Relationships between Δ¹⁴C and the molecular quality of dissolved organic carbon in rivers draining to the coast from the conterminous United States

Dissolved organic carbon (DOC) in natural waters possesses chemical and molecular qualities indicative of its source and age. The apportionment of DOC by age into millennial and decadal pools is necessary to understand the temporal connection between terrestrial and aquatic ecosystems in the global carbon cycle. We measured Δ¹⁴C-DOC and chemical composition indices (specific ultraviolet absorbance (SUVA₂₅₄), fluorescence index (FI), hydrophobic organic acid fraction (HPOA) content) for 15 large river basins in the conterminous United States. Across all rivers the average proportion of HPOA in DOC correlated strongly with SUVA₂₅₄ (r² = 0.93 p Modern) in the Altamaha River for the year 2009. When adjusted by total discharge, these U.S. Rivers export modern carbon at between 34 and 46‰, a signal dominated by the Mississippi River. The variation in Δ¹⁴C correlates to indices of the aromaticity of the DOC measured by the SUVA₂₅₄ (r² = 0.87, p