Amount Effect, Altitude, and Moisture Source Influences on Precipitation Isotopic Variability in the Galápagos Islands

Abstract:

Understanding how climate processes facilitate water isotope variability in precipitation over time and space is critical to interpreting isotope-based paleoclimate proxies, particularly in the eastern tropical Pacific where stable water isotope observations from precipitation (δ¹⁸O_p and δD_p) are sparse. Here we present a new 28-month record of daily δ¹⁸O_p and δD_p 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, low elevation site (7m a.s.l.) has a significantly lower slope than the humid highland site (180m a.s.l.), likely resulting from greater reevaporation of falling rain. An altitude effect is also apparent, based on daily precipitation and δ¹⁸O_p measurements across a 35 km transect of the island, with δ¹⁸O_p decreasing by 0.2‰/100m elevation. HYSPLIT backward trajectory modeling shows air parcels producing rain events with the lowest δ¹⁸O_p values originated over warmer waters to the north of the Galápagos; rain events with the highest δ¹⁸O_p originated to the east. This difference provides a mechanism for changes in seasonal mean isotope ratios and shifts in isotope ratios due to systematic circulation changes, such as in association with ENSO phases. Daily δ¹⁸O_p near sea level was significantly correlated with precipitation amount, as was monthly, amount-weighted δ¹⁸O_p 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 δ¹⁸O_p and precipitation on monthly timescales while the δ¹⁸O_p-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 δ¹⁸O_p may capture a slightly different story than those in the western tropical Pacific, where the amount effect is most strongly manifested on weekly to monthly timescales.