Preservation of organic matter in sediments promoted by iron

Abstract

The biogeochemical cycles of iron and organic carbon are strongly interlinked. In oceanic waters, organic ligands have been shown to control the concentration of dissolved iron1. In soils, solid iron phases shelter and preserve organic carbon2, but the role of iron in the preservation of organic matter in sediments has not been clearly established. Here we use an iron reduction method previously applied to soils3 to determine the amount of organic carbon associated with reactive iron phases in sediments of various mineralogies collected from a wide range of depositional environments. Our findings suggest that 21.5 ± 8.6 per cent of the organic carbon in sediments is directly bound to reactive iron phases. We further estimate that a global mass of (19–45) × 1015 grams of organic carbon is preserved in surface marine sediments as a result of its association with iron4. We propose that these associations between organic carbon and iron, which are formed primarily through co-precipitation and/or direct chelation, promote the preservation of organic carbon in sediments. Because reactive iron phases are metastable over geological timescales, we suggest that they serve as an efficient ‘rusty sink’ for organic carbon, acting as a key factor in the long-term storage of organic carbon and thus contributing to the global cycles of carbon, oxygen and sulphur5.

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Figure 1: Control-corrected percentage of the total sediment organic carbon bound to reactive iron phases.
Figure 2: Carbon isotopic signatures of non-iron-bound and iron-bound organic carbon for all sediment samples.

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Acknowledgements

This work is dedicated to the memory of J. I. Hedges; in addition to being an inspiration to Y.G., he provided several sediment samples used in this work. We thank H. T. Yan for surface area measurements and the captains and crews of RV Coriolis II for their help during sampling missions on the St Lawrence estuary. L. N. Barazzuol is also acknowledged for her work during the first phase of the project. This work was supported by grants (Y.G. and A.M.) and scholarships (K.L.) from NSERC, CFI and FQRNT. This is GEOTOP contribution no. 2012-0001.

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The original hypothesis was formulated by Y.G., and K.L., Y.G. and A.M. designed the project, interpreted the data and wrote the manuscript. K.L. gathered all the data. Groundwork for this study was carried out by A.O., who also contributed to the writing of the manuscript.

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Correspondence to Yves Gélinas.

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The authors declare no competing financial interests.

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This file contains Supplementary Methods, Supplementary Tables 1-3, Supplementary Figures 1-2 and additional references. (PDF 243 kb)

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Lalonde, K., Mucci, A., Ouellet, A. et al. Preservation of organic matter in sediments promoted by iron. Nature 483, 198–200 (2012). https://doi.org/10.1038/nature10855

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