The Triassic-Jurassic (T-J) transition (~201 Ma) was marked by significant environmental and biotic changes, including extreme warming, ocean acidification, anoxia, and a mass extinction. These events are linked to rising atmospheric CO2 levels, potentially from the Central Atlantic Magmatic Province (CAMP). While the magnitude of the CO2 rise is estimated, the source(s) remain debated, with proposals including mantle-derived CO2, sedimentary rock devolatilization, and methane release. Negative carbon isotope excursions (CIEs) support these hypotheses, but their variability complicates analysis. Mercury (Hg) concentrations and isotopes in sediments serve as proxies for volcanic activity. Volcanogenic emissions supply Hg to the atmosphere, and massive inputs can lead to concentration spikes in various facies. Hg isotopes, particularly mass independent fractionation (MIF) of odd isotopes (Δ199Hg), provide provenance information; deep-Earth volcanic emissions have near-zero Δ199Hg, while terrestrial and atmospheric fluxes show negative and positive values, respectively. Previous studies of sedimentary Hg around the T-J transition revealed elevated concentrations in various settings, linked to volcanic sources based on near-zero Δ199Hg values. However, terrestrial and nearshore sections showed multiple sources. This study analyzes a pelagic open-ocean section (Katsuyama, Japan) far from continental influences to isolate the atmospheric Hg flux during the T-J transition, providing a more globally integrated signal.

Numerous studies have investigated mercury (Hg) concentrations and isotopes in sediments across the Triassic-Jurassic (T-J) boundary to understand the role of volcanism in the mass extinction event. Studies in various marine and terrestrial settings have shown elevated Hg concentrations and Hg/TOC ratios during the T-J extinction interval. These enrichments have been attributed to volcanic sources, particularly the Central Atlantic Magmatic Province (CAMP), based on near-zero Δ¹⁹⁹Hg values. However, recent research suggests a more complex picture with multiple Hg sources, including seawater, terrestrial materials, and the atmosphere, especially in nearshore environments. The interpretation of Hg isotope signals is complicated by the influence of various factors like mixing of atmospheric and terrestrial inputs in continental shelf settings. Existing datasets from diverse settings like Nevada, St. Audrie's Bay, Levanto, Haojiagou, and Qilixia offer insights, but a pelagic open-ocean section was needed to isolate the global atmospheric Hg signal effectively.

This study analyzed a biostratigraphically well-dated pelagic radiolarian chert section from the central Panthalassic Ocean (Katsuyama, Japan). The remote location minimizes terrestrial or non-CAMP volcanic influences, allowing for a clearer CAMP signal. Slow pelagic sedimentation rates potentially yield a globally integrated signal. The researchers collected samples from a coherent structural block within the Katsuyama section. Mercury (Hg) concentrations were analyzed using a Direct Mercury Analyzer (DMA80), with results calibrated to MESS-3 standard. Carbon and sulfur concentrations were measured using an Eltra 2000 C-S analyzer. Trace element abundances were measured by inductively coupled plasma mass spectrometry (ICP-MS). A subset of samples were analyzed for Hg isotopes using a double-stage tube furnace coupled with a multiple collector inductively coupled plasma mass spectrometer (Neptune Plus). Hg isotopic results are expressed as delta (δ) values, and mass independent fractionation (MIF) values (Δ199Hg and Δ200Hg) were calculated. The Triassic-Jurassic boundary was identified based on radiolarian and conodont biostratigraphy and a change in chert color.

The Katsuyama section showed low Hg concentrations in background intervals above and below the T-J extinction interval but a dramatic rise (>60 ppb, max 163 ppb) within the extinction interval. Total organic carbon (TOC) and total sulfur (TS) remained low, and thorium (Th) showed limited variations. However, normalized Hg concentrations (Hg/TOC, Hg/TS, Hg/Th) increased significantly within the extinction interval. Importantly, mass independent fractionation (MIF) of odd Hg isotopes (Δ199Hg) showed slightly positive values in background intervals and significantly negative values (-0.14 to -0.05‰) within the extinction interval. Mass independent fractionation of even Hg isotopes (Δ200Hg) showed near-zero values in background intervals and slightly negative values within the extinction interval. The excess Hg loading (57-71x background) at Katsuyama indicates atmospheric loading. The negative Δ199Hg values rule out typical volcanic sources (Δ199Hg ~0‰) and suggest alternative sources that are consistent with combustion of terrestrial organic matter, such as wildfires and/or contact metamorphism of organic-rich sediments by igneous sills. The lack of correlation between Hg and Th also points away from terrestrial inputs.

The negative Δ¹⁹⁹Hg values associated with Hg spikes in the T-J extinction interval at Katsuyama support the hypothesis of thermogenic Hg generated through volatilization of organic matter. Photic-zone euxinia is unlikely given low TS concentrations and redox-sensitive trace elements. Terrestrial inputs are improbable due to the distance from continental sources and lack of correlation between Hg and Th. Wildfires, which were more frequent during the T-J transition, represent a likely source of Hg with negative Δ¹⁹⁹Hg values. Subsurface combustion of organic matter in organic-rich sedimentary rocks (e.g., black shales) heated by CAMP-related igneous sills provides another significant source, analogous to modern anthropogenic fossil fuel combustion. The coupled negative excursions of Δ¹⁹⁹Hg and Δ²⁰⁰Hg further support the hypothesis of combustion of organic matter, as this process generates such isotopic signatures. The findings indicate that the combustion of organic matter through both wildfires and intrusive heating of sediments released large amounts of toxic gases, significantly contributing to the environmental and biotic perturbations of the T-J boundary.

This study provides strong evidence that combustion of organic-rich sediments, both through surface wildfires and subsurface heating by CAMP-related igneous intrusions, released significant amounts of mercury to the atmosphere during the end-Triassic mass extinction. The negative mass independent fractionation (MIF) of mercury isotopes associated with the Hg enrichment supports this conclusion. This process, analogous to modern anthropogenic fossil fuel combustion, is a significant factor in environmental and biotic perturbations. Future research should focus on quantifying the relative contributions of wildfires versus intrusive heating, examining Hg concentrations and isotopes in organic-rich formations proximal to CAMP intrusions, and refining carbon-cycle models to incorporate the impact of organic matter combustion.

This study focuses on a single pelagic location. While this minimizes terrestrial influences and provides a robust atmospheric signal, it may not fully represent regional or global Hg fluxes. Further studies across a broader geographic range are needed to assess the spatial extent and variability of Hg signatures during the T-J transition. The exact relative contributions of wildfires and intrusive heating to Hg release remain uncertain, demanding further investigation. Quantifying the amount of carbon released through combustion of organic matter remains a challenge.