| Abstract： |
The geochemical characteristics of sulfur and nitrogen contents and isotopic compositions in a special coal, the Late Permian bark coal of southern China, were investigated by systematic petrographic and geochemical methods. The results show that barkinite is the major maceral component in the test samples, ranging from 12.4% to 76.4%. Increasing S-org and decreasing S-py contents with rising barkinite contents imply that the precursor plants of bark coal have a significant influence on the sulfur occurrence. The delta S-34 values of organic and pyritic sulfur for bark coal vary from-12.8 parts per thousand to +10.9 parts per thousand and -5.4 parts per thousand to +11.5 parts per thousand, respectively. High sulfur concentration (>2%) but rather variable 834S(org) values for Mingshan and Changguang coal seams suggest different sources of biogenic and non-biogenic secondary sulfur, respectively. Related delta N-15 values in bark coal range from 1.6 parts per thousand to 5.6 parts per thousand, with an average of 3.0 parts per thousand. The delta N-15 values within the same coal seam profile display inhomogeneous features, with a maximum deviation of 2.5 parts per thousand, indicating some degree of microbial degradation during the peat accumulation stage. According to the distributions of delta N-15 vs delta S-34(org) and S-py, the three bark coal seams MS, CG1, and CG2 may have resulted from different coal-forming systems. For the MS coal seam, negative delta S-34(org) values and large positive delta N-15 values resulted from incomplete denitrification coexisting with strong bacterial sulfate reduction. As the coal-forming system altered from an open to a closed state and the water column changed towards anoxic conditions, N-fixation and nitrification, as well as the suppression of bacterial sulfate reduction, occurred for the CG coal seams, causing positive delta S-34(org) values but gradually negative delta N-15 values in the bark coal of these seams. |
