Using Earth system model output to simulate DCF variability in speleothems: Implications for atmospheric 14C calibration

Speleothems have become a cornerstone in atmospheric ¹⁴C reconstruction. In particular, the part of the IntCal20 calibration curve before 34 ka BP (Reimer et al., 2020) heavily relies on a set of speleothems from Hulu Cave in China (Cheng et al., 2018). The interpretation of speleothem ¹⁴C archives, however, is often exacerbated by the so-called dead carbon fraction (DCF) in speleothem carbonate. It quantifies the percentage of old, ¹⁴C-free carbon from dissolved bedrock carbonate or aged soil organic matter, and is controlled by various parameters. Modelling efforts to disentangle these parameters have already been made by previous studies.

Here, we present forward-modelled DCF time series obtained by coupling CaveCalc, a numerical model for speleothem chemistry and isotopes (Owen et al., 2018), with IntCal20 and results from paleoclimate modelling. To compare our coupled model with an extensive DCF measurement record from Sofular Cave in Northern Turkey, we convert time-dependent soil respiration output from the Max Planck Institute Earth System Model version 1.2 (MPI-ESM1.2) to soil pCO₂ via a simplistic soil respiration model and use it as input for CaveCalc. The resulting forward-modelled DCF is in very good agreement with the long-term trends of the measurement record and demonstrates that soil respiration has been the main driver of DCF variability in the Last Glacial Maximum and the Early Holocene at Sofular Cave.

Further, we show that, holding soil respiration and all other climate parameters constant, adding only 10 % of 1000 year old carbon to the soil CO₂ can cause variations of up to 200 years in the DCF. This finding suggests that the DCF variability of only 50 years, which is assumed for Hulu Cave by Reimer et al. (2020), might be significantly higher, and underlines the importance of including additional records, like the one from Sofular Cave, to the next generation of calibration curves.

 

References:

Reimer, P. J., Austin, W. E. N., Bard, E., Bayliss, A., et al.: The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0–55 cal kBP), Radiocarbon, 62(4), 725-757, doi:10.1017/RDC.2020.41, 2020.

Cheng, H., Lawrence Edwards, R., Southon, J., et al.: Atmospheric ¹⁴C/¹²C changes during the last glacial period from Hulu Cave, Science, 362(6420), 1293–1297, doi:10.1126/science.aau0747, 2018.

Owen, R., Day, C. C., and Henderson, G. M.: CaveCalc: A new model for speleothem chemistry & isotopes, Computers & Geosciences, 119, 115–122, doi:10.1016/j.cageo.2018.06.011, 2018.