Phil Shane

Tephrochronology: a New Zealand case study

Abstract Tephrochronology is the study of volcanic ash (tephra) beds for the purpose of correlating and dating volcanic and other geologic events. Large magnitude silicic eruptions can disperse tephra up to thousands of kilometres from the vent, producing a near instantaneous marker horizon. In addition to their geochronological value, tephra beds are a major source of data on the eruption frequency and geochemistry of large rhyolitic volcanoes. The Quaternary Taupo volcanic zone (TVZ) in New Zealand is one of the most frequently active rhyolitic centres on Earth, and for much of the 20th century its tephra beds have been the focus of study. As a result, fingerprinting and dating techniques have been fine-tuned, and tephra beds now under-pin the late Cenozoic chronology for a wide variety of disciplines from volcanology to sequence stratigraphy and archaeology. The key to tephrochronology is the identification and correlation of tephra horizons. In the proximal setting ( 14 C (ca. 40 ka), and are fine-grained and/or lack K-rich phenocrysts, the isothermal plateau fission-track (ITPFT) method using glass can provide an accurate and precise age. 40 Ar – 39 Ar can be employed for high-resolution dating, however this may require single crystal laser fusion to detect relict and partially degassed crystals that are common in pyroclastic deposits, even within single pumice blocks. The New Zealand tephrostratigraphic record is not complete in space or time. Only the post-64 ka record is well established. However, there are chronologically well-constrained tephra beds dating back to 10 Ma. Some tephra beds cover most of the North Island of New Zealand, are found in the South Island, and extend as much as 1400 km from vent into the Pacific Ocean. Widespread and/or stratigraphically important units include: Kaharoa (665 years BP); Kawakawa (22 ka); Rangitawa (0.33 Ma); Potaka (1 Ma) and Pakihikura (1.6 Ma). Ages on these and other tephra beds provide a framework for global climatic and sea-level change recorded in New Zealand, and allow direct correlation between the marine and terrestrial realms.

Rhyolite magma processes of the ∼AD 1315 Kaharoa eruption episode, Tarawera volcano, New Zealand

Abstract Products of the ∼5 km3 (DRE), ∼5-yr duration Kaharoa eruption episode display two main high-silica rhyolite compositions; T1 erupted early (as plinian pyroclastics), and T2 erupted late (mostly as lavas). The T1 and T2 eruptive types are defined by crystal contents and compositional variations in whole rock, glass, plagioclase and biotite. Stratigraphically intermediate pyroclastic deposits have an intermediate composition (T1+2). A small volume of rhyodacite pyroclastics, mingled with injected basalt, was also erupted. The Kaharoa rhyolites were erupted from multiple sources spread along an 8-km linear vent zone, but the changes in eruptive compositions were largely controlled by position in the eruption sequence and magma discharge rates, rather than vent locations. Data from the Kaharoa eruptive types, vent locations, eruption sequence and discharge rates can be combined with concepts of magma chamber evacuation processes to produce a preliminary dimensional model of the pre-eruption rhyolite magma body. Our model magma body is sill-like, ∼8 km long, 1 km wide, 1.4 km thick, and located at ∼6–7 km depth in the upper crust. T1 magma overlay T2 magma in the upper levels of the chamber, with each magma layer internally mixed to a homogeneous composition along an axial extent defined by the vent locations. An underlying third rhyolite magma (T3) is recognised as the silicic end-member that was modified by basalt to form the rhyodacite eruptives. The rhyolite magma stratification survived multiple injections of basalt magma, which primed and finally triggered the Kaharoa eruptions. The T1+2 eruptives resulted from syn-eruption mingling in the conduit of the two main rhyolite magma types. Thickness of the T1 layer in the model can be estimated at 0.25 km; the T2 layer was somewhat thicker. Thicknesses of the underlying T3 and basalt layers are uncertain. Post-eruption geothermal heat flow indicates a residual magma volume of ≥6 km3, suggesting that the pre-eruption magma volume was ≥11 km3.

A 28 000-6600 cal yr record of local and distal volcanism preserved in a paleolake, Auckland, New Zealand

Abstract A 52.5 m core was extracted from Pukaki Crater, an infilled basaltic explosion crater in the Auckland Volcanic Field, for detailed tephra and palynological analysis. The core consists of a lower 6 m of finely laminated lacustrine sediments representing the interval c. 28 000–6600 cal yr overlain by 46.5 m of homogeneous marine silts deposited between c. 7600 and 6600 cal yr. Favourable conditions have preserved at least 40 tephra layers in the sediments. These have been derived from one local and five distal sources and were deposited within the crater lake between c. 28 000 and c. 7600 cal yr. The tephra beds were identified by stratigraphic position, geochemical analyses, and ferro‐magnesian mineral assemblage. This tephrostratigraphic framework is underpinned by three distinctive tephra beds, namely Tuhua (c. 6950 cal yr), Rotoma (c. 9500 cal yr), and Kawakawa (c. 26 500 cal yr). Of the 40 tephra beds, 7 are sourced from the rhyolitic Okataina Volcanic Centre (Mamaku c. 8200 cal yr; Rotoma c. 9500 cal yr; Waiohau c. 13 800 cal yr; Rotorua c. 15 800 cal yr; Rerewhakaaitu c. 17 700 cal yr; Okareka c. 21 400 cal yr; Te Rere c. 25 000 cal yr), 3 from the rhyolitic Taupo Volcanic Centre (Opepe c. 10 200 cal yr; Kawakawa c. 26 500 cal yr; Poihipi c. 27 500 cal yr), 5 from the andesitic Tongariro Volcanic Centre, 14 from the andesitic Taranaki Volcano, 1 from Mayor Island (Tuhua c. 6950 cal yr), and 8 from the basaltic Auckland Volcanic Field. In addition, two previously unidentified rhyolitic tephra (c. 17 100 cal yr and c. 20 720 cal yr) are recorded. The occurrence of numerous andesitic and rhyolitic tephra beds in the Auckland region extends the known dispersal of the units and has implications for the assessment of volcanic hazards from distal sources. Many of the Taranaki‐derived tephra beds do not stratigraphically match those recorded in the Waikato lakes region and this suggests that Taranaki Volcano produced more ash than previously estimated. The distal tephra record preserved at Pukaki provides age constraints for Auckland Volcanic Field basaltic tephra that are otherwise poorly dated. Basaltic fall events are recorded at c. 14 450 cal yr, 15 750 cal yr, 19 380 cal yr, 19 420 cal yr, 23 825 cal yr, 24 175 cal yr, 25 200 cal yr, and 25 700 cal yr. Fresh glass in the basaltic tephra allows them to be chemically fingerprinted and discriminated, and this will open a new avenue to development of a regional basaltic tephrostratigraphy.

New occurrence of Youngest Toba Tuff in abyssal sediments of the Central Indian Basin

Abstract Volcanic glass and pumice found in siliceous abyssal sediments of the Central Indian Basin, south of the Equator, have previously been assigned various origins including intra-basin volcanism, Indonesian arc, and Krakatau. Rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the Youngest Toba Tuff erupted at 74 ka from northern Sumatra. The correlation extends the distribution of coarse (>63 μm) glass shards some 1500 km south of the previously known fallout zone, and into the Southern Hemisphere (reaching latitudes of ca. 14°S). This provides evidence for bi-hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption. Such dispersal could have facilitated the global impact of the eruption.

Upper-ocean-to-atmosphere radiocarbon offsets imply fast deglacial carbon dioxide release

Radiocarbon in the atmosphere is regulated largely by ocean circulation, which controls the sequestration of carbon dioxide (CO2) in the deep sea through atmosphere–ocean carbon exchange. During the last glaciation, lower atmospheric CO2 levels were accompanied by increased atmospheric radiocarbon concentrations that have been attributed to greater storage of CO2 in a poorly ventilated abyssal ocean. The end of the ice age was marked by a rapid increase in atmospheric CO2 concentrations that coincided with reduced 14C/12C ratios (Δ14C) in the atmosphere, suggesting the release of very ‘old’ (14C-depleted) CO2 from the deep ocean to the atmosphere. Here we present radiocarbon records of surface and intermediate-depth waters from two sediment cores in the southwest Pacific and Southern oceans. We find a steady 170 per mil decrease in Δ14C that precedes and roughly equals in magnitude the decrease in the atmospheric radiocarbon signal during the early stages of the glacial–interglacial climatic transition. The atmospheric decrease in the radiocarbon signal coincides with regionally intensified upwelling and marine biological productivity, suggesting that CO2 released by means of deep water upwelling in the Southern Ocean lost most of its original depleted-14C imprint as a result of exchange and isotopic equilibration with the atmosphere. Our data imply that the deglacial 14C depletion previously identified in the eastern tropical North Pacific must have involved contributions from sources other than the previously suggested carbon release by way of a deep Southern Ocean pathway, and may reflect the expanded influence of the 14C-depleted North Pacific carbon reservoir across this interval. Accordingly, shallow water masses advecting north across the South Pacific in the early deglaciation had little or no residual 14C-depleted signals owing to degassing of CO2 and biological uptake in the Southern Ocean.

Paleovegetation of marine isotope stages 4 and 3 in northern new zealand and the age of the widespread rotoehu tephra

Abstract Paleolake sediment, constrained by tephrochronology, from Onepoto basin volcanic crater in Auckland, Northern New Zealand (36° 48′S), provides one of the few uninterrupted records of paleovegetation for marine oxygen isotope stages (MIS) 4 and 3 (76,000–26,000 yr B.P.) in the region. This period was characterized by cool temperate conifer-hardwood forest that lacked some of the warmer taxa typical of the Holocene. The period 64,400–60,500 yr B.P. was marked by opening of forest canopy and expansion of small trees and shrubs, and correlates to the thermal minima of MIS 4. However, the landscape was never as open as the forest-shrubland mosaic of the MIS 2. The beginning of MIS 3 (60,500–50,500 yr B.P.) was marked by the dramatic expansion and then decline of conifer-hardwood forest dominated by Dacrydium cupressinum, a species that prefers wetter conditions. This forest was succeeded by the typically montane Nothofagus at 50,500 yr B.P., corresponding to a thermal decline. Thus, MIS 3 began with an abrupt change to moist cool conditions that lasted about 5000 yr, followed by gradual cooling and dryer conditions. This supports some interpretations from other parts of the southwest Pacific region, that MIS 3 was a period of increased precipitation. The widespread and stratigraphically important Rotoehu tephra, erupted from Okataina Volcanic Centre, has been variously dated at 45,000–65,000 yr B.P. At Onepoto, sedimentation rate and paleovegetation reconstruction imply an age of c. 44,300 yr B.P. The tephra provides a correlation horizon in the marine and terrestrial realms during a period (MIS 3) difficult to date by radiometric methods.

Reactivation of a rhyolitic magma body by new rhyolitic intrusion before the 15.8 ka Rotorua eruptive episode: implications for magma storage in the Okataina Volcanic Centre, New Zealand

The 15.8 ka Rotorua rhyolite eruptive episode from the Okataina Volcanic Centre comprises a plinian pumice fall deposit followed by the extrusion of two rhyolitic lava domes or flows and their associated block-and-ash flows, with a total volume >1 km3 (dense rock equivalent). Variations in mineralogy, whole-rock, glass and mineral chemistry, and calculated magmatic properties suggest that two distinct rhyolitic magmas were sequentially tapped during the eruption. The first magma erupted (T1) is characterized by low SiO2 (c. 76.5 wt% in glass), calcic feldspars (An44), magnesian hornblendes (MgO c. 14.45 wt%), clinopyroxene, and high temperatures (c. 835 °C) and fO2 (1.8–2.1 ΔFMQ (where FMQ is the fayalite–magnetite–quartz buffer)). The second magma (T2) was more evolved and is characterized by higher SiO2 (c. 77.4 wt% in glass), Na-rich feldspars (An24), less magnesian hornblendes (MgO c. 11.8 wt%), biotite, and low temperatures (c. 750 °C) and fO2 (0.65–1.1 ΔFMQ). Both magmas are homogeneous, but evidence for some magma mingling indicates that they were in contact during eruption. However, there was only a minor degree of hybridization, perhaps reflecting the contrasting temperatures and viscosities of the two magmas. The crystal-rich, poorly vesicular nature of the T2 ejecta indicates that it originated from a cooling, high-level magma chamber that was reactivated by intrusion of hotter, volatile-rich T1 magma. The ponding of rhyolite magmas at shallow depth and their subsequent reactivation by later rhyolitic intrusion may be an important process in the compositional evolution and eruption dynamics of many Okataina Volcanic Centre rhyolite magma bodies.

A long late-Quaternary record from Lake Poukawa, Hawke's Bay, New Zealand

Abstract The Lake Poukawa Basin is a large co-seismic depression located at 20 m above mean sea level in Hawke’s Bay in eastern North Island, New Zealand. We present a detailed environmental history of the basin for the last c. 60 ka based on analyses of the top 105 m of a 200-m core record. Dating control is provided by radiocarbon, optically stimulated luminescence (OSL) and U/Th disequilibrium ages. The chronology is supported by nine tephras of inferred age including marker tephras, Kawakawa (22 590±230 yr BP at −18.25 m core datum), Tahuna (c. 35–43 ka) at −33.1 m core datum, and Rotoehu (45–50 ka) at −39.1 m core datum. Disagreements between some of the older tephra ages and the numerical ages from the OSL and U/Th dating mean that more than one age model can be applied. Three major lithostratigraphic units are identified: a basal calcareous silt with lignitic peats between 105.28 and 98.58 m of marine oxygen isotope stage (MIS) 3 age; an extended sequence of detrital shelly sands and silts, between 98.58 and 8 m of MIS 3 and 2 age; and a Holocene peat unit (MIS 1) from 8 to 0 m. Alternatively, but less likely, the basal unit may represent stage MIS 5a and the detrital shelly sands would then contain an amalgam of MIS 4, 3, and 2 deposits. We propose a notably moist phase represented by the peat which our numerical dating model places near the start of isotope stage 3. This suggests the existence of mild conditions during an interstadial in central New Zealand at c. 55–50 ka when a podocarp–beech–broadleaf forest of near-interglacial affinity surrounded the basin. The interstadial is marked by both lake and peat formation in the basin. After 50 ka a thermal decline set in, though the climate remained moist initially. Under these conditions, the Poukawa Basin was rapidly infilled by alluvial fan deposits from the surrounding hills. The floor of the basin was occupied by grasses and sedges, responding to both the highly disturbed environment and swampy conditions in the basin. After the deposition of the Rotoehu Ash, effective precipitation declined markedly and woody shrubs expanded across the previously swampy basin floor. The data suggest an apparent thermal decline of c. 6–7°C for much of MIS 2 and the latter half of MIS 3. The Holocene was marked by the establishment of fen and lake environments on the basin floor. Prior to human disturbance, podocarp–broadleaf forest surrounded the basin.

Holocene vegetation, environment, and tephra recorded from Lake Pupuke, Auckland, New Zealand

Abstract Lake Pupuke provides a near‐complete, high‐resolution environmental record of the Holocene from northern New Zealand. Tephra beds constrain the timing of a range of proxy indicators of environmental change, and demonstrate errors in a radiocarbon chronology. Agathis australis forest progressively increases from c. 7000 yr BP and, in conjunction with indicators of reduced biomass productivity, support a model of long‐term climate change to drier conditions over the Holocene. However, except for Agathis, conifer‐hardwood forest dominated mainly by Dacrydium cupressinum shows little change throughout the pre‐human Holocene, suggesting environmental stability. Dramatic vegetation change occurred only within the last millennium as a result of large‐scale Polynesian deforestation by fire. This happened a short time before the local eruption of c. 638 cal. yr BP Rangitoto Tephra. The identification of two eruptions of tephra from Rangitoto volcano has implications for future hazard planning in the Auckland region, because the volcanoes were previously considered single event centres. Changes in atmospheric circulation since the Late Glacial, possibly causing lower frequency of distal ashfall in Auckland during the Holocene, complicates the use of long‐term records in hazard frequency assessment.

Pre-eruption thermal rejuvenation and stirring of a partly crystalline rhyolite pluton revealed by the Earthquake Flat Pyroclastics deposits, New Zealand

The Earthquake Flat Pyroclastics form a c. 10 km3 rhyolite deposit erupted at c. 50 ka from the margin of Okataina Volcanic Centre, immediately following the caldera-forming eruption of the Rotoiti Pyroclastics (c. 100 km3) from vents c. 20 km to the NE. Earthquake Flat Pyroclastics deposits display textural and compositional complexity on a crystal-scale consistent with rejuvenation of a near-crystalline pluton in the upper crust. Quartz and plagioclase crystals are resorbed, whereas hornblende and biotite are euhedral. Fe–Ti oxides indicate large variations in pre-eruption temperatures (702–805 °C). Differences of up to 70 °C within pumice lapilli show that crystals were chaotically juxtaposed during magma stirring and evacuation. Chemical zoning within hornblende crystals is consistent with rimward increases of c. 50 °C. These features are consistent with a convective self-stirring process. Previous isotope studies demonstrate a long (>100 ka) crystallization history for the magma. Resorption of crystals deep in the magma may have produced a Ca-, Fe- and Mg-enriched rhyolite melt that allowed the growth of reverse-zoned hornblende. Microdiorite lithic fragments in the Earthquake Flat Pyroclastics and Rotoiti deposits and a basaltic eruption that immediately preceded the Rotoiti eruption suggest that mafic underplating beneath Okataina Volcanic Centre provided a major thermal and volatile pulse to drive the caldera eruptions.