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

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-bitum inous stage), coal quality differs significantly. Coals from low-lying mire s (e.g. Fohnsdorf) are characterized by significantly higher ash yields and sulfur contents than coals from raised mires (e.g. Leoben). Organic geoche mical and carbon isotope investigations were performed to assess the differ ences in facies. The Fohnsdorf coals are characterized by higher yields of soluble organic matter (SOM), slightly higher proportions of hydrocarbons i n the SOM, generally lower pristane / phytane ratios, and the occurrence of high C-org-normalized n-alkane concentrations. Only n-alkane patterns in t he Leoben samples show a marked odd over even predominance. These differenc es are indicative of a more intensive, dys- to anaerobic biochemical degrad ation of plant material within the Fohnsdorf mire. Terpenoid biomarkers cha racteristic of conifers and angiosperms were detected in the Leoben coals, whereas the latter are missing in the Fohnsdorf samples. In the Leoben coal s higher saturated to aromatic diterpenoid ratios were detected than in the Fohnsdorf coals. Because of similar rank, enhanced aromatisation in the Fo hnsdorf basin is probably related to microbial activity. Despite this, the Fohnsdorf coals are characterized by lower hopane concentrations. The resul ts imply that the aromatisation of terpenoid biomarkers is governed by the activity of anaerobic rather than aerobic bacteria. An overall negative cor relation between delta C-13 of organic matter and the degree of aromatisati on of diterpenoid biomarkers is observed. It is suggested that the carbon i sotopic composition results from different extents of microbial degradation of the biomass. The Fohnsdorf coals are rich in sulfur. High contents of c oalbed methane were encountered during mining, suggesting high activities o f anaerobic bacteria in a nearly neutral, sulfate-bearing (brackish) enviro nment of the low-lying mire and methanogenesis during or after coatificatio n, Sulfate reduction and methanogenesis are known to increase delta C-13 va lues of residual organic matter. (C) 2001 Elsevier Science Ltd. All rights reserved.