1CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
2Observatoire Midi-Pyrénées, Laboratoire Géosciences Environnement Toulouse, CNRS/IRD/Université de Toulouse, France
Abstract(#br)Coal deposits were important natural Hg sinks in the Late Paleozoic and Middle-Late Mesozoic. Coal combustion since the industrial revolution has emitted more than 35,000t of Hg into the atmosphere. Environmental geochemistry research on Hg in coal is essential to understand the natural Hg cycle throughout Earth history and the present-day Hg cycle under human perturbation. In this review, we summarize the methods of Hg isotope measurement in coals, and compile recently published Hg isotope data of >200 world coal samples according to their locations, formation periods (Carboniferous, Permian, Jurassic, Cretaceous, Early-Middle Cenozoic) and ranks (anthracite, bituminous coal, subbituminous coal, lignite), and illustrate the controls of... sources and biogeochemical processes on Hg isotope compositions in coal deposits. Up to 4.7‰ variation (−3.90 to 0.77‰) in mass dependent fractionation (MDF, represented by δ 202 Hg) and 1.0‰ variation (−0.63 to 0.34‰) in mass independent fractionation (MIF, represented by Δ 199 Hg) are observed in world coal deposit, with an average value of −1.16±0.79‰ (1SD) and −0.11±0.18‰ (1SD) for δ 202 Hg and Δ 199 Hg, respectively. We find that coal δ 202 Hg and Δ 199 Hg are broadly controlled by two source materials: terrestrial plants (biogenic Hg) and crustal rocks (geogenic Hg), accounting for 37–46% and 54–63% of Hg in coals. No clear trends are seen in neither δ 202 Hg vs. coal-forming periods nor δ 202 Hg vs. coal ranks, which we attribute to important overlapping of δ 202 Hg in the source materials of different coal-forming periods and the multidirectional Hg MDF during coalification. Interestingly, a step-wise increase in total Hg and a disappearance of Δ 199 Hg are observed along with increasing coal rank, suggesting the addition of hydrothermal Hg into high-rank coals. The hydrothermal fluids not only upgraded the coal ranks, but also increased the Hg concentrations and Δ 199 Hg of coals. In addition, the addition of hydrothermal Hg into coal deposits was also partly responsible for the comparatively higher total Hg and Δ 199 Hg in the Late Paleozoic coals relative to the Mesozoic and Cenozoic coals. The co-variation of atmospheric oxygen levels and Δ 199 Hg of coals at different coal-forming time widows suggests that the removal of biogenic Hg in response to widespread swamp fires may have increased the Δ 199 Hg in Late Paleozoic coals as well. Our study indicates that coal Hg isotopes provide new insights into the biogeochemical cycle of Hg in coal deposits, and that Δ 199 Hg is a robust indicator to trace Hg addition/removal in coal deposits and biogeochemical processes (magmatic intrusion, hydrothermal fluids penetration, burning of coal swamp) occurring during coal deposition.