Variable composition of particle‐bound organic carbon in steepland river systems

Abstract

The reported range (<10% to >90%) of modern particulate organic carbon (POC) in the suspended load of steepland rivers is at odds with the opinion that this class of river is an important source of ancient organic carbon. Analysis of all available data shows the composition of POC transported by steepland rivers is fundamentally determined by the relative contributions made by shallow hillslope erosion processes that mobilize soil‐derived modern organic carbon and deep‐seated erosion processes that release bedrock‐derived ancient organic carbon. Attribution of sources during a flood with a return period of >20 years in the Waipaoa River basin, New Zealand, serves to emphasize that by no means all of the POC originating from high‐yield steepland drainage basins is derived from sedimentary rocks. POC in the Waipaoa River is expected to be 14C‐depleted because gully erosion into bedrock ordinarily regulates suspended sediment yield across the entire range of flows. However, during this event the topsoil contribution was more important, ranging from 56% ± 9% on the rising limb to 16% ± 5% on the falling limb of the flood hydrograph and modern organic carbon accounted for 71% of the total POC yield. The amount of ancient organic carbon may be overestimated if the contribution topsoil makes to POC yield across the flood hydrograph is underestimated. Furthermore, to quantify the contribution made by different sources of organic matter in steepland drainage basins, it is necessary to consider the time‐varying signatures and relative contributions each erosion process makes to event POC yields.