“Stump Horizon” in the Bílina Mine (Most Basin, Czech Republic) — GC–MS, optical and electron microscopy in identification of wood biological origin

Abstract

Numerous coalified tree stumps remained preserved in-situ in the so-called “Stump Horizon” (the palaeontologic horizon No. 31), which represents clayey overburden of the main coal seam in the Bílina open cast mine in the Most Basin (Czech Republic). The petrological and chemical composition, palaeobotanical origin and preservation of 24 selected tree stumps were studied by optical and scanning electron microscopy and gas chromatography–mass spectrometry (GC–MS). Composition of the fossil wood is dominated by ulminite, particularly texto-ulminite B. Textinite forms up to 38vol.% in the decomposed tree stumps. Corpohuminite dominates in bark and root tissues. Partly gelified and deformed woody tissues contain both corpohuminite and resinite fillings. Random reflectance of ulminite ranges from 0.33% to 0.39% and carbon content from 49wt.% to 78wt.%. Samples represent pure woody material with small admixtures of clay minerals, siderite, and very rare pyrite. Observations by scanning electron microscopy revealed various levels of deformed secondary xylem with more or less swollen cell walls, conspicuous tracheids, uniseriate rays of a different height, and round or cylinder-like resin or corpohuminite bodies. According to our results and other published data the wood might belong to Glyptostroboxylon and Taxodioxylon genera that are supposed to belong to Glyptostrobus and Quasisequoia plants respectively, representatives of the coniferous family Cupressaceae. The biomarker composition in the extracts of the fossil wood includes sesquiterpenoids (α-cedrane, drimane, eudesmane), diterpenoids (abietane, fichtelite, 16α(H)-phyllocladane) and their degraded compounds. The terpenoids are derived from precursors produced by the source plants and microorganisms. The terpenoid signatures support a relationship to the Cupressaceae family with input of microbial species. These characteristics were identical for all studied samples. Significant variations have been observed in sesquiterpenoid α-cedrane and diterpenoid 16α(H)-phyllocladane contents.