The accumulation of Hg in sediments can be influenced by various factors including redox conditions, organic carbon burial fluxes, and clay mineral content16,20,25,26,27. Reducing conditions promote the formation of organic-Hg complexes and Hg-sulfides, which are likely to be the dominant forms of Hg in marine sediments20,26,27. Hg can become enriched in clay minerals under certain chemical conditions (e.g., elevated E h ) through adsorption of sparingly soluble Hg(OH) 2 28,29. However, Hg adsorbed onto organic matter is the dominant form of Hg in most aquatic systems20. Small mass-dependent fractionations (MDF) may result from physical–chemical–biological processes during Hg uptake in marine sediments, but the lack of mass-independent fractionation (MIF) renders Hg isotope systematics (especially MIF) a powerful tracer of Hg provenance30,31. Hg-isotopic MIF variation in Phanerozoic sedimentary successions has been interpreted in terms of source changes rather than diagenetic effects8,15,32.

In most of the study sections, Hg exhibits a stronger correlation to TOC (i.e., r ranging from+0.55 to +0.95 than to sulfur (S) (r mostly <+0.45) or aluminum (Al) (r ranging from +0.20 to +0.84 (Supplementary Fig. 12; note all r values significant at p(a) < 0.01). This strong correlation supports organic matter as the dominant Hg substrate. Although there is pronounced variation in TOC concentrations in most sections, both raw and TOC-normalized Hg concentrations (i.e., Hg/TOC) show systematic stratigraphic trends in the 10 study sections, suggesting that elevated Hg fluxes to the sediment were not simply due to increased organic matter burial. Furthermore, increases in Hg/TOC around the LPME are not related to changes in sediment lithology, as samples containing <1% Al (i.e., carbonates) and those containing >1% Al (i.e., marls and shales) show nearly identical patterns of secular Hg/TOC variation in all profiles despite paleoenvironmental differences (Supplementary Fig. 13). Thus, we infer that the large increases in Hg/TOC observed around the LPME reflect a large increase in Hg fluxes to the ocean followed by rapid Hg removal to the sediment, reflecting the short residence time of Hg in the atmosphere–ocean system.

The sharp peaks of Hg/TOC that first appear near the LPME horizon (~251.94 Ma) continue upsection in each study section for stratigraphic intervals corresponding to ~50–200 kyr. This period also corresponds to the peak of the end-Permian mass extinction, characterized by major perturbations to global biogeochemical cycles and terrestrial and marine ecosystems1,33,34 (Fig. 4). This timeframe is also consistent with the interval of large-scale intrusion of Siberian Traps magmas into organic-rich sediments of the Tunguska Basin during the intrusive sill-complex phase of Burgess et al. 9. The Hg/TOC peaks, therefore, are likely to be tied, in part, to the onset of heating of subsurface organic-rich sediments by sill intrusions of the Siberian Traps LIP rather than to the onset of flood basalt eruptions3,9. However, the relationship of Hg emissions to LIP activity is not well understood at present35.

Fig. 4 Relationship of mercury records to PTB marine ecosystem perturbations. Hg/TOC values from all study sections, biodiversity variations70,71, and inorganic carbon isotopes72. C. Clarkina, cha. C. changxingensis, dien. Neospathodus dieneri, k.-d. Neoclarkina krystyni-N. discreta, ku. Sweetospathodus kummeli, m. C. meishanensis, Nv. Novispathodus, p.-s. Hindeodus parvus-Isarcicella staeschi, w.-s. C. wangi-C. subcarinata, yin. C. yini; Gri. Griesbachian, Dien. Dienerian, LPME latest Permian mass extinction. Geochronologic and biozonation data modified from ref. 19, and Hg/TOC data of Buchanan Lake and Meishan D from ref. 8. Four samples with Hg/TOC ratios > 1000 ppb/% (Buchanan Lake = 3, Meishan D = 1) are marked by an arrow Full size image

Hg/TOC ratios exhibit only a weak relationship to distance from the Siberian Traps LIP but a strong relationship to depositional water depth (Fig. 1). Paleogeographically, sections from NE Panthalassa have higher average Hg/TOC ratios during the enrichment interval (85 ± 67 ppb/%) relative to sections from the Paleo-Tethys (62 ± 40 ppb/%) or Panthalassic oceans (30 ± 21 ppb/%; Fig. 2a, b). With regard to water depths, average Hg/TOC ratios for the pre-enrichment interval are 26 ± 14, 82 ± 60, and 27 ± 19 ppb/% for shallow, intermediate, and deep sections, respectively (Fig. 2a). Thus, intermediate-depth sections show higher background Hg/TOC values (by a factor of nearly 3) than either surface and deep-ocean sections, implying elevated aqueous Hg concentrations in the upper thermocline region (~200–500 m) of Late Permian oceans. Average EFs during the enrichment interval are 3.4 ± 0.7, 4.6 ± 1.8, and 4.9 ± 2.9 for shallow, intermediate, and deep sections, respectively (Fig. 2b), indicating that the pulse of Hg released during the PTB crisis was preferentially transferred out of the surface ocean and into deeper waters. Hg enrichment in shallow-water settings during the Toarcian (~183 Ma) was inferred to have been the result of intense terrestrial runoff36, although this is likely not the case for the present study sections owing to distinctly greater mercury enrichments at intermediate-depth relative to shallow-water settings. Instead, this pattern is similar to the Hg loading in the thermocline of modern oceans, which results from adsorption of Hg onto sinking organic particles and downward transfer through the biological pump37. However, other factors (e.g., the amount and type of organic matter) may also have influenced the depth-dependent distribution of Hg in the study sections.

There is a distinct difference in the timing of initial Hg enrichment relative to the LPME horizon in the shallow-water relative to the deep-water study sites. At shallow-water locales, the spike in Hg enrichments and faunal turnover are nearly synchronous, whereas the deep-water locales show a large time lag between the initial Hg pulse and faunal turnover. Hg/TOC peaks are ~0.5 and 0.3 m below the LPME in the deepwater Akkamori-2 and Ubara sections, representing at least a 50–100 kyr lag (Fig. 1; see Methods for age models). A smaller time gap (~20 kyr) between Hg enrichments and the LPME horizon is inferred for the intermediate-depth Xiakou section.

The synchronicity of the Hg enrichments and the extinction horizon in shallow-water sections might be related to sediment homogenization by bioturbation. However, in key sections Hg enrichments occur predominantly in sediments with limited fabric disruption38,39, indicating that the offsets in Hg enrichments and the extinction horizon are not linked to bioturbation. For instance, sediment homogenization at Meishan is limited to 2–4 cm just below the extinction horizon (Bed 25) and is largely lacking above the LPME40. The pelagic sections from Japan also exhibit strong primary sedimentary fabric preservation with only limited evidence of bioturbation39,41.

Mercury isotopes can be used to track the source and depositional pathways of mercury into marine sediments (see Blum et al. 30 and references therein) given that the two main Hg sources to the oceans, i.e., terrestrial runoff and atmospheric deposition of Hg(II), have different isotopic signatures30,31. Mercury has a complex biogeochemical cycle and undergoes transformations that may induce MDF (δ202Hg) and/or MIF (Δ199Hg) of Hg isotopes30. Volcanogenic Hg has δ202Hg values between ‒2‰ and 0‰42,43, and its MDF can be influenced by a wide range of physical, chemical, and biological processes. MIF, in contrast, occurs predominantly through photochemical processes8,30. Hg emitted by arc volcanoes or hydrothermal systems does not appear to have undergone significant MIF (~0‰), although a relatively limited number of settings have been studied to date. Coal combustion commonly leads to release of Hg with negative δ202Hg and Δ199Hg values43,44. Alternatively, photoreduction of Hg(II) complexed by reduced sulfur ligands in the photic zone can limit negative MIF45. However, Hg enrichments and negative MIF records in the present study units cannot be due exclusively to oceanic anoxia near the PTB, because Hg enrichments are measured in diverse redox environments and the Hg is hosted mainly by organic matter rather than sulfides.

The near-zero Δ199Hg values (mostly 0‰ to +0.10‰) for the pre-LPME interval at Meishan D and Xiakou may reflect photochemical reduction of Hg or the mixing of terrestrial and atmospheric sources of Hg43 (Fig. 3). However, the lower to middle Changhsingian interval at Gujo-Hachiman (the stratigraphic equivalents of which were not sampled in the Meishan D and Xiakou sections) exhibits distinctly elevated Δ199Hg compositions, ranging from +0.10‰ to +0.35‰, which are typical of marine sediments30 and consistent with photoreduction of aqueous HgII26,43. All three sections (especially the pelagic Gujo-Hachiman section) exhibit near-zero, although somewhat variable, Δ199Hg values during and following the LPME, which are consistent with predominantly volcanic and/or thermogenic (i.e., coal-derived) Hg inputs.

MDF (δ202Hg) profiles for the study sections show roughly similar patterns: Meishan D and Xiakou yield background (pre-LPME and post-PTB) values of ca. ‒0.50‰, whereas the stratigraphically older part of the Gujo-Hachiman section shows more negative pre-LPME values, ranging from ‒0.80‰ to ‒2.30‰ with a mean of ‒1.50‰ (Fig. 3). All three sections show increased variability in δ202Hg around the LPME, with Meishan D and Xiakou each possibly displaying two negative spikes. These excursions in MDF support a change in the source or cycling of marine Hg close to the LPME, although the exact nature of the controlling processes is uncertain. For the pre-LPME interval at Gujo-Hachiman, the large positive MIF and negative MDF signatures imply a dominant atmospheric transport pathway30,46. The small positive MIF and negative MDF signatures of the Meishan D and Xiakou sections may indicate mixed atmospheric and terrestrial sources, with possible Hg inputs from land plants owing to increased Hg loadings in terrestrial ecosystems.

Our new Hg-isotopic results yield insights beyond those of earlier Hg studies of the PTB. Grasby et al. 8 inferred that δ202Hg-Δ199Hg values were consistent with Hg sourced mainly from volcanic activity for a deep slope section in the Canadian Arctic (Buchanan Lake), and a combination of atmospheric inputs and terrestrial runoff for a nearshore section in China (Meishan D). Although our minimum MIF values are much less negative than those reported by Grasby et al. 8, our data for Meishan D also support a mixture of terrestrial and atmospheric Hg sources. We infer that changes around the LPME in the deep-ocean Gujo-Hachiman section (near-zero to weakly positive Δ199Hg values, a concurrent increase of MDF, and strong Hg enrichments) are evidence of atmospheric inputs of Hg (i.e., from volcanic emissions as well as volcanic-related thermogenic sources such as coal combustion) to the open ocean thousands of kilometers distant from riverine fluxes. Overall, the trends in δ202Hg-Δ199Hg values are consistent with massive inputs of Hg from volcanic emissions and/or combustion of Hg-bearing organic-rich sediments by the Siberian Traps LIP.

The LPME coincided with the onset of sill complex formation of the Siberian Traps LIP9, indicating that the initial Hg enrichments near the LPME in PTB sections were also coincident with those sills. Hg profiles can provide high-resolution records of volcanic activity given the short residence time of Hg in the atmosphere and oceanic water column (<2 years and <1000 years, respectively)37,47. Compared to the synchronicity of Hg peaks and the LPME in shallow-water sections, the observed time gaps of ~50 to 100 kyr between the initial appearance of Hg peaks and the LPME in pelagic deep-water sections (Akkamori-2 and Ubara) may support a diachronous marine extinction event. This conclusion, however, is dependent on the geological synchronicity of the Hg peaks, which depends on the age model and the placement of the LPME in each section (see Methods). A protracted extinction model has also been proposed based on the differential timing of sponge extinctions relative to the LPME in the Arctic region48 and radiolarian extinctions in the Nanpanjiang Basin49,50.

A diachronous extinction event would provide new insights into the long-debated influence of various ‘kill mechanisms’, e.g., hypercapnia51,52, thermal stress53, and oxygen and sulfide stresses54,55. The effects of hypercapnia and thermal stress should be nearly synchronous, as heat and carbon dioxide are fairly evenly distributed through atmospheric and marine circulation on 1–2 kyr time scales56. Moreover, the effects of hypercapnia should be coincident with peak Hg enrichments and peak outgassing (assuming the two are equivalent) given that silicate and marine weathering will begin to draw down atmospheric carbon dioxide following the onset of a carbon injection (e.g., refs. 57,58). This is consistent with the synchronous increase in atmospheric Hg and CO 2 during the end-Triassic crisis15. In contrast, ocean anoxia can develop over a wide range of time scales, depending on initial local oxygen concentrations, baseline nutrient levels, and the extent and rate of nutrient release into the marine system from enhanced weathering and positive feedbacks associated with the P cycle59,60. For anoxia to develop in deep-ocean settings (e.g., extensive anoxia in deep-marine settings near the LPME24,61), greater nutrient loading (e.g., P, Fe) is needed than for shelf settings62. Thus, the presence of Hg enrichment across different marine environments (assuming a volcanogenic origin) provides new evidence for oxygen stress, rather than extreme temperatures or hypercapnia, as the critical driver of Earth’s largest mass extinction event. It should also be noted that elevated temperatures reduce oxygen saturation levels in seawater and cause the metabolic effects of low oxygen to become more severe63.

Mercury enrichments near the LPME horizon in continental shelf, continental slope, and abyssal marine sections, combined with Hg isotopes (δ202Hg–Δ199Hg), provide evidence for a massive increase in volcanic-related Hg emissions during the Permian–Triassic biotic crisis. This study provides direct geochemical evidence from marine sections for near global-scale volcanic effects linking the Siberian Traps LIP to the PTB crisis. Relative to pre-LPME background values, Hg-EFs rose by factors of 3–8 during the mass extinction event before returning to near-background levels in the Early Triassic. Hg/TOC ratios are significantly higher (by a factor of nearly 3) in intermediate-depth sections relative to surface and deep-ocean sections prior to the PTB crisis, reflecting a general concentration of Hg within the upper thermocline region through the action of the biological pump. Further, with current placements of the LPME horizon in each section, stratigraphic differences between the initial spike of Hg concentrations and the LPME represent a time gap that provides evidence of a globally diachronous mass extinction event. Specifically, the extinction horizon in deep-water sections (e.g., Akkamori-2 and Ubara) postdated peak volcanogenic Hg inputs by ~50 to 100 kyr, whereas it was nearly synchronous in shallow-water sections. Because of feedbacks in the marine oxygen cycle, sulfide and oxygen stresses would have developed over thousands or even tens of thousands of years after the peak of volcanic outgassing. A lag between peak volcanogenic Hg inputs and biotic turnover is likely when ecosystem destabilization is caused by oxygen stress, in contrast to the geologically rapid response expected if extreme temperatures or hypercapnia were the main kill mechanism. In summary, evidence for a protracted extinction interval provides new support for oxygen and sulfide stresses as the main kill mechanism over a large swath of the ocean in response to Siberian Traps LIP volcanism.