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1. The Mercury Isotopic Composition of Earth's Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust

   https://doi.org/10.1029/2021GL094301  

   Moynier, Frédéric; Jackson, Matthew G.; Zhang, Ke; Cai, Hongming; Halldórsson, Sæmundur Ari; Pik, Raphael; Day, James M. D.; Chen, Jiubin (September 2021, Geophysical Research Letters)

   Abstract The element mercury (Hg) can develop large mass‐independent fractionation (MIF) (Δ¹⁹⁹Hg) due to photo‐chemical reactions at Earth's surface. This results in globally negative Δ¹⁹⁹Hg for terrestrial sub‐aerially‐derived materials and positive Δ¹⁹⁹Hg for sub‐aqueously‐derived marine sediments. The mantle composition least affected by crustal recycling is estimated from high‐³He/⁴He lavas from Samoa and Iceland, providing an average of Δ¹⁹⁹Hg = 0.00 ± 0.10, Δ²⁰¹Hg = −0.02 ± 0.0.09,δ²⁰²Hg = −1.7 ± 1.2; 2SD,N = 11. By comparison, a HIMU‐type lava from Tubuai exhibits positive Δ¹⁹⁹Hg, consistent with altered oceanic crust in its mantle source. A Samoan (EM2) lava has negative Δ¹⁹⁹Hg reflecting incorporation of continental crust materials into its source. Three Pitcairn lavas exhibit positive Δ¹⁹⁹Hg which correlate with⁸⁷Sr/⁸⁶Sr, consistent with variable proportions of continental (low Δ¹⁹⁹Hg and high⁸⁷Sr/⁸⁶Sr) and oceanic (high Δ¹⁹⁹Hg and low⁸⁷Sr/⁸⁶Sr) crustal material in their mantle sources. These observations indicate that MIF signatures offer a powerful tool for examining atmosphere‐deep Earth interactions. 

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2. Mercury isotope evidence for marine photic zone euxinia across the end-Permian mass extinction

   https://doi.org/10.1038/s43247-023-00821-6  

   Sun, Ruoyu; Liu, Yi; Sonke, Jeroen E.; Feifei, Zhang; Zhao, Yaqiu; Zhang, Yonggen; Chen, Jiubin; Liu, Cong-Qiang; Shen, Shuzhong; Anbar, Ariel D.; et al (May 2023, Communications Earth & Environment)

   Abstract The driving forces, kill and recovery mechanisms for the end-Permian mass extinction (EPME), the largest Phanerozoic biological crisis, are under debate. Sedimentary records of mercury enrichment and mercury isotopes have suggested the impact of volcanism on the EPME, yet the causes of mercury enrichment and isotope variations remain controversial. Here, we model mercury isotope variations across the EPME to quantitatively assess the effects of volcanism, terrestrial erosion and photic zone euxinia (PZE, toxic, sulfide-rich conditions). Our numerical model shows that while large-scale volcanism remains the main driver of widespread mercury enrichment, the negative shifts of Δ¹⁹⁹Hg isotope signature across the EPME cannot be fully explained by volcanism or terrestrial erosion as proposed before, but require additional fractionation by marine mercury photoreduction under enhanced PZE conditions. Thus our model provides further evidence for widespread and prolonged PZE as a key kill mechanism for both the EPME and the impeded recovery afterward. 

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3. Impacts of Volcanic Emissions on the Global Biogeochemical Mercury Cycle: Insights From Satellite Observations and Chemical Transport Modeling

   https://doi.org/10.1029/2023GL104667  

   Geyman, Benjamin M.; Thackray, Colin P.; Jacob, Daniel J.; Sunderland, Elsie M. (November 2023, Geophysical Research Letters)

   Abstract Volcanism is the largest natural source of mercury (Hg) to the biosphere. However, past Hg emission estimates have varied by three orders of magnitude. Here, we present an updated central estimate and interquartile range (232 Mg a⁻¹; IQR: 170–336 Mg a⁻¹) for modern volcanic Hg emissions based on advances in satellite remote sensing of sulfur dioxide (SO₂) and an improved method for considering uncertainty in Hg:SO₂emissions ratios. Atmospheric modeling shows the influence of volcanic Hg on surface atmospheric concentrations in the extratropical Northern Hemisphere is 1.8 times higher than in the Southern Hemisphere. Spatiotemporal variability in volcanic Hg emissions may obscure atmospheric trends forced by anthropogenic emissions at some locations. This should be considered when selecting monitoring sites to inform global regulatory actions. Volcanic emission estimates from this work suggest the pre‐anthropogenic global atmospheric Hg reservoir was 580 Mg, 7‐fold lower than in 2015 (4,000 Mg). 

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4. The Signature of Metasomatized Subcontinental Lithospheric Mantle in the Basaltic Magmatism of the Payenia Volcanic Province, Argentina

   https://doi.org/10.1029/2021GC010071  

   Chilson‐Parks, Benjamin H.; Calabozo, Fernando M.; Saal, Alberto E.; Wang, Zhengrong; Mallick, Soumen; Petrinovic, Ivan A.; Frey, Frederick A. (January 2022, Geochemistry, Geophysics, Geosystems)

   Abstract The Payenia region of Argentina (34.5–38°S) is a large Pliocene‐Quaternary volcanic province of basaltic compositions in the Andean Cordillera foothills representing the northernmost extent of back‐arc volcanism in the Andean Southern Volcanic Zone (SVZ). Although the chemical diversity of the Payenia basalts has been characterized previously, the processes and sources responsible for such variation remain controversial. Here, we report new whole‐rock major and trace element concentrations, Sr‐, Nd‐, Hf‐, and Pb‐isotope ratios and high‐precision olivine oxygen‐isotope ratios in a suite of 35 alkaline basalts from Payenia. These lavas have major and trace elements that define a compositional range from arc‐influenced to intraplate signature. Variable crustal contamination and/or recent slab‐derived inputs inadequately account for elemental and isotopic systematics and spatial compositional variations of Payenia lavas. We present a simple forward model indicating that early metasomatism and subsequent melting of the metasomatized subcontinental lithospheric mantle (SCLM) has significantly contributed to the Payenia lava compositional range. Isotopic ingrowth calculations of radiogenic Sr, Nd, Hf, and Pb suggest that the SCLM metasomatism occurred at 50–150 Ma, consistent with the timing of the breakup of Gondwana and the development of the proto‐Pacific Andean arc. Variations in δ¹⁸O_olivinevalues from modeled melts indicate that the metasomatism and melting within the SCLM can fractionate oxygen isotopes even when the metasomatizing melt has MORB‐like δ¹⁸O values, providing a different explanation for the low‐δ¹⁸O signatures observed in continental arc settings. 

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5. Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica‐depleted Paleozoic oceans

   https://doi.org/10.1111/gbi.12449  

   Trower, Elizabeth J.; Strauss, Justin V.; Sperling, Erik A.; Fischer, Woodward W. (May 2021, Geobiology)

   Abstract The Phanerozoic Eon marked a major transition from marine silica deposition exclusively via abiotic pathways to a system dominated by biogenic silica sedimentation. For decades, prevailing ideas predicted this abiotic‐to‐biogenic transition were marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ³⁰Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms. We undertook 446 ion microprobe analyses of δ³⁰Si and δ¹⁸O of sponge spicules and radiolarians from Ordovician–Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges living in marine slope and basinal environments displayed small Si isotope fractionations relative to coeval radiolarians. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we found that the concentration of dissolved silica in seawater was less than ~150 μM during early Paleozoic time—a value that is significantly lower than previous estimates. We posit that the topology of the early Paleozoic marine silica cycle resembled that of modern oceans much more closely than previously assumed. 

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