While much of the dissolved organic carbon (DOC) within rivers is destined for mineralization to CO₂, a substantial fraction of riverine bicarbonate (HCO₃^-) flux represents a CO₂ sink, as a result of weathering processes that sequester CO₂ as HCO₃^-. We explored landscape-level controls on DOC and HCO₃^- flux in subcatchments of the boreal, with a specific focus on the effect of permafrost on riverine dissolved C flux. To do this, we undertook a multivariate analysis that partitioned the variance attributable to known, key regulators of dissolved C flux (runoff, lithology, and vegetation) prior to examining the effect of permafrost, using riverine biogeochemistry data from a suite of subcatchments drawn from the Mackenzie, Yukon, East, and West Siberian regions of the circumboreal. Across the diverse catchments that we study, controls on HCO₃^- flux were near-universal: runoff and an increased carbonate rock contribution to weathering (assessed as riverwater Ca:Na) increased HCO₃^- yields, while increasing permafrost extent was associated with decreases in HCO₃^-. In contrast, permafrost had contrasting and region-specific effects on DOC yield, even after the variation caused by other key drivers of its flux had been accounted for. We used ionic ratios and SO₄ yields to calculate the potential range of CO₂ sequestered via weathering across these boreal subcatchments, and show that decreasing permafrost extent is associated with increases in weathering-mediated CO₂ fixation across broad spatial scales, an effect that could counterbalance some of the organic C mineralization that is predicted with declining permafrost.