Abstract

From respiration and DNA synthesis to superparamagnetic nanoparticles, magnetotactic bacteria and old rusty cars, iron is everywhere. Our understanding of iron geochemistry is central to the study of carbon and vice versa as it is nearly impossible to find an environment where these two elements are not conjoined. Iron has a profound effect on the carbon that cycles on geological time scales—in sedimentary rocks, in coal and petroleum deposits, the balance between carbon preservation and remineralization is in part modulated by iron. Approximately 20% of the organic carbon buried in sediments is protected by reducible iron phases, well below the oxic-anoxic limit of the sediment where they are no longer thermodynamically stable. Iron represents a globally important sink for sedimentary organic matter (OM), contributing to maintaining the delicate balance of O2 and CO2 in the atmosphere.
Iron also impacts the carbon cycling in active oceanic, atmospheric and lithospheric reservoirs, for example by linking continental erosion to carbon deposition in sediments, and iron-rich riverine discharge and dust deposition to phytoplankton blooms in the middle of the ocean. The association of iron and OM also influences the photoreactivity (Zepp, 2003) and bioavailability (Mackay and Zirino, 1994; Raiswell and Canfield, 2012a), of both elements in aquatic systems.
In spite of its significance to high-turnover and refractory carbon, the exact mechanism of interaction between iron and OM is not yet known. We postulate the formation of inner-sphere complexes or coagulates at oxic-anoxic interfaces. We observe preferential sheltering of organic molecules with low C:N atomic ratios and enriched isotopic signatures (δ13C). A novel method, coupling a total organic carbon (TOC) analyzer to an isotope ratio mass spectrometer, was developed to determine the δ13C of the dissolved organic matter that is retained by iron and other minerals. We find that iron phases increase the affinity and adhesion of 13C-enriched dissolved molecules to particulate phases – which has been reported to slow bacterial degradation. Further elucidation of the mechanism of interaction between the 2 elements could be achieved through novel instrumental methods, including TEM microscopy and EXAFS spectroscopy which are used to determine the macrostructural arrangement of iron and OM and the chemical environment surrounding iron atoms in sediments.