Hydrological uncertainties in the modelling of cave drip-water δ¹⁸O and the implications for stalagmite palaeoclimate reconstructions

In this paper we review our current understanding of karst drip-water hydrology, emphasising the extent of non-linear and non-stationary process dynamics that render stalagmite palaeoclimate reconstructions using a statistical pseudo-proxy approach difficult to implement. We outline an approach to attribute the uncertainty that arises specifically as a consequence of variable water routing through the overlying soil, epikarst and karst aquifer. This is based upon the development of a monthly lumped parameter karst hydrological model which we use to demonstrate the range of modelled drip-water discharges possible from a single climate input. Refinement of the model, to include precipitation δ¹⁸O, enables us to determine the theoretical range in drip-water and stalagmite δ¹⁸O for three sites with contrasting climates: northern temperate (NW Scotland), monsoonal (Ethiopia), and Mediterranean (Gibraltar). For actual climate (monthly mean temperature; monthly total precipitation; monthly mean precipitation δ¹⁸O), we compare model simulations of karst groundwater storage and drip-water δ¹⁸O to demonstrate our ability to model different climate regimes realistically. We also investigate the δ¹⁸O variability associated with specific karst water reservoirs that differ in their capacity and drainage mechanisms δ¹⁸O variability is then compared to stalagmite δ¹⁸O record from the three regions for the last ∼45 years. We conclude by reviewing the implications of our hydrological model for stalagmite δ¹⁸O Quaternary palaeoclimate reconstructions over different timescales and sampling resolutions.