Isotopic equilibrium between raindrops and water vapor during the onset and the termination of the 2005–2006 wet season in the Bolivian Andes

Abstract

The isotopic equilibrium state between precipitation and low-level water vapor is a common assumption in numerous paleoclimate and atmospheric studies based on water stable isotopes. However, the paucity of field observations limits the validation of this assumption. This study examines the isotopic equilibrium state from event-based precipitation and daily near-surface water vapor samples collected during the onset and the termination of the 2005–2006 wet season in the Bolivian Andes (Zongo valley, 16°09′S, 68°07′W). Our observations show that the observed isotopic composition of precipitation (δDp) deviates from the theoretical isotopic composition of precipitation at equilibrium with water vapor (δDp_eq). Disequilibriums (ΔDp_eq = δDp - δDp_eq) are mostly negative (73%), indicating that precipitation is more depleted than a condensate that would have been formed from surface water vapor, and half of them are between −10 and + 10‰. They are significantly correlated to δDp (r2 = 0.30, n = 70, p < 0.001) suggesting that controls on δDp also impact ΔDp_eq. Although equilibrium state does not prevail at the individual rain event scale, a strong relationship is observed between δDp and δDp_eq over the whole period of field samplings (r2 = 0.86, n = 70, p < 0.001). The review of possible causes to explain the disequilibriums shows that below-cloud rain evaporation and diffusive exchanges are little involved. Other local processes such as rain type, condensation conditions and surface water recycling appear as better candidates to explain ΔDp_eq. Lastly, we explore how local processes affect δDp. We show that large-scale dynamic along air masses history is dominant (nearly 80%) to explain δDp whereas local effects are dominant to explain deuterium excess in precipitation. In consequence, we conclude that δDp is a correct candidate to examine and to reconstruct large-scale atmospheric processes from past to present time scales.