Tank et al. Dissolved C flux from circumboreal catchments Table S2: End-member values used for the mixing model analysis, as described in the main text. Ca/Na Mg/Na HCO3/Na Sr/Na*10^3 Cl/Na FEL Mean 0.35 0.24 2 3 0.001 CARB Mean 50 20 100 75 0.001 EVAP Mean 0.06 0.02 0.3 0.26 0.97 BAS Mean 1.23 0.8 4.43 5 0.001 SGW Mean 0.12 0.005 0.02 15 1.24 FEL SD 0.075 0.06 0.5 0.5 0.00002 CARB SD 10 4 20 12.5 0.00002 EVAP SD 0.03 0.01 0.15 0.105 0.04 BAS SD 0.7 0.54 2.22 1 0.00002 SGW SD 0.06 0.0025 0.01 7.5 0.62 LEGEND AND DATA SOURCES: FEL: Felsic silicates (Yukon, WSib, ESib). Millot et al. [2003] CARB: Carbonates (Yukon, WSib, ESib). Millot et al. [2003] EVAP: Evaporites (Yukon, WSib, Esib). Global average from Shoukhar-Stash et al [2007] and Millot et al [2003; Salt R.] BAS: Basaltic silicates (Yukon): Average of values derived from Dessert et al. [2003] and Pokrovsky et al. [2005] SGW: Saline groundwater (WSib). Values from Kurchikov and Plavnik [2009] MEAN: Mean endmember value SD: Standard deviation of endmember value REFERENCES: Dessert, C., B. Dupre, J. Gaillardet, L. M. Francois, and C. J. Allegre (2003), Basalt weathering laws and the impact of basalt weathering on the global carbon cycle, Chem. Geol., 202, 257-273, doi: 10.1016/j.chemgeo.2002.10.001. Kurchikov, A. R., and A. G. Plavnik (2009), Clustering of groundwater chemistry data with implications for reservoir appraisal in West Siberia, Russian Geology and Geophysics, 50, 943-949, doi: 10.1016/j.rgg.2009.10.003. Millot, R., J. Gaillardet, B. Dupre, and C. J. Allegre (2003), Northern latitude chemical weathering rates: Clues from the Mackenzie River Basin, Canada, Geochim. Cosmochim. Acta, 67, 1305-1329. Pokrovsky, O. S., J. Schott, D. I. Kudryavtzev, and B. Dupre (2005), Basalt weathering in Central Siberia under permafrost conditions, Geochim. Cosmochim. Acta, 69, 5659-5680, doi: 10.1016/j.gca.2005.07.018. Shouakar-Stash, O., S. V. Alexeev, S. K. Frape, L. P. Alexeeva, and R. J. Drimmie (2007), Geochemistry and stable isotopic signatures, including chlorine and bromine isotopes, of the deep groundwaters of the Siberian Platform, Russia, Appl. Geochem., 22, 589-605, doi: 10.1016/j.apgeochem.2006.12.005.