Phytolith analysis and paleoenvironmental reconstruction from Lake Poukawa Core, Hawkes Bay, New Zealand

As part of a multiproxy investigation, phytoliths were extracted from sediments in a 197-m core in Hawkes Bay, New Zealand. They provide a continuous vegetation–climate record spanning the time period from at least the last interglacial (marine oxygen isotope stage 5) to the present. The phytolith record demonstrates that grass/cyperaceae grew during warmer periods and woody taxa dominated the site during colder periods. During the present interglacial, the Poukawa basin is occupied by a shallow lake surrounded by an extensive fen. During colder–drier periods, the floor of the basin dried out and woody taxa occupied the basin floor. This contrasts with the pollen record, which demonstrates a converse pattern. The apparent discrepancy reflects the purely local provenance of the phytolith assemblage. Significant changes in phytolith assemblages occur at the same depth as major tephras, indicating a sharp decline in trees and shrubs and a surge in grass and cyperaceae. A series of successional changes follow each major tephra fall. Initially, the woody taxa are killed off and replaced by grass and cyperaceae that rapidly colonise the fresh surface. Trees and or shrubs succeed the grass and cyperaceae after a significant lag.