The Last Glacial Maximum and Deglaciation in Southern New Zealand: New Pollen-Climate Reconstructions
The project builds upon existing knowledge of late Quaternary palaeoenvironmenta change and tests the recently developed New Zealand INTIMATE (Integration of Ice Marine and Terrestrial archive) climate event stratigraphy (NZ-I CES; 30-8 ka). Four pollen and sediment records from three climatically contrasting regions in the South Island provide a vegetation and climate history for this area between 38-4 ka. In this study, the Last Glacial Cold Period (LGCP; c. 31.4-18.9 ka) is characterised by a two step cooling, with the coldest conditions, reaching possibly >5.3°C cooling, occurring between 21-19 ka, marking the Last Glacial Maximum. A new precipitation proxy using macrophyte pollen concentrations at an eastern South Island site suggests dominantly dry conditions prevailed during the LGCP except for two periods of wetter climate around 26-24 ka and 21 ka. The dry periods correspond with evidence of glacial advance, colder environments and possibly increased intensity of the southern westerlies. Conversely, the wet periods coincide with reduced glacial activity, milder climates and decreased westerly wind intensity. Deglaciation began between 18.9-18.4 ka followed by rapid climate amelioration culminating with Dacrydium cuppressinum-dominant lowland forest at western sites as early as 11.9 ka, indicative of the start of the Holocene. A disturbance in forest development occurs between 13.4-11.9 ka in one record and may be indicative of a minor cooling within the timeframe of a late glacial climate reversal recognised in the NZI-CES. Overall the project results (timing and pattern of climate change) broadly align with the NZ-I CES. However, there are some disparities, in particular during the LGCP, which this study suggests began at least 3-4 ka earlier than concluded in the NZ-I CES. The NZ-I CES oversimplifies the complexity of the LGCP which contains evidence of significant climate variability that may be important for an understanding of the possible forcing factors on climate change. The chronology derived from the current study supports recent evidence that points towards a younger, refined age of 25.4 ka for the Kawakawa/Oruanui Tephra, a key chronostratigraphic marker for the LGCP. Pollen-climate models and Environmental Lapse Rates were used to quantify changes in mean annual temperatures with sometimes conflicting results. This research reveals some limitations of the current New Zealand pollen-climate transfer function when applied to reconstruction of cold climate periods in particular. These include a lack of limitations with modern analogues and a number of wide-ranging pollen taxa that encompass a broad climate envelope. The current research also highlights the potential of regional climate regimes and spatial differences in vegetation and inferred climate reconstructions. These differences pose a major limitation for a New Zealand-wide composite. While the NZ-I CES provides a valuable framework of climate change during a period of large climate variability, results of this study highlight aspects that need further consideration and revision.