Mean annual meteoric precipitation δ18O values varied slightly with elevation, -13.89 ‰ VSMOW at 1253 m and -12.89 ‰ VSMOW at 766 m, and showed distinct seasonal trends (mean weighted seasonal values ranging from -11.89 ‰ VSMOW during autumn to -15.00 ‰ VSMOW during the winter). Carbonate δ18O varied significantly with MAP, ranging from -14.13 (‰ VPDB) at the most arid site to -11.38 (‰ VPDB) at the wettest site, but showed an opposite trend to soil water δ18O values (-4 to -12 ‰ VSMOW for the 70 cm depth). Neither measured (summer) soil water values nor mean meteoric precipitation values generated realistic temperatures of formation for the observed trend in carbonate 18O values. When mean annual temperatures across the gradient were used for temperature of formation, calculated soil water δ18O in equilibrium with carbonates was similar to winter precipitation δ18O at the most arid sites but became increasingly similar to mean annual meteoric precipitation δ18O as MAP increased. We suggest that our unexpected trend in carbonate δ18O values is largely due to the diminishing influence of winter precipitation and/or snowmelt on soil moisture as MAP increases across the climatic gradient, and indicates seasonal changes in isotopic composition of meteoric precipitation and soil water may need to be considered in interpretation of oxygen isotope values for paleoclimatic reconstruction of pedogenic minerals in temperate climates.