Variability and Controls on δ¹⁸O, d‐excess, and ∆′¹⁷O in Southern Peruvian Precipitation

Abstract

The isotopic composition of precipitation is used to trace water cycling and climate change, but interpretations of the environmental information recorded in central Andean precipitation isotope ratios are hindered by a lack of multi‐year records, poor spatial distribution of observations, and a predominant focus on Rayleigh distillation. To better understand isotopic variability in central Andean precipitation, we present a three‐year record of semimonthly δ18Op and δ2Hp values from 15 stations in southern Peru and triple oxygen isotope data, expressed as ∆′17Op, from 32 precipitation samples. Consistent with previous work, we find that elevation correlates negatively with δ18Op and that seasonal δ18Op variations are related to upstream rainout and local convection. Spatial δ18Op variations and atmospheric back trajectories show that both eastern‐ and western‐derived air masses bring precipitation to southern Peru. Seasonal d‐excessp cycles record moisture recycling and relative humidity at remote moisture sources, and both d‐excessp and ∆′17Op clearly differentiate evaporated and non‐evaporated samples. These results begin to establish the natural range of unevaporated ∆′17Op values in the central Andes and set the foundation for future paleoclimate and paleoaltimetry studies in the region. This study highlights the hydrologic understanding that comes from a combination of δ18Op, d‐excessp, and ∆′17Op data and helps identify the evaporation, recycling, and rainout processes that drive water cycling in the central Andes.

Reinterpreting Precipitation Stable Water Isotope Variability in the Andean Western Cordillera Due To Sub‐Seasonal Moisture Source Changes and Sub‐Cloud Evaporation

“…They were, however, lower than mean observations on the Altiplano which characterize up‐wind precipitation: d‐ex = 15‰ at Pamphuta in Aron et al. (2021) and d‐ex = 17‰ at Oruro in Fiorella et al. (2015).…”

“…The atmospheric models overwhelmingly indicate that the dominant moisture transport is from the east, via moist upper‐atmosphere easterlies over high Andean elevations (e.g., Falvey & Garreaud, 2005). However, isotopic studies claim a significant contribution from westerly Pacific moisture influence due to enriched observations (e.g., Aron et al., 2021). Northerly moisture influence is not often discussed in previous studies, either because it has been overlooked or it has only recently become significant due to climate change induced increased convective atmospheric instability in the Western Amazon (Segura et al., 2020).…”

“…Aron et al. (2021) used back trajectory analysis to interpret annual mean rain water isotopes from Arequipa as a mix of enriched moisture from the Pacific and depleted moisture from the east. Earlier observations also assumed that enriched isotopic values indicated Pacific moisture influence (Aravena et al., 1999), and some event‐specific observations confirmed Pacific moisture‐sourced enriched precipitation along the coast (Jordan et al., 2018).…”

“…(2015). Many processes influence d‐ex in this region in addition to sub‐cloud evaporation including changing moisture sources, ice crystal formation in deep convection, condensation from upper troposphere water vapor, and airmass mixing which makes it challenging to identify a baseline d‐ex for unevaporated precipitation and to interpret temporal variability (Aron et al., 2021 and references therein).…”

“…For example, Eastern Cordillera studies have shown that during a strong South American Low‐Level Jet (SALLJ), a southward flow of low‐level winds along the eastern Andes, that easterly winds also strengthen and high altitude precipitation is more depleted in the heavy isotopes (Guy et al., 2019; Vimeux et al., 2011). Precipitation isotopic variation in the Western Cordillera of the Peruvian Andes is less studied with few published datasets from southern Peru and northern Chile (Aravena et al., 1999; Aron et al., 2021; Bershaw et al., 2016; Fritz et al., 1981; Herrera et al., 2018; Jordan et al., 2018; Valdivielso et al., 2020). Processes controlling isotopic fractionation of vapor transport and precipitation in the Western Cordillera are poorly understood and the interplay between easterly, westerly, and northerly sources makes interpreting elevation gradients and inter‐annual and intra‐annual variability more complicated.…”