Sulfur and nitrogen isotopic composition of Late Permian bark coals: Source identification and associated environmental assessment

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 Sorg and decreasing Spy contents with rising barkinite contents imply that the precursor plants of bark coal have a significant influence on the sulfur occurrence. The δ34S values of organic and pyritic sulfur for bark coal vary from −12.8‰ to +10.9‰ and −5.4‰ to +11.5‰, respectively. High sulfur concentration (>2%) but rather variable δ34Sorg values for Mingshan and Changguang coal seams suggest different sources of biogenic and non-biogenic secondary sulfur, respectively. Related δ15N values in bark coal range from 1.6‰ to 5.6‰, with an average of 3.0‰. The δ15N values within the same coal seam profile display inhomogeneous features, with a maximum deviation of 2.5‰, indicating some degree of microbial degradation during the peat accumulation stage. According to the distributions of δ15N vs δ34Sorg and Spy, the three bark coal seams MS, CG1, and CG2 may have resulted from different coal-forming systems. For the MS coal seam, negative δ34Sorg values and large positive δ15N 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 δ34Sorg values but gradually negative δ15N values in the bark coal of these seams.