Dissolved organic matter mobility and retention in a High-energy subterranean estuary
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Permeable sandy sediments beneath high-energy beaches are potent biogeochemical reactors with high organic matter turnover rates. Despite their role in nutrient and trace metal cycles, little is known about their function in trapping and mobilizing dissolved organic matter (DOM). To explore DOM-solid phase interactions, we analyzed the molecular composition of water- and acid-leachable sedimentary DOM down to 24m in a subterranean estuary (STE) on Spiekeroog Island, German North Sea, using fluorescence spectroscopy and FT-ICR-MS. We also examined the coprecipitation of DOM and Fe3+ (oxy)hydroxides in STE porewaters containing reduced Fe2+ exposed to air. About 10% of total organic carbon was leachable as DOC in both treatments. Finer sediments with higher Fe and Al content retained more DOC. Water leachates contained aliphatic, nitrogen-rich compounds likely from marine sources, while acid leachates were more varied, including oxidized aromatic and labile compounds, likely from terrestrial and marine sources. Fe-DOM coprecipitation induced molecular composition changes, but DOC removal was within analytical uncertainties, and molecular fractionation varied across sampling periods. Thus, DOM-Fe coagulation at redox zones of deep beach STEs may be low and influenced by transport-driven changes in porewater chemistry. Overall, terrigenous DOM is preferentially retained in sediments, while the labile fraction is more mobile. This study highlights the role of organo-mineral interactions in DOM cycling at the land-ocean interface, especially in organic-poor, sandy environments.