Palaeo-lake and swamp stratigraphic records of Holocene vegetation and sea-level changes, Mangaia, Cook Islands

Stratigraphy of swamps inside the inner makatea rim of Mangaia was investigated to show Holocene changes in vegetation and sea level. In the mid-Holocene five lakes existed where there are now clay-filled swamps, and lake notches on the makatea wall indicate that sea level was sustained at 1.1 m higher than present. Fine annual laminations in gyttja deposits indicate the greatest lake depth in that period, dated between 6500 and 4500 yr B.P. Pollen evidence of wetland communities also points to a higher sea level at that time. Pollen analyses and charcoal concentrations of cores from two different drainage basins show that the greatest change in terrestrial vegetation of the Holocene on Mangaia was clearance of forest by people, resulting in soil erosion from the inner volcanic cone and clay infilling of the lakes. Humans were present on Mangaia as early as 2500 yr B.P. Although some clearance of forest occurred during that early period of human occupation, systematic island-wide anthropogenic disturbance began ca. 1650 yr B.P., as shown in both cores from a decline in forest pollen and a major and sustained increase in Dicranopteris, a fern that colonizes disturbed land.