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

• Main Authors: Horan, K, Hilton, RG, Dellinger, M, Tipper, E, Galy, V, Calmels, D, Selby, D, Gaillardet, J, Ottley, CJ, Parsons, DR, Burton, KW
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Journal of Science (AJS) 2019
• Subjects: erosion and weathering; carbon cycle; Mackenzie River; petrogenic organic carbon; rhenium; Arctic; Canada; Liard; Tsiigehtchic; Mackenzie river
• Online Access: https://www.repository.cam.ac.uk/handle/1810/315179; https://doi.org/10.17863/CAM.62287

© 2019 American Journal of Science. All rights reserved. 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, OCpetro) oxidation remain poorly constrained. Here, we use rhenium as a proxy to trace and quantify CO2 release by OCpetro 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 OCpetro 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 tro 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 tro ratio of materials undergoing weathering in the basin, and agree well with published rock samples. Dissolved Re yields are combined with river sediment [Re]/[OCpetro] ratios to estimate the CO2 emissions by OCpetro weathering. These are 0.45+019/-0.11 metric tonnes of carbon, tC km2 yr-1 for 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-2yr-1, 0.78+0.35/-0.21tC km-2 yr-1 and 1.01+0.42./-0.25tC km-2 yr-1 for the Peel, Arctic Red and Liard catchments, respectively. When considered alongside published silicate and carbonate weathering rates and ...