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Climate of the Past An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/cp-2020-110
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2020-110
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  24 Sep 2020

24 Sep 2020

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This preprint is currently under review for the journal CP.

Climatic variations during the Holocene inferred from radiocarbon and stable carbon isotopes in a high-alpine cave

Caroline Welte1,2, Jens Fohlmeister3,4, Melina Wertnik1,2, Lukas Wacker1, Bodo Hattendorf5, Timothy I. Eglinton2, and Christoph Spötl6 Caroline Welte et al.
  • 1Laboratory of Ion Beam Physics, ETHZ, Otto-Stern Weg 5, 8093 Zurich, Switzerland
  • 2Geological Institute, ETHZ, Sonnegstrasse 5, 8092 Zurich, Switzerland
  • 3Potsdam Institute for Climate Impact Research, Telegrafenberg, 14473 Potsdam, Germany
  • 4GFZ German Research Centre for Geosciences, Section "Climate Dynamics and Landscape Development", 14473 Potsdam, Germany
  • 5Laboratory of Inorganic Chemistry, D-CHAB, ETHZ, Vladimir-Prelog Weg 1, 8093 Zurich, Switzerland
  • 6Institute of Geology, University of Innsbruck, Innrain 52f, 6020 Innsbruck, Austria

Abstract. A novel technique making use of laser ablation coupled online to accelerator mass spectrometry (LA-AMS) allows analyzing the radiocarbon (14C) concentration in carbonate samples continuously at high spatial resolution within very short analysis times. This new technique can provide radiocarbon data similar to the spatial resolution of stable carbon (C) isotope measurements by isotope-ratio mass spectrometry (IRMS) and, thus, can help to interpret δ13C signatures, which otherwise are difficult to understand due to numerous processes contributing to changes in C-isotope changes ratios. In this work we analyzed δ13C and 14C on the Holocene stalagmite SPA 127 from the high-alpine Spannagel Cave (Austria). Combined stable carbon and radiocarbon profiles allow to identify three growth periods characterized by different δ13C signatures: (i) the period > 8 ka BP is characterized by relatively low δ13C values with small variability combined with a comparably high radiocarbon reservoir effect (expressed as dead carbon fraction, dcf) of around 60 %. This points towards C contributions of host rock dissolution and/or from an old organic matter (OM) reservoir in the karst potentially mobilized due to the warm climatic conditions of the early Holocene. (ii) Between 3.8–8 ka BP a strong variability in δ13C reaching values from −8 to +1 ‰ with a generally lower dcf was observed. The δ13C variability is most likely caused by changes in gas exchange processes in the cave, which are induced by reduced drip rates as derived from reduced stalagmite growth rates. Additionally, the lower dcf indicates that the OM reservoir is contributing less to stalagmite growth in this period possibly as a result of reduced precipitation or because it is exhausted. (iii) In the youngest section between 2.4–3.8 ka BP, comparably stable and low δ13C values combined with an increasing dcf reaching up to 50 % are again hinting towards a contribution of an aged OM reservoir in the karst.

Caroline Welte et al.

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Short summary
Stalagmites are valuable climate archives, but unlike other proxies the use of stable carbon (C) isotopes is still difficult. A stalagmite from the Austrian Alps was analyzed using a new laser ablation method for fast radiocarbon (14C) analysis. This allowed to combine 14C and stable C isotopes showing that gas exchange processes influence the stable C variations in that speleothem and besides soil and bedrock a third source is contributing during periods of warm, wet climate: old organic matter.
Stalagmites are valuable climate archives, but unlike other proxies the use of stable carbon (C)...
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