Maoris, Middens and Dune front accretion rate on the North-East Coast of New Zealand: resilience of sedimentary system after a tsunami / Maoris, dépôts alimentaires et taux d'accrétion du front dunaire sur la côte nord-est de la Nouvelle-Zélande : résilience d'un système sédimentaire après un tsunami

摘要

Abstract Coastal dunes at Henderson Bay and Kowhai Bay on the northeast coast of North Island, New Zealand, extend for 5 km. The dune system displays an eroded front to the sea (to the east) and dominant winds are from the west. A significant part of the aeolian fluxes is moving from the inland part of the dune system to the sea-facing sand cliff of the dunes. This explains how the dune may prograde toward the sea. The face of the dune is cut by modern gullies in which various types of Maori shell middens (including bones, egg shell, tools and ovenstones) are found. The midden, inside of the gully walls, express the topography of the front dune at the time of their deposition. They are all in situ. Radiocarbon ages of four samples range from 775±45 to 270±45 years, with the oldest at the top and the younger at the base. Accumulation of sand is moving from the top of the dune down to the base. The maoris occupation post-dates a tsunami, which is dated between a minimum radiocarbon age of 1180-930 cal yr. BP and a maximum age of 5590-5310 cal yr. BP. A tentative reconstruction of the successive topographies allows calculating the accretion rates. Between the 15th and the 16th century sand may have been accumulating at a rate of one metre per century on the upper slope of the dune. After the 18th century erosion appears on the upper part of the slope, whereas sand has accumulated at the base of the slope. The last part of the paper discusses the resilience of tsunami driven coastal accumulations as they are evolving under conditions where no tsunami has been active again.