Fingerprints and age models for widespread New Zealand tephra marker beds erupted since 30,000 yr ago as a framework for NZ-INTIMATE.

Abstract

The role of tephras in the NZ-INTIMATE project is a critical one because most high-resolution palaeoclimatic records are linked and dated by one or more tephra layers. In this review, first we document eruptive, distributional, and compositional fingerprinting data, both mineralogical and geochemical, for 22 key marker tephras erupted since 30,000 years ago to facilitate their identification and correlation. We include new glass compositional data. The selected marker tephras comprise 10 from Taupo and nine from Okataina volcanoes (rhyolitic), one from Tuhua volcano (peralkaline rhyolitic), and one each from Tongariro and Egmont volcanoes (andesitic). Second, we use four approaches to develop 2σ-age models for the tephras (youngest to oldest): (1) calendar ages for Kaharoa and Taupo/Y were obtained by wiggle-matching log-derived tree-ring sequences dated by 14C; (2) Whakaipo/V was dated using an age–depth model from peat; (3) 14 tephras in the montane Kaipo peat sequence (Waimihia/S, Unit K, Whakatane, Tuhua, Mamaku, Rotoma, Opepe/E, Poronui/C, Karapiti/B, Okupata, Konini, Waiohau, Rotorua, Rerewhakaaitu) were dated by simultaneously wiggle-matching stratigraphic position and 51 independent 14C-age points against IntCal04 using Bayesian probability methods via both OxCal and Bpeat; and (4) the five oldest tephras, erupted before ca 18,000 cal. yr BP, were dated by calibrating limited numbers of 14C ages using IntCal04 (Okareka) or comparison curves of the expanded Cariaco Basin sequence (Te Rere, Kawakawa/Oruanui, Poihipi, Okaia). Kawakawa/Oruanui tephra, the most widely distributed marker tephra, was erupted probably ca 27,097±957 cal. yr BP. Potential dating approaches for the older tephras include their identification in Antarctic ice cores (if present) or annually laminated sediments for which robust calendar-age models have been constructed, high-precision AMS 14C dating on appropriate material from environmentally stable sites, systematic luminescence dating, or new radiometric techniques (e.g. U–Th/He) if suitable minerals are available and errors markedly reduced. Further application of Bayesian age-modelling to stratigraphic sequences of 14C ages, possibly augmented with luminescence ages, may help refine age models for pre-Holocene tephras with the largest errors. Finally, we discuss the critical role these marker tephras play in the ongoing construction of an event stratigraphy for the New Zealand region, which is a key objective of Australasian and Southern Hemisphere INTIMATE projects.