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Volcanic ash deposits (tephra) in western Canada are instrumental in providing independent chronologic control for many archaeological and paleoenvironmental sites. In Alberta, tephra are a key chronologic tool in a region where radiocarbon dates are often unreliable because of the prevalence of carbonate-rich bedrock and other "old carbon" sources, such as coal. However, many studies using tephra for age control, particularly archaeological projects, identify tephra simply through field characteristics or light microscopy. In both Alberta and British Columbia, many radiocarbon dates that were used to date key tephra deposits were bulk conventional ages on peat and lake sediments, which are not always reliable. These factors have led to uncertainty in the age and number of Bridge River and Mount St. Helens (MSH) set Y tephra present in the region and incomplete distribution maps. New major-element geochemical analyses from archaeological and sedimentary sites across south-central Alberta, complemented by new analyses of tephra from British Columbia and Saskatchewan, refine the distribution of the Bridge River, MSH Yn and Mazama tephra. New geochemical data, radiocarbon dates, and a detailed overview of proximal MSH set Y stratigraphy and geochemistry show that only one MSH layer, Yn, is present in this region, rather than two MSH set Y tephra as previously suggested. Additionally, re-assessment of age data combined with new geochemical analyses confirm that there is also only one Bridge River tephra. A Bayesian modelled age estimate is determined for MSH Yn based on new AMS dates on the tephra and vetted existing conventional ages, providing a revised age estimate for MSH Yn of 3805–3535 cal BP (mean of 3660 cal BP). [ABSTRACT FROM AUTHOR]

Tephrochronology is one of the most effective ways to correlate and date Quaternary deposits across large distances. However, it can be challenging to obtain direct ages on tephra beds when they are beyond the limit of radiocarbon dating, do not contain mineral phases suitable for 40K-40Ar (or 40Ar/39Ar) dating, or suitable glass shards for fission-track dating are not available. Zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging technique for dating young (<1 Ma) tephra. Here, we demonstrate that LA-ICP-MS zircon U-Pb dating can produce reliable ages for key tephra beds found in Yukon and Alaska. We assessed five different techniques for calculating tephra maximum depositional ages from zircon U-Pb ages for eight tephra beds. Our preferred zircon U-Pb ages (reported with 2σ uncertainties), based on a Bayesian model for calculating maximum depositional ages, are broadly consistent with previously established chronology constructed from stratigraphy, paleomagnetism, and/or glass fission track and 40Ar/39Ar ages: Biederman tephra (178 ± 17 ka), HP tephra (680 ± 47 ka), Gold Run tephra (688 ± 44 ka), Flat Creek tephra (708 ± 43 ka), PA tephra (1.92 ± 0.06 Ma), Quartz Creek tephra (2.62 ± 0.08 Ma), Lost Chicken tephra (3.14 ± 0.07 Ma), and GI tephra (542 ± 64 ka). We also present newly revised glass fission-track and 40Ar/39Ar ages recalculated from previous determinations using updated ages for the Moldavite tektite and Fish Canyon Tuff standards, and updated K decay constants. For Pleistocene age zircon crystals, corrections for 230Th disequilibrium and common-Pb are significant and must be treated with caution. Similarly, apparent tephra ages are sensitive to the choice of method used to calculate a maximum depositional age from the assemblage of individual crystallization ages. This study demonstrates that LA-ICP-MS zircon U-Pb dating can be successfully applied to numerous Pliocene-Pleistocene Alaskan-Yukon tephra, providing confidence in applying this method to other stratigraphically important tephra in the region. • LA-ICP-MS method is well suited for rapid dating of very young zircon crystals. • Eight key tephra beds from Eastern Beringia were dated by zircon U-Pb method. • Young U-Pb ages are particularly sensitive to common-Pb and 230Th corrections. • Bayesian modeling reduces subjectivity in calculating ages. [ABSTRACT FROM AUTHOR]

The Aleutian Arc-Alaska Peninsula and Wrangell volcanic field are the main source areas for tephra deposits found across Alaska and northern Canada, and increasingly, tephra from these eruptions have been found further afield in North America, Greenland, and Europe. However, there have been no broad scale reviews of the Late Pleistocene and Holocene tephrostratigraphy for this region since the 1980s, and this lack of data is hindering progress in identifying these tephra both locally and regionally. To address this gap and the variable quality of associated geochemical and chronological data, we undertake a detailed review of the latest Pleistocene to Holocene tephra found in interior Alaska and Yukon. This paper discusses nineteen tephra that have distributions beyond southwest Alaska and that have the potential to become, or already are, important regional markers. This includes three ‘modern’ events from the 20th century, ten with limited data availability but potentially broad distributions, and six that are widely reported in interior Alaska and Yukon. Each tephra is assessed in terms of chronology, geochemistry and distribution, with new Bayesian age estimates and geochemical data when possible. This includes new major-element geochemical data for Crater Peak 1992, Redoubt 1989–90, and two andesitic tephra from St Michael Island (Tephra D), as well as revised age estimates for Dawson tephra, Oshetna, Hayes set H, Aniakchak CFE II, and the White River Ashes, northern and eastern lobes. [ABSTRACT FROM AUTHOR]

Abstract: The Variegated (VT) tephra, likely sourced from the eastern Aleutian arc, has a geographic distribution that places it amongst the most widespread tephra beds in eastern Beringia. First identified in the Fairbanks area of interior Alaska, it has been identified at eight additional sites ranging from Togiak Bay in southwestern Alaska, to the Klondike area of west-central Yukon. Correlation of these occurrences is established through the equivalence of glass major and trace-element geochemistry, Fe-Ti oxide geochemistry, stratigraphy, and independent age data. In Yukon and Alaska, VT tephra has a minimum bulk tephra volume estimate of ∼32 km3. Previous age estimates for VT tephra have varied, ranging from a glass fission-track age of 125 ± 30 ka to a weighted mean thermoluminescence (TL) age of 77.8 ± 4.1 ka from bracketing ages on loess. A new infrared stimulated luminescence (IRSL) age of 106 ± 10 ka, paleoenvironmental data, and several TL and IRSL ages from Togiak Bay suggest that the time of deposition is more likely between these previous age estimates: post-marine isotope stage (MIS) 5e but underlying a prominent soil likely associated with MIS 5c, placing it within late MIS 5d. [Copyright &y& Elsevier]

Lakes and bogs in northeastern North America preserve tephra deposits sourced from multiple volcanic systems in the Northern Hemisphere. However, most studies of these deposits focus on specific Holocene intervals and the latest Pleistocene, providing snapshots rather than a full picture. We combine new data with previous work, supplemented by a broad review of the characteristics and ages of potential source regions and volcanoes, to develop the first composite tephrostratigraphic framework covering the last ∼14,000 years for this region. We report new cryptotephra records from three ombrotrophic peat bogs—Irwin Smith (Michigan), Bloomingdale (New York), and Sidney Bog (Maine)—as well as new analyses and age models from previously reported sites, Nordan's Pond Bog (Newfoundland) and Thin-Ice Pond (Nova Scotia). A new tephra (Iliinsky) from the NGRIP and GRIP ice cores is also presented as it can be correlated to new data from these terrestrial records and helps validate radiocarbon age models. We identify 21 new tephra in addition to the 15 already known, several of which cover the entire region – the White River Ash east, Newberry Pumice, Ruppert (NDN-230), and Mazama. For the first time we find Mount St. Helens Yn (ca. 3660 cal yr BP) and a set P tephra (∼3000–2550 cal yr BP), and confirm the presence of Jala Pumice from Volcan Ceboruco, Mexico, and KS 1 from Ksudach volcano, Kamchatka. We describe new "ultra-distal" tephra, including the early Holocene KS 2 eruption, and propose correlations to volcanoes Iliinsky and Shiveluch of Kamchatka, and Ushishir of the Kurile Islands. Not all of these tephra represent large eruptions, with several plausible correlations to sub-Plinian events. Using Bayesian age-modeling, we present new age estimates for the newly described tephra, for tephra with previously poor age control, and for several proximal correlatives. Overall, we demonstrate northeastern North America's importance for providing transcontinental linkages between paleoenvironmental records and providing insights into ash distribution from different styles and sizes of eruptions. • Tephra are abundant in NE North America, with 36 unique units deposited between ∼14,000 and the present day. • Source volcanoes are in the Cascades, Alaska, Kamchatka, Kuriles and potentially Japan. • Updated Bayesian modelled ages are presented for key proximal correlative eruptions and newly described tephra. • The tephra link paleoenvironmental records from this region to the Far East, Greenland and Europe. • Correlated source eruption volumes vary widely; this alone cannot explain recorded ash distribution trends. [ABSTRACT FROM AUTHOR]

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The ‘Lateglacial’ period (∼14.7–11.7 cal ka BP) eruptions of Mount St Helens and Glacier Peak in the Cascade Range deposited ash layers (tephras) within a short time span across much of western North America where they form event-stratigraphic marker layers or isochrons. They were deposited at a time which has long been of interest because it represents the transition between two fundamental states of the climate system: the late Pleistocene glacial world when ice sheets were widespread, and the modern interglacial Holocene world. This transition was marked by rapid changes in the distribution of plants, animals and humans on the landscape, and is characterised by short, rapid climate reversals in the warming trend. Yet despite the importance of understanding this period for many areas of palaeoclimatology, palaeobotany and archaeology it remains one of the most difficult for which to develop accurate chronologies because of fluctuations in atmospheric radiocarbon concentration. Hence, the occurrences of distinctive tephra isochrons are valuable for chronological control. Here, we report the first detection of the Mount St Helens set J and Glacier Peak tephras as closely-spaced ‘cryptotephra’ layers (not visible in stratigraphy to the naked eye) in three eastern seaboard lakes and dated to 13.74–13.45 cal ka BP. The presence of these tephras >4000 km from their sources affords an opportunity for continent-wide correlations by providing a high-precision chronological benchmark that is otherwise often lacking in North American studies of palaeoenvironmental change and deglaciation, megafaunal extinction and palaeoindian colonisation. [ABSTRACT FROM AUTHOR]

Holocene cryptotephras of Alaskan and Pacific Northwestern origin have recently been detected ca. 7000 km away on the east coast of North America. This study extends the emerging North American tephrochronological framework by geochemically characterising seventeen cryptotephra layers from four newly explored peatlands. All detected tephras were deposited during the late Holocene, with no horizons present in the peat between ca. 3000–5000 years ago. The prevalence of the Alaskan White River Ash eastern lobe (AD 847 ± 1) is confirmed across the eastern seaboard from Newfoundland to Maine and a regional depositional pattern from Mount St Helens Set W (AD 1479–1482) is presented. The first occurrences of four additional cryptotephras in eastern North America are described, three of which may originate from source regions in Mexico, Kamchatka (Russia) and Hokkaido (Japan). The possibility of such tephras reaching eastern North America presents the opportunity to link palaeo-archives from the tropics and eastern Asia with those from the western Atlantic seaboard, aiding inter-regional comparisons of proxy-climatic records. [ABSTRACT FROM AUTHOR]

Polar ice-cores have long been recognised as unrivalled repositories of past volcanic events. Although tephra products from local eruptions tend to dominate these records, improvements in micro-sampling and analytical techniques are uncovering a growing number of cryptotephras erupted from exceptionally distant volcanoes. We present a series of nine Middle Holocene cryptotephra deposits detected within the NGRIP ice-core that originate from five different volcanic regions across the Northern Hemisphere (Alaska, Cascades, Iceland, Japan, Kamchatka). Unique compositional signatures are employed to identify ash from three large caldera-forming events in Kamchatka (KS 2 from Ksudach), the Cascades (Mazama) and North East Japan (Mashu), along with ash from the Hekla 4 eruption in Iceland. High-precision ice-core ages (adopting a 1950 CE datum for the GICC05 timescale assigned to the Greenland ice cores) are derived for each eruption: Hekla 4 (4325 ± 8 a b1.95k), KS 2 (7089 ± 26 a b1.95k), Mashu (i-f) (7473 ± 33 a b1.95k) and Mazama (7562 ± 35 a b1.95k), all of which can be employed as chronological fix-points in other proxy records where these deposits are also preserved. Four further cryptotephra deposits and one macro-deposit are also identified and traced to sources in Iceland and Alaska. The cryptotephra originating from Alaska is correlated to a deposit identified in lake records from the Kenai Peninsula, thought to originate from Redoubt Volcano. The remaining four deposits are typical of the products of Katla, Grímsvötn and Veiðivötn in Iceland. This ensemble of Middle Holocene tephra deposits highlights the pivotal position of the Greenland ice-sheet and its ice-cores to capture deposition from the convergence of several far-travelled ash clouds. Precise age estimates derived from the annually resolved ice-core record greatly enhances the value of these tephra isochrons. • 9 Middle Holocene tephras identified inGreenland ice-core records, originate from 5 different volcanic source regions. • 3 tephras originate from caldera-forming eruptions in the Cascades (Mazama), Kamchatka (KS 2) and Japan (Mashu i-f). • Glass shards from the widespread isochron Hekla 4 eruption are detected. • High-precision ice-core age estimates are reported for each tephra enhancing their application as chronological fix-points. • The Greenland ice-sheet is a valuable archive of eruptive products that helps fill gaps in the global volcanic record. [ABSTRACT FROM AUTHOR]

Abstract: Far-travelled volcanic ashes (tephras) from Holocene eruptions in Alaska and the Pacific northwest have been traced to the easternmost extent of North America, providing the basis for a new high-precision geochronological framework throughout the continent through tephrochronology (the dating and correlation of tephra isochrons in sedimentary records). The reported isochrons are geochemically distinct, with seven correlated to documented sources in Alaska and the Cascades, including the Mazama ash from Oregon (∼7600 years old) and the eastern lobe of the White River Ash from Alaska (∼1150 years old). These findings mark the beginning of a tephrochronological framework of enhanced precision across North America, with applications in palaeoclimate, surface process and archaeological studies. The particle travel distances involved (up to ∼7000 km) also demonstrate the potential for continent-wide or trans-Atlantic socio-economic disruption from similar future eruptions. [Copyright &y& Elsevier]