Qualitative transformations of dissolved organic matter along supra-permafrost flow: insights from Arctic subterranean estuaries

Abstract. Increasing coastal erosion and permafrost thaw along the Arctic shoreline represent major lateral sources of dissolved organic matter (DOM) to the Arctic coastal ocean. One key and still underrecognized flow path is non-point source supra-permafrost groundwater flow discharging through sandy beaches in front of coastal bluffs. Here, we explore the qualitative transformations of DOM and dissolved organic carbon (DOC) along this flow path, with a focus on the beach discharge zone, where fresh supra-permafrost groundwater mixes with recirculated seawater. We sampled meltwater, beach groundwater, and seawater along coastal bluff transects extending up to 0.5–1 km offshore. Along these salinity and redox gradients, DOC and chromophoric DOM (CDOM) declined sharply, partially reflecting dilution. Optical indices (a350, SUVA254, HIX) and PARAFAC-based fluorescence analyses revealed a shift from humic-like, high molecular weight (HMW) DOM in meltwaters to protein-like, low molecular weight (LMW) DOM in nearshore waters. The inverse relationship between humic- and protein-like components, combined with reactive Fe-hydroxide coatings on sandy sediments, elevated dissolved iron (Fetot) and high dissolved inorganic carbon (DIC) under low oxygen conditions in beach groundwater, suggests that microbial degradation and mineral-organic interactions likely contribute to DOM transformation. Alongside these transformation processes, mixing governs bulk DOC behaviour. These findings suggest that nearshore and intertidal zones, particularly subterranean estuaries (STEs), are zones of DOM transformation along the Arctic coastline. However, whether they act primarily as transient filters, long-term carbon sinks, and/or dynamic biogeochemical reactors remains unresolved, highlighting the need for further research on their role in transferring permafrost-derived carbon to the ocean.