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Erosion of organic carbon in the Arctic as a geological carbon dioxide sink



Soils of the northern high latitudes store carbon over millennial timescales (thousands of years) and contain approximately double the carbon stock of the atmosphere1,2,3. Warming and associated permafrost thaw can expose soil organic carbon and result in mineralization and carbon dioxide (CO2) release4,5,6. However, some of this soil organic carbon may be eroded and transferred to rivers7,8,9. If it escapes degradation during river transport and is buried in marine sediments, then it can contribute to a longer-term (more than ten thousand years), geological CO2 sink8,9,10. Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers at high latitudes remains poorly constrained. Here, we quantify the source of POC in the Mackenzie River, the main sediment supplier to the Arctic Ocean11,12, and assess its flux and fate. We combine measurements of radiocarbon, stable carbon isotopes and element ratios to correct for rock-derived POC10,13,14. Our samples reveal that the eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5,800 ± 800 years, much older than the POC in large tropical rivers13,14. From the measured biospheric POC content and variability in annual sediment yield15, we calculate a biospheric POC flux of teragrams of carbon per year from the Mackenzie River, which is three times the CO2 drawdown by silicate weathering in this basin16. Offshore, we find evidence for efficient terrestrial organic carbon burial over the Holocene period, suggesting that erosion of organic carbon-rich, high-latitude soils may result in an important geological CO2 sink.

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Figure 1: Source of POC in the Mackenzie River basin.
Figure 2: Transport of POC in the Mackenzie River.
Figure 3: Fate of particulate organic carbon offshore.


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Radiocarbon measurements were funded by the Natural Environment Research Council (NERC), UK (Allocation 1611.0312) to R.G.H and C.B. Fieldwork was funded by CNRS (OXYMORE and CANNIBALT) to J.G. and R.G.H., the Woods Hole Oceanographic Institution Arctic Research Initiative to V.G. and an Early Career Research Grant by the British Society for Geomorphology to R.G.H. V.G. was supported by the US National Science Foundation (OCE-0928582) and H.C. by a Royal Society University Fellowship. The research was carried out under Scientific Research Licence No. 14802 issued by the Aurora Research Centre, who we thank for logistical support (in particular D. Ross and J. Gareis). We also thank I. Peters for preparation of offshore borehole samples, C. Johnson, X. Philippon and M. Bollard for analytical assistance, E. Tipper and K. Hilton for field assistance and discussions and D. Ofukany, G. Lennie, R. Wedel and R. Pilling of Environment Canada for loan of equipment.

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Authors and Affiliations



R.G.H., V.G. and J.G. conceived the study and R.G.H., J.B., D.C., V.G. and M.D. designed the fieldwork and collected the river samples. M.O. and H.C. collected sediment and carbonate data from the offshore borehole. R.G.H., V.G., M.D., C.B. and D.G. processed the samples and carried out the geochemical analyses. R.G.H. wrote the manuscript with input from all co-authors.

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Correspondence to Robert G. Hilton.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The location of river depth profiles collected from the Mackenzie River.

Three locations along the Mackenzie River were sampled (circles) at the delta (black), Tsiigehtchic (grey) and Norman Wells (white) in addition to the major tributaries, the Liard River (red diamond), Arctic Red River (light blue square) and Peel River (dark blue square). The location of the sediment core MTW01 from the Mackenzie trough is shown (triangle). a, Major river channels (black lines) overlain on digital elevation model GMTED 15 arcsec with upstream sediment source catchment areas delineated by flow accumulation and flow direction outputs from the digital elevation model (dotted lines). The Great Slave Lake is indicated upstream of the Liard confluence and acts as an effective sediment trap in the basin15. b, Permafrost zone coverage in the upstream areas of the basin42. White rectangle shows the sample locations near the Mackenzie delta displayed in c, overlain on LANDSAT imagery.

Extended Data Figure 2 Source of particulate organic carbon in the Mackenzie River basin.

a, Radiocarbon content (reported as Fmod) as a function of the stable isotope ratio of organic carbon (δ13Corg) of river sediments for the Mackenzie River (circles) and its major tributaries (diamonds and squares) for suspended load samples from river depth profiles (filled symbols) and river bed materials (open symbols). Dashed lines and shaded regions show hypothetical compositions produced by mixing rock-derived POCpetro40 and POCbiosphere41. b, Fmod as a function of Al/OCtotal. High Al/OCtotal and low Fmod correspond to the petrogenic source of POC (POCpetro). Linear trends are shown for the Peel and Arctic Red rivers (blue, y = (−1.5 ± 0.3 × 10−6)x + (0.85 ± 0.11), r2 = 0.85, P < 0.02), the Mackenzie River at delta (black, y = (−5.9 ± 0.5 × 10−6)x + (0.65 ± 0.03), r2 = 0.95, P < 0.001), and the Mackenzie and Liard rivers (grey, y = (−2.3 ± 0.3 × 10−6)x + (0.56 ± 0.03), r2 = 0.82, P < 0.001). The intercepts at Fmod = 0 for POCpetro are given with uncertainty (±1 s.d.) and are different for each sub-basin, reflecting the distribution of organic carbon-rich rocks in the Mackenzie mountains40. c, Measured δ13Corg versus those predicted by the endmember mixing model (EMM-predicted) (equations (1) and (2); Methods). The good agreement between measured and predicted values within the uncertainty on the measurements suggests that mixing of POCpetro and POCbiosphere can explain the first-order variability in δ13Corg values between catchments and between suspended load and river bed materials.

Extended Data Figure 3 Radiocarbon age of biospheric particulate organic carbon in the Mackenzie River derived from the mixing analysis.

The number of POCbiosphere measurements of a given range of 14C ages is shown for each sampling location as a narrow rectangle. The distribution of published basal peat sample 14C ages for the Mackenzie River basin25 is shown as wide rectangles.

Extended Data Figure 4 River particulate organic carbon in the Mackenzie basin.

Organic carbon concentration as a function of Al/Si, which is a function of grain size in the Mackenzie River basin26. Analytical errors (2 s.d.) are shown as grey lines if larger than the point size.

Extended Data Figure 5 Stable isotope composition and nitrogen to organic carbon ratio of terrestrial and marine sediments.

Suspended sediments from the Mackenzie River (circles) at the delta (black), Tsiigehtchic (grey) and Norman Wells (white) are shown. Marine sediment samples from the MTW01 sediment core (this study, triangles, black <63 µm, grey >63 µm) are shown with published surface sediment samples from the Beaufort Sea (white triangles) and Davis Strait (black squares)7,29. The terrestrial POC field shows an indicative range of values measured in the Mackenzie River. The marine OC field shows values expected for Arctic Ocean marine OC. Analytical errors (2 s.d.) are shown as grey lines if larger than the point size.

Extended Data Table 1 River suspended sediment and bed material samples from the Mackenzie basin in 2009–2011
Extended Data Table 2 Sediment samples from the offshore core MTW01
Extended Data Table 3 River bank samples from the Mackenzie River in 2009

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Hilton, R., Galy, V., Gaillardet, J. et al. Erosion of organic carbon in the Arctic as a geological carbon dioxide sink. Nature 524, 84–87 (2015).

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