burial-thermal history of the Anyue gas field, considering stratigraphy, tectonics, and boundary conditions. Measured temperatures and vitrinite reflectance validated the model. Kinetic modeling of oil cracking, using the SARA TI model in PetroMod, was employed to assess methane generation at various geological heating rates. Finally, a methane emission model, based on pyrobitumen content, rock density, conversion ratio (methane to pyrobitumen yield), and gas reserves, estimated the total volume of methane released.

The Anyue gas field's natural gas is extremely dry (C₁/ΣC₂₋₅ = 583–3019), with high CH₄ content (90.11%–99.87%). δ¹³C and δD values indicate thermogenic origin and thermal equilibrium. Δ¹³CH₂D analysis revealed high methane formation temperatures (246–269 °C), significantly higher than current reservoir temperatures (140–165 °C), peak oil-cracking temperatures (160–180 °C), and modeled Late Cretaceous burial temperatures (200–220 °C). These temperatures align with the highest trapping temperatures of quartz inclusions (~249–319 °C) associated with ELIP-related hydrothermal activity. Kinetic modeling supports the necessity of temperatures above 250 °C to produce the observed extremely dry gas composition, confirming the influence of ELIP-induced hydrothermal activity. Petrological evidence, such as pyrobitumen characteristics resembling mesophase pitch and honeycomb micropores, supports high-temperature coking by rapid heating. Noble gas analysis shows variable mantle contributions in the Sinian reservoirs (Z₂dn), ranging from 4.8% to 38.5%, but near-zero mantle He in Cambrian reservoirs (ε₁l). This heterogeneity suggests heterogeneous upwelling of mantle fluids associated with the ELIP. The ⁴⁰Ar*/⁴He ratios indicate different thermal and hydrothermal activity levels in different reservoirs. Based on pyrobitumen content, the Anyue region's total methane generation (TMG) is estimated at 2.01 × 10¹⁴ m³ (1440 Gt CH₄ equivalent to 40,410 Gt CO₂ considering global warming potential). Considering preserved methane, the total methane emission (TME) is estimated as 2.00 × 10¹⁴ m³ (1440 Gt). The Sichuan Basin could have released ~1440 Gt CH₄, comparable to or exceeding previously estimated CO₂ release from ELIP volcanism. The high methane formation temperatures, petrological evidence, and mantle-derived noble gas signatures strongly link the methane generation to the ELIP's magmatic activity. The Siberian Traps may have generated significantly more CH₄ given its larger magma volume and similar geological settings.

The findings highlight the significant role of ELIP-induced, high-temperature CH₄ emissions in causing the GLE. The high formation temperatures and mantle-derived noble gas signatures strongly implicate the ELIP's hydrothermal fluids in rapidly cracking paleo-oils. The estimated CH₄ emissions are comparable to, and may even exceed, previously estimated CO₂ emissions from the ELIP, suggesting that CH₄ played a primary role in the associated global warming. The similar geological context of the Siberian Traps suggests a similar mechanism for CH₄ generation and release during the PTE, potentially significantly contributing to the most severe mass extinction event in Earth's history. The fact that significant STLIP volcanism postdated the EPME suggests CO₂ alone cannot explain the extinction, and the CH₄ released from oil cracking provides a plausible explanation for early phases of the extinction. The study supports the concept that high-temperature CH₄ emissions from LIPs are crucial in understanding mass extinctions and the carbon cycle. This high-temperature methane formation differs from thermogenic methane from burial processes or abiotic methane from Fischer-Tropsch reactions, but its thermogenic origin traces to geothermal anomalies linked to magmatic activity rather than sedimentary processes.

This study demonstrates that high-temperature methane generated via LIP-induced oil cracking is a key driver of climate change and carbon cycle perturbations, especially during mass extinction events. The ELIP's influence on CH₄ emissions in the Sichuan Basin highlights the importance of considering this mechanism in paleoclimate reconstruction and extinction events. Future research should focus on the global distribution of reservoir pyrobitumen associated with LIPs and the quantification of CH₄ emissions from those locations, expanding our understanding of Earth's history and the global carbon cycle.

The study focuses on the Anyue gas field in the ELIP's outer zone; samples from closer to the center might reveal even higher methane releases. The methane emission model relies on estimations of pyrobitumen distribution and conversion ratios; uncertainties in these estimations could influence the overall TME estimate. The study doesn't fully account for climate feedbacks from methane, which could amplify the warming effect.