22 results on '"Jensen, Britta J.L."'
Search Results
2. Permafrost-preserved wood and bone: Radiocarbon blanks from Yukon and Alaska
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Martinez De La Torre, Hector A., Reyes, Alberto V., Zazula, Grant D., Froese, Duane G., Jensen, Britta J.L., and Southon, John R.
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- 2019
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3. High-resolution age modelling of peat bogs from northern Alberta, Canada, using pre- and post-bomb 14C, 210Pb and historical cryptotephra
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Davies, Lauren J., Appleby, Peter, Jensen, Britta J.L., Magnan, Gabriel, Mullan-Boudreau, Gillian, Noernberg, Tommy, Shannon, Bob, Shotyk, William, van Bellen, Simon, Zaccone, Claudio, and Froese, Duane G.
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- 2018
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4. First evidence of cryptotephra in palaeoenvironmental records associated with Norse occupation sites in Greenland
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Blockley, Simon P.E., Edwards, Kevin J., Schofield, J. Edward, Pyne-O'Donnell, Sean D.F., Jensen, Britta J.L., Matthews, Ian P., Cook, Gordon T., Wallace, Kristi L., and Froese, Duane
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- 2015
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5. A re-examination of the three most prominent Holocene tephra deposits in western Canada: Bridge River, Mount St. Helens Yn and Mazama.
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Jensen, Britta J.L., Beaudoin, Alwynne B., Clynne, Michael A., Harvey, Jordan, and Vallance, James W.
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TEPHROCHRONOLOGY , *VOLCANIC ash, tuff, etc. , *ANALYTICAL geochemistry , *RADIOCARBON dating , *GEOCHEMISTRY , *LAKE sediments , *GEOCHEMICAL modeling - Abstract
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]
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- 2019
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6. Late Pleistocene and Holocene tephrostratigraphy of interior Alaska and Yukon: Key beds and chronologies over the past 30,000 years.
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Davies, Lauren J., Jensen, Britta J.L., Froese, Duane G., and Wallace, Kristi L.
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VOLCANIC ash, tuff, etc. , *PLEISTOCENE Epoch , *HOLOCENE Epoch , *STRATIGRAPHIC geology , *CHRONOLOGY ,INTERIOR Alaska (Alaska) - Abstract
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]
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- 2016
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7. 150,000 years of loess accumulation in central Alaska.
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Jensen, Britta J.L., Evans, Michael E., Froese, Duane G., and Kravchinsky, Vadim A.
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LOESS , *HALFWAY houses , *PALEOENVIRONMENTAL studies , *PALEOMAGNETISM , *STRATIGRAPHIC geology ,INTERIOR Alaska (Alaska) - Abstract
The Halfway House site in interior Alaska is arguably the most studied loess deposit in northwestern North America. The site contains a complex paleomagnetic and paleoenvironmental record, but has lacked the robust chronologic control that would allow its full potential to be exploited. Detailed reexamination of stratigraphy, paleomagnetics and tephrostratigraphy reveals a relatively complete marine isotope stage (MIS) 6 to Holocene record constrained by the Old Crow (124 ± 10 ka), VT (106 ± 10 ka), Sheep Creek-Klondike (ca. 80 ka), Dominion Creek (77 ± 8 ka) and Dawson (ca. 30.2 cal ka BP) tephras. We show two well-developed paleosols formed during Marine Isotope Stages (MIS) 5e and 5a, while MIS 5c and 5b are either poorly represented or absent. The new tephrostratigraphy presented here is the most complete one to date for the late Pleistocene and indicates MIS 5 sediments are more common than previously recognized. A magnetic excursion within the sediments is identified as the post-Blake excursion (94.1 ± 7.8 ka), providing independent age control and adding to the increasing body of evidence that Alaskan loess is a detailed recorder of variations of the Earth's magnetic field over time. A high-resolution magnetic susceptibility profile placed into this new chronostratigraphic framework supports the hypothesis that wind-intensity is the main variable controlling fluctuations in susceptibility. Correlation of the susceptibility record to global marine δ 18 O records is complicated by highly variable accumulation rates. We find the lowest rates of accumulation during peak warm and cold stages, while abrupt increases are associated with periods of transition between marine isotope (sub)stages. Building on previous accumulation models for Alaska, surface roughness is likely a leading variable controlling loess accumulation rates during transitions and peak cold periods, but the negligible accumulation during MIS 5e and 5a suggests that loess production was exceedingly low, negating the role of surface roughness. This interplay of variables leads to optimal conditions for loess accumulation during transitions between isotope stages, and to a somewhat lesser extent, stadials and interstadials. [ABSTRACT FROM AUTHOR]
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- 2016
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8. The Palisades is a key reference site for the middle Pleistocene of eastern Beringia: new evidence from paleomagnetics and regional tephrostratigraphy
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Jensen, Britta J.L., Reyes, Alberto V., Froese, Duane G., and Stone, David B.
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CLIFFS , *PLEISTOCENE Epoch , *PALEOMAGNETISM , *QUATERNARY Period - Abstract
Abstract: The Palisades, in central Alaska, is one of the most prominent exposures of Quaternary sediments on the Yukon River. Perennially-frozen silt and sand at the Palisades are presently thought to preserve paleoenvironmental records from the Holocene to ∼Marine Isotope Stage (MIS) 8 and, beneath a major unconformity, the earliest Pleistocene (∼2 Ma). We present new paleomagnetic and tephrochronologic constraints that substantially revise the age of the sediments at the Palisades. We describe 15 new tephra beds, including five beds below the prominent PAL tephra that correlate to known tephra with independent age control from other sites in eastern Beringia. These five known tephra include Chester Bluff tephra, which is present in east-central Alaska and the Yukon, and the newly named Alyeska Pipeline and Taylor Highway tephra from central Alaska; all are constrained to the middle Pleistocene. Paleomagnetic transects from the base of the bluff to the MIS 5e forest bed yield normal polarity, with the exception of a brief reversal event between Old Crow tephra (124 ± 10 ka) and the MIS 5e forest bed that is likely the first documentation of the Blake paleomagnetic event in Alaskan loess. The detailed tephrostratigraphy and paleomagnetic data collectively suggest that most of the sedimentary record at the Palisades is middle Pleistocene in age. The Palisades thus preserves a rare record of late to middle Pleistocene paleoenvironments with multiple regionally distributed tephra beds. [Copyright &y& Elsevier]
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- 2013
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9. The variegated (VT) tephra: A new regional marker for middle to late marine isotope stage 5 across Yukon and Alaska
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Jensen, Britta J.L., Preece, Shari J., Lamothe, Michel, Pearce, Nick J.G., Froese, Duane G., Westgate, John A., Schaefer, Janet, and Begét, Jim
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VOLCANIC ash, tuff, etc. , *OXYGEN isotopes , *SEA glass , *THERMOLUMINESCENCE - Abstract
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]
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- 2011
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10. A late–Middle Pleistocene (Marine Isotope Stage 6) vegetated surface buried by Old Crow tephra at the Palisades, interior Alaska
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Reyes, Alberto V., Jensen, Britta J.L., Zazula, Grant D., Ager, Thomas A., Kuzmina, Svetlana, La Farge, Catherine, and Froese, Duane G.
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PLEISTOCENE stratigraphic geology , *PALEOBOTANY , *HABITATS , *SEDIMENTS , *PALEOECOLOGY , *CLIFFS - Abstract
Abstract: A 40 cm thick primary bed of Old Crow tephra (131 ± 11 ka), an important stratigraphic marker in eastern Beringia, directly overlies a vegetated surface at Palisades West, on the Yukon River in central Alaska. Analyses of insect, bryophyte, and vascular plant macrofossils from the buried surface and underlying organic-rich silt suggest the local presence of an aquatic environment and mesic shrub-tundra at the time of tephra deposition. Autochthonous plant and insect macrofossils from peat directly overlying Old Crow tephra suggest similar aquatic habitats and hydric to mesic tundra environments, though pollen counts indicate a substantial herbaceous component to the regional tundra vegetation. Trace amounts of arboreal pollen in sediments associated with the tephra probably reflect reworking from older deposits, rather than the local presence of trees. The revised glass fission-track age for Old Crow tephra places its deposition closer to the time of the last interglaciation than earlier age determinations, but stratigraphy and paleoecology of sites with Old Crow tephra indicate a late Marine Isotope Stage 6 age. Regional permafrost degradation and associated thaw slumping are responsible for the close stratigraphic and paleoecological relations between Old Crow tephra and last interglacial deposits at some sites in eastern Beringia. [Copyright &y& Elsevier]
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- 2010
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11. A latest Pleistocene and Holocene composite tephrostratigraphic framework for northeastern North America.
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Jensen, Britta J.L., Davies, Lauren J., Nolan, Connor, Pyne-O'Donnell, Sean, Monteath, Alistair J., Ponomareva, Vera, Portnyagin, Maxim, Booth, Robert, Bursik, Marcus, Cook, Eliza, Plunkett, Gill, Vallance, James W., Luo, Yan, Cwynar, Les C., Hughes, Paul, and Pearson, D. Graham
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PLEISTOCENE Epoch , *HOLOCENE Epoch , *PEAT bogs , *ICE cores , *VOLCANIC ash, tuff, etc. , *BOGS - Abstract
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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- 2021
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12. An extensive middle to late Pleistocene tephrochronologic record from east-central Alaska
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Jensen, Britta J.L., Froese, Duane G., Preece, Shari J., Westgate, John A., and Stachel, Thomas
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CHRONOLOGY , *GEOLOGY , *EARTH sciences - Abstract
Abstract: Chester Bluff (CB), in Yukon Charley Rivers National Preserve (YCNP), Alaska, contains an extensive middle to late Pleistocene tephra record within thick loess deposited on a terrace. The regionally prominent Old Crow (OC) tephra (140±10ka), GI tephra (560±80ka) of Fairbanks, the Preido Hill (PrH) tephra of the Klondike, and VT tephra (77.8±4.1ka) of both the Klondike and Fairbanks were found at CB. The presence of these tephra beds will aid in correlating disparate paleoenvironmental records across eastern Beringia. There are 15 new tephra beds; 11 from the Wrangell volcanic field (WVF) and/or Hayes volcano, one from the Aleutian-Arc Alaska Peninsula (AAAP) and three unclassified beds. CB is normally magnetized, indicating the entire sequence is of Bruhnes age (<780ka). Also present are at least two interglacials, one late–Middle Pleistocene, the other early–Middle Pleistocene. At least five more organic horizons representing interstadials or interglacials are present. Collectively, sediments at CB span most of the middle to late Pleistocene (i.e. ∼780 to <77.8±4.1ka). The bluffs represent the most extensive middle-to-late Pleistocene sedimentary record yet established for Yukon or Alaska. [Copyright &y& Elsevier]
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- 2008
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13. West Coast volcanic ashes provide a new continental-scale Lateglacial isochron.
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Pyne-O'Donnell, Sean D.F., Cwynar, Les C., Jensen, Britta J.L., Vincent, Jessie H., Kuehn, Stephen C., Spear, Ray, and Froese, Duane G.
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VOLCANIC ash, tuff, etc. , *VOLCANIC eruptions , *STRATIGRAPHIC geology , *PLEISTOCENE Epoch - Abstract
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]
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- 2016
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14. A mid to late Holocene cryptotephra framework from eastern North America.
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Mackay, Helen, Hughes, Paul D.M., Jensen, Britta J.L., Langdon, Pete G., Pyne-O'Donnell, Sean D.F., Plunkett, Gill, Froese, Duane G., Coulter, Sarah, and Gardner, James E.
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HOLOCENE Epoch , *TEPHROCHRONOLOGY , *PEATLANDS , *SEDIMENTATION & deposition , *VOLCANIC ash, tuff, etc. - Abstract
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]
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- 2016
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15. Exploiting the Greenland volcanic ash repository to date caldera-forming eruptions and widespread isochrons during the Holocene.
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Davies, Siwan M., Albert, Paul G., Bourne, Anna J., Owen, Sara, Svensson, Anders, Bolton, Matthew S.M., Cook, Eliza, Jensen, Britta J.L., Jones, Gwydion, Ponomareva, Vera V., and Suzuki, Takehiko
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VOLCANIC ash, tuff, etc. , *HOLOCENE Epoch , *GREENLAND ice , *VOLCANIC eruptions , *ICE cores , *EXPLOSIVE volcanic eruptions , *LAKE sediments - Abstract
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]
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- 2024
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16. Middle Pleistocene (MIS 7) to Holocene fossil insect assemblages from the Old Crow basin, northern Yukon, Canada.
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Kuzmina, Svetlana, Froese, Duane G., Jensen, Britta J.L., Hall, Elizabeth, and Zazula, Grant D.
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PLEISTOCENE Epoch , *HOLOCENE Epoch , *FOSSIL insects , *CHRONOLOGY , *ECOLOGY - Abstract
The Old Crow basin, northern Yukon, provides a key record of environmental change in northwestern North America from the late-Middle Pleistocene through the Early Holocene. Site chronologies are based on the presence of the Old Crow tephra (OCt: 124 ± 10 ka) at three sites, and provide a stratigraphic framework for observations. Ecological affinities of fossil insects indicate that Pleistocene environments were dominated by dry tundra and steppe-tundra. Forest insect species are relatively rare even during the last interglaciation (MIS 5e), and it is only by considering the total insect assemblages that interglacial beds can be recognized. Last interglacial insects from these sites indicate a relatively wet and warm tundra environment with evidence for sparse forest vegetation. Rare taxa with strong affinities to warmer temperatures are present and could suggest a warmer than modern climate. Early Holocene deposits yield a greater abundance of forest insects relative to MIS 5 or 7 interglacial deposits. Fossil insect assemblages reflect the position of the sites above the Arctic Circle, and in contrast to central Yukon sites, steppe insects are less common during cold stages and forest insects are less common during warm stages. These data suggest overall that the contrast between cold and warm stages was less pronounced than in other regions of Yukon and Alaska, and may indicate influence of persistent large regional lakes during the Pleistocene. [ABSTRACT FROM AUTHOR]
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- 2014
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17. Permafrost response to last interglacial warming: field evidence from non-glaciated Yukon and Alaska
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Reyes, Alberto V., Froese, Duane G., and Jensen, Britta J.L.
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GLOBAL warming , *PERMAFROST , *STRATIGRAPHIC geology , *GEOPHYSICAL observations , *GEOLOGICAL time scales , *PALEOECOLOGY , *PSEUDOMORPHS , *BIODEGRADATION - Abstract
Abstract: We present stratigraphic observations from three sites in eastern Beringia – Ch’ijee’s Bluff in northern Yukon and nearby exposures on the Old Crow River, the Palisades on the Yukon River in Alaska, and placer mining exposures at Thistle Creek in west-central Yukon – which provide insight into the response of permafrost to regional warming during the last interglaciation. Chronology is based on the presence of Old Crow tephra, an important regional stratigraphic marker that dates to late Marine Isotope Stage 6, supplemented by paleoecology and non-finite 14C ages on wood-rich organic silts. Old Crow tephra overlies several relict ice wedges at the Palisades and Thistle Creek, indicating that permafrost at these sites did not thaw completely during the last interglaciation. Prominent deposits of last interglacial wood-rich organic silt are present at multiple sites in eastern Beringia, and probably represent accumulations of reworked forest vegetation due to thaw slumping or deposition into thermokarst ponds or depressions. Consistent stratigraphic relations between these deposits, Old Crow tephra, and ice wedge pseudomorphs at our three study sites, and at least six other sites in eastern Beringia, suggest that thaw of shallow permafrost was widespread during the last interglaciation. Limited stratigraphic evidence suggests that thaw was probably on the order of meters, rather than 10s of meters. The ubiquity of shallow permafrost degradation during the last interglaciation suggests that current ground warming may foreshadow widespread near-surface thaw under even modest future warming scenarios. However, the persistence of relict pre-last interglacial ice wedges highlights the potential for the regional antiquity of discontinuous permafrost, and provides compelling field evidence for the long-term resilience of deep permafrost during sustained periods of warmer-than-present climate. [ABSTRACT FROM AUTHOR]
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- 2010
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18. New approach to assessing age uncertainties – The 2300-year varve chronology from Eklutna Lake, Alaska (USA).
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Fortin, David, Praet, Nore, McKay, Nicholas P., Kaufman, Darrell S., Jensen, Britta J.L., Haeussler, Peter J., Buchanan, Casey, and De Batist, Marc
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- ALASKA
- Abstract
Abstract Developing robust chronological frameworks of lacustrine sediment is central to reconstructing past environmental changes. We present varve chronologies from five sites extending back 2300 years from Eklutna Lake, in the Chugach Mountains of south-central Alaska. The chronologies are built from image analysis of high-resolution photographs and CT scans of sediment cores. The age uncertainty of each record is tested by three methods. We first present varve chronologies from individual sites and reconcile the difference in varve delimitation from two observers. The varve chronologies from each site are then compared to each other using a series of marker beds that can be traced across the lake basin. Finally, using a new Bayesian probabilistic model, we develop age models that incorporate information regarding age uncertainty from the multiple-observer method and the age distribution of marker layers from multiple cores. To evaluate the accuracy of the Bayesian model output, we used seven radiocarbon ages from terrestrial macrofossils and four tephra layers traceable across the core sites. The major-element geochemistry of the tephra layers and their ages are presented here for the first time. The Bayesian age model offers a new approach to quantifying age uncertainty in inter-correlated cores of varved sediment. Highlights • New cores from Eklutna Lake contain the longest continuous varve chronology published from Alaska, extending back 2300 years. • Bayesian probabilistic model integrates uncertainties from multiple observers and counts between marker beds in multiple cores. • Analyzing more cores from the same lake increases the apparent age uncertainty, but improves understanding of likely errors. • Four tephra layers are now precisely dated; new geochemical data indicate Redoubt and Augustine source volcanoes. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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19. A stable isotope record of late Cenozoic surface uplift of southern Alaska.
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Bill, Nicholas S., Mix, Hari T., Clark, Peter U., Reilly, Sean P., Jensen, Britta J.L., and Benowitz, Jeffrey A.
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HYDROGEN isotopes , *HYDROLOGIC cycle , *ALTIMETRY , *CLIMATE change , *DISTILLATION , *CLIMATOLOGY - Abstract
Although the timing of an acceleration in late-Cenozoic exhumation of southern Alaska is reasonably well constrained as beginning ∼5–∼6 Ma, the surface uplift history of this region remains poorly understood. To assess the extent of surface uplift relative to rapid exhumation, we developed a stable isotope record using the hydrogen isotope composition ( δ D) of paleo-meteoric water over the last ∼7 Ma from interior basins of Alaska and Yukon Territory. Our record, which is derived from authigenic clays ( δ D clay ) in silicic tephras, documents a ∼50–60‰ increase in δ D values from the late Miocene (∼6–∼7 Ma) through the Plio-Pleistocene transition (∼2–∼3 Ma), followed by near-constant values over at least the last ∼2 Ma. Although this enrichment trend is opposite that of a Rayleigh distillation model typically associated with surface uplift, we suggest that it is consistent with indirect effects of surface uplift on interior Alaska, including changes in aridity, moisture source, and seasonality of moisture. We conclude that the δ D clay record documents the creation of a topographic barrier and the associated changes to the climate of interior Alaska and Yukon Territory. [ABSTRACT FROM AUTHOR]
- Published
- 2018
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20. Middle to Late Pleistocene ice extents, tephrochronology and paleoenvironments of the White River area, southwest Yukon.
- Author
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Turner, Derek G., Ward, Brent C., Bond, Jeffrey D., Jensen, Britta J.L., Froese, Duane G., Telka, Alice M., Zazula, Grant D., and Bigelow, Nancy H.
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PLEISTOCENE Epoch , *TEPHROCHRONOLOGY , *PALEOENVIRONMENTAL studies , *GLACIATION - Abstract
Abstract: Sedimentary deposits from two Middle to Late Pleistocene glaciations and intervening non-glacial intervals exposed along the White River in southwest Yukon, Canada, provide a record of environmental change for much of the past 200 000 years. The study sites are beyond the Marine Isotope stage (MIS) 2 glacial limit, near the maximum regional extent of Pleistocene glaciation. Non-glacial deposits include up to 25 m of loess, peat and gravel with paleosols, pollen, plant and insect macrofossils, large mammal fossils and tephra beds. Finite and non-finite radiocarbon dates, and twelve different tephra beds constrain the chronology of these deposits. Tills correlated to MIS 4 and 6 represent the penultimate and maximum Pleistocene glacial limits, respectively. The proximity of these glacial limits to each other, compared to limits in central Yukon, suggests precipitation conditions were more consistent in southwest Yukon than in central Yukon during the Pleistocene. Conditions in MIS 5e and 5a are recorded by two boreal forest beds, separated by a shrub birch tundra, that indicate environments as warm or warmer than present. A dry, treeless steppe-tundra, dominated by Artemisia frigida, upland grasses and forbs existed during the transition from late MIS 3 to early MIS 2. These glacial and non-glacial deposits constrain the glacial limits and paleoenvironments during the Middle to Late Pleistocene in southwest Yukon. [Copyright &y& Elsevier]
- Published
- 2013
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21. High-precision ultra-distal Holocene tephrochronology in North America
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Pyne-O'Donnell, Sean D.F., Hughes, Paul D.M., Froese, Duane G., Jensen, Britta J.L., Kuehn, Stephen C., Mallon, Gunnar, Amesbury, Matthew J., Charman, Dan J., Daley, Tim J., Loader, Neil J., Mauquoy, Dmitri, Street-Perrott, F. Alayne, and Woodman-Ralph, Jonathan
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HOLOCENE Epoch , *TEPHROCHRONOLOGY , *VOLCANIC ash, tuff, etc. , *VOLCANIC eruptions , *GEOLOGICAL time scales , *ARCHAEOLOGICAL geology , *SOCIOECONOMICS - Abstract
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]
- Published
- 2012
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22. A 2300-year record of glacier fluctuations at Skilak and Eklutna Lakes, south-central Alaska.
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Thomas, Joseph E., Kaufman, Darrell S., Praet, Nore, McKay, Nicholas P., Van Daele, Maarten, Jensen, Britta J.L., and De Batist, Marc
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GLACIERS , *ALPINE glaciers , *ICE cores , *RADIATIVE forcing , *LAKES - Abstract
Holocene glacier fluctuations in south-central Alaska reflect hydroclimate changes in a region strongly influenced by large-scale features of North Pacific climate. Glacier fluctuations over the past approximately 2300 years were inferred from multiple geophysical and geochemical properties of a new 13-m-long sediment core from proglacial Skilak Lake. The chronology is based on annual-layer (varve) counting, supported by radiocarbon ages and the identification of the 1912 CE Katmai tephra. Sediment density and accumulation rate show two prominent century-scale increases, one near the base of the cored sequence (around 270-370 BCE) and one near the top (around 1600–1900 CE, Little Ice Age). These two peaks, and the absence of such increases between them, coincide with sedimentation-rate changes as previously measured in multiple cores from Eklutna Lake, located 130 km northeast of Skilak Lake. The similarity between the two sedimentary sequences, despite their contrasting geomorphic settings and spatial scales, implies that regional climate change is the overriding control. Sediment with high bulk density and thick varves was deposited concurrently with the expansion of glaciers in the catchment and elsewhere in the region during the Little Ice Age. We likewise attribute the deposits with similar characteristics to expanded glaciers centered around 320 BCE, and the absence of such sediment during the intervening 1800 years to the lack of a major glacier advance. Climatologically, both periods of high sediment accumulation rate correspond with times of intensified Aleutian Low pressure, as recently reconstructed based on regional syntheses of oxygen-isotope records for the earlier period, and on sea-salt concentration in two Alaskan ice cores for the later period. Both periods also coincide with pronounced multi-decadal downturns in combined solar and volcanic global radiative forcing. The massive buildup of glaciers during the Little Ice Age might reflect the increased frequency of these downturns combined with millennial-scale decrease in orbitally driven insolation. Glaciolacustrine records from these two lakes add a new perspective on the multi-century variability of glaciers and hydroclimate in the North Pacific region. • Available varve chronologies extend back to roughly 370 BCE at Eklutna and Skilak Lakes. • Century-scale pattern of sediment accumulation rate is similar at the two lakes. • Accumulation rate is interpreted as glacier fluctuations as supported by geomorphic evidence. • Glacier expansions are most prominent prior to 270 BCE, and between 1600 and 1900 CE • Large glaciers correspond with downturns in solar-volcanic forcing and strong Aleutian Low. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
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