Sedimentary records from the northwest margin of Pangea and the Tethys show anomalously high Hg levels at the latest Permian extinction boundary. Background δ202Hg values are consistent with normal marine conditions but exhibit negative shifts coincident with increased Hg concentrations. Hg isotope mass-independent fractionation (Δ199Hg) trends are consistent with volcanic input in deep-water marine environments. In contrast, nearshore environments have Δ199Hg signatures consistent with enhanced soil and/or biomass input. We hypothesize that the deep-water signature represents an overall global increase in volcanic Hg input and that this isotope signature is overwhelmed in nearshore locations due to Hg from terrestrial sources. High-productivity nearshore regions may have experienced stressed marine ecosystems due to enhanced Hg loading.
An anomalous spike in Hg concentrations observed at the latest Permian extinction (LPE) boundary is thought to be associated with contemporaneous Siberian Trap eruptions (Grasby et al., 2015a; Sanei et al., 2012). Hg spikes have subsequently been recognized at several other mass extinction boundaries associated with large igneous province (LIP) events (Grasby et al., 2015b; Percival et al., 2015; Sial et al., 2013; Thibodeau et al., 2016). It remains unclear, however, if volcanic eruptions were the sole source of anomalous Hg deposition, or if other Hg sources and pathways related to environmental perturbations by LIPs were also significant. This is critical for tracing Hg fluxes to the environment during mass extinctions, as a marker of volcanism, as well as elucidating potential deleterious impacts on global ecosystems. We examined mercury stable isotopes across the LPE because they display both mass-dependent fractionation (MDF, reported as δ202Hg) and mass-independent fractionation of odd-mass-number isotopes (MIF, reported as Δ199Hg) that yield important information on Hg sources and cycling (Blum et al., 2014).
The LPE was the most severe mass extinction in Earth history (Chen and Benton, 2012; Erwin et al., 2002). The LPE was closely linked with the Siberian Trap eruptions that occurred over 800 k.y., starting ∼300 k.y. prior to the LPE (Burgess and Bowring, 2015). Along with impacts on marine ecosystems (Chen and Benton, 2012), the eruptions caused massive soil erosion (Algeo and Twitchett, 2010) related to denudation of terrestrial plant cover and wildfires (Benton and Newell, 2014). The exact causal connection between eruption and extinction processes is, however, uncertain. One significant impact would be toxic metal release. It is estimated that Hg emission rates from the Siberian Traps were 0.8–10 Gg/yr (Grasby et al., 2015a): 32%–399% above modern geogenic sources and comparable to the ∼2.2 Gg/yr of anthropogenic Hg released at present (Pacyna and Pacyna, 2001).
We examined the extinction boundary from two marine settings of Pangea (Fig. 1): (1) the Buchanan Lake section from the Sverdrup Basin, Canadian Arctic Archipelago (Grasby and Beauchamp, 2009), and (2) the Meishan section, China (Yin et al., 2001). Buchanan Lake records sedimentation from the midlatitude boreal margin of northwest Pangea in a bathyal to near-abyssal environment (deep water far from shore), whereas Meishan represents an equatorial environment within the Tethys Sea (Fig. 1) in a clastic sediment–starved carbonate platform setting. The Meishan section is highly condensed relative to Buchanan Lake (Grasby and Beauchamp, 2009). For comparison, we used the datum for the LPE event boundary as correlated by Grasby and Beauchamp (2009).
Due to the strong bonding of Hg to organic matter and reduced sulfur in sediments, Hg does not show significant fractionation during burial/heating of sedimentary rocks (Smith et al., 2008). Thus, variations of Hg isotopes in sedimentary rocks have been explained by source changes rather than diagenetic effects (Thibodeau et al., 2016). Hg isotopic composition is expressed in δ202Hg notation referenced to the NIST-3133 Hg standard:
MIF is reported in Δ notation (ΔxxxHg), describing the difference between the measured δxxxHg and the theoretically predicted δxxxHg value:
Detailed sampling and laboratory methods are given in the GSA Data Repository1.
The two sections show relatively constant background Hg concentrations, both absolute and when normalized by total organic carbon (TOC) to account for Hg drawdown by organic matter (Fig. 2; Grasby et al., 2013). At the LPE boundary, both sections shift to higher Hg concentrations and Hg/TOC. At Buchanan Lake, earlier spikes in Hg concentrations are related to coal ash deposition (Grasby et al., 2011); however, the main shift in Hg/TOC is at the LPE. Baseline δ202Hg values (taken as the lowest position in the sections) are ∼−0.39‰ at Buchanan Lake and −0.65‰ at Meishan. Both sections show a negative deviation in δ202Hg just prior to the LPE, low values across the extinction, and then a return toward baseline values (Figs. 2B and 2E). Vertical lines in Figure 2 show our defined baseline and minimum values for Meishan to aid comparison.
A small but significant MIF signal was also observed. The overall average Δ199Hg/Δ201Hg value of 1.08 ± 0.27 (Fig. 3A) for both sections is consistent with values for photoreduction of aqueous Hg(II) driven by natural dissolved organic matter (average of 1.02; Bergquist and Blum, 2007). For pre-extinction samples, Δ199Hg values of ∼+0.15‰ were observed in Buchanan Lake and ∼+0.10‰ at Meishan (Figs. 2C and 2F). At the LPE, Buchanan Lake showed a slight positive shift in Δ199Hg values (from 0.12‰ to 0.18‰), whereas Meishan had a significant negative shift (to −0.12‰; Figs. 2C and 2F).
Mercury spikes at the LPE boundary in both sections are coincident with abundant framboidal pyrite rainout related to a switch to euxinic ocean conditions (Grasby and Beauchamp, 2009; Shen et al., 2007). A coincident shift to higher Hg/TOC reflects enhanced Hg loading and sulfide scavenging (Sanei et al., 2012). Thus, Hg loading to the marine environment appears to have been a global event. Stable isotopes of Hg provide insight into the sources and pathways of this Hg (Fig. 4).
Background Hg Source
Background δ202Hg values of ∼–0.50‰ are consistent with that reported for pre-anthropogenic marine sediments (δ202Hg of −0.76‰ ± 0.16‰; Gehrke et al., 2009), and the relatively narrow δ202Hg values (∼−0.60‰) of most geogenic sources (Sherman et al., 2009; Smith et al., 2008; Yin et al., 2016). While Hg released into the environment can undergo complicated geochemical transformation processes resulting in large variations of δ202Hg (>10‰), transport and burial result in mixing and homogenization of Hg, such that marine δ202Hg has a narrow range reflecting the original geogenic sources. Our background values appear to reflect such a signature of normal marine conditions.
Geogenic Hg sources have insignificant Hg-MIF (Δ199Hg ∼0‰) signatures (Smith et al., 2008; Sherman et al., 2009; Yin et al., 2016). However, volcanic plume particles absorb HgII(g) from the atmosphere with positive Δ199Hg (Rolison et al., 2013), providing a plausible explanation for the observed positive Δ199Hg baseline values, particularly at Buchanan Lake. We argue that our background values reflect a dominant input of volcanic-sourced Hg by atmospheric Hg(II) deposition and/or enhanced Hg(II) photoreduction in the water column due to greater water clarity. This is supported by the clastic-starved setting of Meishan, and the deep-water, far-shore setting of Buchanan Lake.
Source of Hg Spikes
During the LPE event, Hg emissions came from volcanic eruptions, as well as burning coal and biomass (Grasby et al., 2015a; Sanei et al., 2012). Active modern volcanic emissions (Zambardi et al., 2009) have δ202Hg = −1.74‰ ± 0.36‰ for gaseous elemental Hg (Hg0g) and δ202Hg = −0.11‰ ± 0.18‰ for particulate Hg (Hg2+p). Terrestrial vegetation accumulates Hg via absorption of wet/dry atmospheric Hg deposition and/or through incorporation of (Hg0g) by stomata of leaves (Demers et al., 2013; Yin et al., 2013). Mercury can undergo mass-dependent fractionation during plant uptake of atmospheric Hg, resulting in foliage δ202Hg values ranging from −2‰ to −4‰ (Carignan et al., 2009; Demers et al., 2013; Yin et al., 2013). Modern soils that mainly receive Hg from atmospheric deposition and litterfall have δ202Hg values of −2.0‰ ± 0.6‰ (Demers et al., 2013; Jiskra et al., 2015; Zhang et al., 2013). Given this, all of these potential Hg sources could contribute to the negative δ202Hg shift observed, but they do not have unique δ202Hg signatures such that they can be distinguished.
Volcanoes, the dominant source of Hg, contribute both gaseous Hg(g) and particulate Hg(p) to the atmosphere with no Hg-MIF (Δ199Hg ∼ 0‰). At the LPE, the negligible Δ199Hg shift from the deep-marine section at Buchanan Lake (Fig. 2C) suggests that the Hg spike does not relate to a change in the background Hg source (volcanoes). The overall slight positive Δ199Hg values in the sediment likely reflect HgII(g) absorbed from the atmosphere by volcanic ash with positive Δ199Hg values (Rolison et al., 2013). For the coal ash layer below the LPE (Grasby et al., 2011), the Δ199Hg values suggest that the Hg spike represents the early onset of volcanic activity, as in Burgess and Bowring (2015), rather than coal combustion itself. This finding is consistent with Thibodeau et al. (2016), who suggested that stable values of Δ199Hg reflected a dominant volcanic source for an Hg spike at the end-Triassic extinction. In contrast, the negative shift in Δ199Hg values at Meishan is more indicative of increased Hg contribution from biomass and soil sources at the LPE. This is consistent with the observed shift in δ202Hg at Meishan being more negative than Buchanan Lake, as terrestrial organics have significantly lower δ202Hg values than volcanic emissions. The negative shift in Δ199Hg also correlates with previously reported increases in polycyclic aromatic hydrocarbons (PAHs) and abundance of black carbon particles, including char, at the LPE in Meishan (Fig. 3B), which were related to ash deposition from massive wildfires (Shen et al., 2011). Wildfires at this site would not only enhance Hg influx from biomass burning (Pirrone et al., 2010), but might also have promoted soil erosion as a result of significant denudation of vegetation, and the Hg from these sources overwhelmed any signature from enhanced volcanic emissions from the Siberian Traps.
Stable isotope data suggest that Hg spikes at the LPE came from two prominent sources: (1) volcanic and (2) terrestrial (soil and/or biomass combustion) emissions (Fig. 4). The MIF signatures from Buchanan Lake record enhanced background volcanic Hg loading and, given its distal deep-water and downwind setting (Grasby et al., 2011), provide direct evidence for widespread volcanic impacts on the global environment. However, stable isotope records from the nearshore, shallower-water setting of Meishan show that this background volcanic signal was overwhelmed by Hg from a terrestrial source. This implies that shallow-marine environments had an even larger Hg flux related to massive soil erosion and/or biomass combustion. This is critical because nearshore shallow-water environments are key areas of primary productivity related to upwelling of nutrient-rich waters along continental margins (Chavez and Messié, 2009). While increased background emissions from volcanic sources at the LPE could have stressed global ecosystems (Sanei et al., 2012), especially high levels of Hg input into shallow-water marine environments could have been particularly severe.
Hg stable isotope data show that prior to the LPE, deep- to shallow-water marine records have similar background δ202Hg values that represent original geogenic sources. At the LPE, a Hg spike, global in extent, is observed. Stable isotope data from distal deep-water locations suggest volcanic emissions had widespread Hg loading impacts on the global environment. In contrast, nearshore, shallower-water settings appear to have received additional Hg loading related to massive soil erosion and/or biomass burning. This may have placed additional ecological stress on these shallow-water environments, which were key areas of marine primary productivity.
We acknowledge the U.S. Geological Survey Wisconsin Mercury Research Laboratory and Wisconsin State Laboratory of Hygiene for the use of their laboratory space for the determination of THg concentrations and stable Hg isotopes. Hua Zhang from the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, helped with collecting the Meishan samples. Marcus Johnson is thanked for his assistance with Hg isotopic analyses at University of Michigan. J. Gleason and J. Blum acknowledge support for this work from National Science Foundation grant OPP-0909264. This is Earth Science Sector, Natural Recourses Canada contribution 20160079.
- Received 15 August 2016.
- Revision received 13 October 2016.
- Accepted 14 October 2016.
- © 2016 Geological Society of America