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

Abstract

The sources of isotopically light carbon released during the end-Triassic mass extinction remain in debate. Here, we use mercury (Hg) concentrations and isotopes from a pelagic Triassic–Jurassic boundary section (Katsuyama, Japan) to track changes in Hg cycling. Because of its location in the central Panthalassa, far from terrigenous runoff, Hg enrichments at Katsuyama record atmospheric Hg deposition. These enrichments are characterized by negative mass independent fractionation (MIF) of odd Hg isotopes, providing evidence of their derivation from terrestrial organic-rich sediments (Δ199Hg < 0‰) rather than from deep-Earth volcanic gases (Δ199Hg ~ 0‰). Our data thus provide evidence that combustion of sedimentary organic matter by igneous intrusions and/or wildfires played a significant role in the environmental perturbations accompanying the event. This process has a modern analog in anthropogenic combustion of fossil fuels from crustal reservoirs.

Mercury evidence from southern Pangea terrestrial sections for end-Permian global volcanic effects

“…Volcanogenic Hg inputs to all Permian–Triassic boundary sections appear to have been synchronous at a temporal resolution of ~10 5 years and, therefore, are likely to have shared a dominant common source (e.g., STLIP). This finding is analogous to that for the Triassic–Jurassic boundary, at which numerous marine and terrestrial sites have yielded Hg anomalies linked to eruptions of the Central Atlantic Magmatic Province at a temporal resolution of ~10 5 years 23 , 30 , 40 – 42 .…”

“…Although Δ 199 Hg has been widely used to track Hg sources during the Permian–Triassic transition 14 , 16 , 18 , interpretation of Hg isotopes in sediments can be complicated. Several factors influence Δ 199 Hg: (1) the proportions of Hg sourced from the mantle (near-zero MIF 22 ) versus organic-rich sediments by sills and/or combustion of vegetation and soil by wildfires (negative MIF 23 ); (2) the magnitude of Hg fractionation during atmospheric transport, which ranges from 0.1 to 0.8‰ in the modern atmosphere 24 ; and (3) mixing of multiple Hg sources, including terrestrial runoff, atmospheric removal, and seawater loading, in terrestrial and nearshore facies 16 , 17 , 25 , 26 . In Permian–Triassic successions, Δ 199 Hg shows large variations in different settings, with near-zero values in deep-shelf to slope settings but negative excursions in continental, shallow-shelf, deep-basinal, and pelagic settings (Fig.…”

“…On the other hand, deep-basinal and pelagic facies (e.g., Shangsi and Gujo-Hachiman, Fig. 3h, i ), in which terrestrial Hg inputs are limited, provide a promising reservoir for tracking atmospheric fluxes of Hg 23 . For the pelagic Gujo-Hachiman section, the negative or near-zero Δ 199 Hg values associated with Hg concentration peaks near the LPME, document elevated fluxes of isotopically light Hg because Δ 199 Hg becomes more positive during atmospheric transport 24 .…”

“…A subset of 43 samples (17 and 26 from Bunnerong and Ripplemead, respectively) was analyzed for mercury isotopes using a Neptune Plus multi-collector inductively coupled plasma mass spectrometer (Thermo Electron Corp, Bremen, Germany). The Bunnerong samples were analyzed at Tianjin University 5 , and the Ripplemead samples at the State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, per methods as reported in Shen et al 23 , 30 . Hg isotopic results are expressed as delta (δ) values in units of per mille (‰) variation relative to the bracketed NIST 3133 Hg standard, as follows: …”

Enhanced Weathering Triggered the Transient Oxygenation Event at ∼1.57 Ga

“…Volcanic eruption, particularly that associated with LIPs, is one of the major causes for Hg enrichment in sedimentary successions (e.g., Grasby et al, 2019). Volcanically emitted Hg(0) has no MIF (e.g., Δ 199 Hg = 0‰); whereas photochemical redox reactions between Hg(0) and Hg(II) in atmosphere generates positive MIF, leaving a negative Δ 199 Hg signal (Δ 199 Hg < 0‰) in the residual Hg(0) pool and a positive Δ 199 Hg signal (Δ 199 Hg > 0‰) in reactive, water-soluble Hg(II) (Blum et al, 2014;Fu et al, 2021;Shen et al, 2022). As a result, marine sediments containing direct atmospheric Hg(II) deposition-such as those from open ocean environments-would have positive ∆ 199 Hg values (Shen et al, 2019(Shen et al, , 2022; whereas terrestrial reservoirs such as plant biomass and soils that primarily accumulate atmospheric Hg(0) would show negative Δ 199 Hg values (Blum et al, 2014).…”