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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]

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]

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]

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]

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]

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]

The increased use of radiocarbon analyses for archeological, paleontological, and paleoclimate research has created a demand for robust and well characterized procedural blanks. Currently, blanks are distributed ad hoc throughout the radiocarbon community in limited quantities and are depleted quickly by high throughput labs. This forces AMS facilities to regularly seek out suitable blanks from other facilities, and the switch to different blank material inhibits assessment of long term procedural blank performance. An excellent source for well-preserved radiocarbon "dead" organic material is the permafrost-rich region of unglaciated Yukon and Alaska. Extensive placer mining operations and river bluffs expose Middle Pleistocene sediments that regularly yield pristine organic material. Here, we present an assessment of laboratory precision and overall measure of performance of two processing blanks from the region. The AVR-07-PAL-37 wood and Hollis Mammoth pelvis bone collected from Alaska and Yukon, respectively. Both are independently dated to the Middle Pleistocene and have been verified by 14C-AMS to be radiocarbon "dead", with mean FmC of 0.0054 ± 0.0002 (n = 714) and 0.0031 ± 0.0002, (n = 219), respectively. Preliminary use at several AMS and stable isotope facilities in the U.S.A., Canada, United Kingdom, and China confirms that AVR-07-PAL-37 and Hollis Mammoth are suitable blanks for radiocarbon analyses. Both materials are available for wide distribution to other interested AMS labs. [ABSTRACT FROM AUTHOR]

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]

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]