Organic geochemistry of Duckmantian (Pennsylvanian) coals from the Ruhr Basin, western Germany

Abstract

The molecular composition of 58 Duckmantian Ruhr coals and adjacent sedimentary rocks has been analyzed using organic-geochemical investigations representing a maturity range from 0.74 to 0.98% vitrinite reflectance. In addition eight samples of higher maturity (1.46 to 3.55% vitrinite reflectance) were studied for comparison. The composition of coals depends primarily on plant input into the peat swamps, but also to a large extent on thermal maturity. Several maturity-related parameters have been applied in this study such as MPI, DMAI, and diverse hopanoid-based maturity ratios. Especially diamantane-based parameters proved to be excellent for characterizing high rank samples, whereas hopane- and phenanthrene-based parameters are well-suited for the lower maturity range.Various specific biomarkers reflect the composition of the Duckmantian coals concerning the floral differences and floral shift during the Westphalian. The dominating n-alkanes, hopanes, and bicyclic sesquiterpanes show only marginal differences and reveal a quite homogenous floral composition within the six coal seams. Tetracyclic diterpanes, as approved markers for modern as well as Paleozoic conifers and "pre-gymnosperms", have been identified in all six coal seams, but in low concentration (one order of magnitude lower than bicyclic sesquiterpanes). A slight increase of the tetracyclic diterpane ent-beyerane in the younger seams indicates an increase in abundance of "pre-gymnosperms" (cordaites) during the Duckmantian. Significantly increased proportions of diverse compounds like bicyclic sesquiterpanes (eudesmane) or tetracyclic terpanes clearly testify an origin from higher land plants. The major part of the Duckmantian coal swamps biomass with its vitrinite rich coals originated presumably from barks of arborescent lycopods. High concentrations of hopanes mirror the input of bacterial organic matter and a strong microbial reworking of the deposited terrestrial plant organic matter.