Photooxidation of dissolved organic carbon plays a large role in its remineralization and loss from lakes and oceans. The susceptibility of particulate organic carbon (POC) to this type of photochemical reaction has not been studied; if it occurs in nature, this process might play a role in the organic carbon loss observed between sediment delivery and burial on continental margins adjacent to large river mouths. To assess the extent of POC photooxidation in these turbid, high-CDOM systems, we must empirically determine the organic carbon-specific spectral absorption coefficient in the frequently resuspended clay-organic aggregates, which are typical of suspended and deltaic river sediments.

In this laboratory study, changes in particulate spectral absorption were related to sorption-induced changes in organic carbon content of suspended sediments. Spectral absorption measurements were collected using a 10-cm pathlength ac-9 and a 25-cm pathlength ac-s (WETLabs). Changes in particulate spectral absorption were isolated by difference, by measuring spectral absorption of the initial CDOM and sediment, CDOM suspensions immediately after sediment addition, and again measuring the suspensions and filtrates after allowing CDOM to repartition onto particles over 24 hours of mixing. Control experiments (CDOM-only and deionized water plus sediment suspensions) allowed isolation of the optical property changes due specifically to movement of chromophores from the dissolved to the particulate phase. CDOM fluorescence measurements were also taken with each absorption/attenuation spectrum.

In suspensions with 10 mg/L (dry weight) of < 63 micrometer sediment, visible (400-750nm) decrease in dissolved absorption is mirrored by the increase in particle absorption. The spectral shape of the absorption change resembles that of CDOM. These results suggest that light absorption by organic matter chromophores may be independent of particle association; i.e., color is conserved when passing from the dissolved to the particulate phase.