Evidence for a prolonged Permian–Triassic extinction interval from global marine mercury records

The latest Permian mass extinction, the most devastating biocrisis of the Phanerozoic, has been widely attributed to eruptions of the Siberian Traps Large Igneous Province, although evidence of a direct link has been scant to date. Here, we measure mercury (Hg), assumed to reflect shifts in volcanic activity, across the Permian-Triassic boundary in ten marine sections across the Northern Hemisphere. Hg concentration peaks close to the Permian-Triassic boundary suggest coupling of biotic extinction and increased volcanic activity. Additionally, Hg isotopic data for a subset of these sections provide evidence for largely atmospheric rather than terrestrial Hg sources, further linking Hg enrichment to increased volcanic activity. Hg peaks in shallow-water sections were nearly synchronous with the end-Permian extinction horizon, while those in deep-water sections occurred tens of thousands of years before the main extinction, possibly supporting a globally diachronous biotic turnover and protracted mass extinction event.

RGM) 1 . Hg is distributed globally mainly in the form of Hg 0 , which compromises 90% of total atmospheric Hg and has an atmospheric residence time of ~6 months to 1 year, allowing for long-distance transport 2,3 . Hg 0 can be oxidized to Hg 2+ , which is removed from the atmosphere through both wet (RGM) and dry (Hgp) deposition 3,4,5 . Hg 2+ can be methylated into neurotoxic and bioaccumulative methylmercury (MMHg) in the aqueous environment 6,7 , which poses a serious threat to human health via fish or rice consumption 8,9 . In addition to atmospheric deposition, Hg is also delivered to the ocean by river-borne particles (e.g., clays and organic matter) 10,11 .
In the aqueous environment, Hg is present mainly as elemental mercury (Hg 0 aq), divalent inorganic mercury (Hg 2+ aq), monomethylmercury (MMHg), dimethylmercury (DMHg), and particle-bound mercury (Hg P aq) 12 . Hg has somewhat variable behavior with water depth in the ocean 13 . In the surface layer, Hg 2+ aq can be reduced to Hg 0 aq and then reemitted to the atmosphere. Hg 2+ aq is also adsorbed onto suspended organic particulates (= Hg P aq) in amounts that are generally proportional to primary production.
Most Hg P aq is released back to the water column during remineralization of organic matter, leading to increased concentrations of Hg 2+ aq within the oceanic thermocline region. Methylation of Hg 2+ aq to MMHg and DMHg occurs mainly in oxygen-minimum zones (OMZs). Only a small fraction of Hg P aq reaches the deep-ocean floor to accumulate in abyssal deposits. In marine sediments, Hg tends to form strong, stable complexes with organic matter and/or HgS minerals and Hg-S complexes that resist remobilization in the burial environment 14 .
The large reservoir of Hg in the ocean (~953 × 10 6 mol) plays an important role in the Earth's Hg cycle 13 . In the open ocean, Hg concentrations are mainly between 0.1 ` and 0.2 pmol in the surface mixed layer, yielding an inventory of ~3 × 10 6 mol Hg 13 .
The intermediate layer (~200-1000 m) often has the higher Hg concentrations (~0.4 pmol) due to remineralization of sinking organic particulates, yielding an inventory of 110 × 10 6 mol Hg. Hg concentrations increase slightly downward to the abyssal seafloor (~0.8 pmol), yielding an inventory in the deep ocean of 840 × 10 6 mol Hg. The residence times of Hg in the deep ocean (1700 yr), and intermediate layer (120 yr) are much longer than that in the surface mixed layer (7 months) and atmosphere (6-12 months) 13 .
Hg has seven stable isotopes (196,198,199,200,201,202  and magnetic isotope effect (MIE), and can be used to explore specific processes such as Hg 0 volatilization, dark Hg(II) reduction, and photochemical processes (see refers.15, 16, and references therein). Nearly all kinetic reactions (e.g., natural sunlight with fulvic acid, dark organically mediated reduction, chemical reduction, photolysis with formic acid, ethylation, volatilization) involving Hg produce products with lower δ 202 Hg and leave a residual pool of reactant with higher δ 202 Hg. Biotic and dark abiotic reactions do not produce significant MIF (i.e., 199 Hg). In contrast, all photochemical reactions that have been studied produce changes in both MDF and MIF (see refers.15, 16, and references therein). The largest positive MIF of Hg isotope is caused by photochemical degradation of methylmercury in water, and the largest negative MIF of Hg isotopes is caused by photochemical reduction of inorganic Hg.

Supplementary Note 2-Study sections `
This study examined eleven sections with a near-global distribution. Two sections are from the NE Panthalassic Ocean (or NW Pangean margin), five from the Paleo-Tethys ocean, and three from the central Panthalassic Ocean (Supplementary Fig. 1).

Central Panthalassic Ocean
The Panthalassic Ocean covered >50% of Earth's surface and was nine times larger than the Tethys Oceans during the Triassic, yet paleoenvironmental conditions within it remain very poorly known owing mainly to the fact that most of its oceanic crust and sediments have been subducted 57,58 . A few slivers of deep-ocean sediments obducted onto the Japanese microcontinent provide the best records of conditions in the central Panthalassic Ocean.

Gujo-Hachiman, central Japan: The Gujo-Hachiman section (35.7355 °N and
136.8489 °E) was located in the central Panthalassic Ocean (Fig. 1), accumulating a thin section of abyssal seafloor sediments at ~5000 m water depth (refers. 59,60 , and references therein). The section comprises 6.9 m of green-gray ribbon chert of biosiliceous origin belonging to the Wuchiapingian Neoalbaillella optima-Albaillella lauta Zone and Changhsingian A. angusta-A. flexa, A. triangularis, A. yaoi, and A. degradans zones 61,62 , overlain by 0.6 m of black shale of Griesbachian age 59,63 ( Supplementary Fig. 9). Previous geochemical studies were focused mainly on productivity and redox changes (e.g., refers 21,22,24,59). The present study is the first and Hindeodus parvus (Conodonta) were recovered in the siliceous claystone and the black shale, respectively. The LPME was placed near the contact of the siliceous claystone and carbonaceous shale, corresponding to an abrupt decrease in microfossil abundance and a negative organic carbon isotope excursion. The PTB was placed ~75 cm above the LPME based on the lowest occurrence of a Hindeodus parvus specimen 65 .