Biogeochemical controls on mercury stable isotope compositions of world coal deposits: A review

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 35,000 t 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 parts per thousand variation (-3.90 to 0.77 parts per thousand) in mass dependent fractionation (MDF, represented by delta Hg-202) and 1.0 parts per thousand variation (-0.63 to 034 parts per thousand in mass independent fractionation (MIF, represented by Delta Hg-199) are observed in world coal deposit, with an average value of -1.16 +/- 0.79 parts per thousand (1SD) and -0.11 +/- 0.18 parts per thousand (1SD) for delta Hg-202 and Delta Hg-199, respectively. We find that coal delta Hg-202 and Delta Hg-199 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 delta Hg-202 vs. coal-forming periods nor delta Hg-202 vs. coal ranks, which we attribute to important overlapping of delta Hg-202 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 Delta Hg-199 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 Delta Hg-199 of coals. In addition, the addition of hydrothermal Hg into coal deposits was also partly responsible for the comparatively higher total Hg and Delta Hg-199 in the Late Paleozoic coals relative to the Mesozoic and Cenozoic coals. The co-variation of atmospheric oxygen levels and Delta Hg-199 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 Delta Hg-199 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 Delta Hg-199 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. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.