A 4300 year palynological and sedimentological record of environmental change and human impact from Wharau Road Swamp, Northland, New Zealand

Abstract

The palynology and sedimentology of the late Holocene Wharau Road Swamp, Northland, are described. Organic sediment began accumulating ca 4300 yr B. P. in a valley as a result of damming by a basaltic lava flow from nearby Mount Te Puke. Mixed conifer-hardwood forest dominated the region until major anthropogenic forest clearance dated by radiocarbon at ca 600 yr B. P. Dacrydium cupressinum was the dominant taxon. Agathis australis was always present until European clearance, with peaks in the pollen record at inferred ages of ca 3700 yr B. P. and ca 1800 yr B. P. Climate changes similar to those registered in other pollen diagrams from northern New Zealand are evident, and suggest that climate was wetter and warmer than at present before 4000 yr B. P. From about 2600 yr B. P. climate became drier and cooler, indicated by a decline in Ascarina lucida and D. cupressinum. A period of milder and wetter climate from ca 2000 yr B.P. is suggested by increases in D. cupressinum, A. lucida and Cyathea. Major forest disturbance at ca 600 yr B.P. is recorded by a sharp decline in all tree and shrub taxa accompanied by increases in herbs and pteridophytes, and a coincident sharp rise in charcoal influx. Also of particular importance at this time is the dramatic rise in the curve for Pteridium esculentun (bracken), which is associated with Polynesian land clearance and cultivation. The date for forest clearance is much later than the widely accepted date of ca 1000 yr B. P for first settlement. Sedimentological evidence, in particular changes in grain-size distribution, supports palynological inferences of anthropogenic disturbance of local vegetation and associated soil instability. Increased rates of erosion are indicated by sharp rises in coarse grainsize fractions from ca 700 yr B. P. These granulometric trends ire accompanied by changes in sediment chemistry, especially potassium and sodium, which show increased concentrations.