Radiocarbon signatures of carbon phases exported by swiss rivers in the Anthropocene

Lateral carbon mobilization processes are particularly prone to anthropogenic perturbations due to human intervention of the land surface as well as aquatic corridors yet remain poorly constrained despite their importance as a key component of the C-cycle. A major source of this uncertainty involves processes modulating the sources and fate of carbon along the freshwater aquatic continuum. This study examines the radiocarbon (14C) signatures of particulate and dissolved organic carbon (POC, DOC, respectively) and dissolved inorganic carbon (DIC) transported by Swiss rivers with a view to assessing controls on the origin and cycling of carbon within corresponding watersheds. Twenty-one rivers were selected that span a range of watershed properties and are monitored by the National Long-Term Surveillance of Swiss Rivers (NADUF) program, enabling radiocarbon data to be interpreted within a broader hydrological and geochemical context.  Samples were collected during high-flow conditions in summer 2021, a year of extreme rain events. Average discharge of our sample set (n=21) was 299.76 +/- 513.87 m³/s, while the annual average discharge of all stations was 184.90 ± 264.92 m³/s. The range of D14C values of POC was -158‰ to - 446‰ (n = 21), while corresponding ranges of  D14C values for DOC and DIC were - 43‰ to - 377‰ and - 40‰ to - 301‰, respectively, indicating the presence of pre-aged carbon in all three pools. Hydrological properties such as discharge, runoff and precipitation did not appear as major significant controlling factors. Except for DI14C where annual average runoff of the past decade showed a significant negative correlation. Instead, based on Multivariate Regression Analysis, “alpine” variables such as mean basin elevation, slope, and barren areas were negatively correlated with D¹⁴C values of all three D14C-phases, while rivers draining lower elevation terrain, where agricultural land-use is more extensive were associated with higher D¹⁴C values in -the organic carbon pools (POC, DOC). Repeated sampling under different hydrological conditions and associated 14C (and 13C) measurements are being used to provide additional insights into the controls on the amount and nature of carbon exported by Swiss rivers, the interconnectivity between different carbon pools within the corresponding drainage basins and ecoregions, as well as to predict long-term trends in the context of changing climate and anthropogenic forcing.