Ion microprobe δ¹⁸O analyses to calibrate slow growth rate speleothem records with regional δ¹⁸O records of precipitation

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.