Distribution, age and chemical composition of tephra layers in deep-sea sediments off western Indonesia

Tephra layers occur in deep-sea sediments of the northeastern Indian Ocean, adjacent to western Indonesian are. The layers range in age from Recent to Late Miocene. Relative abundance of light and heavy mineral species in all tephra layers have been determined, and pure glass shards from representative samples have been analyzed chemically for major oxides. On the basis of the chemical data, three distinct provinces can be recognized: (1) an extensive province of rhyolitic tephra layers, ranging in age back to Late Miocene, is found adjacent to Sumatra; (2) a more restricted province of dacitic layers, adjacent to Sunda Strait and western Java; and (3) a province of andesitic layers, found adjacent to eastern Java and the Lesser Sunda Islands. Chemical composition of tephra layers in each province remains constant with time. As an example, tephra layers from the rhyolitic province are characterized by a high and restricted range of SiO₂ (75–77%) when expressed on an H₂O-free basis.Tephra layers recovered from the study area were examined for chemical evidence of secondary alteration. The analyses revealed that H₂O is the only major oxide in the glass shards which increases progressively with the age of the tephra layers regardless of the bulk composition. H₂O, however, reaches a “saturation point” of 4–5% in the layers 250–400 thousands of years old and remains constant to the oldest recovered tephra layer (7.5 m.y. old).The decrease in silica content in deep-sea tephra layers eastward along the Indonesian volcanic arc coincides with a similar eastward decrease in average silica content in Indonesian lavas. A relatively high silica content in lavas from Sumatra, with associated ignimbrites and their deep-sea ash-fall equivalents is closely linked to thick pre-Cenozoic crust. In the portion of the arc to the east of Sumatra, the crust is Cenozoic and thin. Difference in silica content of both the lavas and deep-sea tephras along the Indonesian arc is considered in regard to the hypothesis of “magma filtering” which is based on the contrasting density gradients of ascending magma and the upper crust.     

Dissolved carbon dioxide in basaltic glasses: concentrations and speciation

Carbon dioxide dissolved in both synthetic Ca±Mg-bearing silicate glasses and natural basaltic glasses has been characterized using infrared spectroscopy. CO₂ is inferred to be dissolved in these glasses as distorted Ca or Mg carbonate ionic complexes that result in unique infrared absorption bands at 1515 cm⁻¹ and 1435 cm⁻¹. This speciation contrasts with the case of CO₂-bearing sodium aluminosilicate glasses, which contain both dissolved molecular CO₂ and dissolved Na-carbonate ionic-complexes. The difference in speciation in Ca±Mg-bearing melts may result in part from a higher activity of oxygens that react with CO₂ molecules to produce carbonate.Dissolved CO₂ contents of natural basaltic glasses can be determined from the intensities of the carbonate absorption bands at 1515 cm⁻¹ and 1435 cm⁻¹. The uncertainty of the method is estimated to be ± 15% of the amount present. The infrared technique is a powerful tool for the measurement of dissolved CO₂ contents in natural basaltic glasses since it is non-destructive, can be aimed at regions of glass a few tens of microns in size, and can discriminate between dissolved carbonate and carbon present as carbonate alteration, contained in fluid inclusions, or adsorbed on the glass.A set of submarine basaltic glasses dredged from a variety of locations contain 0–400 ppm dissolved CO₂, measured using the infrared technique. These concentrations are lower than most previous reports for similar basaltic glasses. No general relationship is observed between dissolved CO₂ content and depth of magmatic eruption, although some correlation might be present in restricted geographic locales.     

Reprint of Glass compositions and tempo of post-17 ka eruptions from the Afar Triangle recorded in sediments from lakes Ashenge and Hayk,Ethiopia

Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region.     

Glass compositions and tempo of post-17 ka eruptions from the Afar Triangle recorded in sediments from lakes Ashenge and Hayk,Ethiopia

Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region.     

Improved age estimates for Holocene Ko-g and Ma-f~j tephras in northern Japan using Bayesian statistical modelling

The Ko-g and Ma-f~j tephras are two key isochronous marker layers in northern Japan, which are from the largest Plinian eruptions of Komagatake volcano (VEI = 5) and Mashu caldera (VEI = 6), respectively. Despite extensive radiocarbon studies associated with the two tephras, individual calibrated results show considerable variations and thus accurate ages of these important eruptions remain controversial. Bayesian statistical approaches to calibrating radiocarbon determinations have proven successful in increasing accuracy and sometimes precision for dating tephras, which is achieved through the incorporation of additional stratigraphic information and the combination of evidence from multiple records. Here we use Bayesian approaches to analyse the proximal and distal information associated with the two tephra markers. Through establishing phase and deposition models, we have taken into account all of the currently available stratigraphic and chronological information. The cross-referencing of phase models with the deposition model allows the refinement of eruption ages and the deposition model itself. Using this we are able to provide the most robust current age estimates for the two tephra layers. The Ko-g and Ma-f~j tephras are hereby dated to 6657-6505 (95.4%; 6586±40, μ±σ) cal yr BP, and 7670-7395 (95.4%; 7532±72, μ±σ) cal yr BP, respectively. These updated age determinations underpin the reported East Asian Holocene tephrostratigraphic framework, and allow sites where the tephra layers are present to be dated more precisely and accurately. Our results encourage further applications of Bayesian modelling techniques in the volcanically active East Asian region.     

Compositions of glass in proximal tephras from eruptions in the Azores archipelago and their links with distal sites in Ireland

The Azores archipelago is one of the most active volcanic areas in the North Atlantic region, with approximately 30 eruptions during the last 600 years. The geochemical composition of associated tephra-derived glass is, however, not well characterized. This study presents major element compositions of glass shards from five major eruptives on the Azores: a trachybasaltic eruptive on the island of Faial (Capelinhos AD, 1957) and four explosive trachytic eruptives on the island of São Miguel (Fogo A c. 5600 cal yrs. BP, Sete Cidades c. AD 1440, Fogo AD 1563 and Furnas AD 1630). The major element compositions suggest that tephras from three active stratovolcanoes on São Miguel, Sete Cidades, Fogo and Furnas, can be distinguished from one another using bi-plots of FeO_tot vs. TiO₂ and FeO_tot vs. CaO. Late Holocene tephras found on Ireland have previously been attributed to eruptions occurring on Jan Mayen but possess a strong geochemical similarity to proximal tephras from the Azores, especially those from the Furnas volcano. The similarity of the proximal tephras on São Miguel, especially Furnas AD 1563 and Furnas AD 1630 and distal tephras in Ireland is demonstrated by strong similarity coefficients (>0.95) and the closeness of major element composition. The dominant wind direction over the Azores is favourable for tephra dispersal to western Europe and we suggest that at least three tephras found in Ireland were erupted from the Furnas volcano, and that trachytic tephras erupted from explosive eruptions on São Miguel have a potential to contribute to the construction of a European-wide tephrostratigraphic framework.     

Compositional heterogeneity of distal tephra deposits from the 1912 eruption of Novarupta, Alaska

Physical and chemical analyses of distal tephra from the 1912 eruption of Novarupta, Alaska, show considerable variations in glass and mineral compositions. A combination of a 150°C range in temperature deduced from iron-titanium oxide geothermometry, and curved patterns in bivariant element plots of glass compositions indicate that a chamber of compositionally zoned magma existed prior to the eruption. Magma-mixing cannot explain these features. The magma chamber may have resembled the model recently proposed by McBirney (1980). A highly silicic, quartz-phyric magma with mean phenocryst compositions of An₂₅ plagioclase, Fs₄₂ orthopyroxene, at a temperature of 880°C and a water pressure of 1.4 kbar, was located above a more mafic, hotter magma, bearing phenocrysts of An₄₅ plagioclase and Fs₃₅, orthopyroxene.Our results on distal tephras compare favorably with those from a recently completed study at source by Hildreth (1983), suggesting that useful petrologic information about distant volcanoes can be obtained from both types of deposits. Compositionally heterogeneous abyssal tephra layers are common in the Gulf of Alaska. Eruptions from chambers of zoned magma may account for many of these layers.     

Imposex levels and concentrations of organotin compounds (TBT and its metabolites) in Nassarius nitidus from the Lagoon of Venice

Specimens of Nassarius nitidus were collected in seven stations of the Venice Lagoon to assess the levels of tributyltin (TBT) and its metabolites monobutyltin and dibutyltin in the tissues and monitor their effect on organisms, in particular the phenomenon of imposex (superimposition of male sexual characteristics on females). The following values of population indices were found: vas deferens sequence: 1.2 ± 0.7–4.0 ± 0.5; relative penis length: 6–47%. The least impacted station was situated in the northern part of the Lagoon, where females without imposex were found and Butyltin (BuTs) concentrations in the organisms (average sum of BuTs = 43 ± 14 ng Sn g⁻¹ w.) were significantly lower than in the other stations (range of average sum of BuTs: 101 ± 22–217 ± 27 ng Sn g⁻¹ d.w.). Population indices were found to be related to the TBT content in the tissues. In particular VDSI had a significant logarithmic correlation: r = 0.95, n = 8, p < 0.05.     

Late Quaternary tephrostratigraphy and pollen stratigraphy of Uwa Formation,Shikoku Island,SW Japan: Reconsidering the MIS 11 super-interglacial horizon

We investigated a late Quaternary terrestrial sedimentary sequence (Uwa Formation) in core IC2, from a site adjacent to that of the reported core IC on NW Shikoku Island, SW Japan, and developed its tephra and pollen stratigraphy to refine the age model of the formation. First, we identified 19 horizons with high glass shard concentrations in the IC2 core sediments as possible tephras or cryptotephras, and correlated them with reported tephras on the basis of the major- and trace-element compositions of their glass shards. All correlated widespread tephras and cryptotephras were products of volcanoes in the Kyushu volcanic zone (Aso, Kakuto, Aira, Ata, and Kikai calderas). Second, we confirmed the presence in core IC2 of two pollen zones dominated by Quercus subgen. Cyclobalanopsis, which is an indicator of very warm interglacial vegetation. In the Japanese Islands, these two vegetation zones have usually been considered to characterize marine isotopic stages (MISs) 1 and 11. A previous study of the Uwa Formation correlated the upper pollen zone to MIS 1, but the lower zone was not correlated to MIS 11; rather, it was inferred to be older than MIS 12 because it was stratigraphically below the “Oda” tephra (equivalent to a distal Kasamori 5 [Ks5] tephra [MIS 12]). In this study, however, noting that the Naruohama-IV tephra (Nh-IV; MIS 10d) and Ks5 cannot be distinguished by their shard chemistries, we inferred that the suggested “Oda” tephra actually correlates to Nh-IV, rather than to the Ks5 tephra. By re-assigning the “Oda” tephra to Nh-IV, we could correlate the underlying Quercus subgen. Cyclobalanopsis-abundant zone to MIS 11 and, consequently, a pair of pollen zones indicating cool and warm conditions below the MIS 11 pollen zone to MISs 12 and 13, respectively. The resulting age model whereby tephra and pollen constraints are integrated showed a roughly constant sedimentation rate from MIS 13, without any long-term gaps; further, our MIS 13 horizon in core IC2 corresponds to the reported 1 Ma tephra horizon in core IC. Therefore, these findings represent a dramatic change in the Uwa Formation age model and validate the Uwa Formation as one of the most useful terrestrial archives of Quaternary tephrostratigraphy and paleoclimatic fluctuation in SW Japan.     

Explosive eruptive record in the Katmai region,Alaska Peninsula: an overview

At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin and from Shelikof Strait to Bristol Bay (∼8,500 km²). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka; the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy Mountain summit dome (<250 ¹⁴C years B.P.). East Mageik has erupted most frequently during Holocene time with seven explosive events (9,400 to 2,400 ¹⁴C years B.P.) preserved as tephra layers. Mount Martin erupted entirely during the Holocene, with lava coulees (>6 ka), two tephras (∼3,700 and ∼2,700 ¹⁴C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time (mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 ¹⁴C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite ash from Kaguyak’s caldera-forming event (5,800 ¹⁴C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 ¹⁴C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500 ¹⁴C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 ¹⁴C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available.     

Optically stimulated luminescence dating of single and multiple grains of quartz from perennially frozen loess in western Yukon Territory, Canada: Comparison with radiocarbon chronologies for the late Pleistocene Dawson tephra

Optically stimulated luminescence (OSL) dating of perennially frozen loess was tested on quartz grains extracted from deposits associated with the late Pleistocene Dawson tephra in western Yukon Territory, Canada. OSL samples were obtained from ice-rich loess bracketing the Dawson tephra, while radiocarbon (¹⁴C) samples were collected from the bulk sediments directly underlying the tephra and from a ground-squirrel burrow 2.7 m below the tephra. Here we report the OSL characteristics and ages of the extracted quartz grains, as well as additional radiocarbon ages for samples described in Froese [2002. Age and significance of the late Pleistocene Dawson tephra in eastern Beringia. Quaternary Science Reviews 21, 2137–2142; 2006. Seasonality of the late Pleistocene Dawson tephra and exceptional preservation of a buried riparian surface in central Yukon Territory, Canada. Quaternary Science Reviews 25, 1542–1551]. We refine the time of Dawson tephra deposition to between 25,420±70 and 25,290±80 ¹⁴C a BP. Bayesian analysis of constraining radiocarbon ages places the deposition of the Dawson tephra at between 30,433 and 30,032 cal a BP. Linear modulation (LM) OSL analysis of multi-grain aliquots of quartz showed that the initial part of the decay curve is dominated by a rapidly bleached (‘fast’) component; these samples, however, had relatively dim continuous wave (CW) OSL signals at the multi-grain aliquot (each composed of 80 grains) and single-grain scales of analysis. The single-aliquot regenerative-dose protocol was applied to multi-grain aliquots and single grains to obtain equivalent dose (D_e) values for samples collected from below and above the Dawson tephra. The D_e values were examined graphically and numerically, the latter using the central age, minimum age, and finite mixture models. For multi-grain aliquots, the central age model gave weighted mean D_e values between 30 and 50 Gy, which greatly underestimated the expected D_e of 74–81 Gy for both samples studied. Possible reasons for these underestimations are discussed, and a solution proposed based on single-grain analysis. Measurements of single grains produced D_e values in agreement with the expected D_e, and yielded OSL ages of 28±5 and 30±4 ka for the samples taken from above and below the Dawson tephra, respectively. Examination of individual grains with differing luminescence behaviors showed that a significant number of the measured quartz grains exhibited anomalous luminescence properties that would have compromised the results obtained from multi-grain aliquots. We therefore recommend analysis of individual grains to overcome the age-shortfall from multi-grain analysis of these and similar samples of quartz.     

Origin and stratigraphy of phreatomagmatic deposits at the Pleistocene Sinker Butte Volcano, Western Snake River Plain, Idaho

Sinker Butte is the erosional remnant of a very large basaltic tuff cone of middle Pleistocene age located at the southern edge of the western Snake River Plain. Phreatomagmatic tephras are exposed in complete sections up to 100 m thick in the walls of the Snake River Canyon, creating an unusual opportunity to study the deposits produced by this volcano through its entire sequence of explosive eruptions. The main objectives of the study were to determine the overall evolution of the Sinker Butte volcano while focusing particularly on the tephras produced by its phreatomagmatic eruptions. Toward this end, twenty-three detailed stratigraphic sections ranging from 20 to 100 m thick were examined and measured in canyon walls exposing tephras deposited around 180° of the circumference of the volcano.Three main rock units are recognized in canyon walls at Sinker Butte: a lower sequence composed of numerous thin basaltic lava flows, an intermediate sequence of phreatomagmatic tephras, and a capping sequence of welded basaltic spatter and more lava flows. We subdivide the phreatomagmatic deposits into two main parts, a series of reworked, mostly subaqueously deposited tephras and a more voluminous sequence of overlying subaerial surge and fall deposits. Most of the reworked deposits are gray in color and exhibit features such as channel scour and fill, planar-stratification, high and low angle cross-stratification, trough cross-stratification, and Bouma-turbidite sequences consistent with their being deposited in shallow standing water or in braided streams. The overlying subaerial deposits are commonly brown or orange in color due to palagonitization. They display a wide variety of bedding types and sedimentary structures consistent with deposition by base surges, wet to dry pyroclastic fall events, and water saturated debris flows.Proximal sections through the subaerial tephras exhibit large regressive cross-strata, planar bedding, and bomb sags suggesting deposition by wet base surges and tephra fallout. Medial and distal deposits consist of a thick sequence of well-bedded tephras; however, the cross-stratified base-surge deposits are thinner and interbedded within the fallout deposits. The average wavelength and amplitude of the cross strata continue to decrease with distance from the vent. These bedded surge and fall deposits grade upward into dominantly fall deposits containing 75–95% juvenile vesiculated clasts and localized layers of welded spatter, indicating a greatly reduced water-melt ratio. Overlying these “dryer” deposits are massive tuff breccias that were probably deposited as water saturated debris flows (lahars). The first appearance of rounded river gravels in these massive tuff breccias indicates downward coring of the diatreme and entrainment of country rock from lower in the stratigraphic section. The “wetter” nature of these deposits suggests a renewed source of external water. The massive deposits grade upward into wet fallout tephras and the phreatomagmatic sequence ends with a dry scoria fall deposit overlain by welded spatter and lava flows.Field observations and two new ⁴⁰Ar–³⁹Ar incremental heating dates suggest the succession of lavas and tephra deposits exposed in this part of the Snake River canyon may all have been erupted from a closely related complex of vents at Sinker Butte. We propose that initial eruptions of lava flows built a small shield edifice that dammed or disrupted the flow of the ancestral Snake River. The shift from effusive to explosive eruptions occurred when the surface water or rising ground water gained access to the vent. As the river cut a new channel around the lava dam, water levels dropped and the volcano returned to an effusive style of eruption.     

Explosive silicic volcanism in Iceland and the Jan Mayen area during the last 6 Ma: sources and timing of major eruptions

Fifty-three major explosive eruptions on Iceland and Jan Mayen island were identified in 0–6-Ma-old sediments of the North Atlantic and Arctic oceans by the age and the chemical composition of silicic tephra. The depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 Ma and paleoclimatic proxies (δ¹⁸O, ice-rafted debris) for the last 1 Ma. Major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off-rift volcanic zones in Iceland, and to the Jan Mayen volcanic system. The tholeiitic central volcanoes along the Iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4.9 Ma. They were the source of at least 26 eruptions of dominant rhyolitic magma composition, including the late Pleistocene explosive eruption of Krafla volcano of the Eastern Rift Zone at about 201 ka. The central volcanoes along the off-rift volcanic zones in Iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition, with three distinct episodes recorded at 4.6–5.3, 3.5–3.6, and 0–1.8 Ma. The longest and last episode recorded 11 Pleistocene major events including the two explosive eruptions of Tindfjallajökull volcano (Thórsmörk, ca. 54.5 ka) and Katla volcano (Sólheimar, ca. 11.9 ka) of the Southeastern Transgressive Zone. Eight major explosive eruptions from the Jan Mayen volcanic system are recorded in terms of the distinctive grain-size, mineralogy and chemistry of the tephra. The tephra contain K-rich glasses (K₂O/SiO₂>0.06) ranging from trachytic to alkali rhyolitic composition. Their normative trends (Ab–Q–Or) and their depleted concentrations of Ba, Eu and heavy-REE reflect fractional crystallisation of K-feldspar, biotite and hornblende. In contrast, their enrichment in highly incompatible and water-mobile trace elements such as Rb, Th, Nb and Ta most likely reflect crustal contamination. One late Pleistocene tephra from Jan Mayen was recorded in the marine sequence. Its age, estimated between 617 and 620 ka, and its composition support a common source with the Borga pumice formation at Sør Jan in the south of the island.     

Zircon U–Pb dating using LA-ICP-MS: Quaternary tephras in Boso Peninsula,Japan

Zircon U–Pb dating using LA-ICP-MS was applied to six Quaternary tephras in Boso Peninsula, central Japan: J1, Ks4, Ks5, Ks10, Ks11, and Ch2 in descending order. Accurate age determination of these tephras is of critical importance because they are widespread tephras in Japan and also relevant to a candidate site for the global boundary stratotype section and point of the early–middle Pleistocene boundary. Twenty grains were dated for each tephra and the following results were obtained. The J1 tephra had only 5 grains that yielded <2 Ma. The obtained age was ∼0.2 m.y. older than the stratigraphic age. No Quaternary ages were obtained from the Ks4 tephra. The Ks5 and Ks10 tephras had 10–12 grains that were ∼0.1–0.3 m.y. older than the stratigraphic age. The Ks11 tephra had 14 grains that yielded a weighted mean age of 0.52 ± 0.04 Ma (error reported as 95% confidence level), which was in agreement with the stratigraphic age. The Ch2 tephra had 16 grains that yielded a weighted mean age of 0.61 ± 0.02 Ma, which was also in agreement with the stratigraphic age. The good agreement between zircon U–Pb ages and the stratigraphy for Ks11 and Ch2 tephras validates the reliability of the established stratigraphy and our dating approach. The other tephras that yielded ∼0.1–0.3 m.y. older ages than the stratigraphy may indicate that the analyzed zircons were antecrysts that crystallized before eruption or they were detrital zircons incorporated during deposition.     

Derivation and test of ionospheric activity indices from real-time ionosonde observations in the European region

New ionospheric activity indices are derived from automatically scaled online data from several European ionosonde stations. These indices are used to distinguish between normal ionospheric conditions expected from prevailing solar activity and ionospheric disturbances caused by specific solar and atmospheric events (flares, coronal mass ejections, atmospheric waves, etc.). The most reliable indices are derived from the maximum electron density of the ionospheric F 2-layer expressed by the maximum critical frequency foF 2. Similar indices derived from ionospheric M(3000)F 2 values show a markedly lower variability indicating that the changes of the altitude of the F 2-layer maximum are proportionally smaller than those estimated from the maximum electron density in the F 2-layer. By using the ionospheric activity indices for several stations the ionospheric disturbance level over a substantial part of Europe (34°N–60°N; 5°W–40°E) can now be displayed online.     

Late Pleistocene and Holocene activity of the Atacazo–Ninahuilca Volcanic Complex (Ecuador)

The Atacazo–Ninahuilca Volcanic Complex (ANVC) is located in the Western Cordillera of Ecuador, 10 km southwest of Quito. At least six periods of Pleistocene to Holocene activity (N1 to N6) have been preserved in the geologic record as tephra fallouts and pyroclastic flow deposits. New field data, including petrographic and whole-rock geochemical analyses of over forty soil and tephra sections, 100 pumice and lithic samples, and 10 new ¹⁴C ages allow us to constrain: (1) the tephra fall isopachs and detailed characteristics of the last two events (N5–N6) including volume estimates of the tephra and pyroclastic flow deposits and the corresponding volcanic explosivity index (VEI); (2) the petrographical and geochemical correlations between domes, tephras, and pyroclastic flow deposits; and, (3) the timing of the last 4 eruptive events and a period of quiescence that endured a few thousand years (1000–4000).     

Frequent eruptions of Mount Rainier over the last ∼2,600 years

Field, geochronologic, and geochemical evidence from proximal fine-grained tephras, and from limited exposures of Holocene lava flows and a small pyroclastic flow document ten–12 eruptions of Mount Rainier over the last 2,600 years, contrasting with previously published evidence for only 11–12 eruptions of the volcano for all of the Holocene. Except for the pumiceous subplinian C event of 2,200 cal year BP, the late-Holocene eruptions were weakly explosive, involving lava effusions and at least two block-and-ash pyroclastic flows. Eruptions were clustered from ∼2,600 to ∼2,200 cal year BP, an interval referred to as the Summerland eruptive period that includes the youngest lava effusion from the volcano. Thin, fine-grained tephras are the only known primary volcanic products from eruptions near 1,500 and 1,000 cal year BP, but these and earlier eruptions were penecontemporaneous with far-traveled lahars, probably created from newly erupted materials melting snow and glacial ice. The most recent magmatic eruption of Mount Rainier, documented geochemically, was the 1,000 cal year BP event. Products from a proposed eruption of Mount Rainier between AD 1820 and 1854 (X tephra of Mullineaux (US Geol Surv Bull 1326:1–83, 1974)) are redeposited C tephra, probably transported onto young moraines by snow avalanches, and do not record a nineteenth century eruption. We found no conclusive evidence for an eruption associated with the clay-rich Electron Mudflow of ∼500 cal year BP, and though rare, non-eruptive collapse of unstable edifice flanks remains as a potential hazard from Mount Rainier. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. T. W. Sisson and J. W. Vallance contributed equally to this study.     

Towards a detailed distal tephrostratigraphy in the Central Mediterranean: The last 20,000 yrs record of Lago Grande di Monticchio

A detailed compilation of distal tephrostratigraphy comprising the last 20,000 yrs is given for the Central Mediterranean region. A total of 47 distinct ash layers identified in the maar lake sediments of Lago Grande di Monticchio (Basilicata, Southern Italy) are compared with proximal and distal terrestrial-marine tephra deposits in the circum-central Mediterranean region. The results of these studies provide valuable information for reconstructing the Late Pleistocene and the Holocene dispersal of pyroclastic deposits from south Italian explosive volcanoes, in particular Somma-Vesuvius, the Campi Flegrei caldera, Ischia Island and Mount Etna. Prominent tephras are discussed with respect to their reliability as dating and correlation tools in sedimentary records. Ashes from Plinian eruptions of Somma-Vesuvius (i.e. Avellino, Mercato, Greenish, Pomici di Base), for instance, are well-defined by their distribution patterns and their unique composition. The widespread Y-1 tephra from Mount Etna, on the other hand, derived most likely from two distinct Plinian events with changing wind conditions, and therefore becomes a less reliable stratigraphic marker. Statistical–numerical calculations are presented in order to discriminate between Holocene tephras from the Campi Flegrei caldera (i.e. Astroni 1–3, Agnano Monte Spina, Averno 1, Lagno Amendolare), since these ashes are characterized by an almost indistinguishable chemical fingerprint. As a highlight, numerous Campanian eruptions of proposed low-intensity have been identified in the distal site of Monticchio suggesting a revision of existing tephra dispersal maps and re-calculation of eruptive conditions. In summary, the tephra record of Monticchio is one of the key successions for linking both, terrestrial records from Central-southern Italy and marine sequences from the Tyrrhenian, Adriatic and Ionian Seas.     

Vertical seismic profile at Pike's Peak, Saskatchewan, Canada: turning rays and velocity anisotropy

First-arrival traveltimes from a multi-offset vertical seismic profile (VSP) were used to estimate velocity anisotropy in the presence of a vertical velocity gradient. A numerical model consisting of two layers with vertical velocity gradients of 3.1 and 1.2 s⁻¹, respectively, and global anisotropy parameters of =0.12±0.02 and δ=0.30±0.06 yielded first-arrival traveltimes that matched the observed traveltimes well. Shallow receivers were found to be crucial for constraining the vertical velocity field and for determining the parameters of anisotropy at depth.     

Merging eruption datasets: building an integrated Holocene eruptive record for Mt Taranaki,New Zealand

Acquiring detailed eruption frequency datasets for a volcano system is essential for realistic eruption forecasts. However, accurate datasets are inherently difficult to compile, even if one or more well-dated eruption records are available. A single record typically under-represents the eruption frequency, while combining two or more records may result in an overrepresentation. Although glass compositions have proven to be successful in tephrochronological studies of dominantly rhyolitic tephras; microlitic growth and thin glass shards inhibit their application to andesitic tephras. A method consisting of a combination of two techniques for correlating syn-eruptive deposits is demonstrated on data from the typical andesitic stratovolcano of Mt. Taranaki, New Zealand. Firstly, tentative matches are identified using the radiocarbon age and associated error of each event. Secondly, the compositions of titanomagnetite micro-phenocrysts are used as an independent check, and shown to be a useful correlation tool where age data is available. Using two lake-core records containing tephra layers in an overlapping time-frame, the radiocarbon age-correlation procedure suggested 31 tephra matches. Geochemistry data were available for 15 of these pairs. In three of these cases, the titanomagnetite compositions did not match. Hence, these “paired” tephras were from compositionally distinct magmas and therefore likely represent separate events. An additional three matches were reassigned within the temporal uncertainty limits of the dating procedure, based on better geochemical pairing. The final combined dataset suggests that there have been at least 138 separate ash fall-producing eruptions between 96 and 10 150 years B.P. from Taranaki. Using the combined dataset the mixture of Weibulls renewal model forecasts a probability of 0.52 for an eruption occurring in the next 50 years at this volcano. The present annual eruption probability is estimated at 1.6%. This likelihood is almost double that obtained when relying on a single stratigraphic record.