John A. Westgate

Eruptive history of Earth's largest Quaternary caldera (Toba, Indonesia) clarified

Single-grain laser-fusion {sup 40}Ar/{sup 39}Ar analyses of individual sanidine phenocrysts from the two youngest Toba (Indonesia) tuffs yield mean ages of 73{plus minus}4 and 501{plus minus}5 ka. In addition, glass shards from Toba ash deposited in Malaysia were dated at 68{plus minus}7 ka by the isothermal plateau fission-track technique. These new determinations, in conjunction with previous ages for the two oldest tuffs at Toba, establish the chronology of four eruptive events from the Toba caldera complex over the past 1.2 m.y. Ash-flow tuffs were erupted from the complex every 0.34 to 0.43 m.y., culminating with the enormous (2500-3000 km{sup 3}) Youngest Toba tuff eruption, caldera formation, and subsequent resurgence of Samosir Island. Timing of this last eruption at Toba is coincident with the early Wisconsin glacial advance. The high-precision {sup 40}Ar/{sup 39}Ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions.

Correlation Techniques in Tephra Studies

Distinctive tephra layers constitute important time-parallel markers, which if widespread, offer the potential for reliable correlation over long distances. Confident correlations require a multiple criterion approach to tephra characterisation and equivalence of samples should only be considered firmly established if their stratigraphic, palaeontologic, palaeomagnetic, and radiometric age relations are compatible, and the physicochemical properties of their glass shards and phenocrysts agree. The strong susceptibility of tephra to reworking further argues for use of several stratigraphic controls in order to safeguard against gross errors.

Isothermal plateau fission-track ages of hydrated glass shards from silicic tephra beds

Abstract The age of silicic tephra beds that have experienced a simple thermal history can be accurately and precisely determined by application of the isothermal plateau fission-track technique to hydrated glass shards. Age estimates obtained by this means compare very well with those based on co-existing mineral phases using the K—Ar,40Ar/39Ar, external detector fission-track and thermoluminescence techniques. This concordance contradicts the popular belief that hydrated glass shards give unreliable ages. Distal tephra beds—many of which are difficult to date because of their fine grainsize, low abundance of crystals and common presence of detrital grains—are readily dated by the isothermal plateau glass-fission-track method. This new advance should greatly facilitate the development of detailed chronologies of tephra-bearing sedimentary sequences located far from volcanic centres.

A 3 M.Y. Record of Pliocene-Pleistocene Loess in Interior Alaska

Many distal tephra beds exist in the thick, fossiliferous loess deposits near Fairbanks interior Alaska. Isothermal plateau fission-track ages, determined on glass shards from tephra beds, in conjunction with tephrostratigraphic and magnetostratigraphic techniques, indicate that loess deposition began in the late Pliocene-an antiquity previously unsuspected. Hence, there is the opportunity now to reconstruct a detailed, well-dated record of environmental changes in interior Alaska during the past 3 m.y.

Ancient Permafrost and a Future, Warmer Arctic

Climate models predict extensive and severe degradation of permafrost in response to global warming, with a potential for release of large volumes of stored carbon. However, the accuracy of these models is difficult to evaluate because little is known of the history of permafrost and its response to past warm intervals of climate. We report the presence of relict ground ice in subarctic Canada that is greater than 700,000 years old, with the implication that ground ice in this area has survived past interglaciations that were warmer and of longer duration than the present interglaciation.

Geochemistry of Santorini tephra in lake sediments from Southwest Turkey

A layer of volcanic ash (4 cm maximum recorded thickness) is present at ∼250 cm depth in littoral sediments of Golhisar Golu; a small intramontane lake in Southwest Turkey. The major-element glass chemistry, determined by electron microprobe, is characterised by a rhyolitic composition. Trace-element data, determined by solution inductively coupled plasma mass spectrometry (icp-ms) match previously reported results for proximal pumice samples from Santorini and Crete. Peat immediately below the tephra deposit has yielded radiocarbon ages of 3330±70 yr bp and 3225±45 yr bp (calibrated age range 1749–1406 bc).

Tephrochronology of late Cenozoic loess at Fairbanks, central Alaska

Nineteen distinctive, distal tephra beds from the Gold Hill Loess in the Fairbanks region, central Alaska, are divided into two groups based on their color, mineralogy, glass-shard morphology, and the major- and trace-element composition of their glass shards. This dichotomy in tephra composition is believed to reflect the two source regions, namely, the eastern Aleutian arc–Alaska Peninsula region and the Wrangell volcanic field. Despite the fragmentary nature of the preserved loess record, which contains many local unconformities, these tephra beds can be used to link the numerous loess exposures in the Fairbanks region in their proper time sequence, establishing thereby a comprehensive, reliable, time-stratigraphic framework. Because these distal tephra beds are related to large-magnitude volcanic eruptions, given the long distance to the nearest possible source vent, they will also enable correlation of the late Cenozoic sedimentary sequence at Fairbanks to those of adjacent regions of Alaska and the Yukon. Tephra beds with bubble-wall or chunky glass shards have been dated by the isothermal plateau fission-track method. Results demonstrate that the Gold Hill Loess spans a long interval of time from late Pliocene time to the last interglacial, about 125 ka. However, the age of the basal part of this formation is not well constrained due to the apparent absence of tephra beds. Paleomagnetic studies suggest that loess deposition in the Fairbanks region began about 3 Ma.

Revision of the marine chronology in the Wanganui Basin, New Zealand, based on the isothermal plateau fission-track dating of tephra horizons

Abstract The occurrence of tephra horizons in basins adjacent to volcanic arcs provide an excellent opportunity for establishing a reliable chronostratigraphic framework for detailed sedimentological studies. In this study, three widespread and stratigraphically important rhyolitic tephra horizons interbedded in Plio/Pleistocene strata of the Wanganui Basin, New Zealand, are dated by application of the isothermal plateau fission-track (ITPFT) technique to hydrated glass shards. All glass samples were corrected for annealing and consequently yield reliable ages. Rangitawa Tephra yielded statistically indistinguishable ages from three localities that are in excellent agreement with recently determined zircon fission-track age estimates of ca. 0.35 Ma. ITPFT ages of 1.05 ± 0.05 and 1.63 ± 0.15 Ma for Potaka Pumice and Pakihikura Pumice, respectively, are considerably older than previous FT estimates but consistent with new magnetostratigraphic data that places the Potaka within the Jaramillo Subchron, and Pakihikura within the Matuyama Chron between the Cobb Mountain and Olduvai Subchrons. Combining our fission-track ages with the magnetostratigraphy, the true age of sediments within the Wanganui Basin is found to be significantly underestimated. Sedimentation rates of between ca. 680-630 m/Ma from 1.63 Ma to 0.35 Ma are calculated in the eastern part of the basin and are much lower than those calculated using the previous FT chronology. This new ITPFT-age data demonstrates that the existing Plio/Pleistocene marine chronology in New Zealand will require age revision and has important implications when considering the evolution of several other sedimentary basins in southern North Island that contain the same ITPFT-dated tephra horizons.

Early to middle Pleistocene tephrochronology of North Island, New Zealand: Implications for volcanism, tectonism, and paleoenvironments

Thick volcaniclastic sequences of early to middle Pleistocene age in southern North Island, New Zealand, contain rhyolitic tephra beds that record the early history of the Taupo volcanic zone (TVZ). At least 54 different tephra beds are recorded, and their chronology is defined by glass fission-track ages and paleomagnetism. The tephra beds span the interval ca. 2.0−0.6 Ma and provide an event frequency of 1/19 k.y., significantly higher than the frequency of sheet-forming ignimbrites preserved in the TVZ at this time (≈1/100 k.y.). The distal tephra beds thus provide a record of volcanism not revealed in the proximal volcanic region and suggest a major period of explosive activity at ca. 1.79−1.60 Ma. Several important marker horizons are identified: Pakihikura tephra (1.63 Ma), Mangatewaiiti tephra (1.24 Ma), Potaka tephra (1.00 Ma), Kidnappers B tephra (1.00 Ma), and Kaukatea tephra (0.88 Ma). These tephra beds allow direct correlation between (1) marine and nonmarine facies and (2) the fore-arc and back-arc regions of New Zealand. The tephra beds provide a framework for a paleoenvironmental reconstruction of the southern North Island. Volcaniclastic transport routes from the TVZ to basins in the south and southeast, and through the site of present mountain ranges, supplied material to a terrestrial lowland fore-arc area in the interval 1.64−0.7 Ma. Uplift and deformation since 0.7 Ma have disrupted paleodrainage routes, diverting them to the north and southwest.

The application of laser ablation ICP-MS to the analysis of volcanic glass shards from tephra deposits: bulk glass and single shard analysis

Abstract Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been applied to the chemical analysis of fine-grained (125–250 μm) volcanic glass shards separated from tephra deposits. This has been used both for bulk sample analysis and for the analysis of individual shards. Initial work concentrated on the use of an infra-red (IR) laser operating at 1064 nm, which gave craters of the order of 200 μm and was suitable for the analysis of bulk samples. This technique requires of the order of 80 μg of sample to determine a full suite of trace elements. Modification of the laser optics to enable operation in the ultra-violet (UV, at 266 nm) enables craters between 5 and 50 μm diameter to be produced, and the UV laser couples better with glass than the IR laser. We have applied this UV laser system to the analysis of single shards from Miocene tephra deposits from the Ruby Range in south-west Montana. Detection limits are below 1 ppm for a wide range of petrogenetically significant elements, but are critically dependent upon operating conditions. Calibration is achieved using synthetic multi-element glasses, with internal standardisation provided from electron probe analyses. Analysis of single shards provides a wide range of data from a single sample, enabling (i) magmatic evolution to be discerned within one eruption and (ii) the identification of separate populations of shards within one deposit which may not be apparent from the electron probe data. In this paper we will present a summary of the techniques used for both bulk sample and single shard analysis and compare some new bulk analyses with analyses of glass derived from other analytical methods.