Quaternary tephra marker beds and their potential for palaeoenvironmental reconstruction on Chatham Island,east of New Zealand,southwest Pacific Ocean

Tephras provide one of the most reliable methods of time control and synchronisation within Quaternary sequences. We report on the identification of two widespread rhyolitic tephras – the Kawakawa and Rangitawa tephras – preserved in extensive peat deposits on Chatham Island ～900 km east of New Zealand. The tephras, both products of supereruptions from the Taupo Volcanic Zone, occur as pale, fine‐ash dominated layers typically 10–150 mm thick. Mineralogically they are dominated by rhyolitic glass, together with subordinate amounts of quartz, feldspar, hypersthene, hornblende, Fe–Ti oxides and zircon. Phlogopite/biotite was identified additionally in Rangitawa Tephra. Ages for each tephra were obtained via mineralogical and major element glass composition‐based correlation with well‐dated equivalent deposits on mainland New Zealand, and we also obtained a new zircon fission‐track age for Rangitawa Tephra (350 ± 50 ka) on Chatham Island. Both tephras were erupted at critical times for palaeoenvironmental reconstructions in the New Zealand region: the Kawakawa at ca. 27 cal. ka, near the beginning of the ‘extended’ LGM early in marine isotope stage (MIS) 2; and the Rangitawa at ca. 350 ka near the end of MIS 10. The time constraints provided by the tephras demonstrate that Chatham Island peats contain long‐distance pollen derived from mainland New Zealand, which provides a reliable proxy for identifying glacial–interglacial climate conditions, in this case during the MIS 11–10 and MIS 2–1 cycles. The two tephras thus provide important chronostratigraphic tie‐points that facilitate correlation and synchronisation not only across the Quaternary deposits of the Chatham Islands group but also with climatically significant terrestrial and marine records in the wider New Zealand region. Copyright © 2010 John Wiley & Sons, Ltd.     

A Younger Dryas Ash Bed in western Norway,and its possible correlations with tephra in cores from the Norwegian Sea and the North Atlantic

A bed of volcanic ash up to 23 cm thick is found in lacustrine and marine sediments in western Norway. It is formally mamed the Vedde Ash Bed, and its age is approximately 10,600 yr B.P., i.e., mid-Younger Dryas. The bed consits of pure glass having a bimodal basaltic and rhyolitic somposition. The geochemistry of the glass shards suggests an Icelandic source. By means of stratigraphic position and geochemistry, the ash is correlated with ash zones found in cores from the continental shelf, the Norwegian Sea, and the North Atlatic.     

High‐resolution study of Icelandic tephras in the Kangerlussuaq Trough,southeast Greenland,during the last deglaciation

Tephra abundance data and geochemistry in Late‐glacial and Holocene sediments on the East Greenland shelf are presented. Two well‐known tephras were identified from electron microprobe analysis of tephra shards picked from ash peaks in the cores. These are the Vedde Ash and Saksunarvatn Ash, which probably were deposited on the shelf after transport on drifting ice. The radiocarbon dates (marine reservoir corrected by −550 yr) that constrain the timing of deposition of the tephra layers compare well with the terrestrial and ice‐core ages of the tephras without requiring additional reservoir correction to align them with the known tephra ages. Several prominent tephra layers with a composition of Ash Zone 2 tephra punctuate the deglacial sediments. These tephra peaks coincide with significant light stable isotope events (signifying glacial meltwater) and fine‐grained sediments poor in ice‐rafted detritus. We interpret the Ash Zone 2 tephra peaks as sediment released from the Greenland Ice Sheet during strong melting pulses of the deglaciation. Copyright © 2002 John Wiley & Sons, Ltd.     

Onshore–offshore correlation of Pleistocene rhyolitic eruptions from New Zealand: implications for TVZ eruptive history and paleoenvironmental construction

Taupo Volcanic Zone (TVZ), in the North Island, New Zealand, is arguably the most active Quaternary rhyolitic system in the world. Numerous and widespread rhyolitic tephra layers, sourced from the TVZ, form valuable chronostratigraphic markers in onshore and offshore sedimentary sequences. In deep-sea cores from Ocean Drilling Program (ODP) Leg 181 Sites 1125, 1124, 1123 and 1122, located east of New Zealand, ca 100 tephra beds are recognised post-dating the Plio-Pleistocene boundary at 1.81 Ma. These tephras have been dated by a combination of magnetostratigraphy, orbitally tuned stable-isotope data and isothermal plateau fission track ages. The widespread occurrence of ash offshore to the east of New Zealand is favoured by the small size of New Zealand, the explosivity of the mainly plinian and ignimbritic eruptions and the prevailing westerly wind field.Although some tephras can be directly attributed to known TVZ eruptions, there are many more tephras represented within ODP-cores that have yet to be recognised in near-source on-land sequences. This is due to proximal source area erosion and/or deep burial as well as the adverse effect of vapour phase alteration and devitrification within near-source welded ignimbrites. Despite these difficulties, a number of key deep-sea tephras can be reliably correlated to equivalent-aged tephra exposed in uplifted marine back-arc successions of Wanganui Basin where an excellent chronology has been developed based on magnetostratigraphy, orbitally calibrated sedimentary cycles and isothermal plateau fission track ages on tephra. Significant Pleistocene tephra markers include: the Kawakawa, Omataroa, Rangitawa/Onepuhi, Kaukatea, Kidnappers-B, Potaka, Unit D/Ahuroa, Ongatiti, Rewa, Sub-Rewa, Pakihikura, Ototoka and Table Flat Tephras. Six other tephra layers are correlated between ODP-core sites but have yet to be recognised within onshore records.The identification of Pleistocene TVZ-sourced tephras within the ODP record, and their correlation to Wanganui Basin and other onshore sites is a significant advance as it provides: (1) an even more detailed history of the TVZ than can be currently achieved from the near-source record, (2) a high-resolution tephrochronologic framework for future onshore-offshore paleoenvironmental reconstructions, and (3) well-dated tephra beds correlated from the offshore ODP sites with astronomically tuned timescales provide an opportunity to critically evaluate the chronostratigraphic framework for onshore Plio-Pleistocene sedimentary sequences (e.g. Wanganui Basin, cf. Naish et al. [1998. Quaternary Science Reviews 17 695–710].     

Identification and significance of a visible,basalt‐rich Vedde Ash layer in a Late‐glacial sequence on the Isle of Skye,Inner Hebrides,Scotland

This paper reports the discovery of a visible, tephra horizon of Late‐glacial age from the site of Loch Ashik in the Isle of Skye, the Inner Hebrides, Scotland. Although the tephra shards have a bimodal geochemical composition identical to that of the Vedde Ash (a well known marker horizon within Late‐glacial sequences. The horizon at Ashik is dominated by basaltic shards and devitrified tephra shards, giving the layer its characteristic black colour. Only rhyolitic shards have previously been reported from Vedde Ash horizons in the British Isles. This new evidence raises some important questions about the factors that govern the distribution and accumulation of basaltic tephra, and about the methods used to detect ash shards in basins distal to centres of volcanic activity. Copyright © 2001 John Wiley & Sons, Ltd.     

Late Holocene tephrochronology of the northern Antarctic Peninsula

Andesitic and basaltic andesitic tephra layers are abundant in Holocene deposits from the Antarctic Peninsula. Visually discernible tephra horizons occur in three lakes on Livingston Island. Tephra in two other lakes and in a moss bank on Elephant Island, with very low ash concentrations, were detected magnetically. Deception Island is the most likely volcanic source for the tephra. With direct ¹⁴C dating, age/depth curves, and cross-correlations at least 14 tephra horizons dating to between ca. 4700 and 250 yr B.P. were identified and now form the basis for a preliminary regional tephrochronology that will be a valuable dating tool for investigating the Holocene climatic history of Antarctica.     

Extending the known distribution of the Vedde Ash into Siberia: occurrence in lake sediments from the Timan Ridge and the Ural Mountains,northern Russia

Tephra shards from the Vedde Ash eruption have been identified in two lakes from northwestern Russia and the Polar Ural Mountains. This is the most distal and easternmost occurrence of this regional tephra marker horizon found so far and it extends the area of the Vedde Ash tephra more than 1700 km further east than previously documented. This means that particles the size of fine sand have travelled more than 4000 km from the Katla volcano source, south Iceland. These findings offer a new possibility to correlate archives over a very long distance in the time period around the Younger Dryas.     

Variation in rhyolitic magma composition linked with fractionation from a common source: insights from the Rotoiti eruption,Taupo Volcanic Zone,New Zealand

ABSTRACT

The Taupo Volcanic Zone (TVZ), New Zealand, is a well-documented volcanic arc characterized by explosive rhyolitic magmas within a series of caldera complexes that include the Okataina Volcanic Centre (OVC). New quartz melt inclusion and volcanic glass data from the 45 ka caldera-forming Rotoiti eruption within the OVC are compared to published studies. The new data are characterized by low K₂O (~1.5–3.5 wt.%), Rb (~30–70 ppm), Sr (~40–90 ppm), U (~0.5–2.5 ppm), and Ba (~300–1000 ppm) ranges that differ significantly from other OVC systems (~3.0–4.5 wt.% K₂O, ~80–150 ppm Rb, and ~2.5–5.0 ppm U). Most interestingly, the Rotoiti melt inclusion data measured in this study show a decrease in Rb, Sr, and U, although the fractionation trends originate from the same source point as published OVC data. This progressive decreasing trend is interpreted as an interaction with a less enriched rhyolitic melt (represented by the low Rb, Sr, and U of glasses) during fractionation processes from a common TVZ source. The established model for TVZ rhyolites is that they are extracted from a middle or upper crustal source (‘mush’ zone) prior to eruption. Adding to this model, new melt inclusion data suggest that all TVZ rhyolites are fractionated from this common TVZ source and, prior to eruption, the Rotoiti system was rejuvenated by this source (evidenced by the low REE glasses). Exactly what triggers the common TVZ source to fractionate remains unclear, but a proposed mechanism to account for this involves the successive melting of the upper crust by upwelling mantle induced by incremental subduction.     

Marine tephrochronology of the Mt. Edgecumbe Volcanic Field, Southeast Alaska, USA

The Mt. Edgecumbe Volcanic Field (MEVF), located on Kruzof Island near Sitka Sound in southeast Alaska, experienced a large multiple-stage eruption during the last glacial maximum (LGM)-Holocene transition that generated a regionally extensive series of compositionally similar rhyolite tephra horizons and a single well-dated dacite (MEd) tephra. Marine sediment cores collected from adjacent basins to the MEVF contain both tephra-fall and pyroclastic flow deposits that consist primarily of rhyolitic tephra and a minor dacitic tephra unit. The recovered dacite tephra correlates with the MEd tephra, whereas many of the rhyolitic tephras correlate with published MEVF rhyolites. Correlations were based on age constraints and major oxide compositions of glass shards. In addition to LGM-Holocene macroscopic tephra units, four marine cryptotephras were also identified. Three of these units appear to be derived from mid-Holocene MEVF activity, while the youngest cryptotephra corresponds well with the White River Ash eruption at ∼ 1147 cal yr BP. Furthermore, the sedimentology of the Sitka Sound marine core EW0408-40JC and high-resolution SWATH bathymetry both suggest that extensive pyroclastic flow deposits associated with the activity that generated the MEd tephra underlie Sitka Sound, and that any future MEVF activity may pose significant risk to local population centers.     

Late Quaternary volcanism in New Zealand: Towards an integrated record using distal airfall tephras in lakes and bogs

Studies on distal airfall tephra layers preserved in lake sediments and peats in northern New Zealand have documented the stratigraphic, chronologic, and compositional relationships of 46 eruptives, aged c. 17000–700yr BP, which originated from six North Island volcanic centres: Taupo (9 tephras), Okataina (8), Maroa (1) (rhyolitic); Mayor Island (2) (peralkaline); Tongariro (11), Egmont (15) (andesitic). Sources were distinguished by mineralogy and composition, field relations, and ¹⁴C chronology. All known rhyolitic tephra-producing eruptions from Taupo, Okataina, and Maroa volcanoes since c. 17000yr BP are represented, but only a small proportion of the known tephras erupted from Tongariro, Egmont, or Mayor Island volcanoes is recorded. The distal tephras from these latter volcanic centres may thus reflect atypically powerful (or oblique) eruptions, or dispersal by strong winds. An improved record of volcanism for the Tongariro, Egmont, and Mayor Island centres might be obtainable from suitable lakes or bogs more proximal to them.     

Rerewhakaaitu Tephra,a land–sea marker for the Last Termination in New Zealand,with implications for global climate change

The Rerewhakaaiutu Tephra erupted from Okataina Volcanic Centre, North Island, New Zealand, at 14,700±95 ¹⁴C yr BP (ca 17,600 cal yr BP) at a time of rapid re-organisation of Earth's climate system at the end of the Last Glacial (Termination I). It provides a distinctive isochron in a range of different environments in North Island and in adjacent South Pacific Ocean sediments. Terrestrial evidence, based on fluvial aggradation and downcutting relationships, loess accumulation rates, palaeovegetation patterns, and buried soil development and mineralogy, shows that marked amelioration of climate occurred shortly before the Rerewhakaaitu Tephra was deposited. Similarly, marine evidence from around this time shows major changes in accumulation rates of sediment and aeolian quartz and in the abundance of various marine organisms, while foraminiferal oxygen and carbon isotope records indicate that the arrival of the glacial meltwater signal occurred close to or just after the deposition of the Rerewhakaaitu Tephra. These changes are discussed in relation to controls on climate by oceanic and atmospheric mechanisms. The re-organisation of climate commencing at ca 15,000–14,500 ¹⁴C yr BP (ca 18,000–17,400 cal yr BP) is detected elsewhere in the Southern Hemisphere and evidently was linked to orbitally forced warming which is thought to have initiated ice retreat in both hemispheres.     

Three new distal tephras in sediments spanning the Last Glacial–Interglacial Transition in Scotland

Three new microtephras are reported from a number of lake sites from the Inner Hebrides and Scottish mainland. One occurs stratigrapically in the middle of Greenland Interstadial 1 (GI‐1) and has been named the Penifiler Tephra. It is rhyolitic and possesses a geochemical signature that is very similar to that of the Borrobol Tephra, which also occurs in three of the sequences reported here, but which lies close to the lower boundary of GI‐1. The second occurs stratigraphically in the early Holocene below the Saksunarvatn Ash and is named the Ashik Tephra. This tephra is geochemically bimodal, with a rhyolitic component comparable to the An Druim Tephra that occurs later in the Holocene, and a basaltic component which is similar to the Saksunarvatn Ash. A third tephra occurs stratigraphically above the Saksunarvatn Ash and is provisionally named the Breakish Tephra. The consistent inter‐site correlation demonstrated for these new tephras at several sites enhances the regional tephrostratigraphic framework, and increases the potential for correlating palaeoenvironmental events during GI‐1 and the early Holocene. However, the occurrence of multiple tephras with similar geochemistry in close stratigraphic and temporal proximity has implications for the rigour with which tephrostratigraphic investigations must be performed. Copyright © 2006 John Wiley & Sons, Ltd.     

Occurrence of gold in hydrothermal pyrite,western Taupo Volcanic Zone,New Zealand

The Taupo Volcanic Zone (TVZ) on the North Island of New Zealand is located above the subducting Pacific slab and hosts hydrothermal systems related to subduction, arc magmatism and crustal extension. In these systems, gold is transported primarily as sulphide complexes, with gold being deposited in response to boiling and mixing of the deep geothermal fluids. Conglomerate cobbles and hydrothermal fumaroles from the upper Retaruke River in the western TVZ are mineralised deposits that have been eroded from the nearby Miocene alluvial sedimentary plateau. Abundant gold-bearing pyrite was precipitated in the conglomerates and fumaroles by late hydrothermal process, primarily occurring as veinlets, disseminations and fine-grained aggregates which consists discrete euhedral microcrystals. Scanning electron microscopy combined with X-ray energy disperses spectroscopy shows that pyrites are commendably affected by late hydrothermal fluids, possibly by the carbonate fluid. Electron probe microanalysis revealed that the pyrite contains 20–120 ppm Au (averaging 60 ppm). Wavelength dispersive spectral elemental mapping suggests that gold is distributed uniformly in pyrite, indicating structurally bound gold (solid solution) in pyrite. Gold mineralisation recognised in the conglomerates and fumaroles demonstrates that the upper Retaruke River is a promising target for future gold exploration.     

日本樱岛火山灰玻璃碎屑形态特征研究

火山灰年代学是控制地层年代最简单可靠的直接手段,近年来在磁性地层、构造演化和古气候环境、考古等研究中被广泛重视和应用,经常成为最关键的要素。然而对其在地层中的识别常常产生较大分歧,制约了对年代的控制及与此相关的许多科学问题的解决。火山玻璃作为火山灰中的常见和标签物质,是识别火山灰最直接可靠的材料。日本强烈的第四纪火山活动和清晰连续的火山灰沉积在这方面具有代表性。本文利用光学显微镜和扫描电镜,对日本樱岛第四纪晚期典型火山灰中的火山玻璃形态和表面结构特征进行观察分析,总结出火山玻璃碎屑的外部形貌为独特、醒目的各种玻璃碎片状,呈各种大小和形态的带有气孔的曲面或弧面尖棱角状颗粒,如锋利的长条状、各种多边形粒状、片状（月牙状、三角状、多边形状等）和三交面状等,表面特征为各种与火山喷发炸裂作用相关的光滑或弱贝壳状断口的曲面,及与火山喷气有关的各种孔洞、管槽、小圆坑、椭圆形坑和眼状坑。其中眼状坑和三交面状这两种特殊表面形态分别只在安山质和流纹质火山灰中出现,为准确识别中-酸性火山灰及其类型提供了微观形态学鉴定标志。     

Multiple glaciations and marine transgressions, western Kennedy Channel, Northwest Territories, Canada

Along a 70 km section of western Kennedy Channel three prominent weathering zones are identified and serve to differentiate major events in the Quaternary landscape. The oldest zone (Zone 111b) is characterized by a deeply weathered, erratic-free terrain which extends from the mountain summits down to ca. 470 m above sea level. This zone shows no evidence of former glacierization. Zone 111a extends from ca. 470 to 370m above sea level and is characterized by sparse granite, gneiss and quartzite erratics amongst weathered bedrock and extensive, oxidized colluvium. The Precambrian provenance and uppermost profile of these erratics reflect the maximum advance of the northwest Greenland Ice Sheet onto northeastern Ellesmere Island. These uppermost erratics along western Kennedy Channel decrease in elevation southward and suggest that the former Greenland ice was thickest in the direction of the major outlet of Petermann Fiord. No evidence of a former ice ridge in Nares Strait was observed. Zone II is marked by the moraines of the outermost Ellesmere Island ice advance which form a prominent morpho-stratigraphic boundary where they cross-cut the zone of Greenland erratics at ca. 250–350 m above sea level. These moraines show advanced surface weathering and ice recession from them is associated with a pre-Holocene shoreline at 162 m above sea level. Late Wisconsin/Würm glacial deposits, equivalent to Zone I, were not observed in the lower valleys bordering Kennedy Channel. The outermost Ellesmere Island ice advance (Zone II) is radiometrically bracketed by ¹⁴C dates on in situ shells from subtill and supratill marine units which are 40,350±750 and>39,000 B.P., respectively. Amino acid age estimates on the same shell samples and others from similar stratigraphic positions all suggest ages of >35,000 B.P. Stratigraphically and chronologically this ice advance is correlated with the outermost Ellesmere Island ice advance 20–40 km to the north which formed small ice shelves when the relative sea level was ca. 175 m above sea level. The Holocene marine transgression along western Kennedy Channel occurred in an ice-free corridor maintained between the separated margins of the northwest Greenland and northeast Ellesmere Island ice sheets during the last glaciation. Initial emergence may have begun ca. 12,300 B.P., however, sea level had dropped only 15 m by ca. 8000 B.P. after which glacio-isostatic unloading of the corridor was rapid. The implications of these data are discussed in the context of existing models on high latitude glaciation and paleoclimatic change     

Extending the known distribution of the Younger Dryas Vedde Ash into northwestern Russia

The known distribution of wind‐blown Vedde Ash (ca. 10.3 ka BP) has been extended to the Karelian Isthmus in northwestern Russia. This has been possible as the result of a density separation technique that separates the rhyolitic Vedde Ash shards from the minerogenic host sediment. The Vedde Ash occurs in the middle of a pollen zone with high percentages of, for example, Artemisia and Chenopodiaceae, suggesting that the Younger Dryas (or GS‐I in the GRIP ice‐core event stratigraphy) was cold and dry throughout its duration. This is in agreement with sites in south Sweden where the Vedde Ash also occurs in the middle of a pollen zone dominated by Artemisia, Chenopodiaceae and Cyperaceae. Copyright © 2000 John Wiley & Sons, Ltd.     

Early to middle Holocene silicic tephra horizons from the Katla volcanic system,Iceland: new results from the Faroe Islands

Comparatively few Icelandic tephra horizons dated to the early part of the Holocene have so far been detected outside Iceland. Here, I present several tephra horizons that have been recorded in a Holocene peat sequence on the Faroe Islands. Geochemical analyses show that at least two dacitic and one rhyolitic tephra layers were erupted from the Katla volcanic system on southern Iceland between ca. 8000 and 5900 cal. yr BP. The upper two layers can be correlated with the SILK tephras described from southern Iceland, whereas the third, dated to ca. 8000 cal. yr BP, has a geochemistry virtually identical to the rhyolitic component of the Vedde Ash. The results suggest that the Late Weichselian and early Holocene eruption history of the Katla volcano was probably more complex than inferred from Iceland. A new, early Holocene rhyolitic tephra dated to ca. 10 500 cal. yr BP probably originates in the Snæfellsnes volcanic centre in western Iceland. These new findings may play an important role in developing a Holocene tephra framework for northwest Europe. Copyright © 2002 John Wiley & Sons, Ltd.     

Farola Monte Hermoso: Fission-track Dating of Darwin's mammals deposit in Argentina

The Montehermosan Land Mammal Age has been etablished by fission-track dating of the mammal deposit of Farola Monte Hermoso. The fossil bones were found in the lowest part of the outcrop, consisting of fluvially derived clay and silt. The erosional surface between the lowest portion and the overlying aeolian sands is charcterised by a compact unit of rounded volcanic glass shards; a second fresh volcanic ash level is about 1 m above. EDS analyses and fission-track age analyses were performed on the two levels. The age determined for the lower volcanic ash level, 1.7 ± 0.4 Ma, is a minimum age for the Chapadmalalan mammal assemblage and a maximum age for the Uquian mammal assemblage in the Bonaerense area.     

Non‐synchronous deposition of North Atlantic Ash Zone II in Greenland ice cores,and North Atlantic and Norwegian Sea sediments: an example of complex glacial‐stage tephra transport

Tephra provides regional chronostratigraphical marker horizons that can link different climate archives with highly needed accuracy and precision. The results presented in this work exemplify, however, that the intermittent storage of tephra in ice sheets and during its subsequent iceberg transport, especially during glacial stages, constitutes a potential source of serious error for the application of tephrochronology to Nordic Seas and North Atlantic sediment archives. The peak shard concentration of the rhyolitic component of the North Atlantic Ash Zone II (NAAZ‐II) tephra complex, often used to correlate marine and ice core records in Marine Isotope Stage (MIS) 3, is shown to lag the eruption event by ca. 100–400 years in some North Atlantic and Norwegian Sea cores. While still allowing for a correlation of archives on millennial timescales, this time delay in deposition is a major obstacle when addressing the lead–lag relationship on short timescales (years to centuries). A precise and accurate determination of lead–lag relationships between archives recording different parts of the climate system is crucial in order to test hypotheses about the processes leading to abrupt climate change and to evaluate results from climate models. Copyright © 2011 John Wiley & Sons, Ltd.