Vegetational and climatic history during oxygen isotope stage 9, Wanganui district, New Zealand, and correlation of the Fordell Ash

Abstract

Cover beds of a marine terrace, exposed near Fordell, Wanganui district, New Zealand, contain a polleniferous, mostly alluvial sequence, overlying interglacial marine pebbly sand, deposited after the cutting of the Braemore Marine Terrace about 340,000 yr ago during oxygen isotope substage 9c. The rhyolitic Fordell Ash lies within terrestrial lignite above the marine deposits. Palynology of the terrestrial cover beds indicates that Nothofagus forest became widespread in the eastern Wanganui district during a stadial period (oxygen isotope substage 9b) that followed cutting of the terrace. Vegetation of the district then developed into podocarp — hardwood forest, initially with common Prumnopitys taxifolia, then later mixed with Dacrydium cupressinum, as the climate wanned. The interglacial peak of oxygen isotope substage 9a coincides with high pollen percentages recorded for the small trees Ascarina lucida and Dodonaea viscosa. This sequence of middle Pleistocene vegetation is similar to the post-glacial to mid Holocene vegetation record for this region. Fordell Ash probably fell during early oxygen isotope substage 9a, when a dense Dacrycarpus dacrydioides semi-swamp forest was growing locally at the Fordell Site and the climate was warming. The tephra fall affected the local vegetation by providing a new surface for colonisation by pioneering plant communities. Regional forests appear to have been partly disturbed, because some short-lived serai communities developed. Chemical correlation of a tephra associated with Fordell Ash suggests it is younger than Mt Curl Tephra, and Mt Curl Tephra may be older than previously thought.