ENHANCING THE RECORD OF VOLCANIC ASH-FALL EVENTS IN NEW ZEALAND FOR PALAEOENVIRONMENTAL AND ARCHAEOLOGICAL APPLICATIONS

Tephra, the unconsolidated pyroclastic products of explosive volcanic eruptions, provide marker horizons for linking and dating sedimentary and archaeological sequences and a dossier of past volcanic activity. In North Island, New Zealand, frequent and sometimes voluminous activity from rhyolitic and andesitic volcanoes has resulted in an extensive tephra record. This record provides key time-lines or isochrons for palaeoenvironmental and archaeological studies. For example, the rhyolitic Kaharoa eruption of Mt Tarawera in c. AD 1314 produced a widespread tephra deposit (Kaharoa tephra) now known as the ‘settlement layer' because it approximately marks the start of permanent settlement, and extensive environmental impacts, by Polynesians in northern New Zealand.

Nevertheless, it is evident that the New Zealand tephra record, based on the visible spectrum of deposits, underestimates the number and types of eruptions in the past. In particular, ash-fall events from smaller-magnitude eruptions from andesitic volcanoes are poorly documented but do represent significant natural events as shown by the 1995-1996 Ruapehu eruptions. This paper demonstrates potential enhancement of the conventional tephra record by detection and analysis of cryptotephra - zones or concentrations of sparse, fine-grained glass shards concealed within sediments, i.e. not visible as a layer. The term originates from the Greek kryptein, to hide, to convey this hidden, diminutive nature. In our study we use techniques, now well established in European studies, to detect and source concentrations of tephra-derived glass concealed within peat bogs and lake sediments in northern New Zealand. We show a much more comprehensive record of tephra fallout at all sites investigated, and suggest that cryptotephras can be used to enhance conventional tephra records in New Zealand by (1) providing additional marker horizons, e.g. from smaller andesitic eruptions, for time periods poorly represented by visible tephras, and by (2) extending the distribution of key stratigraphic markers, such as the Kaharoa tephra, over much larger areas than previously possible.