The mid- to late-Holocene palaeoenvironmental history of a low island adjacent to the southern Papuan coast is reconstructed from sedimentary and pollen analysis of swamp stratigraphies, supported by conventional and AMS radiocarbon dating, in an effort to constrain dates for prehistoric horticultural activity. Extensive prehistoric relict mound-and-ditch horticultural field systems located on low, flat clayland areas adjacent to the swamps appear to have been constructed after 2500 yr BP, but before 19th century European contact, based on archaeological and ethnographic evidence. Facies changes in swamp basin infill stratigraphy indicate conformable deposition within tidal lagoonal mangrove environments until c. 3000 yr BP. Then shallowing water conditions resulted in a transition to brackish-freshwater facies, and a vegetation change to sedge-dominated swamps. The observed shift from mangrove to sedge-dominated communities occurred during a falling trend in local relative sea level which may have initiated mangrove dieback. Onset of allocthonous deposition of clayland-derived sediments, related to horticulture on swamp-marginal clayland, significantly post-dates the mangrove to sedge community change in the pollen record. Close temporal coincidence of radiocarbon dates for human occupation and sandy facies deposition at swamp edges implies significant anthropic disturbance on the clayland around 1200 yr BP, but evidence for a process-link remains equivocal.
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