[B-BG01] 地球惑星科学　生命圏フロンティア

[BBG01-03] The end-Triassic mass extinction: Molecular fossil and isotopic insights

★Invited Papers

*Calum Peter Fox^1,2,3、Jessica H Whiteside⁴、Paul E Olsen⁵、Xingqian Cui⁶、Roger E Summons⁷、Erdem Idiz⁸、Alex I Holman²、Manuel Rigo⁹、Aisha Al Suwaidi³、Kliti Grice² (1.Japanese Agency for Marine-Earth Science and Technology, Yokosuka, Japan、2.Curtin University, Perth, Australia、3.Khalifa University of Science and Technology, Abu Dhabi, UAE、4.University of Southampton, Southampton, UK、5.Columbia University in the City of New York, New York, USA、6.Shanghai Jiao Tong University, Shanghai, China、7.Massachusetts Institute of Technology, Cambridge, USA、8.University of Oxford, Oxford, UK、9.Università degli Studi di Padova, Padova, Italy)

キーワード：end-Triassic, Mass Extinctions, Biomarkers, Compound-Specific Isotopes

Mass extinction events in Earth history are important for the evolutionary trajectories of life. Many, but not all, of these events are driven by large igneous provinces (LIPs). One such event, driven by Earth’s most laterally extensive LIP termed the Central Atlantic Magmatic Province (CAMP), is the end-Triassic mass extinction event (ETE) that occurred ~201 million years ago. Through voluminous outgassing the CAMP is expected to have caused a cascade of environmental catastrophes that resulted in the ETE. One method to compare CAMP activity and the timing of the ETE at the global scale is to correlate negative carbon isotope excursions (CIEs) in the organic carbon isotope (δ¹³C_org) record. Such CIEs are reportedly driven by massive fluxes of isotopically light carbon into the atmosphere from exogenous sources linked to the CAMP. A key locality in ETE discussions and to which many δ¹³C_org records and CIEs are compared to is the St. Audrie’s Bay section in the SW UK. At this locality we undertook a comprehensive investigation of biomarkers (molecular fossils) and their stable carbon isotopes that help detail origins of organic matter, ecological shifts, and environmental changes. At a prominent CIE reported by some to represent the extinction event and termed the initial CIE, biomarker and compound-specific carbon isotopic data show that the emergence of microbial mats, influenced by an influx of fresh to brackish water, provided isotopically light carbon to both organic and inorganic carbon pools in centimeter-scale water depths, leading to the negative CIE. Thus, the initial CIE and its associated disappearance of marine biota at St. Audrie’s Bay are the result of local environmental change and do not mark either the global extinction event or input of exogenous light carbon into the atmosphere. Below the initial CIE, another CIE routinely used in chemostratigraphic correlations and termed the precursor CIE is also recognized. A similar geochemical approach reveals that the precursor CIE is unrelated to CAMP activity, but instead results from the increased input of terrestrially derived ¹³C-depleted plant material based on compound-specific isotopes. These results highlight the caution that should be taken when correlating δ¹³C_org records on the global scale since factors other than the CAMP may also produce CIEs. Above the initial CIE, during a lithological change representing a flood event and return to fully marine conditions that is corroborated by biomarkers, another CIE is observed. During this CIE, biomarker distributions reveal an episode of persistent photic zone euxinia (PZE; toxic hydrogen sulfide poisoning in the sunlit region of the water column) that extended further upward into the surface waters than typically reported for the ETE. In the same interval, shelly taxa almost completely disappear in addition to the last occurrences of conodonts and phytosaurs. Here, a Lilliput assemblage is preserved consisting of only rare calcitic oysters (Liostrea) and ghost fossils of decalcified aragonitic bivalves. This ‘double whammy’ of PZE and acidification driving marine extinction is also coupled with polycyclic aromatic hydrocarbon (PAHs) evidence for soil erosion, linking terrestrial and marine ecological stressors. Elsewhere in Europe, Greenland, and China, PAHs also support widespread biomass burning events through increases in smoke signals, indicating that wildfire events were a prominent feature of the ETE. Although ecological stressors leading to the extinction are becoming clearer, much work is needed to fully resolve the ETE and help determine environmental and ecological shifts at the regional versus global scale as well as to establish which chemostratigraphic correlations are most robust.