δD, δ18O AND HYDROLOGICAL VARIATIONS IN ADJACENT DRIPS IN THREE CAVES AT INCREASING ELEVATION: VANCOUVER ISLAND, BC

The δD and δ¹⁸O of cave drip waters broadly reflect that of the local meteoric precipitation and the mean annual temperature, relationships that allow for paleoclimate records to be derived from speleothem calcite. However, the hydrology of the karst vadose zone is increasingly recognised for its complexity due to storage in the epikarst and mixing of fast and slow flowing waters along fractures and conduits. In order to better understand cave drip hydrology and the paleoclimate records obtained from speleothems, we have monitored 9 cave drips for 16 months in 2004-06 in 3 caves located at increasing altitude (0, 550, 750 m ASL) on the west coast of Vancouver Island. The monitored drips are all actively depositing calcite and located > 30 m from each other within each cave. Drip rates generally increase with elevation. Most drips have high frequency (15 minute) variations in drip rates and electrical conductivity ( ≈ TDS), but neither variable fluctuates seasonally. The meteoric precipitation at Victoria (~250 km south, 20 m ASL, GNIP data for 1978-82) has maximum δD and δ¹⁸O from June-September, a period corresponding to the North Pacific High being at its northern limit, and minimum values in January-February when the Aleutian Low dominates the northern Pacific. The cave drip water δD and δ¹⁸O also varies seasonally, but 6 months out of phase with the Victoria precipitation. The seasonal drip amplitudes for all 9 drips (δD = ~8.2 ‰, δ¹⁸O = ~1.0 ‰) are damped compared to the meteoric waters (δ¹⁸O = 5.9 ‰, δD = 45 ‰). Differences up to 2 ‰ are observed in δ¹⁸O between adjacent drips. The isotopic lapse rate between the caves is 0.2 ‰ 100 m^-1, about 60% of that for rain and snow. It is not possible to interpret the phase lag between meteoric and drip water δ values as simple temporal lag due to storage, or from piston flow through the vadose zone. The 15-minute instrumental drip data show rapid response in rate, chemistry, and temperature to recharge events indicating that some proportion of the drip waters are fast recharge water.