Main controls on the stable carbon isotope composition of speleothems

Fohlmeister, Jens; Voarintsoa, Ny Riavo G.; Lechleitner, Franziska A.; Boyd, Meighan; Brandtstätter, Susanne; Jacobson, Matthew J.; L. Oster, Jessica (2020). Main controls on the stable carbon isotope composition of speleothems. Geochimica et cosmochimica acta, 279, pp. 67-87. Elsevier Science 10.1016/j.gca.2020.03.042

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The climatic controls on the stable carbon isotopic composition (δ13C) of speleothem carbonate are less often discussed in the scientific literature in contrast to the frequently used stable oxygen isotopes. Various local processes influence speleothem δ13C values and confident and detailed interpretations of this proxy are often complex. A better understanding of speleothem δ13C values is critical to improving the amount of information that can be gained from existing and future records. This contribution aims to disentangle the various processes governing speleothem δ13C values and assess their relative importance. Using a large data set of previously published records we examine the spatial imprint of climate-related processes in speleothem δ13C values deposited post-1900 CE, a period during which global temperature and climate data is readily available. Additionally, we investigate the causes for differences in average δ13C values and growth rate under identical climatic conditions by analysing pairs of contemporaneously deposited speleothems from the same caves. This approach allows to focus on carbonate dissolution and fractionation processes during carbonate precipitation, which we evaluate using existing geochemical models. Our analysis of a large global data set of records reveals evidence for a temperature control, likely driven by vegetation and soil processes, on δ13C values in recently deposited speleothems. Moreover, data-model intercomparison shows that calcite precipitation occurring along water flow paths prior to reaching the top of the speleothem can explain the wide δ13C range observed for concurrently deposited samples from the same cave. We demonstrate that using the combined information of contemporaneously growing speleothems is a powerful tool to decipher controls on δ13C values, which facilitates a more detailed discussion of speleothem δ13C values as a proxy for climate conditions and local soil-karst processes.

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