Pleistocene palynology of the Petone and Seaview drillholes, Petone, Lower Hutt Valley, North Island, New Zealand

A mid to late Pleistocene coastal sedimentary sequence drilled to a depth of 311.2 m (299 m of which was composed of Quaternary sediments) at Petone, near Wellington, New Zealand, consists of fossiliferous sands, silts, clays, and occasional peaty and slightly carbonaceous horizons, and provides a discontinuous pollen profile extending from the Postglacial to as far back as about 350,000 years ago. The sequence includes four major cold periods and four major warm periods, although minor fluctuations are recognised within some of these periods. At Seaview, near Petone, another drillhole encountered similar sediments, with 175.3 m of sediment spanning the same period of time and representing a marked eastward thinning from the deeper part of the Lower Hutt/Port Nicholson basin penetrated by the Petone Drillhole.

Large‐scale climatic shifts from warm to cool in the Petone area were represented by periods of substantial erosion and/or deposition of coarse gravel resulting in long breaks between pollen‐bearing sequences. Nine biozones can be recognised in the pollen profiles; these are related to the lithology, local and regional surface formations and local climatic stages, and tentatively correlated with oxygen isotope stages. Biozone P6 (late Pleistocene Oturi or Kaihinu Interglacial) contains taxa restricted today to the north of North Island, representing the period of maximum warmth recognised in the study, and may correlate with oxygen isotope stage 5. This comparison and the radiocarbon‐dated Postglacial (Biozone P9), form the two tie points required for an attempt to correlate the pollen sequence with the oxygen isotope scale. From the top of the sequence, the other biozones are correlated as follows: ‐ Biozone P8 with the peak of the Last Glaciation, Biozone P7 the earlier part of the Last Glaciation (Otira Glacial), Biozone P5 the Waimea Glacial, Biozone P4 the Terangi or Karoro Interglacial, Biozone P3 the Waimaunga Glacial, Biozone P2 the “Normandale” Interglacial and Biozone PI the Nemona Glacial and older stages.

A discriminant function analysis of the complete sequence, starting with a comparison between the Postglacial and Last Glaciation spore and pollen assemblages, provides a graph indicating relative fluctuations in the vegetation, which bears a close similarity to the oxygen isotope scale for the same period of time. Abrupt changes in climate are indicated between, and occasionally within each glacial and interglacial. These changes are interpreted as representing periods of environmental instability of quite variable length, but possibly as short as 100 years.

Keywords:

• Palynology
• pollen diagrams
• vegetation changes
• paleoclimate
• biozones
• oxygen isotopes
• radiocarbon dating
• stratigraphy
• recycling
• discriminant function analysis
• Petone
• Hutt Valley
• Petone Drillhole
• Seaview Drillhole
• Pleistocene
• Last Glaciation
• Postglacial
• Pleistocene climatic stages

Notes

Institute of Geological & Nuclear Sciences Limited, PO Box 30–368, Lower Hutt.