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Massive and prolonged deep carbon emissions associated with continental rifting

Abstract

Carbon from Earth’s interior is thought to be released to the atmosphere mostly via degassing of CO2 from active volcanoes1,2,3,4. CO2 can also escape along faults away from active volcanic centres, but such tectonic degassing is poorly constrained1. Here we use measurements of diffuse soil CO2, combined with carbon isotopic analyses to quantify the flux of CO2 through fault systems away from active volcanoes in the East African Rift system. We find that about 4 Mt yr−1 of mantle-derived CO2 is released in the Magadi–Natron Basin, at the border between Kenya and Tanzania. Seismicity at depths of 15–30 km implies that extensional faults in this region may penetrate the lower crust. We therefore suggest that CO2 is transferred from upper-mantle or lower-crustal magma bodies along these deep faults. Extrapolation of our measurements to the entire Eastern rift of the rift system implies a CO2 flux on the order of tens of megatonnes per year, comparable to emissions from the entire mid-ocean ridge system2,3 of 53–97 Mt yr−1. We conclude that widespread continental rifting and super-continent breakup could produce massive, long-term CO2 emissions and contribute to prolonged greenhouse conditions like those of the Cretaceous.

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Figure 1: Distribution of CO2 flux samples and seismicity.
Figure 2: Carbon isotope compositions and concentrations of diffuse CO2 in the Magadi–Natron basin.
Figure 3: Summary of CO2 fluxes throughout the Magadi–Natron basin.

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Acknowledgements

This work was funded by the NSF EAR Tectonics Program, grant numbers 1113066 (T.P.F.), 1113355 (C.J.E.), and 1113677 (S.A.K.). Additional support was provided by Fulbright New Zealand (J.D.M.). CNRS and INSU-supported CoLIBREA project (C.J.E.). We thank the Tanzania COSTECH and the Kenyan National Council for Science and Technology for granting research permits. We acknowledge M. Songo and the Nelson Mandela African Institute of Science and Technology for support. Aerial photographs were provided by the Polar Geospatial Center, University of Minnesota. Aster GDEM is a product of METI and NASA. We gratefully acknowledge support from the Center for Stable Isotopes, UNM. We thank N. Thomas, S. Goldstein, K. Lehnert, B. Onguso, M. Maqway, K. Kimani and the Masai people for help during fieldwork, and A. Van Eaton for helpful comments. We also thank A. Weinstein for earthquake depth analyses, and N. Thomas for entering the geochemical data into the IEDA EarthChem Library.

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H.L., J.D.M., T.P.F. and S.A.K. planned the field campaign to the Magadi–Natron basin; H.L., J.D.M., T.P.F., C.J.E. and G.K. carried out the field work; H.L., J.D.M., T.P.F. and G.K. conducted CO2 flux measurement and collected gas samples; H.L. carried out gas chemistry and carbon isotope analyses; J.D.M. and S.A.K. performed fault analyses to estimate total CO2 flux; C.J.E. analysed broadband seismic data; Z.D.S. supported carbon isotope analyses at the Center for Stable Isotopes, University of New Mexico; H.L., J.D.M., T.P.F., C.J.E. and S.A.K. collaboratively wrote the paper.

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Correspondence to Hyunwoo Lee.

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Lee, H., Muirhead, J., Fischer, T. et al. Massive and prolonged deep carbon emissions associated with continental rifting. Nature Geosci 9, 145–149 (2016). https://doi.org/10.1038/ngeo2622

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