BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-10-6115-2013 The role of microorganisms in the formation of a stalactite in Botovskaya Cave, Siberia – paleoenvironmental implications Pacton M. 1 6 Breitenbach S. F. M. 1 Lechleitner F. A. https://orcid.org/0000-0002-1697-0429 1 Vaks A. 2 Rollion-Bard C. 3 Gutareva O. S. 4 Osintcev A. V. 5 Vasconcelos C. 1 Geological Institute, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK Centre de Recherches Pétrographiques et Géochimiques, UMR 7358, Université de Lorraine, 54500 Vandoeuvre-lès-Nancy, France Institute of the Earth's Crust, Russian Academy of Sciences, Siberian Branch, 128 Lermontova Street, Irkutsk 664033, Russia Arabica Speleological Club, Mamin-Sibiryak Street, P.O. Box 350, Irkutsk 554082, Russia now at: Laboratoire de Géologie de Lyon : Terre, Planètes, Environnement Campus de la Doua, Universite Lyon 1 2 Rue Raphael Dubois, 69622 Villeurbanne Cedex, France 27 09 2013 10 9 6115 6130 Copyright: © 2013 M. Pacton et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/10/6115/2013/bg-10-6115-2013.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/10/6115/2013/bg-10-6115-2013.pdf

Calcitic speleothems in caves can form through abiogenic or biogenic processes, or through a combination of both. Many issues conspire to make the assessment of biogenicity difficult, especially when focusing on old speleothem deposits. This study reports on a multiproxy 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 δ<sup>13</sup>C data suggest that microbially mediated speleothem formation occurred repeatedly at 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 substance (EPS)-influenced organomineralization processes. The latter reflect paleoenvironmental 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 δ<sup>13</sup>C excursions. The microscale biogeochemical processes imply that microbial activity has only negligible effects on the bulk δ<sup>13</sup>C signature in speleothems, which is more strongly affected by CO<sub>2</sub> degassing and the host rock signature.

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