Nearly half of the global seafloor is overlain by sediment oxygenated to the basement. Yet, despite the availability of oxygen to fuel aerobic respiration, organic carbon persists over million-year timescales. Identifying the controls on organic carbon preservation requires an improved understanding of the composition and distribution of organic carbon within deep oligotrophic marine sediments. Here we show that organic carbon in sediment from the oligotrophic North Atlantic and South Pacific is low (<0.1%), yet stable to depths of 25 m and ages of 24 million years. This organic carbon is not bound in biomass and has a low carbon/nitrogen ratio. X-ray imaging and spectroscopic analyses reveal that the chemical composition of this old, deep organic carbon is dominated (40–60%) by amide and carboxylic carbon with a proteinaceous nature. We posit that organic carbon persists in oxic oligotrophic sediment through a combination of protective processes that involve adsorption to mineral surfaces and physical inaccessibility to the heterotrophic community. We estimate that up to 1.6 × 1019 g of organic carbon are sequestered on million-year timescales in oxic pelagic sediment, which constitutes an important, previously overlooked carbon reservoir.
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The data generated by this study are available from the corresponding author upon request and will be deposited in the Biological and Chemical Oceanography Data Management Office (https://www.bco-dmo.org/data).
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The authors thank D. Repeta, B. Kocar and J. Biddle for their insight as well as C. Anderson, A. Dunlea, C. McKinley and R. Scudder for meaningful conversations. We thank C. Johnson at the WHOI Mass Spectrometry Facility for his work in analysing organic carbon samples and the captain, crew, coring crew and science party of R/V Knorr expedition 223 for facilitating the sample acquisition. Portions of this material are based on work supported while R.W.M. was serving at the National Science Foundation. Portions of this research were conducted at the Stanford Synchrotron Radiation Lightsource and Advanced Light Source. The use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515. The use of the Advanced Light Source is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-05CH11231. This research was supported in part by NSF GRF 1122374 to E.R.E., a NSF Center for Dark Energy Biosphere Investigations (C-DEBI, OCE-0939564) graduate fellowship to E.R.E. and C-DEBI research grant no. CH20655 awarded to C.M.H. Expedition KN223 was funded by the NSF Division of Ocean Sciences (grant no. 1433150 to A.J.S., S.D’H. and R.P.). This work is C-DEBI contribution 450.
The authors declare no competing interests.
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A correction to this article is available online at https://doi.org/10.1038/s41561-019-0423-6.
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Estes, E.R., Pockalny, R., D’Hondt, S. et al. Persistent organic matter in oxic subseafloor sediment. Nature Geosci 12, 126–131 (2019) doi:10.1038/s41561-018-0291-5
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