Abstract
Interactions between organic matter and mineral matrices are critical to the preservation of soil and sediment organic matter. In addition to clay minerals, Fe(III) oxides particles have recently been shown to be responsible for the protection and burial of a large fraction of sedimentary organic carbon (OC). Through a combination of synchrotron X-ray techniques and high-resolution images of intact sediment particles, we assessed the mechanism of interaction between OC and iron, as well as the composition of organic matter co-localized with ferric iron. We present scanning transmission x-ray microscopy images at the Fe L3 and C K1 edges showing that the organic matter co-localized with Fe(III) consists primarily of C=C, C=O and C-OH functional groups. Coupling the co-localization results to iron K-edge X-ray absorption spectroscopy fitting results allowed to quantify the relative contribution of OC-complexed Fe to the total sediment iron and reactive iron pools, showing that 25–62% of total reactive iron is directly associated to OC through inner-sphere complexation in coastal sediments, as much as four times more than in low OC deep sea sediments. Direct inner-sphere complexation between OC and iron oxides (Fe-O-C) is responsible for transferring a large quantity of reduced OC to the sedimentary sink, which could otherwise be oxidized back to CO2.
Highlights
As the largest sink for organic carbon (OC) on Earth, marine sediments play a major role in the global carbon cycle[1]
In conjunction with the X-ray Absorption Near Edge Structure (XANES) spectroscopy, key organic functional groups co-localized with ferric iron were identified using X-ray spectromicroscopy at the C K-edge and Fe L3-edge
Iron K-edge XANES spectra were collected for the pure iron materials and the three synthetic Fe-OC complexes, precipitated from solutions with intial OC:Fe ratios of 1, 3 and 10, respectively (Fig. 3a)
Summary
Free iron oxide contribution to total sediment Fe (wt%) 10.8 ± 2.2(c) 26.5 ± 7.1. recalcitrance, and physical protection by biominerals1) to transfer large quantities of reduced OC from the active surface of the globe to its slowly cycling interior where it remains locked on geological time scales. Sequential extraction methods used to probe iron-OC interactions involve harsh chemical treatments that target operationally defined iron fractions[14,15,16], with the OC:Fe molar ratio of the extracted material being used to infer molecular-level interaction mechanisms[7, 17]. The use of ratios assumes that the entire iron pool is bound to OC, neglecting the possibility that only a fraction of the extracted reactive Fe is associated with OC. We use Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy to quantify the proportion of reactive Fe involved in inner-sphere complexation to OC in contrasting sedimentary environments. We probed for the first time the actual preservative interactions occurring between iron and OC in chemically unaltered sediments
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