The last glacial maximum in central and southern North Island, New Zealand: a paleoenvironmental reconstruction using the Kawakawa Tephra Formation as a chronostratigraphic marker
Abstract Kawakawa Tephra Formation, comprising Oruanui Ignimbrite flow member and Aokautere Ash airfall member, represents the products of an exceptionally large and widespread volcanic eruption from Taupo Volcanic Centre in the North Island of New Zealand. The eruption occurred during the Last Glacial Maximum, and is radiocarbon dated at c. 22.6 ka B.P. Thermoluminescence ages are in broad agreement with the radiocarbon age. The presence of Aokautere Ash in loess deposits, in alluvial gully-fills, on river terraces, and its absence from unstable sites, permits a detailed assessment of geomorphic activity during the Last Glacial Maximum. Widespread erosion of regolith, aggradation of river valleys, and deposition of loess, particularly in the period following eruption of the Kawakawa, point to a cold, dry, variable climate. A collation of pollen data for sediments containing Aokautere Ash, and those 14C dated in the range 17–23 ka, shows that tall forest was highly restricted in the central and southern parts of the North Island. An apparently subalpine grassland/shrubland was present at sites from present sea level to over 800 m elevation, suggesting that factors other than lower temperatures, such as exposure to wind and frost, fire and reduced rainfall, were important in controlling vegetation patterns. We conclude that the interval 23-13 ka B.P., broadly equivalent to oxygen isotope stage 2, represents the period of greatest environmental change in the North Island.