The role of microorganisms in the formation of a stalactite in Botovskaya Cave, Siberia – paleoenvironmental implications

摘要

Calcitic speleothems in caves can form through abiogenic or biogenicprocesses, or through a combination of both. Many issues conspire to makethe assessment of biogenicity difficult, especially when focusing on oldspeleothem deposits. This study reports on a multiproxy analysis of a Siberianstalactite, combining high-resolution microscopy, isotope geochemistry andmicrobially enhanced mineral precipitation laboratory experiments.The contact between growth layers in a stalactite exhibits a biogenicisotopic signature; coupled with morphological evidence, this supports amicrobial origin of calcite crystals. SIMS δ13C data suggest thatmicrobially mediated speleothem formation occurred repeatedly at shortintervals before abiotic precipitation took over. The studied stalactite alsocontains iron and manganese oxides that have been mediated by microbialactivity through extracellular polymeric substance (EPS)-influencedorganomineralization processes. The latter reflect paleoenvironmentalchanges that occurred more than 500 000 yr ago, possibly related to thepresence of a peat bog above the cave at that time.Microbial activity can initiate calcite deposition in the aphotic zone ofcaves before inorganic precipitation of speleothem carbonates. This studyhighlights the importance of microbially induced fractionation that canresult in large negative δ13C excursions. The microscalebiogeochemical processes imply that microbial activity has only negligibleeffects on the bulk δ13C signature in speleothems, which ismore strongly affected by CO₂ degassing and the host rock signature.