Iron diagenesis controls CDOM sedimentary accumulation and optical properties in near-surface sediments of river-dominated continental shelves

Colored dissolved organic matter (CDOM) is a critical component of the carbon cycle that governs many processes in marine environments, including photobiology and photochemistry, trace metal availability, and heat transfer. While fluvial inputs of terrestrial CDOM are diluted with seawater in estuarine and coastal environments, several researchers have reported elevated CDOM absorption signals and depressed spectral slopes relative to those expected from conservative saline mixing in the sub-surface water column, suggestive of sediment-derived CDOM inputs. Indeed, direct measurements of sediment CDOM fluxes have revealed they are significant and optically distinct, although the controls of the optical characteristics remain poorly understood. Here, analyses of sediment cores from a river-dominated ocean margin (RiOMar) reveal that CDOM with amplified absorptions and low CDOM spectral slopes (< 0.005 for S350-400) is generated during diagenetic Fe-mineral dissolution processes, with associations between the organics and Fe(III) responsible for these optical phenomena. Active resuspension of this dissolved pool with different optical properties may affect water column optics after suspended particulates resediment. Researchers should heed caution when inferring dissolved organic carbon (DOC) transfer from land-to-sea via CDOM proxy in RiOMars given a potential confounding effect from saline sediment pore water CDOM. Finally, the absorption characteristics of CDOM generated in Fe-rich sedimentary environments appears to be decoupled from respiration-linked organic carbon degradation and are instead more dependent on the intensity of Fe-related diagenetic processes (i.e. mineral dissolution).