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An optical design study of a bending‐magnet beamline, based on multi‐bend achromat storage ring lattices, at the High Energy Photon Source, to be built in Beijing, China, is described. The main purpose of the beamline design is to produce a micro‐scale beam from a bending‐magnet source with little flux loss through apertures. To maximize the flux of the focal spot, the synchrotron source will be 1:1 imaged to a virtual source by a toroidal mirror; a mirror pair will be used to collimate the virtual source into quasi‐parallel light which will be refocused by a Kirkpatrick–Baez mirror pair. In the case presented here, a beamline for tender X‐rays ranging from 2.1 keV to 7.8 keV, with a spot size of approximately 7 µm (H) × 6 µm (V) and flux up to 2 × 1012 photons s−1, can be achieved for the purpose of X‐ray absorption fine‐structure (XAFS)‐related experiments, such as scanning micro‐XAFS and full‐field nano‐XAFS. [ABSTRACT FROM AUTHOR]

The interactions of organic carbon (OC) with ferric iron (Fe) oxides play an important role in OC stabilization, but reductive dissolution of Fe(III) oxides under anoxic conditions potentially compromises the stabilization of Fe-bound OC (Fe-OC). Up to date, there are very limited studies on the fate of Fe-OC in sulfidic conditions commonly encountered in OC-rich marine sediments. Herein, multi-pronged studies were implemented to quantitatively characterize the capacity of metastable iron sulfide (FeS) to sequester OC extracted from marine sediments and to unravel the fate of ferrihydrite-bound OC complexes (Fh-OCs) when exposed to dissolved sulfide. Results show that, for a given mechanism of OC association (adsorption or coprecipitation), FeS has DOC retention capacity generally comparable to Fh, implying that the metastable phase could serve as an important OC sink in anoxic marine sediments. During reduction of Fh-OCs by dissolved sulfide, Fh reduction was progressively retarded with increasing OC pre-adsorption, while Fh reduction was promoted after DOC pre-coprecipitation. Despite high extents of Fe reduction by dissolved sulfide, OC release from Fh-OC coprecipitates was low due to efficient re-sequestration of released DOC by coprecipitation with FeS, and an increase in OC pre-loading could facilitate Fe2+ sulfidization and thus re-sequestration of released DOC by FeS. When Fh-OC adsorption complexes were exposed to dissolved sulfide, substantial OC release was restricted to the complexes with lower OC pre-adsorption (C/Fe ≤ 1.5). Overall, reactive Fe reduction by dissolved sulfide may not lead to substantial release of associated OC as long as reduced Fe could be sulfidized to efficiently re-sequester concomitantly released DOC. Results of carbon-1 s near-edge X-ray absorption fine structure spectroscopy indicate that, upon reduction of Fh-OCs by dissolved sulfide and subsequent FeS formation, carboxyl-C was selectively released into the solution but aromatic-C selectively remained on the solid matrixes after Fe reduction, suggesting that OC sink switching from Fe oxides to Fe sulfides could result in molecular fractionations of porewater DOC to some extent, with potential implications for OC biogeochemical cycling. Ferrihydrite reduction by dissolved sulfide and concomitant release of bound DOC, FeS formation, and re-sequestration of DOC by coprecipitation with FeS. [Display omitted] [ABSTRACT FROM AUTHOR]

Marine sediments are the largest sink for organic carbon (OC) on Earth, and iron (Fe) oxides play an important role in stabilization of sedimentary OC. However, the roles of Fe oxides in OC stabilization during prolonged burial, for example, up to tens of thousands of years or more are still poorly constrained. In this study, we used traditional chemical extraction and near-edge X-ray absorption fine structure (NEXAFS) spectroscopic technique to characterize bulk OC and Fe-associated OC (Fe-OC) through depth in gravity cores collected from three sites near the Yangtze River Estuary (YRE), in the South Yellow Sea (SYS), and in the middle Okinawa Trough, which have contrasting depositional environments. Results show that depositional environments have exerted quite different influences on sources and burial of sedimentary OC, and thus on OC degradation during prolonged burial at the three sites. Reactive Fe (Fe R) contents at the three sites are greatly influenced by sediment sources, the history of its transport, and its reworking intensity, with Fe R contents near the highly dynamic YRE much higher than at the central SYS and the middle Okinawa Trough. The fractions of Fe-OC in total OC (f Fe-OC) displayed no clear or consistent trends with depth or by site, probably due to the dual roles of Fe redox cycling in OC protection versus its oxidation. As indicated by the f Fe-OC , reactive Fe plays a limited role in OC preservation in margin sea sediments of East China. A combination of NEXAFS spectra and isotopic compositions of bulk OC and Fe-OC indicates that main OC functionalities have not experienced differential alterations and/or no specific OC moieties have been selectively stabilized/released during prolonged burial in the three contrasting depositional environments. • Depositional regimes control sources and burial of sedimentary organic carbon (OC). • Sediment source, transport history and reworking intensity affect Fe R contents. • There is no clear selectivity of OC retention by reactive iron (Fe R) along depth. • Fe R plays limited roles in OC preservation in sediments of east China margin seas. [ABSTRACT FROM AUTHOR]