Distal occurrence of mid-Holocene Whakatane Tephra on the Chatham Islands, New Zealand, and potential for cryptotephra studies
Introduction
In New Zealand, the Taupo and Okataina volcanic centres in the central Taupo Volcanic Zone (TVZ, Fig. 1A) are the most productive Late Quaternary rhyolitic centres in the world (Wilson et al., 1995). They have produced >60 rhyolitic eruptions within the past ca. 60,000 years (Wilson et al., 2009, Lowe, 2011). Tephras from these eruptions form a comprehensive chronostratigraphic framework for Quaternary stratigraphic and palaeoclimate records in the New Zealand region including the NZ-INTIMATE project (e.g., Alloway et al., 1993, Alloway et al., 2007, Pillans et al., 1993, Pillans et al., 2005, Newnham et al., 1999a, Newnham et al., 2007, Lowe et al., 2008).
Tephra layers from some of the largest-volume eruptions are commonly encountered in distal settings. For example, tephra from the ca. 27,100 calendar (cal.) BP Kawakawa/Oruanui eruption (∼530 km3 dense rock equivalent, DRE) from Taupo Volcanic Centre (Wilson, 2001) has been identified >1500 km from source (Carter et al., 1995, Lowe et al., 2008). Another less voluminous but nevertheless widespread tephra is the ca. 17,600 cal. BP Rerewhakaaitu Tephra (∼5 km3 DRE) from the Okataina Volcanic Centre, known to occur at least 500 km from source (Newnham et al., 2003, Lowe et al., 2008). These and numerous other Quaternary-aged tephras have been described in cores of marine sediments around New Zealand, mainly to the east or northeast in the Pacific Ocean (e.g., Pillans and Wright, 1992, Carter et al., 1995, Carter et al., 2003, Alloway et al., 2005, Shane et al., 2006, Allan et al., 2008) and also, much less frequently, to the west in the Tasman Sea (e.g., Nelson et al., 1985, Carter et al., 2004).
Although Holocene-aged tephras have been described from a number of marine cores taken from sites within ∼300 km of the TVZ, such as P69, MD97–2121, H209–H215, S794, S803, and the RR series (e.g., Kohn and Glasby, 1978, Stewart and Neall, 1984, Pillans and Wright, 1992, Carter et al., 1995, Carter et al., 2008, Shane et al., 2006), such tephras are rarely documented in cores taken from waters beyond ∼300 km. Only two Late-Holocene eruptives, the Waimihia (ca. 3400 cal. BP) and Taupo (ca. 232 AD) tephras, have been recorded ∼560 km and ∼660 km, respectively, from TVZ thus far (Carter et al., 1995). Allan et al. (2008) examined three cores from ODP Site 1123, ∼1100 km east of New Zealand (Fig. 1C), but reported no Holocene tephras despite recording 70 visible tephra layers (each ∼4 cm thick on average) within sediments dating back to ca. 1.8 Ma. Yet it is evident from observations of historic and modern ash clouds that such clouds, even from eruptions of quite low volume, are very commonly blown long distances ranging from ∼750 km–∼2500 km (e.g., Mt Tarawera, 1886; Mt St Helens, 1980; Mt Pinatubo, 1991; Mt Ruapehu, 1995-96), and ash from the eruptions of Mt Spurr (1992) and Klyuchevskoy (1994) travelled at least ∼5000 km (Lowe, 2011). Most recently, tephra from the Icelandic Eyjafjöll eruptions of April–May in 2010, relatively minor in terms of volume, was distributed over much of Scandinavia and Europe as far as Siberia, more than ∼8000 km from source (Davies et al., 2010a).
Thus it seems highly likely that ash from many of the Holocene rhyolitic eruptions from central TVZ volcanoes (additional to Waimihia and Taupo tephras) would have been delivered to and deposited in distal settings well over ∼300 km from New Zealand. Consequently, the Chatham Islands, lying ∼900 km directly east of Christchurch, New Zealand (Fig. 1A), and blanketed by extensive peat deposits, would seem to provide a very suitable, stable archive for recording distal tephra deposits either as thin, visible layers or as cryptotephras. Two relatively thick (≥∼10 cm), visible tephra layers are already known: the Rangitawa (ca. 340 ka) and Kawakawa/Oruanui tephras (Holt et al., 2010). A third visible tephra layer, the mid-Holocene Whakatane Tephra, ∼5 mm in thickness, is preserved in peat on Pitt Island in the Chatham Islands group (Fig. 1B). This paper describes the identification of this distal tephra using compositional analysis of constituent glass shards via the electron microprobe and by dating peat contiguous with the layer using the radiocarbon (14C) method. It discusses the significance of this occurrence, the most-distal Holocene tephra recorded for the New Zealand region, with respect to potential cryptotephra studies of marine cores around New Zealand and on the Chatham Islands. Also considered are some of the implications for ash dispersal and for volcanic hazards in distal locations such as the Chatham Islands, and some reasons for the current paucity of Holocene tephras in the distal marine record.
Section snippets
Whakatane Tephra and its occurrence on Pitt Island
The Whakatane Tephra (WT) was erupted during the Whakatane eruption episode from the Haroharo Linear Vent Zone (HLVZ, Fig. 1A) within the Okataina Volcanic Centre (Nairn, 2002, Kobayashi et al., 2005, Smith et al., 2006) at ca. 5500 cal. BP. With a total volume estimated at 11.3 km3 (DRE), the eruption involved three different batches of magma (Kobayashi et al., 2005). Ash erupted during the plinian phases was distributed mostly to the north, east, and southeast of source, with the main axis of
Age of Whakatane Tephra and correlation by electron microprobe analysis of glass
A radiometric 14C age was obtained on a sample of peat enclosing the tephra (i.e., slices of peat ∼1-cm thick were taken from directly beneath and above the layer and combined into a single sample). The sample was processed (acid-base-acid pre-treatment) and dated at the University of Waikato Radiocarbon Dating Laboratory using benzene-based liquid scintillation spectrometry. It returned a conventional 14C age of 4806 ± 41 14C BP (Wk-28291). Calibration of the age using SHCal04 (McCormac
Discussion and implications
The discovery of WT on Pitt Island, even at ∼5 mm in thickness, is of considerable significance. Its occurrence in such a distal location ∼850 km from source has implications for several different fields of study including eruption dynamics and volcanic hazard interpretation as well as informing meteorological conditions during its eruption. But perhaps of most significance is that the distal occurrence of WT demonstrates that new insight into documenting the dispersal of tephras across the
Conclusions
The Whakatane Tephra, erupted ca. 5500 cal. BP from the North Island’s Okataina Volcanic Centre, has been identified as a ∼5-mm-thick visible layer of very fine-ash-sized, highly siliceous (rhyolitic) glass shards within peat on Pitt Island. The first Holocene tephra to be recognised in the Chatham Islands, WT was identified using both 14C dating and major element analysis of glass shards by electron microprobe. The Pitt Island occurrence of WT, ∼850 km from source, is the most-distal Holocene
Acknowledgements
KAH and RCW are particularly grateful for the hospitality and assistance of Bill and Di Gregory-Hunt during field work on Pitt Island. RCW is also very grateful to all in the Gregory-Hunt and Lanauze families on Pitt Island, and to the Department of Conservation on the Chatham Islands. We appreciated helpful comments on the manuscript provided by Prof. Brad Pillans (ANU), another (anonymous) reviewer, and the guest editor Dr Siwan Davies (Swansea University). Dr Anja Moebis (Massey University)
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