The Campanian Ignimbrite (Y5) tephra at Crvena Stijena Rockshelter, Montenegro

Clearly defined distal tephras are rare in rockshelter sediment records. Crvena Stijena, a Palaeolithic site in Montenegro, contains one of the longest (> 20 m) rockshelter sediment records in Europe with deposits ranging in age from Middle Pleistocene to mid-Holocene. A distinctive tephra is clearly exposed within the well stratified record approximately 6.5 m below the present land surface. We present geochemical data to confirm that this tephra is a distal equivalent of the Campanian Ignimbrite deposits and a product of the largest Late Pleistocene eruption in Europe. Originating in the Campanian volcanic province of southwest Italy, this tephra has been independently dated to 39.3 ka. It is a highly significant chronostratigraphic marker for southern Europe. Macrostratigraphic and microstratigraphic observations, allied with detailed particle size data, show that the tephra layer is in a primary depositional context and was transported into the rockshelter by aeolian processes. This site is unique because the tephra forms an abrupt boundary between the Middle and Upper Palaeolithic records. Before they can be used as chronostratigraphic markers in rockshelter and cave-mouth environments, it is essential to establish the stratigraphic integrity of distal tephras and the mechanisms and pathways involved in their transport and deposition.     

A 70 ka record of explosive eruptions from the TALDICE ice core (Talos Dome,East Antarctic plateau)

The new Antarctic TALDICE ice core (72° 49′ S, 159° 11′ E, 1620 m depth), containing abundant primary tephras, provides the opportunity to elucidate the late Quaternary volcanic history of the south polar region, as well as to broaden the East Antarctic tephrostratigraphic framework. Here grain size and glass compositional data for representative tephra layers from the last 70 ka core section are used for source identification. Results point to origin of layers from centres of the Melbourne Volcanic Province (McMurdo Volcanic Group), located ～250 km from the coring site. Occurrence of tephra layers within the ice core record suggests that explosive activity in the identified source was not constant over the considered period, with a minimum of activity between 20 and 35 ka, and increased activity back to 65 ka. In addition to palaeovolcanic implications, the TALDICE tephra layers offer prospects for firm correlations between diverse widely separated palaeoarchives and for accurate dating of the Antarctic climatic record. Copyright © 2010 John Wiley & Sons, Ltd.     

Towards a comprehensive distal andesitic tephrostratigraphic framework for New Zealand based on eruptions from Egmont volcano

The chronology and glass composition of 43 andesitic tephra layers in palaeolake sediments in northern New Zealand provide the basis for a fine‐resolution tephrostratigraphy of the interval 10–70 cal. ka. Their ages are constrained by 14 interbedded, (mostly) well‐dated rhyolitic tephra layers. The andesitic tephra have the potential to subdivide time intervals (1–5 kyr) bracketed by well known rhyolitic layers, including periods of rapid climate change such as the last glacial–interglacial transition and the Younger Dryas. The source of the distal andesitic tephra is identified as Egmont volcano (some 270 km S‐SW) on the basis of glass shard composition. The tephra contain high‐K₂O (3–6 wt%) andesitic‐dacitic (SiO₂ = 60–73 wt%) glass, with commonly heterogeneous shard populations (2–10 wt% SiO₂). Within stratigraphic intervals of < 10 kyr, individual tephra layers can be distinguished on the basis of their SiO₂ and K₂O contents, and variability in these contents can also be a distinguishing characteristic. The tephra record greatly extends the dated pyroclastic and geochemical record of Egmont volcano, and demonstrates that the volcano has frequently produced widely dispersed tephra over the last 70 kyr at a generally constant rate. Copyright © 2005 John Wiley & Sons, Ltd.     

Correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation ICP‐MS analyses: current status and future potential

Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) is a high spatial resolution analytical method which has been applied to the analysis of silicic tephras. With current instrumentation, around 30 trace elements can be determined from single glass shards as small as ～ 40 µm, separated from tephra deposits. As a result of element fractionation during the ablation process using a 266 nm laser, a relatively complex calibration strategy is required. Nonetheless, such a strategy gives analyses which are accurate (typically within ±5%) and have an analytical precision which varies from ～ ±2% at 100 ppm, to ～ ±15% at 1 ppm. Detection limits for elements used in correlation and discrimination studies are well below 1 ppm. Examples of the application of trace element analysis by LA‐ICP‐MS in tephra studies are presented from the USA, New Zealand and the Mediterranean. Improvements in instrumental sensitivity in recent years have the potential to lower detection limits and improve analytical precision, thus allowing the analysis of smaller glass shards from more distal tephras. Laser systems operating at shorter wavelengths (e.g. 193 nm) are now more widely available, and produce a much more controllable ablation in glasses than 266 nm lasers. Crater sizes of <10 µm are easily achieved, and at 193 nm many of the elemental fractionation issues which mar longer wavelengths are overcome. By coupling a short wavelength laser to a modern ICP‐MS it should be possible to determine the trace element composition of glass shards as small as 20 µm and, providing sample preparation issues can be overcome, the determination of the more abundant trace elements in glass shards as small as 10 µm is within instrumental capabilities. This will make it possible to chemically fingerprint tephra deposits which are far from their sources, and will greatly extend the range over which geochemical correlation of tephras can be undertaken. Copyright © 2007 John Wiley & Sons, Ltd.     

吉林金龙顶子火山空降堆积物体积估算及其地质意义

用梯形法、对数坐标直线交叉法、厚度对数-面积平方根法对吉林龙岗金龙顶子火山四海期空降堆积物的体积进行了估算,结果分别为0.096 km3,0.138 km3,0.109 5 km3,与Los Alamos National Laboratory开发的火山喷发模拟软件模拟的结果(0.105 km3)相近。对比分析认为;梯形法存在的主要问题是忽略了最后一条等厚线与零点等厚线间的体积,从而低估了实际喷出量;对数直线交叉法虽然减少了厚度与面积间的非线性相关性,但获得的两条直线具有相当大的主观性;厚度对数-面积平方根法是在空降堆积物随着远离火山口呈指数性减薄的事实基础上提出的,是比较准确的体积估算方法。     

Revised age estimate of the Mjáuvøtn tephra A on the Faroe Islands based on Bayesian modelling of 14C dates from two lake sequences

Tephra horizons are potentially perfect time markers for dating and cross‐correlation among diverse Holocene palaeoenvironmental records such as ice cores and marine and terrestrial sequences, but we need to trust their age. Here we present a new age estimate of the Holocene Mjáuvøtn tephra A using accelerator mass spectrometry ¹⁴C dates from two lakes on the Faroe Islands. With Bayesian age modelling it is dated to 6668–6533 cal. a BP (68.2% confidence interval) – significantly older and better constrained than the previous age. Copyright © 2010 John Wiley & Sons, Ltd.     

Toward a revised hazard assessment at Merapi volcano, Central Java

Of 1.1 million people living on the flanks of the active Merapi volcano, 440,000 are at relatively high risk in areas prone to pyroclastic flows, surges, and lahars. For the last two centuries, the activity of Merapi has alternated regularly between long periods of viscous lava dome extrusion, and brief explosive episodes at 8–15 year intervals, which generated dome-collapse pyroclastic flows and destroyed part of the pre-existing domes. Violent explosive episodes on an average recurrence of 26–54 years have generated pyroclastic flows, surges, tephra-falls, and subsequent lahars. The 61 reported eruptions since the mid-1500s killed about 7000 people. The current hazard-zone map of Merapi (Pardyanto et al., 1978) portrays three areas, termed ‘forbidden zone’, ‘first danger zone’ and ‘second danger zone’, based on successively declining hazards. Revision of the hazard map is desirable, because it lacks details necessary to outline hazard zones with accuracy, in particular the valleys likely to be swept by lahars, and excludes some areas likely to be devastated by pyroclastic gravity-currents such as the 22 November 1994 surge. In addition, risk maps should be developed to incorporate social, technical, and economic factors of vulnerability.Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, on eruptive behavior and scenarios, and on existing models and preliminary numerical modeling. Firstly, the reconstructed eruptive activity, in particular for the past 7000 years and from historical accounts of eruptions, helps to define the extent and recurrence frequency of the most hazardous phenomena (Newhall et al., 2000; Camus et al., 2000). Pyroclastic flows traveled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down the radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Newhall et al., 2000; Camus et al., 2000). Secondly, four eruption scenarios are portrayed as hazardous zones on two maps and derived from the past eruptive behavior of Merapi and from the most affected areas in the past. Thirdly, simple numerical simulation, based on a Digital Elevation Model, a stereo-pair of SPOT satellite images, and one 2D-orthoimage helps to simulate pyroclastic and lahar flowage on the flanks and in radial valley channels, and to outline areas likely to be devastated.Three major threats are identified: (1) a collapse of the summit dome in the short-to mid-term, that can release large-volume pyroclastic flows and high-energy surges towards the south–southwest sector of the volcano; (2) an explosive eruption, much larger than any since 1930, may sweep all the flanks of Merapi at least once every century; (3) a potential collapse of the summit area, involving the fumarolic field of Gendol and part of the southern flank, which can contribute to moderate-scale debris avalanches and debris flows.     

A review of the environmental history of Iceland, 13 000-9000 yr BP

A synthesis of the main environmental changes that are interpreted to have occurred in Iceland during the period 13-9 ka BP is presented. Most of the evidence available relates to variations in the position of ice margins, although some limited information on vegetation history and soil stability is also referred to. Only qualitative and limited climatic inferences can be made for this period because of the lack of detailed evidence. A summary curve of the relative extent of ice cover is presented.     

Tephrological implications of beam size—sample‐size effects in electron microprobe analysis of glass shards

This paper concerns the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe. Application of electron probe microanalysis to tephrostratigraphical methods has increasingly facilitated the resolution and refinement of Quaternary chronology associated with records of proxy‐environmental or proxy‐climatic change. The geographical range over which tephras are recovered has expanded significantly with the identification and analysis of crypto (or hidden) tephras in areas far removed from tephra sources. These tephras are dominated by glass shards, which, in many distal environments, may be either small in size (μm) or may be highly pumiceous with low glass:void ratios and thin (<10 μm) shard walls. We demonstrate that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls. This approach distorts the geochemical data, creating analytical differences that may generate inappropriate tephrogeochemical fingerprints. Additional distortion of the geochemical fingerprint in the form of hybrid analyses may be encountered in glass fragments containing micron‐sized crystalline phases such as feldspar. Copyright © 2001 John Wiley & Sons, Ltd.