The age of river-transported carbon: A global perspective

The role played by river networks in regional and global carbon (C) budgets is receiving increasing attention. Despite the potential of radiocarbon measurements (Delta C-14) to elucidate sources and cycling of different riverine C pools, there remain large regions for which no data are available and no comprehensive attempts to synthesize the available information and examine global patterns in the C-14 content of different riverine C pools. Here we present new C-14 data on particulate and dissolved organic C (POC and DOC) from six river basins in tropical and subtropical Africa and compiled > 1400 literature Delta C-14 data and ancillary parameters from rivers globally. Our analysis reveals a consistent pattern whereby POC is progressively older in systems carrying higher sediment loads, coinciding with a lower organic carbon content. At the global scale, this pattern leads to a proposed global median Delta C-14 signature of -203%, corresponding to an age of similar to 1800 years B. P. For DOC exported to the coastal zone, we predict a modern (decadal) age (Delta C-14 = +22 to +46%), and paired data sets confirm that riverine DOC is generally more recent in origin than POC-in contrast to the situation in ocean environments. Weathering regimes complicate the interpretation of C-14 ages of dissolved inorganic carbon, but the available data favor the hypothesis that in most cases, more recent organic C is preferentially mineralized.