Hydrochemical and Isotopic Variability of Cave Drip Water with Increasing Elevation, Vancouver Island, Canada

Coeval paleoclimate records from adjacent speleothems sometimes display significant differences in isotope and trace element geochemistry. These differences have been

ascribed to modification of the external climate signal by complex hydrogeochemical processes in the epikarst and vadose zone. We focus on inter and intra differences

between trios of neighbouring drips in each of three caves located at increasing elevation at the head of Tahsis Inlet, a fiord on the Pacific coast of Vancouver Island,

Canada. Custom drip monitoring stations provided records of discharge, temperature and specific electrical conductivity (SpC) at 15 minute intervals, while monthly bulk water samples were collected over more than 18 months. The setting is coastal temperate coniferous rainforest, with mean annual temperature of 9.8oC and

precipitation of 4000 mm. The summers are dryer while snow packs form above 500 m ASL in the winter.

The drip discharges were generally low, averaging 50 mL/day, and non-seasonal although some trending suggests response to lower frequency inter-annual patterns.

While SpC was similarly non-seasonal, most drips do display high-frequency variations in discharge, SpC, and temperature in response to more transient events.

Clear seasonality was observed in the drip water D/H and O isotope ratios with average amplitude of 8.2 permil and 1.0 permil respectively for all drips, which is damped compared to precipitation in this region. In all cases, drip seasonality was 5-6 months out of phase with the meteoric water, despite recharge-responses observed in the

15-minute data indicating that some proportion of the drip waters are fast recharge. Altitudinal gradients are evident: the drip water isotopic 'lapse rate' is 0.2

permil 100 m which is 60% of that anticipated for meteoric water.