The mass of carbon stored as organic matter in terrestrial systems is sufficiently large to play an important role in the global biogeochemical cycling of CO2 and O2. Field measurements of radiocarbon-depleted particulate organic carbon (POC) in rivers suggest that terrestrial organic matter persists in surface environments over millennial (or greater) timescales, but the exact mechanisms behind these long storage times remain poorly understood. To address this knowledge gap, we developed a numerical model for the radiocarbon content of riverine POC that accounts for both the duration of sediment storage in river deposits and the effects of POC cycling. We specifically target rivers because sediment transport influences the maximum amount of time organic matter can persist in the terrestrial realm and river catchment areas are large relative to the spatial scale of variability in biogeochemical processes.
Our results show that rivers preferentially Erode young deposits, which, at steady state, requires that the oldest river deposits are stored for longer than expected for a well-mixed sedimentary reservoir. This geometric relationship can be described by an exponentially tempered power-law distribution of sediment storage durations, which allows for significant aging of biospheric POC. While OC cycling partially limits the effects of sediment storage, the consistency between our model predictions and a compilation of field data highlights the important role of storage in setting the radiocarbon content of riverine POC. The results of this study imply that the controls on the terrestrial OC cycle are not limited to the factors that affect rates of primary productivity and respiration but also include the dynamics of terrestrial sedimentary systems.
Bibliographical noteFunding Information:
Acknowledgements. All authors thank James Pizzuto and one anonymous reviewer for providing helpful comments that greatly improved the overall quality of the paper. Mark A. Torres acknowledges support from a Caltech Texaco postdoctoral research fellowship and the California Alliance for the Graduate and Professoriate. Ajay B. Limaye acknowledges support from National Center for Earth-surface Dynamics 2 (NCED2) Synthesis Postdoctoral Program. Vamsi Ganti acknowledges support from the Imperial College London’s Junior Research Fellowship. This work was supported by an award from the National Science Foundation to Michael P. Lamb (EAR 1427177) and the Caltech Discovery Fund. All authors acknowledge the participants of the GE126 course at Caltech (co-taught by Woodward W. Fischer, Michael P. Lamb, and A. Joshua West) for providing some of the early ideas that led to this work. Helpful criticism of a draft version of this paper was supplied by Joel Schiengross.
© 2017 Author(s).