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Pacific carbon cycling constrained by organic matter size, age and composition relationships


Marine organic matter is one of Earth’s largest actively cycling reservoirs of organic carbon and nitrogen1,2. The processes controlling organic matter production and removal are important for carbon and nitrogen biogeochemical cycles, which regulate climate. However, the many possible cycling mechanisms have hindered our ability to quantify marine organic matter transformation, degradation and turnover rates3,4. Here we analyse existing and new measurements of the carbon:nitrogen ratio and radiocarbon age of organic matter spanning sizes from large particulate organic matter to small dissolved organic molecules. We find that organic matter size is negatively correlated with radiocarbon age and carbon:nitrogen ratios in coastal, surface and deep waters of the Pacific Ocean. Our measurements suggest that organic matter is increasingly chemically degraded as it decreases in size, and that small particles and molecules persist in the ocean longer than their larger counterparts. Based on these correlations, we estimate the production rates of small, biologically recalcitrant dissolved organic matter molecules at 0.11–0.14 Gt of carbon and about 0.005 Gt of nitrogen per year in the deep ocean. Our results suggest that the preferential remineralization of large over small particles and molecules is a key process governing organic matter cycling and deep ocean carbon storage.

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Figure 1: Least squares regressions indicate quantitative organic matter size, Δ14C value and C:N ratio relationships.


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We gratefully acknowledge B. Phillips, the staff of the Granite Canyon Marine Pollution Studies Laboratory (GCMPSL) and the Natural Energy Laboratory of Hawaii Authority (NELHA) for providing facilities capable of large-volume seawater DOM and suspended POM isolations. K. Okimura, J. Walker, L. Roland, K. Walker, G. V. Reixach, and M. Calleja (UC Santa Cruz) aided with fieldwork and sample collection. S. Griffin (UCI) and P. Zermeno (LLNL) aided with sample analysis. F. Primeau (UCI) aided with error analysis and Matlab scripts. This work was funded by the Friends of Long Marine Lab Student Research Awards (to B.D.W.), the UC Santa Cruz STEPS Institute for Innovation in Environmental Research (to B.D.W.), the UC Santa Cruz Center for the Dynamics and Evolution of the Land-Sea Interface (to B.D.W.), the Earl H. Myers and Ethel M. Myers Oceanographic and Marine Biology Trust (to B.D.W.), the UC Santa Cruz Institute of Geophysics and Planetary Physics (to B.D.W. and M.D.M.), NSF OCE-1358041 and NSF OCE-0623622 (M.D.M.) and NSF ARC-1022716 (E.R.M.D.). A portion of this work was performed under the auspices of the US Department of Energy (contract W-7405-Eng-48 and DE-AC52-07NA27344) and a Keck Carbon Cycle AMS Laboratory Postdoctoral Scholarship (B.D.W.).

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B.D.W. conceived the research; B.D.W., S.R.B., T.P.G., M.D.M. and E.R.M.D. performed research; S.R.B., T.P.G., and E.R.M.D. contributed new reagents/analytical tools and models; B.D.W., S.R.B., T.P.G., E.R.M.D. and M.D.M. analysed data; B.D.W. wrote the paper with inputs from S.R.B., T.P.G., M.D.M. and E.R.M.D.

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Correspondence to Brett D. Walker.

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The authors declare no competing financial interests.

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Walker, B., Beaupré, S., Guilderson, T. et al. Pacific carbon cycling constrained by organic matter size, age and composition relationships. Nature Geosci 9, 888–891 (2016).

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