Forests, rain and runoff: riverine particulate organic matter in the Pacific North-West and its impact on the Earth's thermostat

Export and burial of carbon recently fixed from the atmosphere by plants and soils (as opposed to fossil carbon eroded from bedrock) transfers carbon dioxide from the atmosphere into geological storage. Recent studies suggest that storm-driven erosion of terrestrial biomass (principally through large, deep-seated landslides) can effectively sequester carbon in tectonically and climatically extreme regimes. However, as the contribution of more typical continental terrain remains poorly constrained, it is difficult to evaluate the importance of biomass erosion on a global scale. Moreover, there is insufficient understanding of the processes which mobilise particulate organic matter (POM), its sources and initial pathways and their variation under different hydrologic conditions. We address these issues in the temperate montane forests of Oregon's Coast and Cascade Ranges. We have obtained the C and N concentrations and stable isotope ratios of ~200 samples of POM in riverine suspended sediment from four watersheds, varying in size, geology, climate and vegetation. According to our measurements, the riverine POM comes from multiple carbon stores, which are characterised by distinct values of N/C, δ13C and δ15N, and is mixed during mobilisation and transport. Considerable amounts of POM are mobilised under moderate flow conditions (occurring several times a year), which do not trigger major landslides. Instead, runoff erosion is the principal mobilisation mechanism. We compare the composition of POM in Oregon headwaters to a similar dataset from the Swiss Alps, showing that suspended POM from each location contains both non-fossil and fossil components. Moreover, we show that vegetation is the primary non-fossil end member for the Oregon samples. This contrasts with the Swiss location, where all standing biomass appears to be homogenised through a soil 'window' before erosion. Our findings demonstrate the potential for significant export of POM (in particular, non-fossil POM) from steep uplands wherever there is rain on soil and vegetation. Given the vast area meeting these conditions, we suggest that erosion of continental biomass may be more important to the global carbon cycle than previously thought, and explore potential feedbacks in the Earth's climate system in light of the links between POM transfer, runoff and rainfall.