Quaternary tephra marker beds and their potential for palaeoenvironmental reconstruction on Chatham Island, east of New Zealand, southwest Pacific Ocean

Abstract

Tephras provide one of the most reliable methods of time control and synchronisation within Quaternary sequences. We report on the identification of two widespread rhyolitic tephras - the Kawakawa and Rangitawa tephras - preserved in extensive peat deposits on Chatham Island 900 km east of New Zealand. The tephras, both products of supereruptions from the Taupo Volcanic Zone, occur as pale, fine-ash dominated layers typically 10-150 mm thick. Mineralogically they are dominated by rhyolitic glass, together with subordinate amounts of quartz, feldspar, hypersthene, hornblende, Fe-Ti oxides and zircon. Phlogopite/biotite was identified additionally in Rangitawa Tephra. Ages for each tephra were obtained via mineralogical and major element glass composition-based correlation with well-dated equivalent deposits on mainland New Zealand, and we also obtained a new zircon fission-track age for Rangitawa Tephra (350 ± 50 ka) on Chatham Island. Both tephras were erupted at critical times for palaeoenvironmental reconstructions in the New Zealand region: the Kawakawa at ca. 27 cal. ka, near the beginning of the extended LGM early in marine isotope stage (MIS) 2; and the Rangitawa at ca. 350 ka near the end of MIS 10. The time constraints provided by the tephras demonstrate that Chatham Island peats contain long-distance pollen derived from mainland New Zealand, which provides a reliable proxy for identifying glacial-interglacial climate conditions, in this case during the MIS 11-10 and MIS 2-1 cycles. The two tephras thus provide important chronostratigraphic tie-points that facilitate correlation and synchronisation not only across the Quaternary deposits of the Chatham Islands group but also with climatically significant terrestrial and marine records in the wider New Zealand region.