Climatic and Topographic Control of the Stable Isotope Values of Rivers on the South Island of New Zealand

Abstract

We show that climate and topography control the spatial distribution of stable isotope values on the South Island of New Zealand, based on a spatially dense (n = 193) river isotopic survey. Our data show a δ18O minimum in isotope values east of the Southern Alps that demonstrates topographically driven continentality associated with the Southern Alps, which intersect the prevailing, moisture-laden westerlies. Our data define a South Island surface water line of δ2H = 8.17 (±0.26) × δ18O + 10.57 (±2.04), which is identical within 95% confidence intervals to the global and New Zealand meteoric water lines established from monthly precipitation samples. The observed river δ18O values are strongly correlated with annual temperature range and winter temperature. Strongest correlations are between δ18O and mean minimum winter temperatures (r > 0.7 for June, July, August), with gradients of 0.58–0.66‰ /°C. Based on a multiple regression analysis of δ18O against climate data, we present a river δ18O model and isoscape that demonstrate the control of continentality and moisture source on New Zealand surface water isotope spatial patterns. Model validation against previously published river samples shows skill in predicting river δ18O values (root-mean-square error = 0.83), confirming that the spatial variations in river δ18O (and δ2H) are robust to sampling period and reflect continental, precipitation source and temperature effects. Our data suggest that oxygen or hydrogen isotope paleoclimate proxies derived from rivers or open-system lakes on the South Island should be sensitive to winter temperature.