River-derived suspended particulate matter (SPM) in the northern Gulf of Mexico is an important vector for the delivery of particulate organic carbon (POC) to the Louisiana shelf region. Optical proxies for SPM and POC, such as in situ and remotely-sensed absorption, allow the higher resolution and autonomous measurements necessary for study of carbon cycling in spatially- and temporally-variable river-influenced coastal regions. In this study, the relationship between absorption by particles (300-800nm) and their organic carbon content is investigated.

The mass-specific absorption properties of a variety of suspended and sedimentary particulate matter samples collected from the sediments and water column of the Atchafalaya River and the surrounding inner continental shelf were determined in aqueous suspension from 300-800nm on a spectrophotometer equipped with an integrating sphere. POC was then oxidatively removed from the samples using a variety of chemical and photochemical techniques, and absorption properties remeasured. Prior to oxidation, samples exhibited similar mass-specific absorption spectra, which is consistent with the comparable iron and organic carbon contents of marine sediments in the region. After organic carbon removal, samples were bleached at visible wavelengths. Mass-specific absorption consistently decreased, with spectral changes on the order of 1 m^2 per g organic carbon removed. Changes in the ultraviolet region of the spectrum were larger and more variable, with increases and decreases on the order of 0.01-1 m^2 per g sediment. Part of the variability in the observed mass-specific UV absorption was specific to heat-driven oxidation methods. Changes in UV absorption also appeared consistent with simultaneous changes in mineral composition, crystallinity, and organic carbon content and quality.