A 26-year stable isotope record of humidity and El Nino-enhanced precipitation in the spines of saguaro cactus, Carnegiea gigantea

Seasonal and annual variations of rainfall and humidity are recorded in the carbon and oxygen stable isotope ratios of sequentially grown spines found on the columnar cactus, Carnegiea gigantea. A 26-year long composite δ^18 O and δ^13 C isotope record from the spines of five saguaro cacti was created using bomb radiocarbon and semi-annual variations in δ^13 C. Once dating errors in the composite record are corrected, mean annual spine δ^18 O is negatively correlated (P2 parts per thousand) in the maximum annual spine δ^18 O are positively correlated (P<0.01) with the Southern Oscillation Index (SOI). We attribute these decreases to enhanced winter rainfall associated with the El Nino phase of the El Nino-Southern Oscillation. Minimum annual δ^13 C is negatively correlated with TAP (P<0.05) and mean nighttime VPD (P