Spatio-temporal dimensions of organic carbon-mineral interactions in a source-to-sink system

Rivers play a key role in the global carbon cycle by transferring organic carbon (OC) from the terrestrial biosphere to marine sediments, which act as an important long-term carbon sink. Associations between biospheric OC and mineral phases can alter OC stability and hydrodynamic properties, thereby influencing its transport and storage patterns within a river basin and dictating the fate of terrestrial biospheric OC discharged to the ocean. While research has mainly investigated the formation of these associations within soils, open questions remain on how these interactions evolve over space and time.

The Fraser River Basin in British Columbia, Canada drains regions with distinctive lithological and climatic gradients allowing the simultaneous study of leaf-wax-specific isotopic compositions (δ²H, δ¹³C) and inorganic geochemical signatures (εNd) of sediments as tracers of the provenance of biospheric OC and detrital mineral phases, respectively. In addition, bulk radiocarbon (D¹⁴C) serves as a tool to constrain biospheric OC residence times. Here, to investigate seasonal variations in the geochemical signatures of OC and its mineral host in sediments exported by the Fraser River to the Strait of Georgia using samples from a time-series sediment trap deployed cover the course of a year adjacent to the river mouth.

Both εNd and D¹⁴C follow a seasonal pattern by which aged OC (-176 to -140‰) is mostly transported during high discharge events such as the freshet in June and heavy rainstorms occurring in the upper basin in fall. During the former event, the geochemical signature (ε_Nd: -9.2 to -7.2) points towards the Coastal Range affecting the detrital mineral composition more strongly, which shifts towards a greater proportion of Rocky Mountains-sourced sediment (ε_Nd: -11.7 to -9.1) during the second high discharge event. Biomarker specific δ2H will further elucidate the extent to which the provenance of biospheric OC coincides with the mineral detrital load. Further, comparison with geochemical signatures of fluvial sediments within the Fraser basin, together with corresponding signatures in river-proximal sediments deposited in the Strait of Georgia allow for OC-mineral interactions to be assessed from a source-to-sink perspective.

This coupled investigation of organic and inorganic tracers provides new insights into terrestrial organic carbon export and the role of organo-mineral interactions on riverine organic carbon dynamics. These findings also have important ramifications for the interpretation of sedimentary archives.