FOSSIL BACTERIAL CELLS IN EARLY MIOCENE SHALES – SYNSEDIMENTARY COMMUNITIES OR POSTSEDIMENTARY INVADERS?

Mineralized bacterial cells have been found in Early Miocene (Aquitanian) fossil-rich shales in the Central Paratethys Sea (Austria, Central Europe). Out of a 3-m-thick succession an 8-cm-thick, laminated rock sequence within a Konservat-Lagerstätte has been studied by high resolution sampling (one sample per 0.5-1 cm). A combination of SEM-imaging, qualitative EDX-analyses and geochemical analyses (TOC, S) was conducted in order to describe and interpret the preserved bacterial cells. The cells occur as single microspheres with a diameter of 1-2 µm or arranged to chains along filaments. Elemental composition of the cell walls indicates calcitic mineralization.

The data suggest a homogenous organic-rich and poorly oxygenated paleoenvironment. The mineralized cells are restricted to several centimetres of sediment above a huge sunfish fossil recovered from the rock. As no bacteria were found around other similarly well preserved fossils of cephalopods, bryozoans and bivalves in the same section it is unlikely that they represent a microbial community autochthonously bound to the decaying carcasses. These microbes are rather interpreted as postsedimentary invaders of the deposits and could therefore represent members of a (sub)fossil deep biosphere. Similar reports of allochthonous bacterial cells from the Middle Miocene Monterey Formation (California) further support the postsedimentary interpretation. In contrast, a synsedimentary community of anaerobic Fe- and sulphate reducing bacteria is indicated by the frequent occurrence of framboidal pyrite in the samples.

These time-averaged mixed bacterial assemblages call for careful consideration of a possible contamination by a much younger deep biosphere when interpreting bacterial fossils in sedimentary rocks.