Lignin phenol abundances and ratios are modulated by their interactions with iron hydroxides in sediments

Iron hydroxides play a key role in the preservation of organic matter in soils and sediments, yet the specificity and extent of their interactions with lignin-derived phenols-important source indicators for terrestrial organic matter-remain poorly understood in aquatic systems. In this study, we analyzed surface and downcore sediments along the terrestrial-to-marine continuum of the St. Lawrence Estuary and Gulf, as well as from a boreal lake (Lake Brock), to better characterize iron-lignin associations. Using CuO oxidation (lignin) and buffered dithionite (iron), we quantified lignin oxidation products before and after the reductive dissolution of reactive iron with dithionite and evaluated association patterns through multivariate analyses. Our results show that lignin is consistently associated with reactive iron, with losses upon iron dissolution ranging from ∼20 % for the marine sites to over 40 % in terrestrial and freshwater sediments. Strikingly, 3,5-dihydroxybenzoic acid (3,5-Bd) showed the highest sensitivity to iron reduction, suggesting a unique and possibly distinct origin or sorptive behavior. Despite substantial lignin oxidation products losses, source-indicating S/V and C/V ratios remained relatively stable, and acid-to-aldehyde ratios-proxies for lignin degradation-were not significantly affected by iron binding. Principal component analysis confirmed that compositional shifts following iron hydroxides reduction are small and limited to one terrestrially influenced sample. These findings reveal that iron hydroxides not only shield lignin-rich organic matter from degradation but also act as selective shuttles for phenolic terrestrial compounds across dynamic redox boundaries in aquatic sediments.