Export and sources of organic carbon in the Lena River basin, Northeastern Siberia

Sammanfattning: Permafrost areas are considered to be one of the largest terrestrial storages of carbon. In a warming climate these areas are expected to experience changes in carbon transport to rivers and the oceans due to permafrost thawing, which could enhance erosion, change water flow pathways and increase greenhouse gas emissions. Large amounts of the carbon transported from the terrestrial environment to rivers are in the form of natural organic matter (NOM). The Lena River basin in northeastern Siberia, which is mainly underlain by continuous permafrost, is the largest contributor of NOM to the Arctic Ocean. In this study we present a spatial data set of NOM, including concentrations and stable carbon isotope values (δ13C) of dissolved (DOC) and particulate organic carbon (POC) as well as carbon and nitrogen ratios (C/N) from 77 sample stations in the Lena River and its tributaries. The samples were collected during two field seasons in July 2012 and June 2013.The results from this study showed large spatial variations in concentrations, annual export and fluxes of organic carbon. These variations were primarily due to variations in discharge and topography. The δ13C and C/N indicated that terrestrial sources such as plants and soil organic matter (SOM), were the main sources of the dissolved organic matter (DOM), while particulate organic matter (POM) was mainly derived from aquatic produced material or SOM. There were clear differences in δ13C and C/N of DOM between sampling years, indicating more surficial flow pathways in samples collected earlier in the summer compared to samples collected later in the summer. The δ13C of POM was correlated with water temperatures and topography, showing that tributaries with origin in mountainous areas in general had soil derived POM and lower water temperatures, while tributaries from lowland areas had higher water temperatures and more influence of aquatic sources. We suggest that this pattern is probably due to differences in water flow pathways. Shifts in export of NOM from drainage areas underlain by permafrost will likely be dependent of spatial changes in hydroclimate and the depth of the active layer in a warming climate.