The finding that reactive iron species may have a role in stabilizing organic matter in ocean sediments underlines the tight coupling between the biogeochemical cycles of carbon and iron. See Letter p.198
The sequestration of organic matter in marine sediments is a primary mechanism by which carbon is removed from the atmosphere and stored over geological periods of time. Only a tiny fraction of organic matter produced by the biosphere arrives at, and is eventually buried in, the sediment bed; the vast majority is recycled back to carbon dioxide. But how and why does any organic carbon remain to be incorporated into the sedimentary record? On page 198 of this issue, Lalonde and co-workers1 report findings that might help to solve this puzzle. They reveal that the intimate association of organic matter with iron species in sediments — including some of the iron compounds found in rust — may exert a strong influence on carbon burial.
Despite decades of research, our understanding of the mechanisms by which organic matter is stabilized and sequestered in aquatic sediments remains far from complete2, hindering our ability to develop robust theories of carbon burial. In recent years, a model has emerged proposing that the intrinsic reactivity of the organic matter supplied to soils and sediments is less important than its distribution in the sedimentary matrix. This, along with a few other factors3, controls its accessibility to the organisms that degrade it into simple organic compounds, and, ultimately, to carbon dioxide. In soils, the formation of complexes of iron salts and organic matter has been identified as one mechanism for stabilizing organic carbon. But despite evidence of a close coupling of organic carbon and iron in marine sediments, this relationship has not previously been explored as a mechanism for preserving organic matter in aquatic environments.
Lalonde et al.1 used a simple chemical procedure4 to release reactive forms of iron from sediment samples and to quantify the amount of organic carbon mobilized during this treatment. Their findings suggest that, on average, more than 20% of the organic carbon in aquatic sediments from a wide range of depositional environments — which vary in salinity, proximity to land, water depth, organic carbon content and oxygen availability — is associated with reactive iron. The authors also measured similar proportions of iron-bound organic carbon in older, deeper sediments, suggesting that such organic carbon exhibits marked stability and prolonged resistance to microbial attack.
Because the available surface area of iron minerals in sediments is insufficient to accommodate the observed organic-carbon loadings, Lalonde et al. conclude that adsorption of organic matter to iron-containing matter is insufficient to explain their observations. Instead, they propose that iron salts must have co-precipitated with organic carbon or that iron–organic-carbon complexes must have formed, or both, in diagenetic processes — those involving iron species made available near the sediment surface as a result of biogeochemical or physical changes that occurred shortly after sedimentation.
More specifically, the authors envisage the formation of nanometre-scale domains consisting of macromolecular complexes formed from organic carbon and iron oxide(hydroxide) minerals, and posit that these domains are sufficiently stable to persist for many thousands of years. Their results indicate that the carbon loading of these complexes may decrease during long-term oxygen exposure, but the overall apparent stability of the complexes, particularly under anoxic conditions, implies that they can provide a mechanism for the long-term sequestration of organic carbon.
Further work is needed to confirm that Lalonde and colleagues' chemical analysis specifically detects only the organic matter bound in iron complexes, given that the method has previously been applied only to studies of soil matrices4. But assuming the observations are robust, the authors' findings are impressive from a variety of perspectives. First, they represent a notable advance in our understanding of how organic matter can be preserved in aquatic sediments, one that draws parallels with similar organic-carbon stabilization processes that are prevalent in soils5.
Second, the chemical characteristics of the organic matter associated with the iron complexes suggest that it is enriched in biochemical components that are typically considered to be highly degradable. This observation may help to explain the apparent paradox of how intrinsically labile biomolecules can be preserved in the sedimentary record.
Third, if the overall fraction of organic carbon bound in iron complexes is similar to that observed for the samples measured in this study, then such complexes represent a globally important contributor to the long-term sedimentary sink of organic carbon. Taken together, the findings underline the tight coupling between the biogeochemical cycles of carbon and iron.
However, the authors' study1 leaves many open questions. For example, the precise mechanism by which iron stabilizes organic matter remains unknown, as does the nature and evolution of iron–organic-carbon complexes during early diagenesis and longer-term burial. The selectivity of the stabilization processes for different types of organic compound involved in the formation of iron–organic-carbon complexes, and the impact of this on the legacy of past biological activity preserved as organic signatures in the rock record, also deserve further attention. We can only speculate on how these processes have influenced organic-carbon burial during periods when oceanic conditions were significantly different from today's — for example, because of changes in sea level or concentrations of dissolved oxygen. Overall, however, the recognition of a 'rusty sink' for organic carbon in ocean sea-floor sediments represents a major advance in our understanding of this enigmatic component of the carbon cycle.
Lalonde, K., Mucci, A., Ouellet, A. & Gélinas, Y. Nature 483, 198–200 (2012).
Hedges, J. I. & Keil, R. G. Mar. Chem. 49, 81–115 (1995).
Schmidt, M. W. I. et al. Nature 478, 49–56 (2011).
Mehra, O. P. & Jackson, M. L. Clays Clay Miner. 7, 317–327 (1958).
Kaiser, K. & Guggenberger, G. Org. Geochem. 31, 711–725 (2000).
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