Carbon and other volatiles in the form of gases, fluids or mineral phases are transported from Earth’s surface into the mantle at convergent margins, where the oceanic crust subducts beneath the continental crust. The efficiency of this transfer has profound implications for the nature and scale of geochemical heterogeneities in Earth’s deep mantle and shallow crustal reservoirs, as well as Earth’s oxidation state. However, the proportions of volatiles released from the forearc and backarc are not well constrained compared to fluxes from the volcanic arc front. Here we use helium and carbon isotope data from deeply sourced springs along two cross-arc transects to show that about 91 per cent of carbon released from the slab and mantle beneath the Costa Rican forearc is sequestered within the crust by calcite deposition. Around an additional three per cent is incorporated into the biomass through microbial chemolithoautotrophy, whereby microbes assimilate inorganic carbon into biomass. We estimate that between 1.2 × 108 and 1.3 × 1010 moles of carbon dioxide per year are released from the slab beneath the forearc, and thus up to about 19 per cent less carbon is being transferred into Earth’s deep mantle than previously estimated.

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Data availability

All raw data needed to make the plots are available in Supplementary Tables 1 and 2 as well as in the Source Data file provided. All data are archived through EarthChem (https://doi.org/10.1594/IEDA/111271 at http://get.iedadata.org/doi/111271).

Code availability

The freely distributed software PhreeqC (United States Geological Survey) was used to calculate geochemical solubilities, and is available for download at https://www.usgs.gov/software/phreeqc-version-3.

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This work was principally supported by grant G-2016-7206 from the Alfred P. Sloan Foundation and the Deep Carbon Observatory to P.H.B., K.G.L., D.G., K.P., T.L., J.M.d.M. and D. R. Hummer. In addition, P.H.B. was supported by NSF grant 1144559 during a portion of this project. D.G. was supported by an NSF grant (MCB 15–17567), a Deep Life Modelling and Visualization Fellowship from the Deep Carbon Observatory and an ELSI Origins Network (EON) Research Fellowship, which is supported by a grant from the John Templeton Foundation. The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. This work was further supported in part by JSPS KAKENHI grants JP17K14412, JP17H06105 and JP17H02989 (awarded to M.N.), an NSF grant OCE-1431598 and a NASA Exobiology grant NNX16AL59G (awarded to K.G.L.), NSF grants 0206113, 0711533 and 1049713 (awarded to T.P.F.), and NSF grants 0003628 and 1049748 (awarded to D. R. Hilton). M.Y. was supported by DEKOSIM grant BAP-08-11-DPT.2012K120880, financed by the Ministry of Development of Turkey. M.N. produced the most data. We thank P. Barcala Dominguez for assistance with figure illustration. We thank B. Deck, M. Wahlen and K. Blackmon for analytical assistance at Scripps Institution of Oceanography. We thank B. Marty, G. Alvarado, M. Broadley, D. Byrne, D. Bekaert, J. Libidi and J. Wade for discussions about the project.

Reviewer information

Nature thanks Lorraine Ruzie and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Author notes

    • P. H. Barry

    Present address: Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA, USA

    • D. Giovannelli

    Present address: Department of Biology, University of Naples Federico II, Naples, Italy

    • K. Iacovino

    Present address: Johnson Space Center, NASA, Houston, TX, USA

  1. These authors contributed equally: J. M. de Moor, D. Giovannelli, K. G. Lloyd

  2. Deceased: D. R. Hilton


  1. Department of Earth Sciences, University of Oxford, Oxford, UK

    • P. H. Barry
    •  & C. J. Ballentine
  2. California Water Science Center, USGS, San Diego, CA, USA

    • P. H. Barry
    •  & J. T. Kulongoski
  3. Observatorio Volcanológico y Sismológico de Costa Rica (OVSICORI), Universidad Nacional, Heredia, Costa Rica

    • J. M. de Moor
    • , M. Cascante
    •  & M. Martínez
  4. Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, USA

    • J. M. de Moor
    •  & T. P. Fischer
  5. Institute for Marine Biological and Biotechnological Resources, National Research Council of Italy (CNR-IRBIM), Ancona, Italy

    • D. Giovannelli
    • , G. d’Errico
    • , E. Manini
    •  & F. Smedile
  6. Department of Marine and Coastal Science, Rutgers University, New Brunswick, NJ, USA

    • D. Giovannelli
    • , S. Patwardhan
    • , F. Smedile
    •  & C. Vetriani
  7. Earth-Life Science Institute, Tokyo Institute for Technology, Tokyo, Japan

    • D. Giovannelli
    •  & M. Nakagawa
  8. Department of Earth and Environmental Sciences, Michigan State University, East Lansing, MI, USA

    • M. Schrenk
    •  & H. Miller
  9. Department of Geology, Southern Illinois University, Carbondale, IL, USA

    • D. R. Hummer
  10. Geophysical Institute, University of Alaska, Fairbanks, AK, USA

    • T. Lopez
  11. Graduate School of Oceanography, University of Rhode Island, Kingston, RI, USA

    • C. A. Pratt
  12. Volcanes Sin Fronteras (VSF), San Jose, Costa Rica

    • Y. Alpízar Segura
    • , G. González
    •  & C. J. Ramírez
  13. Department of Earth and Marine Sciences, Università degli Studi di Palermo, Palermo, Italy

    • A. Battaglia
  14. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

    • P. Beaudry
    •  & S. Ono
  15. Department of Earth Sciences, University of Florence, Florence, Italy

    • G. Bini
  16. Dipartimento di Scienze della Vita e dell’Ambiente (DISVA), Università Politecnica delle Marche (UNIVPM), Ancona, Italy

    • G. d’Errico
    • , M. di Carlo
    • , D. Fattorini
    •  & F. Regoli
  17. CoNISMa, Consorzio Nazionale Interuniversitario Scienze del Mare, Rome, Italy

    • D. Fattorini
    •  & F. Regoli
  18. Department of Microbiology, University of Tennessee, Knoxville, TN, USA

    • K. Fullerton
    •  & K. G. Lloyd
  19. Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA

    • E. Gazel
  20. NordVulk, Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland

    • S. A. Halldórsson
  21. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

    • K. Iacovino
  22. Geosciences Research Division, Scripps Institution of Oceanography, University of California, San Diego, CA, USA

    • J. T. Kulongoski
    •  & D. R. Hilton
  23. Department of Earth and Planetary Sciences, Washington University in St Louis, St Louis, MO, USA

    • S. Turner
  24. Institute of Marine Sciences, Middle East Technical University, Erdemli, Turkey

    • M. Yücel


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P.H.B. originally conceived the idea for the project, was lead Principal Investigator (PI) on the Sloan (Deep Carbon Observatory) grant that supported the work, and prepared the first draft of the manuscript. J.M.d.M., D.G. and K.G.L. were co-PIs on the grant and contributed (and equally) to modelling these data and to the writing process. M.S. contributed to modelling and writing. D. R. Hummer, T.L. and C.A.P. were co-PIs on the Sloan grant and contributed to the writing process. Noble gas analysis was conducted in the laboratory of C.J.B. at Oxford. DIC and DOC isotope analysis were conducted by M.N. in the ELSI laboratories (Japan). T.P.F. and D. R. Hilton were the senior PIs who first brought P.H.B. and J.M.d.M. to Costa Rica as PhD students, and were instrumental in the conception of this project. In addition, a portion of the data reported in this contribution was generated from those early expeditions. All other authors (listed alphabetically) provided comments on the manuscript and either assisted in sample collection (as part of the ‘Biology Meets Subduction’ team or on previous expeditions) and/or analysed samples in their respective laboratories. This project was inspired by D. R. Hilton, who was a great mentor and friend.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to P. H. Barry.

Extended data figures and tables

  1. Extended Data Fig. 1 CO2/3He versus δ13C for all samples collected, together with mixing lines between mantle (M), organic sediment (S) and carbonate (C) endmembers.

    We argue that such mixing relationships cannot easily explain the water data, and that instead carbon isotope fractionation associated with calcite precipitation and chemolithoautotrophy introduces the observed δ13C variations. Source data

  2. Extended Data Fig. 2 Helium isotopes (3He/4He) versus X values.

    The X values are air-normalized 4He/20Ne; considering solubility in water for fluid samples23. The majority of samples have high (>5) X values, indicating minimal air-contributions to samples. Source data

  3. Extended Data Fig. 3 Relationship between DC and DIC concentrations and δ13C.

    Values for northern Costa Rica (a and b) are shown with blue symbols and central Costa Rica (c and d), with yellow symbols. Strong correlations allow prediction of DC concentrations and δ13C values for the sites for which DIC compositions are lacking. The slope of the concentration plots (y) is used to calculate the fraction of DIC and DOC in the sample suites. Source data

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