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 < 0.001). Individual Δ¹⁴C-DOC ranged from a low of −92.9‰ (726 y.b.p.) in the Colorado River to 73.4‰ (>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 < 0.001), and FI (r² = 0.6; p < 0.001) as well as differences in annual river discharge (r² = 0.46, p < 0.006). SUVA₂₅₄ was further correlated to broad scale vegetation phenology estimated from the Enhanced Vegetation Index derived from the NASA Moderate Resolution Imaging Spectrometer (MODIS). We show that basins with high discharge, high proportions of vegetation cover, and low human population densities export DOC enriched in aromatic material that corresponds to recently fixed atmospheric CO₂. Conversely old DOC is exported from low discharge watersheds draining arid regions, and watersheds more strongly impacted by humans. The potential influence from fossil carbon from human inputs to aquatic systems may be important and requires more research.