Mercury evidence for combustion of organic-rich sediments during the end-Triassic crisis

The Triassic–Jurassic (T–J) transition (~201 Ma) was marked by extreme warming, ocean acidification and anoxia, and a mass extinction, widely linked to rising atmospheric CO2 associated with emplacement of the Central Atlantic Magmatic Province (CAMP). While CO2 increases of ~2–4× have been inferred, the sources of excess CO2 remain controversial, including mantle-derived emissions, thermogenic release from organic- and carbonate-rich sediments, and methane release from permafrost or clathrates. Negative carbon-isotope excursions (CIEs) across diverse settings indicate major carbon-cycle disruption but vary in magnitude, complicating source apportionment. Mercury concentrations and isotopes in sediments provide a tool to fingerprint volcanic activity and Hg provenance. Prior T–J studies in continental to shelf settings show Hg enrichments, but mixed sources obscure interpretation. This study targets a pelagic, open-ocean chert section (Katsuyama, Japan) to isolate atmospheric Hg deposition during the T–J, test for CAMP-related signals, and evaluate whether Hg sources included combustion of organic-rich sediments and/or wildfires.

Previous work documented Hg spikes and near-zero Δ199Hg values in continental and shelf–slope T–J sections, interpreted as volcanic inputs from CAMP. However, nearshore records may mix multiple sources (terrestrial vegetation/soils with negative Δ199Hg, atmospheric wet deposition with positive Δ199Hg, and volcanogenic Hg with near-zero Δ199Hg), complicating provenance. Studies across Nevada, St. Audrie’s Bay, and Levanto exemplify varied background and transitional Δ199Hg signals influenced by seawater, atmospheric deposition, and terrestrial runoff. Broader reviews highlight Hg as a volcanism proxy but emphasize complexities in Hg removal in marine systems and the need for pelagic records far from continental influence to better reconstruct atmospheric Hg fluxes during LIP events. Carbon-cycle modeling and geochronology have also linked CAMP sill emplacement in Brazilian basins to thermogenic CO2 and negative CIEs, suggesting heating of organic-rich sediments as a key process.

Study site and stratigraphy: The pelagic Katsuyama section (Mino Terrane, central Honshu, Japan; ~35.4228°N, 136.9708°E) comprises bedded radiolarian cherts deposited several thousand kilometers from major detrital sources at very low sedimentation rates below the calcite compensation depth. Biostratigraphy (radiolarian and conodont zones) constrains the T–J boundary at ~2.2 m in the section. Sampling and preparation: Samples were trimmed to remove veins/weathered surfaces and powdered (~200 mesh). Aliquots were prepared for different analyses. Hg concentrations: n=157 chert samples analyzed by Direct Mercury Analyzer (DMA-80) at Yale University (∼150 mg per analysis). Calibration to MESS-3 (80 ppb Hg); QA/QC with replicate and standard every 10 samples; precision (2σ) ±0.5%. Carbon and sulfur: n=259 measured with Eltra 2000 C–S analyzer (University of Cincinnati). Carbonates removed by 2 N HCl (50 °C, 12 h) for TOC; TIC by difference. Precision (2σ): ±2.5% (C), ±5% (S). Trace elements: n=61 measured by ICP-MS (Agilent 7500a) at China University of Geosciences (Wuhan). HF–HNO3 digestion in Teflon bombs at 190 °C; standards BHVO-2 and BCR-2; RSD <5%. Hg isotopes: Subset n=14 preconcentrated using a double-stage tube furnace with 40% anti-aqua regia traps, then analyzed by Neptune Plus MC-ICP-MS. NIST SRM 997 Tl (50 ng/mL) used as internal standard for mass bias correction; tuning with NIST 3133 Hg. Isotopic results reported as δ values (per mil) relative to NIST 3133, with MIF (ΔxxxHg) calculated as ΔxxxHg = δxxxHg − β×δ202Hg (β=0.2520 for 199Hg; 0.5024 for 200Hg). QA/QC with NIST-3177 every 10 samples and GSS-4 (soil) full procedural standards. Data interpretation: Chemostratigraphic profiles of Hg, TOC, TS, Th, and ratios (Hg/TOC, Hg/TS, Hg/Th), and Δ199Hg and Δ200Hg were evaluated to identify mercury-enriched intervals (MEI), assess provenance (atmosphere vs terrestrial vs mantle), and test hypotheses regarding contributions from volcanism, wildfires, and thermogenic release from organic-rich sediments.

- Hg concentrations are low (<5 ppb) outside the extinction interval but increase to >60 ppb (max 163 ppb) within the T–J extinction interval. - TOC and TS remain low (<0.2%) throughout, and Th varies modestly (<3 ppm), indicating Hg spikes are not driven by organic content, sulfur, or detrital inputs. - Normalized Hg ratios increase sharply in the extinction interval: Hg/TOC >500 ppb/%, Hg/TS >2000 ppb/%, Hg/Th >20 ppb/ppm (background values: <50, <500, <5, respectively). - Odd-isotope MIF (Δ199Hg) is slightly positive in background intervals (0 to +0.11‰) and becomes negative during the extinction interval (−0.14 to −0.05‰). - Even-isotope MIF (Δ200Hg) is near-zero in background and shows slightly negative excursions in the extinction interval (down to around −0.08‰, with small values up to ~+0.06‰ reported). - Excess Hg loading in the extinction interval exceeds background by factors of approximately 57× (raw Hg), 71× (Hg/TOC), 60× (Hg/TS), and 49× (Hg/Th). - Multiple Hg peaks near the extinction interval are consistent with pulsed CAMP activity. - Lack of correlation between Hg and Th and the pelagic, remote location argue against significant terrestrial detrital or riverine Hg input.

The pelagic Katsuyama record isolates atmospheric deposition and shows large Hg enrichments with negative Δ199Hg and slightly negative Δ200Hg during the extinction interval. These isotopic signatures are inconsistent with direct mantle-sourced volcanic Hg alone (Δ199Hg ~ 0‰) and with dominant atmospheric wet deposition of Hg(II) to the ocean (typically positive Δ199Hg), and they strongly argue for a source characterized by negative Δ199Hg: combustion of terrestrial organic matter. Alternative mechanisms such as photic-zone euxinia are unlikely given persistently low TS and redox-sensitive element concentrations and the lack of correlation between Δ199Hg and TS. The remote setting and short marine Hg residence time further reduce the likelihood of significant terrestrial runoff contributing to the pelagic signal. The negative Δ199Hg coupled with negative excursions in Δ200Hg, which is generated exclusively in the atmosphere, supports mobilization of Hg stored in vegetation and soils (taken up as gaseous Hg(0) with negative Δ199Hg) via widespread wildfires, and/or thermogenic release from contact metamorphism and devolatilization of organic-rich sediments (coal, black shales) during CAMP sill emplacement (notably in Brazilian basins). The multiple Hg peaks imply pulsed magmatic activity consistent with U–Pb geochronology. Together, the data indicate that combustion of organic-rich sediments and biomass was a significant driver of atmospheric Hg and CO2 release during the end-Triassic crisis, offering an ancient analog to modern fossil-fuel combustion and highlighting the environmental importance of volatile-rich sedimentary reservoirs during large igneous province events.

A pelagic Triassic–Jurassic boundary section from central Panthalassa records large atmospheric Hg loading during the extinction interval, with negative odd-isotope MIF (Δ199Hg) and slightly negative Δ200Hg. These signatures implicate thermogenic and wildfire-derived Hg sourced from combustion of terrestrial organic matter and organic-rich sediments heated by CAMP intrusions, rather than direct volcanic Hg alone. The findings strengthen the view that interaction between magmatism and volatile-rich sedimentary reservoirs can amplify environmental and biotic crises. Future work could target direct Hg measurements in suspected source formations (e.g., Brazilian organic-rich units), expand pelagic isotope datasets for improved temporal resolution, and refine models of Hg cycling and transport during LIP events.

- Pelagic Hg cycling and removal pathways are complex, potentially introducing uncertainties in source apportionment despite the remote setting. - Hg isotope dataset size is limited (n=14), which constrains temporal resolution across the extinction interval. - Mixing of multiple Hg sources (volcanic, atmospheric, terrestrial biomass/soils, thermogenic from sediments) cannot be entirely excluded. - Direct Hg concentration and isotope data from the proposed organic-rich source formations (e.g., Brazilian basins) are lacking. - Eruptive rates and pulsing of CAMP are imperfectly constrained, adding uncertainty to precise correlation with Hg peaks.