24 Nov 2021

24 Nov 2021

Review status: this preprint is currently under review for the journal CP.

Investigating oxygen and carbon isotopic relationships in speleothem records over the last millennium using multiple isotope-enabled climate models

Janica C. Bühler1,, Josefine M. Axelsson2,, Franziska A. Lechleitner3, Jens Fohlmeister4,5, Allegra N. LeGrande6, Madhavan Midhun7,8, Jesper Sjolte9, Martin Werner10, Kei Yoshimura11, and Kira Rehfeld1,12 Janica C. Bühler et al.
  • 1Institute of Environmental Physics, Heidelberg University, Heidelberg, D-69120, Germany
  • 2Department of Physical Geography and Bolin Centre for Climate Research, Stockholm University, Stockholm, SE-106 91, Sweden
  • 3Department of Chemistry, Biochemistry and Pharmaceutical Sciences and Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland
  • 4Potsdam Institute for Climate Impact Research, Telegrafenberg, 14473 Potsdam, Germany
  • 5GFZ German Research Centre for Geosciences, Section “Climate Dynamics and Landscape Development”, 14473 Potsdam, Germany
  • 6NASA Goddard Institute for Space Studies and Center for Climate Systems Research, Columbia University, New York, USA
  • 7Department of Atmospheric Sciences, Cochin University of Science and Technology, India
  • 8Earth Research Institute, University of California Santa Barbara, CA, USA
  • 9Department of Geology, Lund University, Lund, SE-223 62, Sweden
  • 10Alfred Wegener Institute, Helmholtz-Centre for Polar and Marine Research, Bremerhaven, D-27515, Germany
  • 11University of Tokyo, Tokyo, Japan
  • 12Geo- und Umweltforschungszentrum, Schnarrenbergstr. 94-96, 72074 Tübingen, Germany
  • These authors contributed equally to this work.

Abstract. The incorporation of water isotopologues into the hydrology of general circulation models (GCMs) facilitates the comparison between modelled and measured proxy data in paleoclimate archives. However, the variability and drivers of measured and modelled water isotopologues, and indeed the diversity of their representation in different models are not well constrained. Improving our understanding of this variability in past and present climates will help to better constrain future climate change projections and decrease their range of uncertainty. Speleothems are a precisely datable paleoclimate archive and provide well preserved (semi-)continuous multivariate isotope time series in the lower and mid-latitudes, and are, therefore, well suited to assess climate and isotope variability on decadal and longer timescales. However, the relationship between speleothem oxygen and carbon isotopes to climate variables also depends on site-specific parameters, and their comparison to GCMs is not always straightforward.

Here we compare speleothem oxygen and carbon isotopic signatures from the Speleothem Isotopes Synthesis and AnaLysis database version 2 (SISALv2) to the output of five different water-isotope-enabled GCMs (ECHAM5-wiso, GISS-E2-R, iCESM, iHadCM3, and isoGSM) over the last millennium (850–1850 common era, CE). We systematically evaluate differences and commonalities between the standardized model simulation outputs. The goal is to distinguish climatic drivers of variability for both modelled and measured isotopes.

We find strong regional differences in the oxygen isotope signatures between models that can partly be attributed to differences in modelled temperatures. At low latitudes, precipitation amount is the dominant driver for water isotope variability, however, at cave locations the agreement between modelled temperature variability is higher than for precipitation variability. While modelled isotopic signatures at cave locations exhibited extreme events coinciding with changes in volcanic and solar forcing, such fingerprints are not apparent in the speleothem isotopes, and may be attributed to the lower temporal resolution of speleothem records compared to the events that are to be detected. Using spectral analysis, we can show that all models underestimate decadal and longer variability compared to speleothems, although to varying extent.

We found that no model excels in all analyzed comparisons, although some perform better than the others in either mean or variability. Therefore, we advise a multi-model approach, whenever comparing proxy data to modelled data. Considering karst and cave internal processes through e.g. isotope-enabled karst models may alter the variability in speleothem isotopes and play an important role in determining the most appropriate model. By exploring new ways of analyzing the relationship between the oxygen and carbon isotopes, their variability, and co-variability across timescales, we provide methods that may serve as a baseline for future studies with different models using e.g. different isotopes, different climate archives, or time periods.

Janica C. Bühler et al.

Status: open (until 19 Jan 2022)

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Janica C. Bühler et al.

Janica C. Bühler et al.


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Short summary
We collected and standardized output of five isotope-enabled simulations for the last millennium and assess differences and similarities to records from a global speleothem database. Modelled isotope variations mostly arise from temperature differences. While lower resolution in speleothems does not capture extreme changes to the extent of models, they show higher variability on multi-decadal timescales. As no model excels in all comparisons, we advise a multi-model approach where possible.