LAUSR

Abstract Estuaries are dynamic interfaces between land, rivers and the ocean that play major roles in the global carbon cycle. These coastal wetlands store huge amounts of organic carbon (OC), commonly known as “blue carbon” and excellent places to study C cycling. The Guadiana river estuary is among the most important tidal salt marshes in the South - Iberian coastal margin. Here, a detailed organic geochemical study is described that includes the identification of sedimentary OM composition at a molecular and isotopic level. Total organic carbon content (TOC) of core sediments ranged from 0.39 to 2.23% and stable carbon isotope composition (δ13C) also showed a wide range between −22.4 and −27.0‰. A13C depletion trend observed from the surface to bottom in the core sediments profiles, reflects loss of labile biogenic 13C enriched compounds i.e. polysaccharides and a selective preservation of more depleted compounds with depth i.e. lignin and lipids in the core sediments. Series of n-alkanes were found in the range from C10 to C31. Carbon preference index ratio (CPI) calculated for long-chain n-alkanes (C24–C31) that ranged between 1.17 and 1.94 reflecting diverse OM inputs to the sediments. A study of the lignin-derived phenolic composition pointed to a recalcitrant OM derived from both gymnosperm and angiosperm plants. Moreover, high abundance of vinyl phenol and vinyl guaiacol points to a dominant contribution of lignins from grasses and aquatic macrophytes to the sediments. A well-resolved series of long-chain linear alkyl benzenes (LABs from C4 to C22) were recorded in all core sediment samples indicating direct discharge of untreated domestic and/or industrial effluents to the estuary. Polycyclic aromatic hydrocarbons (PAHs) with more than 3 rings reflects the pyrogenic origin of a portion of the sedimentary OM. This study highlights the importance of different OM sources to the lower Guadiana estuarine sediments and contributes to a better knowledge about its origin, dynamics and fate.