Anomalous fractionation of mercury isotopes in the Late Archean atmosphere

This work was funded by a Natural Environment Research Council (NERC) Fellowship NE/H016805/2 and Standard Grant NE/J023485/2 (to A.L.Z.). R.Y. was funded by the Chinese Academy of Sciences through the Hundred Talent Plan. G.J.I. recognizes continued support from R. Summons under the auspices of the Simons Collaboration on the Origin of Life. We thank J. Kirschvink, J. Grotzinger, A. Knoll, and the Agouron Institute for organizing and funding the Agouron Drilling Project, and the Council for Geoscience in South Africa, specifically those at the National Core Library in Donkerhoek, for facilitating access to the core materials.Earth’s surface underwent a dramatic transition ~2.3 billion years ago when atmospheric oxygen first accumulated during the Great Oxidation Event, but the detailed composition of the reducing early atmosphere is not well known. Here we develop mercury (Hg) stable isotopes as a proxy for paleoatmospheric chemistry and use Hg isotope data from 2.5 billion-year-old sedimentary rocks to examine changes in the Late Archean atmosphere immediately prior to the Great Oxidation Event. These sediments preserve evidence of strong photochemical transformations of mercury in the absence of molecular oxygen. In addition, these geochemical records combined with previously published multi-proxy data support a vital role for methane in Earth’s early atmosphere.Publisher PDFPeer reviewe

Sulfur and mercury MIF suggest volcanic contributions to Earth’s atmosphere at 2.7 Ga

This study received funding from a Natural Environment Research Council Standard Grant NE/M001156/1 (ALZ, EGN), and from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Grant 678812 to MWC).The Archean eon is associated with large-scale changes in Earth’s geosphere and biosphere, including the onset of plate tectonics and the expansion of oxygenic photosynthesis, although the full impacts of these changes on the atmosphere remain unclear. Here we present coupled records of mass independent fractionation of sulfur (S-MIF) and mercury (Hg-MIF) isotopes from well preserved sediments of the ∼2.7 billion year old (Ga) Manjeri Formation, Belingwe Greenstone Belt, Zimbabwe. These palaeoatmospheric proxies record different trends for S-MIF and odd number Hg-MIF versus even number Hg-MIF, providing novel constraints on atmospheric chemistry during this time. S-MIF and odd number Hg-MIF values are muted in comparison to values preserved in later Archean sediments, representing a combination of enhanced volcanic input and local mixing. Even number Hg-MIF is absent from these sediments, consistent with complete photo-oxidation of gaseous Hg0, which could have been driven by increased halogen emissions from arc volcanism. When considered within a global geodynamic context, these MIF data suggest an important role for subduction zone-related volcanism associated with early plate tectonics in modulating the ∼2.7 Ga atmosphere.Publisher PDFPeer reviewe

Enhanced volcanic activity and long-term warmth in the middle Eocene revealed by mercury and osmium isotopes from IODP Expedition 369 Site U1514

Rapid plate reorganization may have influenced global climate during the Eocene; however, its linkage remains poorly constrained, particularly during the middle Eocene. To elucidate this tectonic–climatic relationship, here, we conducted a comprehensive analysis based on high-resolution mercury (Hg) and osmium (Os) abundance and isotope data obtained from the complete Eocene sedimentary sequence of Site U1514, drilled in the Mentelle Basin off southwest Australia. The Hg signals in this sedimentary sequence, which are characterized by significantly high enrichment and insignificant mass-independent fractionation (Δ199Hg) signal, confirm that the middle Eocene (∼45–38 Ma) was a period of persistent, increased volcanism, accompanied by intense tectonic activity. In particular, a remarkable seafloor volcanic eruption persisted for approximately 1.5 million years (∼42.0–40.5 Ma), immediately preceding the Middle Eocene Climate Optimum (MECO). Contemporaneously, the trends toward a slightly more radiogenic seawater 187Os/188Os (Osi) composition denote the prevalence of intensified continental weathering under a warm, humid climate during the middle Eocene, a phenomenon particularly evident during the MECO. Importantly, the Hg and Os records from Site U1514 reveal the occurrence of a multi-million-year warming reversal amid the long-term Eocene cooling trend, which likely contributed to significant CO2 reduction during the late Eocene. These findings significantly enhance our understanding of Eocene climate dynamics, which are fundamentally linked to intensive tectonic-driven volcanic activity and associated continental chemical weathering

Bioaccumulation of Hg in rice leaf facilitates selenium bioaccumulation in rice (Oryza sativa L.) leaf in the Wanshan mercury mine

Mercury (Hg) bioaccumulation in rice poses a health issue for rice consumers. In rice paddies, selenium (Se) can decrease the bioavailability of Hg through forming the less bioavailable Hg selenides (HgSe) in soil. Rice leaves can directly uptake a substantial amount of elemental Hg from the atmosphere, however, whether the bioaccumulation of Hg in rice leaves can affect the bioaccumulation of Se in rice plants is not known. Here, we conducted field and controlled studies to investigate the bioaccumulation of Hg and Se in the rice-soil system. In the field study, we observed a significantly positive correlation between Hg concentrations and BAFs of Se in rice leaves (r2 = 0.60, p < 0.01) collected from the Wanshan Mercury Mine, SW China, suggesting that the bioaccumulation of atmospheric Hg in rice leaves can facilitate the uptake of soil Se, perhaps through the formation of Hg-Se complex in rice leaves. This conclusion was supported by the controlled study, which observed significantly higher concentrations and BAFs of Se in rice leaf at a high atmospheric Hg site at WMM, compared to a low atmospheric Hg site in Guiyang, SW China

Sequestering Carbon in China’s Forest Ecosystems: Potential and Challenges

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