INTRA- AND INTER-CAVE DRIP δD AND δ18O ISOTOPIC LAGS AND DAMPING FROM MONITORING IN THREE CAVES ALONG AN ALTITUDINAL GRADIENT, VANCOUVER ISLAND, BRITISH COLUMBIA, CANADA
The expected δ18O depletion of -0.15 to -0.5‰/100 m of ascent was not observed, and instead there was overlap in isotopic ranges (at 1 SD) between all three caves. Notwithstanding, the drips at 740 m ASL are tightly aligned to the GMWL and 18O-depleted; the drips at 550 m ASL and at sea level plot along the GMWL, or between it and the Victoria LMWL, with the exception of the slowest drip (sea level). The δD and δ18O drip amplitudes were damped on average 74% and 73% respectively compared to the Victoria data. A first conclusion from this study is that the substantial intra- and inter-cave variation of δD and δ18O in drips was sufficient to mute the altitudinal isotopic lapse rate in this local coastal rainforest environment.
The δD and δ18O drip isotopes lagged the Victoria record by 155±26 days and 165±50 days respectively. The slowest drip had the longest lag times. The shortest lag (87 days for δ18O, cave at 550 m) implies a shortest residence time in the aquifer. When the calendar dates of the drip isotope records were shifted to fit the time lags there was excellent agreement with local sea level air temperatures. A second conclusion, is that in these caves, most or all of the volumes being discharged have a residence time of less than one climatic year for all components of the source reservoir combined – forest, litter, soil, epikarst and the bedrock transmission zone to the drip point. This reflects the negligible matrix porosity of the host rock and the super-humid conditions. Drip recharge was evidently by simple piston flow.