Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin

Abstract

The exposure of organic carbon in rocks to oxidative weathering can release carbon dioxide (CO₂) to the atmosphere and consume atmospheric oxygen. Alongside volcanism, metamorphism, and the weathering of carbonate minerals by sulfuric acid, this is a major source of atmospheric CO₂ over million year timescales. The balance between CO₂ release and CO₂ drawdown by silicate weathering and organic carbon burial sets the net geochemical carbon budget during weathering and erosion. However, the rates of rock-derived organic carbon (petrogenic organic carbon, OC_petro) oxidation remain poorly constrained. Here, we use rhenium as a proxy to trace and quantify CO₂ release by OC_petro oxidation in the Mackenzie River Basin, Canada, where the other carbon fluxes have been well constrained previously. River water and sediment samples were collected between 2009 and 2013 at gauging stations along the Mackenzie River and its main tributaries (Liard, Peel and Arctic Red). To assess rhenium inputs from silicate, sulfide and OC_petro mineral phases we normalize dissolved rhenium concentrations, [Re]_diss, to sodium and sulfate ion concentrations. This approach suggests that >85 percent of [Re]_diss is derived from OC_petro in the main river channels. [Re]_diss and water discharge measurements are used to quantify dissolved Re yields. River sediments provide a measure of the Re to OC_petro ratio of materials undergoing weathering in the basin, and agree well with published rock samples. Dissolved Re yields are combined with river sediment [Re]/[OC_petro] ratios to estimate the CO₂ emissions by OC_petro weathering. These are 0.45 ^+0.19/_−0.11 metric tonnes of carbon, tC km⁻² yr⁻¹for the Mackenzie River at Tsiigehtchic (3.8 ^+1.5/_−0.9 × 10⁴ moles km⁻² yr⁻¹), and 0.94 ^+0.41/_−0.26 tC km⁻² yr⁻¹, 0.78 ^+0.35/_−0.21 tC km⁻² yr⁻¹ and 1.01 ^+0.42/_−0.25 tC km⁻² yr⁻¹ for the Peel, Arctic Red and Liard catchments, respectively. When considered alongside published silicate and carbonate weathering rates and the sedimentary burial of biospheric organic carbon, these data suggest that the upper part of the Mackenzie River Basin presently acts as an atmospheric CO₂ sink of ∼1 tC km⁻² yr⁻¹ (∼8 × 10⁴ moles km⁻² yr⁻¹) as a result of the carbon transfers by weathering and erosion. During the Last Glacial Maximum, it is possible that the net geochemical carbon balance may have been very different: potential increases in CO₂ emissions from oxidative weathering of OC_petro and carbonate minerals, coupled with reduced biospheric carbon burial, may have tipped the balance to a net source of CO₂.