Oxygen and hydrogen isotopic variations between adjacent drips in three caves at increasing elevation in a temperate coastal rainforest, Vancouver Island, Canada

Abstract

The interpretation of speleothem paleoenvironmental records requires understanding of spatial–temporal variations in vadose drip water chemistry and isotopic composition. This study reports on intra- and inter-cave differences in δD, δ18O and electrical conductivity, using 18 monthly water samples from three adjacent drips (<20m apart) in each of three caves at increasing elevation (0, 550, and 740m ASL) on very steep ground at the head of Tahsis Inlet fjord on the Pacific coast of Vancouver Island, British Columbia.All drips showed isotopic seasonal signals, despite varied patterns of drip hydrology. There was overlap in isotopic ranges (at 1 SD) between all three caves, in contrast with the expected δ18O depletion of −0.15 to −0.5‰/100m of ascent observed in standard precipitation. The isotopic seasonality was approximated with sine curves, and compared to a GNIP data set from Victoria ∼300km to the south. The δD and δ18O drip isotopes lagged the Victoria record by 155±26days and 165±50days respectively. The longest lag was at the slowest drip (sea level), while the shortest lag (87days for δ18O, 550m ASL) implies a short residence time, paradoxically from the drip with the highest mean electrical conductivity. Vadose residence time was less than one climatic year, reflecting a combination of negligible matrix porosity in the host rock and super-humid climatic conditions. Beneath the epikarst, drip hydrology was evidently by simple piston flow.Phase-shifted drip isotope records showed excellent agreement with sea level mean monthly air temperatures at the Tahsis meteorological station over the study period. The δD and δ18O drip amplitudes were damped on average 74% and 73% respectively compared to the Victoria data. The drips at 740m ASL are tightly aligned to the global mean meteoric water line (GMWL) and 18O-depleted; the drips at 550m 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) which displays some seasonal evaporitic enrichment.The findings quantify substantial intra- and inter-cave variation of δD and δ18O in drips having a wide range of drip type and volume of flow, sufficient to mute the altitudinal isotopic lapse rate in this local coastal rainforest environment.