Sedimentary evidence of soil organic matter input to the Curuai Amazonian floodplain

•The aim was to characterize the sedimentary OM in the Curuai floodplain.•Analysis of a 110cm core recording the last 100yr was carried out.•Data revealed four periods.•Sedimentary OM origin alternated between land-derived soil and alluvial vegetation.•Variation was related to variation in hydrodynamics. A multi-proxy study has been performed on a sediment core from the Curuai floodplain, Central Amazonia. The combination of elemental, isotopic and molecular analysis of a 110cm core (a record of ca. the last 100yr) allowed reconstruction of the hydrological conditions of organic matter (OM) deposition. Two units could be delineated. The first (UI) was composed of three sub-units: UIa (0–15cm), composed of highly degraded organic particles originating from the surrounding soil and indicative of restricted transport; UIb (15–48cm), during which the region was permanently flooded and the material stored came from soil runoff, mainly from alluvial forest; and UIc (48–88cm) composed of material from Amazon River suspended sediment, itself originating from OM degradation in forest soil. In UII (88–111cm), the OM originated mainly from the forest soil and other plant remains in the floodplain. The data reveal that, during the four distinct depositional periods, the sedimentary OM alternated between land derived soil and alluvial vegetation due to changes in hydrodynamics.