Radiocarbon Data Reveal Contrasting Sources For Carbon Fractions In Thermokarst Lakes And Rivers Of Eastern Canada (Nunavik, Quebec)

Greenhouse gas emissions from permafrost organic carbon decomposition in lakes and rivers can accelerate global warming. We used radiocarbon (C-14) measurements to determine the predominant sources of dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), and methane (CH4) in five thermokarst lakes and three rivers in an area of widespread permafrost degradation in Northern Quebec to assess contributions from thawing permafrost and other old carbon (fixed before CE 1950) reservoirs. We compared emission pathways (dissolved gas and ebullition), seasons (summer and winter), and surface soil type (mineral and peat soils). Modern carbon (fixed after CE 1950) was the dominant source of DOC, DIC, and CH4 of non-peatland aquatic systems, while POC and sediment carbon were predominantly fixed in the last millennia. In the peatland systems, modern and permafrost carbon were important sources of DOC, lake DIC, lake ebullition CO2, and lake dissolved CH4. In contrast, POC, lake ebullition CH4, and river DIC were dominated by millennial-old carbon. In winter, the C-14 age of DOC, DIC, and POC in the peatland lakes increased, but the C-14 age of dissolved CH4 did not change. Our results point to a clearly older overall carbon source for ebullition CH4 relative to dissolved CH4 in the peatland lakes but not the non-peatland lakes. The younger ages of dissolved CH4 and DIC relative to DOC and POC in all lakes suggest that recent primary productivity strongly influences the large total lake CH4 and CO2 emissions in this area, as diffusion fluxes greatly exceed ebullition fluxes.