| Abstract: | Kimmeridgian bituminous laminites from Orbagnoux (France) contain abundant amorphous organic matter (AOM). Previous studies have shown that the vulcanization pathway was the dominant preservation mechanism of AOM in these laminites, and led to its structureless aspect (a process called amorphization) at the nanoscale. In contrast, new observations in scanning electron microscopy and transmission electron microscopy demonstrate that this AOM exhibits typical cyanobacterial structures (exopolymeric substances, filamentous and coccoid bacteria) and ultralaminae. This identification is supported by a comparison with a recent cyanobacterial biofilm considered as an analogue. Moreover, this comparison demonstrates that ultralaminae in the Orbagnoux environment cannot solely be attributed to microalgal cell walls, but also to constituents of cyanobacteria. The microscopic identification of a ubiquitous cyanobacterial imprint demonstrates that the selective preservation pathway has been largely underestimated in Orbagnoux AOM and/or that the vulcanization process does not lead to the amorphization of organic matter automatically. |
