Abstract
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.
Change history
12 November 2019
An Amendment to this paper has been published and can be accessed via a link at the top of the paper.
28 June 2019
Change history: In this Article, the original affiliation 2 was not applicable and has been removed. In addition, in the Acknowledgements there was a statement missing and an error in a name. These errors have been corrected online.
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Acknowledgements
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. J.M.d.M. acknowledges funding from Universidad Nacional Costa Rica, the World Bank, and the Costa Rican Ley Transitorio 8933 used to acquire a laser carbon isotope system in collaboration with R. Sánchez-Murillo and G. Esquivel-Hernandez. 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. Labidi and J. Wade for discussions about the project.
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Contributions
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). Gas compositional and C isotope analyses were conducted by J.M.d.M and T.I. in the UNA laboratories (Costa Rica). 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.
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Extended data figures and tables
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.
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.
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.
Supplementary information
41586_2019_1131_MOESM1_ESM.xlsx
Supplementary Table Supplementary Table 1 – Sample collection information, as well as helium and carbon isotope data. X-values represent air-normalized 4He/20Ne values (considering solubility in water for fluid samples23), which are used to determine air-corrected 3He/4He values (RC/RA) of the samples24. CO2/3He is calculated using raw He-isotope values (R/RA).
41586_2019_1131_MOESM2_ESM.xlsx
Supplementary Table Supplementary Table 2 – Sample location information, cell counts and polycyclic aromatic hydrocarbon (PAH) data.
Supplementary Table Supplementary Table 3 – Flux calculations used in the text and to make Figure 4.
41586_2019_1131_MOESM4_ESM.xlsx
Supplementary Table Supplementary Table 4 – Water chemistry data, PhreeqC calculations and saturation index (SI) values.
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Barry, P.H., de Moor, J.M., Giovannelli, D. et al. Forearc carbon sink reduces long-term volatile recycling into the mantle. Nature 568, 487–492 (2019). https://doi.org/10.1038/s41586-019-1131-5
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DOI: https://doi.org/10.1038/s41586-019-1131-5
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