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

Horan, Kate and Hilton, Robert G. and Dellinger, Mathieu and Tipper, Ed and Galy, Valier and Calmels, Damien and Selby, David and Gaillardet, JérÔme and Ottley, Chris J. and Parsons, Daniel R. and Burton, Kevin W. (2019) Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin. American Journal of Science, 319 (6). pp. 473-499. ISSN 0002-9599 EISSN: 1945-452X DOI

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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 OCpetro 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 OCpetro 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 +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 OCpetro and carbonate minerals, coupled with reduced biospheric carbon burial, may have tipped the balance to a net source of CO2.

Item Type: Article
Uncontrolled Keywords: 2019AREP; IA76
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Journal or Publication Title: American Journal of Science
Volume: 319
Page Range: pp. 473-499
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 16 Apr 2020 00:52
Last Modified: 16 Apr 2020 00:52

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