The role of microorganisms on the formation of a stalactite in Botovskaya Cave, Siberia -- palaeoenvironmental implications.

Calcitic speleothems in caves can form through abiogenic, biogenic, or a combination of both processes. Many issues conspire to make the assessment of biogenicity difficult, especially when focusing on old speleothem deposits. This study reports a multi-proxy analysis of a Siberian stalactite, combining high-resolution microscopy, isotope geochemistry and microbially enhanced mineral precipitation laboratory experiments. The contact between growth layers in a stalactite exhibits a biogenic isotopic signature; coupled with morphological evidence this supports a microbial origin of calcite crystals. SIMS δ¹³C data suggest that microbially mediated speleothem formation occurred repeatedly for short intervals before abiotic precipitation took over. The studied stalactite also contains iron and manganese oxides that have been mediated by microbial activity through extracellular polymeric substances (EPS)-influenced organomineralization processes. The latter reflect palaeoenvironmental changes that occurred more than 500 000 yr ago, possibly related to the presence of a peat bog above the cave at that time. Microbial activity can initiate calcite deposition in the aphotic zone of caves before inorganic precipitation of speleothem carbonates. This study highlights the importance of microbially induced fractionation that can result in large negative δ¹³C excursions. The micro-scale biogeochemical processes imply that microbial activity has only negligible effects on the bulk δ¹³C signature in speleothems, which is more strongly affected by CO₂ degassing and the hostrock signature [ABSTRACT FROM AUTHOR]