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Contributed by Peter B. Kelemen, April 23, 2015 (sent for review August 7, 2014; reviewed by Jay J. Ague, James Connolly, Rajdeep Dasgupta, and Dimitri Sverjensky)
Significance
This paper reviews carbon fluxes into and out of subduction zones, using compiled data, calculations of carbon solubility in aqueous fluids, and estimates of carbon flux in metasedimentary diapirs. Upper-bound estimates suggest that most subducting carbon is transported into the mantle lithosphere and crust, whereas previous reviews suggested that about half is recycled into the convecting mantle. If upper-bound estimates are correct, and observed output from volcanoes and diffuse outgassing is smaller, then the mantle lithosphere is an important reservoir for carbon. If the subduction carbon cycle remains in balance, then outgassing from ridges and ocean islands is not balanced, so that the carbon content of the lithosphere + ocean + atmosphere has increased over Earth history.
Abstract
Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5–10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory.
Footnotes
- ↵1To whom correspondence may be addressed. Email: peterk{at}ldeo.columbia.edu or manning{at}epss.ucla.edu.
This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2014.
Author contributions: P.B.K. and C.E.M. designed research, performed research, contributed new reagents/analytic tools, analyzed data, and wrote the paper.
Reviewers: J.J.A., Yale University; J.C., ETH Zurich; R.D., Rice University; and D.S., Johns Hopkins University.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1507889112/-/DCSupplemental.
Freely available online through the PNAS open access option.
Reevaluating carbon fluxes in subduction zones
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