Riverine export of particulate organic carbon (POC) to the ocean affects the atmospheric carbon inventory over a broad range of timescales1,2,3,4,5. On geological timescales, the balance between sequestration of POC from the terrestrial biosphere and oxidation of rock-derived (petrogenic) organic carbon sets the magnitude of the atmospheric carbon and oxygen reservoirs6,7. Over shorter timescales, variations in the rate of exchange between carbon reservoirs, such as soils and marine sediments, also modulate atmospheric carbon dioxide levels1. The respective fluxes of biospheric and petrogenic organic carbon are poorly constrained, however, and mechanisms controlling POC export have remained elusive, limiting our ability to predict POC fluxes quantitatively as a result of climatic or tectonic changes. Here we estimate biospheric and petrogenic POC fluxes for a suite of river systems representative of the natural variability in catchment properties. We show that export yields of both biospheric and petrogenic POC are positively related to the yield of suspended sediment, revealing that POC export is mostly controlled by physical erosion. Using a global compilation of gauged suspended sediment flux, we derive separate estimates of global biospheric and petrogenic POC fluxes of and megatonnes of carbon per year, respectively. We find that biospheric POC export is primarily controlled by the capacity of rivers to mobilize and transport POC, and is largely insensitive to the magnitude of terrestrial primary production. Globally, physical erosion rates affect the rate of biospheric POC burial in marine sediments more strongly than carbon sequestration through silicate weathering. We conclude that burial of biospheric POC in marine sediments becomes the dominant long-term atmospheric carbon dioxide sink under enhanced physical erosion.
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We thank Y. Godderis, J. Hemingway and G. Soulet for comments on early versions of the manuscript. G. Fiske generated the NPP data. Support for this project was provided by US National Science Foundation (NSF) grant OCE-0851015 (to B.P.-E., T.E. and V.G.), NSF grant OCE-0928582 (to V.G. and T.E.) and Swiss National Science Foundation grant 200021_140850 (to T.E.).
The authors declare no competing financial interests.
Extended data figures and tables
Basin-averaged NPP estimates were derived from the MOD17 database24.
Extended Data Figure 2 Global relationship between long-term CO2 sequestration yield and sediment yield.
CO2 sequestration through terrestrial biospheric POC burial (black dots; YCorg) is more sensitive to sediment yield than CO2 sequestration through silicate weathering (grey crosses; YCsil). At high physical erosion rates (that is, high sediment yield), the burial of terrestrial biospheric POC becomes the dominant long-term atmospheric CO2 sink. The dotted lines show CO2 sequestration through terrestrial biospheric POC burial for the entire set of biospheric POC export data (Fig. 2), assuming constant burial efficiencies (BE) of 30 and 100%. CO2 sequestration data through silicate weathering are from Gaillardet et al.40. P = 0.001; r2 = 0.80.
Extended Data Figure 3 Organic carbon and radiocarbon contents of bulk suspended sediments and grain size fractions in the Mississippi River.
Results are expressed as modern organic carbon (that is, the product of modern fraction (Fm) and organic carbon content). The linear best fit gives the absolute petrogenic OC content (0.05%) as well as the Fm of the biospheric POC (0.88). Data from Wakeham et al.31 and Rosenheim et al.39. P = 0.001; r2 = 0.99.
Catchment size, suspended sediment yield, petrogenic organic carbon yield, biospheric organic carbon yield, net primary productivity and fraction of the net primary productivity exported annually. The source of the data used to calculate petrogenic organic carbon yield and biospheric organic carbon yield from geochemical constraints is in column K. The methods are described in column K. (XLSX 42 kb)
Catchment size, suspended sediment yield, petrogenic organic carbon yield, biospheric organic carbon yield, net primary productivity and fraction of the net primary productivity exported annually. Petrogenic organic carbon yield calculated using the relationship between suspended sediment yield and petrogenic organic carbon yield presented Figure 1. Biospheric organic carbon yield calculated by difference between total organic carbon yield and petrogenic organic carbon yield. The source of the total organic carbon yield data is in column K. (XLSX 38 kb)
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Galy, V., Peucker-Ehrenbrink, B. & Eglinton, T. Global carbon export from the terrestrial biosphere controlled by erosion. Nature 521, 204–207 (2015). https://doi.org/10.1038/nature14400
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