Triple oxygen isotope variability of precipitation in a tropical mountainous region

Abstract

We present one year of δD, δ¹⁸O, d-excess, and Δʹ¹⁷O data from monthly precipitation at a Caribbean coastal site in Panama and from tap waters across the country to constrain geographic, climate, and moisture source controls on isotopic variability and better understand the sources and mechanisms of precipitation in Central America, a region facing significant modifications to the annual rainfall cycle due to climate change. Monthly precipitation δD ranged from –52.2 to +14.3 ‰, δ¹⁸O from –7.6 to +0.4 ‰, d-excess from +7.1 to +11.6 ‰, and Δ′¹⁷O from +11 to +29 per meg. Rainy season precipitation samples were found to have lower δD, δ¹⁸O, and d-excess due to Rayleigh distillation during the condensation and rainout of Pacific moisture over the central cordilleras, which results in decoupling between d-excess and Δ′¹⁷O. Outlier Δ′¹⁷O values during peak dry and rainy months may reflect seasonal changes in water vapor sourcing, from Caribbean to Pacific and/or locally recycled moisture, or may be a result of organic contamination. Tap water δD ranged from –82.3 to –14.3 ‰, δ¹⁸O from –11.6 to –2.4 ‰, d-excess from +4.3 to +12.2 ‰ and Δ′¹⁷O from –2 to +84 per meg. Tap water δD and δ¹⁸O values increase eastward due to lower orographic effects and Pacific and locally recycled moisture contributions to rainfall and greater secondary evaporation. Tap water d-excess and Δ′¹⁷O values are also de-coupled but lack clear spatial trends and controls. The results of this study indicate the promise of adding Δ′¹⁷O to the isotopic toolkit in tropical mountainous regions with complicated water cycling dynamics and provide a baseline for future triple oxygen isotope investigations.