Bridget A. Bergquist

Mercury anomalies and the timing of biotic recovery following the end-Triassic mass extinction.

The end-Triassic mass extinction overlapped with the eruption of the Central Atlantic Magmatic Province (CAMP), and release of CO2 and other volcanic volatiles has been implicated in the extinction. However, the timing of marine biotic recovery versus CAMP eruptions remains uncertain. Here we use Hg concentrations and isotopes as indicators of CAMP volcanism in continental shelf sediments, the primary archive of faunal data. In Triassic–Jurassic strata, Muller Canyon, Nevada, Hg levels rise in the extinction interval, peak before the appearance of the first Jurassic ammonite, remain above background in association with a depauperate fauna, and fall to pre-extinction levels during significant pelagic and benthic faunal recovery. Hg isotopes display no significant mass independent fractionation within the extinction and depauperate intervals, consistent with a volcanic origin for the Hg. The Hg and palaeontological evidence from the same archive indicate that significant biotic recovery did not begin until CAMP eruptions ceased.

Mercury Isotope Fractionation during Aqueous Photoreduction of Monomethylmercury in the Presence of Dissolved Organic Matter

Monomethylmercury (MMHg) is a toxic pollutant that bioaccumulates in aquatic food webs. A major mechanism that limits MMHg uptake by biota is photodemethylation in surface waters. Recently, the extent of mass-independent fractionation (MIF) of Hg isotopes preserved in fish is being used to quantify this MMHg sink. Here, the effects of different types and amounts of DOM on Hg MIF during MMHg photodemethylation were investigated to assess how variable MIF enrichment factors may be with respect to changing DOM binding sites. From experiments conducted with varying amounts of reduced organic sulfur (S(red)-DOM), the extent and signature of MIF is likely dependent on whether MMHg is dominantly bound to S(red)-DOM. Similar enrichment factors were observed for low MMHg:S(red)-DOM experiments, where S(red)-DOM was in far excess of MMHg. In contrast, significantly lower and variable enrichment factors were observed for experiments with higher MMHg:S(red)-DOM ratios. Additionally the relationship between the two odd Hg isotopes that display MIF (Δ(199)Hg/Δ(201)Hg) was consistent for the low MMHg:S(red)-DOM experiments, while lower Δ(199)Hg/Δ(201)Hg relationships were observed for the higher MMHg:S(red)-DOM experiments. These results suggest that both the extent and signature of MMHg MIF are sensitive to different ligands that bind MMHg in nature.

Mercury isotope compositions across North American forests

Forest biomass and soils represent some of the largest reservoirs of actively cycling mercury (Hg) on Earth, but many uncertainties exist regarding the source and fate of Hg in forest ecosystems. We systematically characterized stable isotope compositions of Hg in foliage, litter, and mineral soil horizons across 10 forest sites in the contiguous United States. The mass independent isotope signatures in all forest depth profiles are more consistent with those of atmospheric Hg(0) than those of atmospheric Hg(II), indicating that atmospheric Hg(0) is the larger source of Hg to forest ecosystems. Within litter horizons, we observed significant enrichment in Hg concentration and heavier isotopes along the depth, which we hypothesize to result from additional deposition of atmospheric Hg(0) during litter decomposition. Furthermore, Hg isotope signatures in mineral soils closely resemble those of the overlying litter horizons suggesting incorporation of Hg from litter as a key source of soil Hg. The spatial distribution of Hg isotope compositions in mineral soils across all sites is modeled by isotopic mixing assuming atmospheric Hg(II), atmospheric Hg(0) and geogenic Hg as major sources. This model shows that northern sites with higher precipitation tend to have higher atmospheric Hg(0) deposition than other sites, whereas drier sites in the western U.S. tend to have higher atmospheric Hg(II) deposition than the rest. We attribute these differences primarily to the higher litterfall Hg input at northern wetter sites due to increased plant productivity by precipitation. These results allow for a better understanding of Hg cycling across the atmosphere-forest-soil interface.

Mercury Stable Isotopes in Ornithogenic Deposits As Tracers of Historical Cycling of Mercury in Ross Sea, Antarctica.

Production of methylmercury (MeHg) in ocean waters and its bioaccumulation in marine organisms are critical processes controlling the fate and toxicity of mercury (Hg). However, these processes are not well understood in the Antarctic, where high levels of MeHg are observed in the subsurface ocean (100-1000 m). We explored the use of Hg stable isotope compositions in historical and modern biological deposits as a new approach for discerning Hg sources and tracing MeHg cycling in the ocean and bioaccumulation in marine biota. We found similar mass independent isotope fractionation (MIF) of Hg between a sediment profile containing historical penguin and seal feces deposits from coastal Antarctica and modern penguin and seal feces, suggesting that penguin and seal feces were the dominant sources of Hg to the sediments at different time periods. Furthermore, sediments dominated by seal feces displayed a significantly lower MIF slope (Δ(199)Hg/Δ(201)Hg) than those dominated by penguin feces despite similar extents of MIF. Since seals forage at greater depths (>400 m) than penguins (<100 m), the high MIF values and lower Δ(199)Hg/Δ(201)Hg in seal feces suggest that a significant fraction of MeHg accumulated by seals was produced in situ in the subsurface ocean from residual inorganic Hg(II) that sank from the euphotic zone after partial photoreduction. Our results suggest that in situ Hg methylation can be an important source of MeHg for marine biota, and Hg isotope compositions in biological archives can be valuable tracers of MeHg cycling.

Effects of mercury and thallium concentrations on high precision determination of mercury isotopic composition by Neptune Plus multiple collector inductively coupled plasma mass spectrometry

Thallium (Tl) has been widely used as an internal standard for mass bias correction during high precision mercury (Hg) isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). However, a recent study by Georg and Newman indicated the potential for Hg hydride formation (HgHx, x = 1, 2) during Hg isotope measurements using an X skimmer cone with a Neptune Plus MC-ICP-MS. Mercury hydride formation could result in an artificial change in 205Tl/203Tl. Due to this observation, the applicability of using Tl as an internal standard for instrumental mass bias correction during high precision Hg isotope measurements has been questioned. In this study, using an adapted gas/liquid phase separator for Hg introduction and the NIST SRM 997 Tl standard for mass bias correction, mercury isotope measurements were performed using a Neptune Plus MC-ICP-MS. While we confirm Georg and Newmans observations, we show that Hg hydride formation is less important when Hg isotope measurements are conducted with high Tl and low Hg concentrations. With careful sample-standard bracketing (with Hg concentration matching within 10%), we demonstrate that measuring 20 to 50 ng mL−1 of Tl and 0.5 to 3.0 ng mL−1 of Hg, high precision Hg isotope ratio measurements are achievable. We caution researchers using other Hg inlet systems to recognize the importance of Hg and Tl concentrations and encourage the optimization of these values during their Hg isotope measurements.

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 … [↵][1]1Email: bergquist{at}es.utoronto.ca. [1]: #xref-corresp-1-1

An investigation of mercury sources in the Puyango-Tumbes River: Using stable Hg isotopes to characterize transboundary Hg pollution

Mercury (Hg) concentrations and stable isotopes along with other trace metals were examined in environmental samples from Ecuador and Perus shared Puyango-Tumbes River in order to determine the extent to which artisanal- and small-scale gold mining (ASGM) in Portovelo-Zaruma, Ecuador contributes to Hg pollution in the downstream aquatic ecosystem. Prior studies investigated the relationship between ASGM activities and downstream Hg pollution relying primarily on Hg concentration data. In this study, Hg isotopes revealed an isotopically heavy Hg signature with negligible mass independent fractionation (MIF) in downstream sediments, which was consistent with the signature observed in the ASGM source endmember. This signature was traced as far as ∼120 km downstream of Portovelo-Zaruma, demonstrating that Hg stable isotopes can be used as a tool to fingerprint and trace sources of Hg over vast distances in freshwater environments. The success of Hg isotopes as a source tracer in fresh waters is largely due to the particle-reactive nature of Hg. Furthermore, the magnitude and extent of downstream Hg, lead, copper and zinc contamination coupled with the Hg isotopes suggest that it is unlikely that the smaller artisanal-scale activities, which do not use cyanidation, are responsible for the pollution. More likely it is the scale of ores processed and the cyanide leaching, which can release other metals and enhance Hg transport, used during small-scale gold mining that is responsible. Thus, although artisanal- and small-scale gold mining occur in tandem in Portovelo-Zaruma, a distinction should be made between these two activities.