Nature, Chemistry, and Origin of Late Cenozoic Megascopic Tephras in Leg 90 Cores from the Southwest Pacific

Abstract

Several thin (1-10 cm) megascopic vitric tephras occur in the late Cenozoic calcareous oozes on Lord Howe Rise in the Tasman Sea and off eastern South Island, New Zealand. Of the 18 tephras analyzed 15 are silicic (75-78% SiO 2 ) with abundant clear glass shards and a biotite ± hypersthene ± green hornblende ferromagnesian mineralogy. The Neogene silicic tephras were derived from the now-extinct Coromandel volcanic area in New Zealand, and the Quaternary ones from the presently active Central Volcanic Region of New Zealand. On the basis of glass chemistry and age, several of the Quaternary tephras are probably correlatives, and at least two can be matched to the major on-land Mt. Curl tephra ( -0.25 m.y.). The occurrence of correlative silicic tephras both northwest and southeast of New Zealand may result from particularly violent eruptions, the ash below and above an altitude of -20 km being dispersed in opposite directions toward the Pacific Ocean and Tasman Sea, respectively. Ash drifting eastward into the southeasterly trade wind belt off northeastern New Zealand could also be carried into the central and northern Tasman Sea.Three megascopic tephras consist of altered basic shards and common labradorite crystals. They record Neogene explosive basaltic to andesitic activity from nearby ocean island or ridge sources in the Ontong-Java Plateau and Vanuatu regions.The megascopic tephras are a very incomplete and biased record of late Cenozoic explosive volcanism in the southwest Pacific because the innumerable, thin, green argillaceous layers in the cores (Gardner et al., this volume) probably represent devitrified intermediate to basic tephras derived mainly from oceanic arc volcanism along the Pacific/Australia plate boundary. In contrast to the New Zealand-derived silicic glass shards, the preservation potential of these more basic shards in Leg 90 calcareous sediments was low.

Subdividing the Pleistocene using the Matuyama–Brunhes boundary (MBB): an Australasian perspective

“…The MBB and the Potaka Tephra are also identified at DSDP Site 593 (Nelson et al, 1985;Barton and Bloemendal, 1986), where revised oxygen isotope measurements (Nelson et al, 1993) show that the Potaka Tephra was erupted during MOIS 28.…”

An occurrence of Tuhua Tephra in deep‐sea sediments from offshore eastern North Island, New Zealand

“…At heights above c. 20 km altitude, stratospheric easterlies occur in the summer months of September through to March (Reid & Penney 1982). Thus, temporal and/or altitudinal variation in wind strength and direction may have "fractionated" the eruption column, carrying ash of smaller grain size to the southeast (e.g., Nelson et al 1996). During separate phases of the eruption, atmospheric pressure regimes may also have varied, and resulted in transport in different directions.…”

Marine tephrochronology: a personal perspective

“…3). A startling find was that tephra deposits could be markedly thicker 'upwind' of source: ash ascending above 20 km altitude was carried directly westward towards the Tasman Sea by easterly stratospheric winds, while coeval lower-level ash was blown eastward ('downwind') of New Zealand into the southern Pacific Ocean (Nelson et al 1985b). A second interesting result was the recognition that the visible tephras were only a fragmentary record of Quaternary volcanism in the southwest Pacific because New Zealand-derived silicic shards were a ubiquitous but disseminated minor component in many of the sediments in the cores.…”

“…time synchroneity between long-term glacial and interglacial signals of both hemispheres and a 'genetic link' between alpine glaciation and the marine isotope record (from analyses of core sediments from the DSDP site 594) (Nelson et al 1985a(Nelson et al , 1986, another documenting a new spectral analysis procedure for dating Quaternary deep-sea sediments (Black et al 1988), and several articles on the visible ('megascopic') tephra record preserved in the marine sediments (Nelson et al 1985b;Froggatt et al 1986) (Fig. 3).…”

“…As recognised by Nelson et al (1985b), cores of marine sediment have the potential to provide a detailed stratigraphic record of tephra fallout because large proportions of volcanic eruptives are deposited in the sea (Cassidy et al 2013).…”