A Late Quaternary record of natural change and human impact from Rangihoua Bay, Bay of Islands, northern New Zealand

Abstract

A multi-proxy analysis of back-dune sediment cores from Rangihoua Bay, northern New Zealand, provides an environmental history of the Late Quaternary, placing human impacts on the site into a geomorphological and ecological context. The inferred palaeo-ecological significance of the trends is generally coeval between proxies. The history commences with a Late Pleistocene deposit that formed part of a river terrace during lower sea level. The dryland vegetation at that time was dominated by Fuscospora forest. The record recommences at c. 7400 yr B.P., by which time Fuscospora had been replaced by podocarp-hardwood forest, comprising mainly Dacrydium and Prumnopitys taxifolia emerging through a Metrosideros canopy. One of the core sites was a lagoon fringed with mainly Cyperaceae, Leptospermum, and Dodonaea. Redox-sensitive elements reflect phases of anoxia related to variation in lagoon depth. Transition from lagoon to peat swamp, due to natural infilling and/or climate change, occurred after c. 5500 yr B.P.

Human impacts were of high magnitude, and include deforestation of the catchment and drainage of the wetland by early Polynesians. Errors in the radiocarbon and tephra chronologies preclude an accurate date for this. Microfossils of introduced Polynesian plants and a thick gravel bed in one of the cores suggest that parts of the wetland were used for prehistoric horticulture.

Keywords

Tephra, sedimentology, geochemistry, diatoms, pollen, estuaries, lagoons, human impact.