Studies on Holocene sea level change along the coast of northeastern South America can assist in understanding how the region's coastal environments might respond to future sea level change. A freshwater wetland along a wave-dominated pocket bay yielded a 980-cm-long sediment core, which records sediment and vegetation change over the past 7000 years. Above basal sand, peaty mud <450-cm depth is overlain by mangrove peat extending <100 cm and dark, peaty freshwater mud <35 cm. Three radiocarbon dates provide a chronological framework and estimate rates of sedimentation. Loss-onignition analysis shows a shift from basal silicates to organic matter at 4000 YBP that indicates a reduction in marine influences and the establishment of a mangrove habitat. Rhizophora dominates the fossil pollen record. Spores of the tree ferns Cnemidaria and Cyathea indicate an adjacent humid forest whereas Polypodium-type spores and Cyperaceae pollen in the upper part of the core indicate freshwater conditions. The reduction in the sedimentation rate from 1.99 mm y−1 before 4000 YBP to 1.05 mm y−1 after 4000 YBP reflects reduced delivery of external sediments to the wetland and the addition of authocthonous organic matter, whereas the further reduction to 0.61 mm y−1 after 3000 YBP suggests declining rates of peat formation and reduced sediment inputs from the forested watershed. We conclude that the stratigraphy and plant succession was the result of long-term building of a beach ridge. Brackish water peat and then freshwater peat formed behind the bar.
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Vol. 2006 • No. 223