Examining bulk and iron-associated organic carbon through depth in margin sea sediments (China) under contrasting depositional settings: Chemical and NEXAFS spectral characterization

Abstract

Abstract 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 (FeR) contents at the three sites are greatly influenced by sediment sources, the history of its transport, and its reworking intensity, with FeR 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 (fFe-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 fFe-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.