Mercury, volcanism, and mass extinctions.

Understanding the causes and timings of mass extinctions are important for our understanding of the evolution of life on Earth and how major biogeochemical cycles have been and can be perturbed. Four of the five biggest mass extinctions (1) are associated with large igneous provinces (LIPS), which are the most voluminous volcanic events on Earth, but whether LIPS triggered the extinctions is disputed due to the difficulty in correlating evidence for the onset and duration of LIPS with mass extinction records (2). These challenges arise largely because direct evidence of LIPS is generally absent from the sedimentary records that contain the fossil extinction records; causal relationships must instead rely on comparing radiometric dating of LIPS with biostratigraphic ages in fossil records. A new tool in understanding the relationship between volcanism and mass extinctions is mercury chemostratigraphy (3). Because large amounts of Hg are emitted from volcanism, it is argued that increases in Hg preserved in sedimentary records can be used as a proxy for increased volcanic inputs. If true, Hg concentration changes measured in the same sedimentary records that preserve biotic and environmental crises allow for detailed insights into the timing of volcanism and changes in the fossil and sedimentary records (3–11). One of the most extensive examples of this new proxy is the study by Percival et al. (12) in PNAS, which finds Hg anomalies in five different records (both marine and terrestrial and from both hemispheres) that span the end-Triassic mass extinction, demonstrating both the global extent of the increased Hg and the pulsed nature of the LIP volcanism. The end-Triassic extinction is one of the top five mass extinctions and is associated with major perturbations to the carbon cycle that are thought to be driven by the Central Atlantic Magmatic Province (CAMP) (13, 14). Percival et al. (12) build upon a study (11) of Hg and Hg isotopes in a single record that observed elevated Hg throughout the extinction and depauperate intervals with biotic recovery only occurring after Hg levels returned to background levels. Earlier studies demonstrated that Hg anomalies are associated with many of the mass extinctions (3–11) but were limited by questions about how representative single records were of increases in the global Hg pool, whether elevated Hg is linked to volcanism, or whether changes in Hg concentration were due to preservation or diagenetic effects. Increased concentrations of Hg in sediments can either be due to (i) an increase in the overall input of Hg to the atmospheric– ocean–terrestrial system or (ii) increased preservation of Hg within sediments either through increases in deposition via scavenging/absorption onto particles or post depositional migration of Hgwithin the sediments. The biogeochemical cycle of Hg is complicated (Fig. 1), with primary sources of Hgmaking up less than one-half of the Hg emissions to the atmosphere even in the modern world (15, 16). Most emissions are secondary sources, with Hg being reemitted after deposition from waters, soils, and biomass burning. Primary natural sources are dominated by volcanic sources, and the two largest primary anthropogenic sources are artisanal and small-scale gold mining and coal burning. Mercury is often emitted to the atmosphere in its reduced form, gaseous elemental Hg. Gaseous Hg is relatively stable and has a long residence time (∼0.5–1 y) with respect to atmospheric mixing, allowing it to be distributed globally. Removal of atmospheric gaseous Hg happens either by direct uptake and absorption or by oxidation to more particle reactive forms. Terrestrial systems remove Hg via uptake by plants and trees or by absorption onto organic matter in soils. Once in plants and soils, it is oxidized to Hg(II) species. Gaseous Hg can also be oxidized to Hg(II) species in the atmosphere that have short residence times (hours to weeks), which are deposited to terrestrial and aquatic systems via wet or dry deposition. One of the major complexities of the Hg cycle is that, after deposition, significant portions of the Hg can be reduced back to gaseous Hg and be emitted back to the atmosphere. Massive volcanism is a simple way to increase Hg input to the atmosphere–ocean–terrestrial system and perturb the carbon cycle at the same time, but other mechanisms could also result in simultaneous changes in Hg and the carbon cycle. Because Hg is stored in