Palaeoenvironment assessment of Pliocene Lom lignite (Bulgaria) from bitumen analysis and preparative off line thermochemolysis

The Dacian coal bearing province is located in NW Bulgaria. Coal formation is thought to be connected with desiccation of the Dacian Basin during a prolonged regression in the Late Pontian and Pliocene (Dacian–Romanian age). A Lom lignite sample was characterized via micropetrography, bulk chemical analysis, spectral methods and thermochemolysis. The pollen record was poor, suggesting coal formation under oxygen-rich conditions during peatification. Straight chain alkanes, alkan-2-ones, alkanols, des-A-triterpenoids, aromatized triterpenoids, aromatized hopane, hopene/hopanes and triterpenoids with a strong dominance of oleanane-derived structures were present in the extracted bitumen. The residue after extraction was subjected to preparative off line tetramethylammonium hydroxide (TMAH) thermochemolysis. The aliphatic products released were C22–C27 fatty acid (FA) methyl esters, i.e. α-methoxymethyl esters, C22–C28 even numbered α,ω-diacids dimethyl esters with a smooth distribution and traces of i- and ai-C15 acid methyl esters. Trapped and bound FAs were distinguished using tetraethylammonium acetate (TEAAc) treatment. The main thermochemolysis products were phenols structurally related to lignin. Vanillyl and syringyl units were produced in similar amounts, indicating nonwoody angiosperm vegetation as the main plant input to the palaeo-swamp, probably originating from marsh or grassland sources. The bitumen provided unequivocal information for an angiosperm contribution to the mire, while the results of the TMAH pyrolysis revealed a concomitant angiosperm and gymnosperm input. The biomarker assemblage makes the Lom lignite unique in Bulgaria, all the other Bulgarian Neogene lignites being dominated by coniferous vegetation, with only traces of angiosperm contributions.