A terrestrial palynological record for the last two glacial cycles from southwestern New Zealand

Abstract

A pollen profile from Okarito Pakihi Bog in south Westland, New Zealand extending from near present back to Marine Isotope Stage (MIS) 6 provides a continuous record of vegetation and climate change for the past two glacial cycles. Independent chronological control was obtained by AMS radiocarbon dating of organic sediments in the upper part of the sequence and OSL dating of inorganic silts in the lower part, with a unique tie point provided by the ca 26.5 cal ka Kawakawa Tephra. As was probably a common occurrence in this region, the basin developed as a moraine-dammed proglacial lake and remained lacustrine until the early Holocene, when a peat bog developed. Survival of the depositional site through subsequent multiple ice advances, unusual in a glaciated landscape, was probably assisted by lateral displacement of the basin relative to its source area, across the Alpine Fault.

There is good correspondence between inferred periods of substantial treeline depression in the pollen profile and the record for ice advance in this region. More cooling events are evident in the pollen record, however, presumably due to the fragmentary nature of glacial geomorphology. The pollen record also shows broad consistency with the MIS record and hence with the Milankovitch orbital forcing model, but with some departures, including an early onset to the last glacial maximum (LGM). Several sub-Milankovitch scale events are also evident, including a mid-LGM warming and Lateglacial reversals during both the last and the penultimate deglaciation.

Introduction

The search for long continuous palynological records from New Zealand that extend from the present back through and beyond the last glaciation has met with mixed success (Newnham et al., 1999). Widespread opportunity for preservation of organic sedimentary sequences under a predominantly humid temperate climate is offset by the prospect of interrupted deposition posed by a dynamic geophysical environment and, in far northern and eastern regions, potential for prolonged drought. As a consequence, current understanding of the last glacial cycle is dependent upon a number of lengthy but discontinuous sequences (e.g., McGlone and Topping, 1983; McGlone et al., 1984; Newnham, 1992; Mildenhall, 1995; Shulmeister et al., 2001; Soons et al., 2002; Newnham and Alloway, 2004; Newnham et al., 2004) or the synthesis of composite records from discrete truncated records in key regions (McGlone, 1980; Bussel, 1990; Moar and Suggate, 1996). Several marine palynological records, which overcome the problem of discontinuity, have contributed important advances (Heusser and van de Geer, 1994; Wright et al., 1995; Mildenhall et al., 2004) but bring attendant problems of pollen provenance, resolution and chronology.

This paper presents a high-resolution palynological record from Okarito, South Westland that under exceptional circumstances extends continuously from the present back through the complete last glacial cycle to the penultimate glaciation. Extensive independent dating is provided by accelerator mass spectrometry (AMS) radiocarbon, optically stimulated luminescence (OSL) and tephrochronology, so that orbital tuning has been circumvented as a primary means of establishing chronological control and correlation with other key records. This combination of high-resolution sampling, continuity, and independent chronostratigraphic framework underpins our contention that the Okarito palynological record provides a benchmark not only for the New Zealand Late Quaternary but also for the terrestrial mid-latitiudes of the Southern Hemisphere. A summary version of the Okarito palynological record and its implications for understanding mechanisms of global climate change has recently been presented (Vandergoes et al., 2005). The purposes of this paper are to present the full Okarito palynological record and to provide a more comprehensive discussion of the observed vegetation and climate changes.