Millennial-length forward models and pseudoproxies of stalagmite δ18O: an example from NW Scotland

The stable oxygen isotope parameter delta O-18 remains the most widely utilised speleothem proxy for past climate reconstructions. Uncertainty can be introduced into stalagmite delta O-18 from a number of factors, one of which is the heterogeneity of groundwater flow in karstified aquifers. Here, we present a lumped parameter hydrological model, KarstFor, which is capable of generating monthly simulations of surface water - ground water - stalagmite delta O-18 for more than thousand-year time periods. Using a variety of climate input series, we use this model for the first time to compare observational with modelled (pseudoproxy) stalagmite delta O-18 series for a site at Assynt, NW Scotland, where our knowledge of delta O-18 systematics is relatively well understood. The use of forward modelling allows us to quantify the relative contributions of climate, peat and karst hydrology, and disequilibrium effects in stalagmite delta O-18, from which we can identify potential stalagmite delta O-18 responses to climate variability. Comparison of the modelled and actual stalagmite delta O-18 for two stalagmites from the site demonstrates that, for the period of overlapping growth, the two series do not correlate with one another, but forward modelling demonstrates that this falls within the range explicable by differences in flow routing to the stalagmites. Pseudoproxy delta O-18 stalagmite series highlight the potential significance of peat hydrology in controlling stalagmite delta O-18 over the last 1000 yr at this site.