Characterizing moisture origins and ionic contributions in wet deposition samples from the Río de la Plata basin

Abstract

Regional patterns and sources of wet atmospheric deposition are an important aspect of biogeochemical cycling. The isotopic composition of wet atmospheric deposition samples can serve as a characteristic marker, dependent on local geographical and meteorological factors, providing valuable information about the spatial and temporal origins of rainwater and its chemical load. This study aimed to characterize the δ18O and δ2H isotopic composition of rainfall events to evaluate the relative contribution of different possible origins of precipitated moisture and its relationship with the potential origin of ions (Na+, Cl−, Mg2+, SO42−, Ca2+, K+, NO3− and NH4+) in wet atmospheric deposition samples collected from four sites within the Río de la Plata basin region in South America during 2007. Our results revealed significant variability in the isotopic and ionic concentrations among different rainfall events at each site (temporal variability overwhelmed spatial variability). Most ions appeared to share a common origin with the moisture that carried them before deposition. The isotopic values of δ18O and δ2H were primarily influenced by the distance from the ocean, the origin of humidity sources, and, to a lesser extent, the magnitude of rainfall. The average deuterium excess values (10.95‰) suggested that a substantial portion of the rainfall during the study period originated in the Atlantic Ocean rather than from recycling from the Amazon basin. Events with more negative δ18O-values (i.e. more depleted events) appeared to be predominantly oceanic, characterized by higher amounts of Na+, Cl− and Mg2+. Conversely, events with more positive δ18O-values (i.e. more enriched events) exhibited increased concentrations of terrestrial ions, including K+, NO3− and NH4+, indicating an inland moisture source. No clear association was found for SO42− and Ca2+ and rainfall isotopic composition. Hysplit back trajectory analyses showed that air masses traveling across the ocean contain higher amounts of marine ions like Na+ and Cl−. Conversely, air masses transported across the continent incorporated higher amounts of ions with terrestrial origins, such as NO3− and NH4+.Our findings suggest a tight coupling between moisture, ion sources and air mass trajectories in the Río de la Plata basin region.