Articles | Volume 5, issue 4
Research article
03 Nov 2017
Research article |  | 03 Nov 2017

Model predictions of long-lived storage of organic carbon in river deposits

Mark A. Torres, Ajay B. Limaye, Vamsi Ganti, Michael P. Lamb, A. Joshua West, and Woodward W. Fischer

Abstract. 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.

Short summary
In this paper, we describe a new model for the storage times of sediments and organic carbon (OC) in river deposits. Comparisons between our model predictions and field data show good agreement, which suggests that our model accurately captures the relevant time and space scales. An implication of our model is that OC is stored in river deposits over geologic timescales and, as a result, we propose that fluvial storage plays a larger role in the carbon cycle than previously recognized.