Particulate organic carbon mobilisation and exportfrom temperate forested uplands

The transfer of organic carbon stored in continental biomass to geologi- cal storage is an important pathway in the global carbon cycle, with the potential to sequester signiﬁcant amounts of carbon dioxide. Despite an initial focus on active mountain belts as prime locations for such erosion, temperate forested uplands also have a signiﬁcant role to play. This the- sis investigates the origins, mobilisation and export of particulate organic carbon (POC) in the headwaters of two representative temperate areas, the Swiss Prealps and Western Oregon, and addresses the signiﬁcance of the results in a global context. Broadly, organic carbon concentration as a percentage of the suspended load is inversely correlated with clastic yield. Mean values in natural catchments range from 1.5% in Switzerland to 10% in Oregon. By chem- ically ﬁngerprinting this POC and major organic carbon stores within each catchment, using carbon and nitrogen elemental and stable isotopic compositions, its provenance is determined. By monitoring its changing concentration and composition over a range of discharge, the processes by which it is mobilised are elucidated. In Switzerland, additional meth- ods including radiocarbon analysis, Raman spectroscopy and biomarker geochemistry add further insights into sources and pathways. Riverine POC in Switzerland derives from binary mixing between bedrock and modern biomass with a soil-like composition, with little direct input of plant matter. The hillslope and actively incising channel are strongly coupled, allowing overland ﬂow to deliver biogenic material directly to the stream beyond a moderate discharge threshold. At this point, the broad trend is reversed; the proportion of organic carbon now increases with discharge and suspended sediment concentration. At higher ﬂows, more biomass is mobilised and the fraction of modern organic carbon inthe suspended load reaches 0.70, increased from 0.30 during background conditions. In Oregon, little fossil organic carbon enters the suspended load even where it is present. Instead, riverine POC derives from mixing between soil-like material and foliage from a variety of plants. Overland ﬂow rarely develops and hillslopes are isolated from channels. Material comes instead from the channel itself and immediately adjacent areas. There is no systematic switch to POC addition; instead, continued dilution by clastic material is observed as discharge increases. Signiﬁcant amounts of non-fossil organic carbon are thus mobilised in both areas without the need for extreme events such as landsliding. Precipita- tion is key: as soon as the rain stops, biomass supply ceases and fossil carbon again dominates in Switzerland, while Oregon streams run clear once more. Relationships with discharge are used where possible to calculate long- term export ﬂuxes of total and non-fossil POC. In the most active Swiss catchment, rating curves are integrated over 29-year discharge records, giving ﬂuxes of 23±6 t km −2 yr −1 and 14±5 t km −2 yr −1 respectively. On the order of 6 t km −2 yr −1 of total POC are exported from the Oregon Cas- cades, of which ∼100% is non-fossil. These represent near-end members of POC export in temperate forested uplands, with the other catchments forming a continuum between them. One Oregon catchment subjected to intensive recent logging shows dramatically diﬀerent behaviour, marked by high clastic yield and a much-reduced fraction of modern organic carbon. Ecosystem biology is shown to be the principal control on POC export style, with lithology having a lesser inﬂuence. Yields of non-fossil POC from the temperate forested uplands studied are comparable to those from active mountain belts, yet the processes responsible for them are much more widely applicable. Their collective contribution to global land-ocean POC discharge may be greater than previously thought, and their role in the carbon cycle—including potential Earth–climate feedbacks—more signiﬁcant.