Paleoclimate reconstructions based on speleothems require a robust interpretation of their proxies. Detailed transfer functions of external signals to the speleothem can be obtained using models supported by monitoring data. However, the transferred signal may not be stationary due to complexity of karst processes. Therefore, robust interpretations require the calibration of speleothem records with instrumental time series lasting no less than a decade. We present the calibration of a speleothem δ18O record from Postojna Cave (Slovenia) with the regional record of δ18O composition of precipitation during the last decades. Using local meteorological data and a regional δ18O record of precipitation, we developed a model that reproduces the cave drip water δ18O signal measured during a two-year period. The model suggests that the average water mixing and transit time in the studied aquifer is 11 months. Additionally, we used an ion microprobe to study the δ18O record of the top 500 μm of a speleothem from the studied cave gallery. According to U–Th dates and 14C analyses, the uppermost section of the speleothem was formed during the last decades. The δ18O record of the top 500 μm of the speleothem has a significant correlation (r2=0.64; p-value <0.001) with the modelled δ18O record of cave drip water. Therefore, we confirm that the top 500 μm of the speleothem grew between the years 1984 and 2003 and that the speleothem accurately recorded the variability of the δ18O values of regional precipitation filtered by the aquifer. We show that the recorded speleothem δ18O signal is not seasonally biased and that the hydrological dynamics described during monitoring period were stationary during recent decades. This research demonstrates that speleothems with growth rates <50 μm/yr can also be used for calibration studies. Additionally, we show that the fit of measured and modelled proxy data can be used to achieve annually resolved chronologies in speleothems that were not actively growing at the time of collection and/or that do not record annual laminae.
Bibliographical noteFunding Information:
We thank Postojnska jama d.d. and the Ministry of Environment and Spatial Planning of Slovenia for the permissions to access the cave and to sample the speleothem. We appreciate the support of Stanislav Glažar, who joined us in every visit to the cave during the monitoring period. We are grateful to Andrej Mihevc from the Karst Research Institute in Postojna who supervised the speleothem collection in the cave. Ion probe analyses were conducted in WiscSIMS laboratory at the University of Wisconsin-Madison thanks to the essential support and advice of John W. Valley. We thank Ian J. Orland and Kathleen A. Wendt for their discussion and support in the analytical design and the fluorescence imaging of PO2 stalagmite. We thank Stojan Žigon for water stable isotope analyses at Jožef Stefan. Radiocarbon analyses were supported by the NERC radiocarbon facility (allocation number 1404.0409) thanks to the collaboration of Andy Baker. One of the authors (SL) received funds from the Slovenian Research Agency (research programme P1-0143 and and project N1-0054 ) the EU Horizon 2020 project MASSTWIN (grant agreement No. 692241 ).
© 2017 Elsevier B.V.
- ion microprobe