Organic matter sources for tidal marsh sediment over the past two millennia in the Minho River estuary (NW Iberian Peninsula)

Environmental changes during the last 2 millennia in the Minho River tidal marsh (NW Portugal-Spain border) were reconstructed. Changes in the sources of organic matter (OM) delivered to the marsh were evaluated from elemental, isotopic and molecular composition using a 1 m sediment core. Carbon isotopic composition (delta13C) and organic carbon to total nitrogen ratio (Corg/N) provided valuable information concerning the origin of the OM. These parameters indicated a major input from land plants, reaching a maximum at 1100–1200 and 1750–1850 AD. These periods match with major flood events in the NW of the Iberian Peninsula, as reported by several authors. A significant reduction in the terrestrial signature occurred at 6–4 cm (ca. 1960–1985 AD), which is contemporaneous with the construction of several major dams on the Minho River. The distribution of selected lipid biomarkers, including n-alkanes, n-fatty acids and n-alkan-2-ones and specific parameters derived from the molecular distributions, were useful for refining bulk geochemical results. Long chain n- alkanes with an odd number of carbons are indicators of soil-and vascular plant derived terrestrial OM and were dominant throughout the core. In addition, a greater contribution of plankton-derived lipids was observed in the sections corresponding to ca. 1960–1985 AD (6–4 cm) and ca. 100–200 AD (96–94 cm). Although different degradation rates for individual compounds might have partly affected biomarker assemblages, the variations could be attributed to a sharp decrease in the freshwater contribution to the Minho River Estuary (dam construction) and a possible marine highstand, respectively. In addition, several parameters suggested changes in land use (including deforestation and farming) and probably the effects of mining exploitation during the Roman occupation of the area.