Modelling the speleothem δ18O proxy using an isotope-enabled dual-porosity groundwater model.

Speleothems are increasingly used in composite paleoclimate records of the last millennia. The stable oxygen isotope (δ18O) values in speleothems reflect the history of the incorporated drip water, and are increasingly being used as proxies of past climates. However, the δ18O values obtained from speleothems have been influenced by atmospheric effects on rainfall inputs, subsequent evaporation from within and above soil profiles, volumetric changes due to transpiration by overlying vegetation, mixing within the karst aquifer, and incorporation of water drips into the speleothem. The role of each of these influences needs to be thoroughly understood in order to confidently use δ18O values obtained from speleothems as palaeo-proxies.
Here, the focus is on the role that flow dynamics of karst systems have on the final δ18O values that speleothems receive from drip waters. To investigate this, an isotope-enabled groundwater flow model for karst aquifers has been enhanced from previous versions to include dual porosity flow paths (fracture and diffuse flow) and with the capacity for calibration against observed drip water isotopic measurements. The model consists of five stores that are connected by fracture and diffuse flowpaths, and will be tested against cave drip water δ18O data collected at 4-6 week intervals over a period of more than five years from the Golgotha Cave in southwest Western Australia.
The model can potentially be used to investigate how mixing in a karst aquifer affects the isotopic composition of water deposited on and incorporated into stalagmites, and subsequently quantify uncertainties associated with climate records derived from speleothems. This paper presents the first attempts to calibrate this isotope-enabled groundwater model.