Export of Carbon From the Yukon River and Some of its Major Tributaries

The fate of carbon (C) in the Yukon River, select headwater streams, and major tributaries is being investigated as part of a five-year study of the water quality of the Yukon basin. Preliminary calculations of C exports from the basin indicate that the Yukon River discharged an estimated 7.37 Tg of C during 2001, as measured at Pilot Station, just upstream of the Yukon delta. Approximately 70% of the C load was transported as inorganic C and 30% as organic C. The inorganic C load was almost entirely comprised of dissolved inorganic C (DIC). The relative importance of suspended particulate carbonates in the inorganic C load increases upstream, with the corresponding carbonate content of bar sediment increasing from about 2% by weight at Pilot Station to about 12% at Eagle, AK, about 2400 km upstream, near the Canadian border. Downstream reduction of sediment carbonate content, depletion of dissolved CO₂ in Yukon River water during summer, and under-saturation of river water with respect to calcite suggest dissolution of particulate carbonates in the river. Organic C load at Pilot Station is nearly equally divided between dissolved organic C (DOC) and suspended organic C (SOC). The relatively high proportion of organic C associated with suspended material is most likely due to adsorption of DOC onto suspended sediment and not to particulate biomass. DOC:SOC ratios are much greater in tributaries of the Yukon, such as the Porcupine River, that have relatively small suspended-sediment concentrations. C loads at Pilot Station for 2001 indicate a yield of about 0.46 moles DIC m^-2 yr^-1, 0.10 moles DOC m^-2 yr^-1, and 0.10 moles SOC m^-2 yr^-1 from the Yukon basin as a whole.