Carbon isotopic characterization of dissolved organic carbon in rainwater: Terrestrial and marine influences

This study presents the first published 14C and 13C isotopic data for dissolved organic carbon (DOC) in rainwater. The 14C fraction modern (Fm) values for rain DOC ranged from 0.9051 to 1.0569. Between 4% and 24% of the rainwater DOC was of fossil fuel origin based on a two end member calculation assuming biogenic carbon has the same 14C content as the contemporary atmosphere (Fm=1.11–1.19) and fossil fuel carbon is devoid of 14C. The δ13C content of rain DOC ranged from −21.8‰ to −28.2‰ spanning the terrestrial and fossil fuels average (∼−26‰) to marine (−18‰ to −20‰) values. Rain events with more continental influences had δ13C values closer to the terrestrial and fossil fuels signal and higher concentrations of fossil fuel derived DOC. Rain events with stronger oceanic influences had δ13C values shifting towards those typical of marine systems and had lower concentrations of fossil fuel derived DOC. The influence air mass back trajectory has on the terrestrial and marine characteristics of rainwater DOC identifies air mass recent history as an important factor controlling the sources of DOC in precipitation. It also indicates water soluble volatile organic carbon (VOC's) have relatively short residence times on the order of days with minimal global transport. However, residence times of days, and the presence of both marine and terrestrial DOC in rainwater, suggests that precipitation represents an important exchange of carbon between land and the oceans especially in coastal regions.