Abstract
Abstract. 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 δ13C 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 δ13C excursions. The microscale 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 host rock signature.
Highlights
The growth of speleothems such as stalactites and stalagmites through the precipitation of calcite has commonly been viewed as an abiogenic process (e.g., Kendall and Broughton, 1978; Broughton, 1983a, b, c)
Several δ13C and δ18O situ isotope geochemistry (SIMS) profiles along with scanning electron microscopy (SEM) including porous cracks confirm that the 13C depletion is typically associated with the porous cracks, which are characterized by small low-magnesium calcite rhombs and local microbial filaments
Since δ13C and δ18O ratios are routinely used as paleoclimate proxies, we investigate if microbial activity might have a significant influence on the isotopic composition of the carbonate, complicating the interpretation of isotope time series in terms of climatic variations
Summary
The growth of speleothems such as stalactites and stalagmites through the precipitation of calcite has commonly been viewed as an abiogenic process (e.g., Kendall and Broughton, 1978; Broughton, 1983a, b, c). A variety of precipitation and dissolution processes results in the deposition of carbonate speleothems, silicates, iron and manganese oxides, sulfur compounds, and nitrates, and in the breakdown of limestone host rock. Destructive processes include microbially influenced corrosion or dissolution of mineral surfaces that can occur through mechanical attack, secretion of exoenzymes, organic and mineral acids (e.g., sulfuric acid), and a variety of other mechanisms (for a summary please refer to Sand, 1997). Of particular interest in cave dissolution processes are reactions involving iron-, sulfur-, and manganese-oxidizing bacteria (Northup and Lavoie, 2001). Several descriptive studies have established the association of bacteria with iron deposits in caves, but experimental evidence for an active microbial role in the formation of iron deposits in caves is still lacking (Northup and Lavoie, 2001)
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