Sarah J. Beith, Calum P. Fox, John E. A. Marshall, Jessica H. Whiteside

Global and Planetary Change (2023)

Compound-specific carbon isotopic analysis of biomarkers and palynological studies from sections across the end-Triassic mass extinction in the northwestern Tethys provide constraints on the origin of the bulk organic carbon ‘initial’ isotope excursion. This excursion is commonly used to correlate among geographically disparate sections that span this pivotal event associated with the rise of modern fauna. Palynological analysis is used to identify intervals of Liassic-equivalent strata in south Wales and northeast England for isotopic study. Compound-specific carbon isotope analysis of n-alkanes and regular isoprenoids from these intervals reveals a trend different than that displayed by bulk organic matter, suggesting the latter signal is not derived from large perturbations in the global carbon cycle but from localized source changes. The most pronounced changes in compound-specific isotope profiles are negative excursions in pristane and phytane, which occur within the positive phase of the bulk carbon isotope excursion and correspond to shifts in total organic carbon and increased heterotrophy. These findings indicate that a re-evaluation of temporal relationships between positive climate feedbacks and their effects is necessary to more accurately understand the likely global nature of the end-Triassic extinction, and more generally, carbon cycle perturbations associated with other mass extinction events.

Calum P. Fox, Alex I. Holman, Manuel Rigo, Aisha Al Suwaidi, Kliti Grice

Global and Planetary Change (2022)

Polycyclic Aromatic Hydrocarbons (PAHs) are routinely used as proxies for wildfire in geological sediments associated with large igneous province (LIP) driven CO2 increases and mass extinction events. One example is the end-Triassic mass extinction event (ETE) driven by Earth's most laterally extensive LIP, the Central Atlantic Magmatic Province (CAMP). However, many PAH records often lack critical information including identifying specific source(s) of PAHs (e.g., pyrogenic vs. petrogenic), intensity of paleowildfire events, whether PAHs represent predominant smoke signals that can travel substantial distance from the burn origin, and if evidence of PAH as markers for soil erosion exists. To better understand ETE wildfire events, a detailed evaluation of PAH distributions from the Italcementi section in the Lombardy Basin, Italy covering the latest Rhaetian was undertaken. We report the best evidence of wildfire activity occurs above the initial carbon isotope excursion (CIE) which is routinely used to chemostratigraphically correlate between ETE sections, rather than within the initial CIE as evidenced at other sections. This wildfire event was intense, short-lived, and occurred during a calcification crisis and δ13Corg anomaly, thereby linking terrestrial and marine ecosystem stress. Evidence of a more prolonged but less intense wildfire event and/or evidence for smoke signals takes place above this interval before the onset of a second calcification crisis. By comparing PAH records from Italy, Greenland, Poland, the UK, and China, during the ETE, few sections show evidence for intense (i.e., higher-temperature) wildfire activity during the initial CIE. However, these investigated PAH records show prolonged increases in the low-molecular-weight (LMW) combustion-derived PAH phenanthrene. We interpret this to represent widespread (and possibly more intense) wildfire activity further from the deposition sites, since LMW combustion-derived PAHs are the major PAHs in smoke aerosols that can travel vast distances, and/or less intense wildfire activity that characteristically produce LMW combustion-derived PAHs. In comparing PAH data, we find widespread wildfire activity across multiple basins supporting wildfire activity was an important ecological stressor in the terrestrial realm during the ETE.

Calum P. Fox, Jessica H. Whiteside, Paul E. Olsen, Kliti Grice

Earth and Planetary Science (2022)

Global warming induced-wildfires of the 21st century reveal the catastrophic effects that widespread biomass burning has on flora and fauna. During mass extinction events, similar wildfire episodes are considered to play an important role in driving perturbations in terrestrial ecosystems. To better evaluate the record of biomass burning and potential carbon cycle feedbacks at the end-Triassic mass extinction (~202 Ma; ETE), we investigated the relative abundances of a range of polycyclic aromatic hydrocarbons (PAHs) and the 13C values of regular isoprenoids and n-alkanes at key sections in the SW UK. These data reveal little evidence for intensive wildfire activity during the extinction event, in contrast to what has been reported further afield in European, Chinese, and Greenland ETE sections. Herein, PAHs instead reflect greater contributions from an episode of soil erosion that we attribute to Large Igneous Province (LIP)-driven acid rain, and possible distal sources of smoke, suggestive of fire elsewhere in the UK/European basins. This terrestrial ecosystem perturbation is coincident with those in the marine realm, indicating ecosystem perturbations occurred across multiple habitats throughout the latest Rhaetian in the SW UK. Additionally, this geochemical approach reveals that the precursor carbon isotope excursion (CIE) routinely used in chemostratigraphic correlations is unrelated to LIP activity, but instead results from the increased input of terrestrially derived 13C-depleted plant material. Furthermore, we find the initial CIE (commonly used to mark the extinction level, but which is now known to precede the ETE) is also unrelated to biomass burning. Collectively, these data reveal that processes other than combustion of terrestrial material are important for the terrestrial phase of the ETE in the SW UK. Similar investigations are required on other ETE sections, both those in close proximity to the LIP driving the extinction and those further afield, to more clearly determine the negative effect(s) of LIPs and their geographic extent in the terrestrial realm.

Calum P. Fox, Jessica H. Whiteside, Paul E. Olsen, Xingqian Cui, Roger E. Summons, Erdem Idiz and Kliti Grice

Geology (2022)

High-resolution biomarker and compound-specific isotope distributions coupled with the degradation of calcareous fossil remnants reveal that intensive euxinia and decalcification (acidification) driven by Central Atlantic magmatic province (CAMP) activity formed a twopronged kill mechanism at the end-Triassic mass extinction. In a newly proposed extinction interval for the basal Blue Lias Formation (Bristol Channel Basin, UK), biomarker distributions reveal an episode of persistent photic zone euxinia (PZE) that extended further upward into the surface waters. In the same interval, shelly taxa almost completely disappear. Beginning in the basal paper shales of the Blue Lias Formation, a Lilliput assemblage is preserved consisting of only rare calcitic oysters (Liostrea) and ghost fossils of decalcified aragonitic bivalves. The stressors of PZE and decalcification parsimoniously explain the extinction event and inform possible combined causes of other biotic crises linked to emplacement of large igneous provinces, notably the end-Permian mass extinction, when PZE occurred on a broad and perhaps global scale.

Sarah J. Beith, Calum P. Fox John E. A. Marshall, Jessica H. Whiteside

Palaeogeography, Palaeoclimatology, Palaeoecology (2021)

The end-Triassic extinction (ETE) is associated with rapid atmospheric CO2-driven warming amplified by positive feedbacks involving weathering, nutrient oversupply, water column anoxia and sul.fate reduction. These conditions culminated in photic zone euxinia (PZE) at least locally in the Western Tethys, but its broader extent across northwest Europe has yet to be identified. Here we present new biomarker and bulk δ13Corg isotopic data that document redox and palaeoecological changes from two end-Triassic sites in the Western Tethys: Felixkirk, in the Cleveland Basin, northeast England, and Lavernock Point, in the Bristol Channel Basin, S Wales. The presence of aryl isoprenoids and Chlorobi-derived isorenieratane indicates shoaling of anoxia and toxic H2S to the photic zone caused by flooding in restricted or semi-enclosed basins. Oscillating redox conditions and severe PZE prevented a swift recovery of at least the benthic ecosystem across several British basins. Additionally, in concert with recent discoveries in the Bristol Channel Basin, in the Cleveland Basin we find that the ‘initial’ negative carbon isotope excursion (CIE) is related to a localized change in organic matter sources. PZE in British and other European basins points towards H2S toxicity as an extinction driver for the Western Tethys, highlighting the need for a global characterization of redox changes across the end-Triassic and other extinction events.

Calum P. Fox, Xingqian Cui, Jessica H. Whiteside, Paul E. Olsen, Roger E. Summons, Kliti Grice

Proceedings of the National Academy of Science (PNAS; 2020)

The negative organic carbon isotope excursion (CIE) associated with the end-Triassic mass extinction (ETE) is conventionally interpreted as the result of a massive flux of isotopically light carbon from exogenous sources into the atmosphere (e.g., thermogenic methane and/or methane clathrate dissociation linked to the Central Atlantic Magmatic Province [CAMP]). Instead, we demonstrate that at its type locality in the Bristol Channel Basin (UK), the CIE was caused by a marine to nonmarine transition resulting from an abrupt relative sea level drop. Our 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 iconic CIE and the disappearance of marine biota at the type locality 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. Instead, the main extinction phase occurs slightly later in marine strata, where it is coeval with terrestrial extinctions and ocean acidification driven by CAMP-induced increases in PCO2; these effects should not be conflated with the CIE. An abrupt sea-level fall observed in the Central European basins reflects the tectonic consequences of the initial CAMP emplacement, with broad implications for all extinction events related to large igneous provinces.