Climatic and Topographic Control of the Stable Isotope Values of Rivers on the South Island of New Zealand
Matthew S. Lachniet, Christopher M. Moy, Christina R. Riesselman, Haroon Stephen, Andrew M. Lorrey
Abstract
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 δ 18 O 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 δ 2 H = 8.17 (±0.26) × δ 18 O + 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 δ 18 O values are strongly correlated with annual temperature range and winter temperature. Strongest correlations are between δ 18 O 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 δ 18 O against climate data, we present a river δ 18 O 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 δ 18 O values (root‐mean‐square error = 0.83), confirming that the spatial variations in river δ 18 O (and δ 2 H) 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.