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Efficient export of carbon to the deep ocean through dissolved organic matter


Oceanic dissolved organic carbon (DOC) constitutes one of the largest pools of reduced carbon in the biosphere. Estimated DOC export from the surface ocean represents 20% of total organic carbon flux to the deep ocean1,2,3, which constitutes a primary control on atmospheric carbon dioxide levels4. DOC is the carbon component of dissolved organic matter (DOM) and an accurate quantification of DOM pools, fluxes and their controls is therefore critical to understanding oceanic carbon cycling. DOC export is directly coupled with dissolved organic nitrogen and phosphorus export. However, the C:N:P stoichiometry (by atoms) of DOM dynamics is poorly understood. Here we study the stoichiometry of the DOM pool and of DOM decomposition in continental shelf, continental slope and central ocean gyre environments. We find that DOM is remineralized and produced with a C:N:P stoichiometry of 199:20:1 that is substantially lower than for bulk pools (typically >775:54:1), but greater than for particulate organic matter (106:16:1—the Redfield ratio). Thus for a given mass of new N and P introduced into surface water, more DOC can be exported than would occur at the Redfield ratio. This may contribute to the excess respiration estimated to occur in the interior ocean5. Our results place an explicit constraint on global carbon export and elemental balance via advective pathways.

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Figure 1: DOC, DON and DOP stoichiometry from field studies.
Figure 2: Decomposition stoichiometry determined from laboratory incubations.
Figure 3: A simplified conceptualization of oceanic DOM pools using a conventional two-pool model.


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This research was funded by grants from the USDOE Ocean Margins Program and the National Science Foundation. A. Nolin conducted thousands of chemical analyses and with I. Buffam spent many days at sea collecting, incubating and filtering water. We thank the crews of the RV Endeavor, RV Oceanus and RV Seward Johnson. R. Chen provided intercomparison and blank checks on DOC determinations during cruises. D. Karl provided access to the HOT data sets that were incorporated into this report.

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Correspondence to Charles S. Hopkinson Jr.

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Supplementary Data

This file lists triplets of depth-DOC-DOP, depth-DOC-DON and depth-DON-DOP data used in calculating labile DOM stoichiometry, Concentration unit is µM. Depth unit is metres for all sites but HOTS, which is in mbar. (XLS 63 kb)

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Hopkinson, C., Vallino, J. Efficient export of carbon to the deep ocean through dissolved organic matter. Nature 433, 142–145 (2005).

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