Relationship between oxygen isotopes in rainfall, cave percolation waters and speleothem calcite at Waitomo, New Zealand

Paul W. Williams
Anthony Fowler

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The relationship between the δ¹⁸O values of rainfall, vadose percolation water, and speleothem calcite was investigated in a cave at Waitomo. Water samples were obtained approximately monthly for two years from a storage rain gauge on the surface and from stored seepage from three stalactites underground. Rain water δ¹⁸OSMOW values varied considerably throughout the observation period, with a precipitation-weighted mean of -5.3‰. Seasonal variability was evident, with winter values being more negative than summer values. Cave seepage waters had a mean of about -5‰ and showed very little variability and no discernible annual variation. This is explained by thorough mixing in the soil and subcutaneous zone stores. Given the average cave temperature (12.8°C) and the δ¹⁸OSMOW value determined for seepage water, the δ¹⁸OPDB value of calcite that is actively depositing in isotopic equilibrium on speleothems at Waitomo should fall in the range of -4.1 to -4.6‰. Observed δ-values of modern speleothem calcites overlap the positive end of this range of theoretical values, indicating that some growing speleothems are not in isotopic equilibrium with seepage waters, but are experiencing either evaporation or kinetic fractionation.