CAPTURING CLIMATE VARIABILITY IN STABLE ISOTOPIC PROXIES FROM THE EASTERN EQUATORIAL PACIFIC: WHAT CAN AND CAN’T STABLE ISOTOPES RECORD?

Hydrogen isotopes in organic molecules from lake sediments in the Galápagos Islands have recently been used as a proxy for precipitation amount in the eastern equatorial Pacific (EEP) (Atwood and Sachs, 2014; Zhang et al., 2014). Such isotope based climate proxies hold tremendous promise because of their strong correlation with the stable isotopic composition of lake water. Unfortunately, mechanisms which influence the stable isotopic composition of precipitation from the EEP are uncertain. This study presents a 28-month record of daily δ¹⁸O and δD from precipitation at sea level on Santa Cruz, Galápagos to investigate how local and regional climate may influence variability in the stable isotopic composition of EEP precipitation at daily to monthly timescales. We compare our data to a prior 13-year record of monthly precipitation δ¹⁸O and δD at 180 m on Santa Cruz. Local daily precipitation amount was found to be significantly correlated with δ¹⁸O near sea level, as was monthly amount weighted δ¹⁸O at sea level and 180 m. However, accounting for the non-normality of the data reveals a stronger amount effect on the daily timescale than the monthly timescale. At monthly timescales, the strength of the precipitation amount and δ¹⁸O correlation is dependent on a period of extreme rainfall during the strong 1997-98 El Niño event. The correlation between monthly amount weighted δ¹⁸O and precipitation amount is greatest during the event (r = -0.85, p<0.001) and is insignificant when the 1997-98 event is removed (r = -0.21, p=0.45). Our results suggest that paleoclimate proxies for rainfall amount which use stable oxygen or hydrogen isotopes are biased toward recording extreme rainfall amount related to strong El Niño events.

Atwood, A.R., and Sachs, J.P., 2014, Separating ITCZ- and ENSO-related rainfall changes in the Galápagos over the last 3 kyr using D/H ratios of multiple lipid biomarkers: Earth and Planetary Science Letters, v. 404, no. 0, p. 408-419.

Zhang, Z., Leduc, G., and Sachs, J.P., 2014, El Nino evolution during the Holocene revealed by a biomarker rain gauge in the Galapagos Islands: Earth and Planetary Science Letters, v. 404, p. 420-434.