Organic geochemical and stable carbon isotopic investigation of coals formed in low-lying and raised mires within the Eastern Alps (Austria)

Abstract

Up to 16 m thick coal seams formed in Miocene pull-apart basins within the Eastern Alps in low-lying and raised mires. Despite similar rank (sub-bituminous stage), coal quality differs significantly. Coals from low-lying mires (e.g. Fohnsdorf) are characterized by significantly higher ash yields and sulfur contents than coals from raised mires (e.g. Leoben). Organic geochemical and carbon isotope investigations were performed to assess the differences in facies. The Fohnsdorf coals are characterized by higher yields of soluble organic matter (SOM), slightly higher proportions of hydrocarbons in the SOM, generally lower pristane / phytane ratios, and the occurrence of high C org-normalized n-alkane concentrations. Only n-alkane patterns in the Leoben samples show a marked odd over even predominance. These differences are indicative of a more intensive, dys- to anaerobic biochemical degradation of plant material within the Fohnsdorf mire. Terpenoid biomarkers characteristic of conifers and angiosperms were detected in the Leoben coals, whereas the latter are missing in the Fohnsdorf samples. In the Leoben coals higher saturated to aromatic diterpenoid ratios were detected than in the Fohnsdorf coals. Because of similar rank, enhanced aromatisation in the Fohnsdorf basin is probably related to microbial activity. Despite this, the Fohnsdorf coals are characterized by lower hopane concentrations. The results imply that the aromatisation of terpenoid biomarkers is governed by the activity of anaerobic rather than aerobic bacteria. An overall negative correlation between δ 13C of organic matter and the degree of aromatisation of diterpenoid biomarkers is observed. It is suggested that the carbon isotopic composition results from different extents of microbial degradation of the biomass. The Fohnsdorf coals are rich in sulfur. High contents of coalbed methane were encountered during mining, suggesting high activities of anaerobic bacteria in a nearly neutral, sulfate-bearing (brackish) environment of the low-lying mire and methanogenesis during or after coalification. Sulfate-reduction and methanogenesis are known to increase δ 13C values of residual organic matter.