A forward model of cave dripwater δ18O and application to speleothem records

Abstract

Speleothem oxygen isotope records in arid regions are typically interpreted as indicators of the total precipitation amount and/or its seasonal balance. Such studies rarely address the potential influence of groundwater mixing processes on δ18O variability of cave dripwater. Here, we develop a model of oxygen isotopes in dripwater, which we compare to water and stalagmite measurements from Cave of the Bells, Arizona. We simulate moisture flux from surface to cave as a two-layer "leaky-bucket" model. In observations and the model, fed with modern climate data, dripwater δ18O is most comparable to that of winter precipitation. We show that seasonality and duration of the regional summer monsoon affect how much summer precipitation reaches the cave. We employ a Monte Carlo method to specify statistically realistic ranges for input climate variables and produce time series and variance spectra of cave drips. The spectra of our synthetic δ18O series exhibit a high degree of variance at decadal to multidecadal frequencies, despite being driven by synthetic data that includes only a seasonal cycle. This suggests that some background level of variance in speleothem δ18O records could be due to nonclimatic processes, such as subsurface water storage and mixing. Interpreting climatic vs. nonclimatic controls on speleothem δ18O variance could be achieved by replicating records from different caves.