Precipitation amount, altitude, and moisture trajectory effects on the stable isotopic composition of precipitation in the Galapagos Islands

Understanding how climate processes facilitate stable water isotope variability in precipitation over time and space is critical to interpreting stable isotope-based paleoclimate proxies, particularly in the eastern equatorial Pacific where stable water isotope observations from precipitation (δ18Op and δDp) are sparse. Here we present a new 28-month record of daily δ18Op and δDp from Santa Cruz, Galápagos. With a prior 13-year record of monthly averaged precipitation isotope data from the island, these new data reveal valuable information on how meteorology, altitude, and source region characteristics influence the stable isotopic composition of precipitation in the region. Two sampling locations on Santa Cruz Island exhibit distinct local meteoric water lines; the drier, lower elevation site (7 m a.s.l.) has a significantly lower slope than the humid highland site (180 m a.s.l.), likely resulting from greater re-evaporation of falling rain. An altitude effect is also apparent, based on daily precipitation and δ18Op measurements across a 35 km transect of the island, with δ18Op decreasing by 0.2‰/100 m elevation. HYSPLIT backward trajectory modeling reveals important differences in how the path of atmospheric moisture affects stable isotope ratios in precipitation in the eastern equatorial Pacific and western equatorial Pacific. No significant relationship between moisture source area and δ18Op or deuterium excess is observed in our samples. We also find a lack of a predictable seasonal shift in moisture sources which explains the very small seasonal variation in δ18Op. Daily δ18Op near sea level was significantly correlated with precipitation amount, as was monthly, amount-weighted δ18Op and precipitation at sea level and 180 m. However, accounting for the non-normality of the data substantially reduces the strength of the correlation between δ18Op and precipitation on monthly timescales while the δ18Op-precipitation relationship on daily timescales remained strong. Overall, we observe a stronger daily, rather than monthly amount effect in the Galápagos dataset. This result suggests Galápagos paleoclimate records which reflect δ18Op may capture a slightly different story than those in the western equatorial Pacific, where the amount effect is most strongly manifested on weekly to monthly timescales