A widespread ca. 1.1 Ma TVZ silicic tephra preserved near Wellington, New Zealand: Implications for regional reconstruction of mid-Pleistocene vegetation

Abstract

A sequence of terrestrial pollen-bearing sediments exposed in Transmission Gully (TG), near Wellington, New Zealand contains a prominent ca. 20-cm-thick silicic tephra bed dated at 1.09±0.12 Ma based on ITPFT dating of glass shards. Such occurrences are rare in the Wellington region. The associated palynology, overlying TG tephra, represents vegetation totally different from any modern assemblage with both warm climate taxa, e.g. Lygodium articulatum, Toronia toru, and cool climate taxa, e.g. Acaena, Apiaceae, Epilobium, Gentiana, Lycopodium australianum, Taraxacum, and Wahlenbergia, occurring together. A pollen assemblage 0.4 m above the base of TG tephra comes from a southern beech forest existing under warm temperate conditions, with both Fuscospora and Nothofagus menziesii beeches, and an abundance of ferns and lycopods represented. Beeches rapidly disappeared from around the site of deposition until at 0.6 m above the base of TG tephra beech pollen were rare and the pollen assemblages were dominated by grass. Ferns and lycopods were still well represented but the dramatic increase in grass pollen suggests that the environment outside the immediate depositional site had become much drier. A ca. 0.64-m-thick gravel layer was then deposited which impeded drainage. Associated pollen assemblages were then dominated by sedges and grasses in approximately equal percentages, although conditions may have still been relatively dry. Overall, the pollen assemblages are indicative of a sedge swamp surrounded by grassland with distant beech forest. Since the glass ITPFT-age range (±1 SD) for TG tephra extends over Oxygen Isotope Stages (OIS) 37 to 26, the fossiliferous sequence overlying the tephra should be any of the five periods of relative warmth, in spite of the lack of tree pollen and dominance in the profile of herbaceous taxa. However, geochemical comparison of TG tephra with those preserved in ODP site 1124, and two equivalent-age sequences in Auckland region (Patiki Core Site, Okariha) suggest a correlation with OIS 35. On this basis, the presence of Beaupreaidites diversiformis 0.4 m above the base of TG tephra provides a date for the final extinction of the modern New Caledonian genus Beauprea in New Zealand. Pollen from Toronia toru occurs throughout the sequence, far further south than in any other Pleistocene sequence so far analysed.