Fate of dissolved organic matter across the permafrost–nearshore water continuum: role of the intertidal sediments

Abstract. Increasing rates of coastal erosion and permafrost thaw along the Arctic coastline represent a major lateral source of dissolved organic matter (DOM) to the coastal environment, where it can meet multiple fates depending on its origin and composition. Along the (ground)water flow path, Iron (Fe)-hydroxides play an important role in the retention of terrestrial organic matter, but its role on DOM released from coastal thawing permafrost specifically remains poorly understood. To address this gap, we sampled permafrost meltwater, beach groundwater, and seawater samples from several coastal bluffs transects up to 2 km from the shoreline. Across the salinity gradient – from permafrost meltwater to nearshore waters - we found that dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) concentrations decreased drastically, indicating significant removal processes along this continuum. Optical indices (aCDOM350, SUVA254, HIX) reflected changes in DOM composition and aromaticity, suggesting microbial degradation and mineral-organic interactions occur to transform DOM. Furthermore, a PARAFAC analysis of fluorescent DOM indicated that permafrost-derived DOM had a high molecular weight (HMW), humic, and terrigenous origin, while coastal ocean-derived FDOM was protein-rich, low molecular weight (LMW), and from microbial (autochthonous) origin. The optical signature of permafrost meltwater faded along the permafrost-nearshore water continuum. Controlled experiments with excess Fe²⁺ along constant oxygen bubbling showed a rapid (within 6 hours) and major decrease in DOC and CDOM, suggesting interaction with reactive Fe-hydroxides, acting as a permanent or temporary trap of permafrost-derived DOM. Overall, our findings highlight the role of intertidal and nearshore zones where subsurface flows regulate the persistence and reactivity of terrestrial DOM as it transits from permafrost to marine environments in the Arctic.