Western Yukon Research Articles

Relict permafrost preserves megafauna, insects, pollen, soils and pore-ice isotopes of the mammoth steppe and its collapse in central Yukon

In eastern Beringia (unglaciated Alaska and western Yukon), the Pleistocene-Holocene transition was characterised by rapid changes in plant, insect and mammal communities as the mammoth steppe ecosystem was replaced, first by shrub tundra and later boreal forest. These changes indicate a transition from well drained terrain with deep active layers to wetter, cooler soils, to which steppe-tundra vegetation was poorly adapted. The nature and precise timing of these events is not well resolved, particularly in central Yukon where regional climate may have been strongly affected by the retreating Cordilleran-Laurentide ice sheet complex. Resolving this uncertainty is not only important for understanding past ecosystems, but also provides a long-term perspective for contemporary environmental change and shrub expansion that affects large areas of northern high-latitudes today. Here, we report chronology (41 radiocarbon dates), stratigraphy, pore-ice δ2H/δ18O measurements, pollen data, megafauna remains and fossil insect assemblages from a permafrost-preserved loessal sequence in central Yukon, named Lucky Lady. The site spans the interval from ca. 17,000 to 8000 cal yr BP (calibrated years before C.E. 1950) and records the Pleistocene-Holocene transition in exceptional resolution. Full glacial environments (ca. 16,500 cal yr BP) supported elements of steppe-tundra vegetation and an insect fauna dominated by Connatichela artemisiae - an endemic weevil indicating warm soil temperatures. The collapse of the mammoth steppe ecosystem began with slowing of loess accumulation and development of paleosol ca. 13,480 cal yr BP. At this time, C. artemisiae remains become infrequent and Artemisia pollen decline to be replaced by Cyperaceae (ca. 13,220), before mesic, shrub taxa (likely dwarf Betula and Salix) becomes dominant ca. 13,210 cal yr BP. The establishment of shrub tundra is associated with rapid changes in δ2H/δ18O measurements, suggesting that ecological turnover coincided with a shift in atmospheric conditions and moisture availability. Finally, boreal vegetation communities became established ca. 10,680 cal yr BP. The replacement of steppe-tundra vegetation in central Yukon lagged other areas of eastern Beringia by as much as 1000 years and coincided with rapid deglaciation during the Bølling–Allerød time-period (14,600–12,900 cal yr BP). Turnover in insect and vegetation communities took place in ca. 40 years as shrub tundra became dominant. This rapid turnover has parallels with contemporary Arctic greening and re-emphasises the sensitivity of high-latitude environments to climate change.

Raising the Resurrection plate from an unfolded-slab plate tectonic reconstruction of northwestern North America since early Cenozoic time

AbstractThe configuration of mid-ocean ridges subducted below North America prior to Oligocene time is unconstrained by seafloor isochrons and has been primarily inferred from upper-plate geology, including near-trench magmatism. However, many tectonic models are permitted from these constraints. We present a fully kinematic, plate tectonic reconstruction of the NW Cordillera since 60 Ma built by structurally unfolding subducted slabs, imaged by mantle tomography, back to Earth’s surface. We map in three-dimensions the attached Alaska and Cascadia slabs, and a detached slab below western Yukon (Canada) at 400–600 km depth that we call the “Yukon Slab.” Our restoration of these lower plates within a global plate model indicates the Alaska slab accounts for Pacific-Kula subduction since ca. 60 Ma below the Aleutian Islands whereas the Cascadia slab accounts for Farallon subduction since at least ca. 75 Ma below southern California, USA. However, intermediate areas show two reconstruction gaps that persist until 40 Ma. We show that these reconstruction gaps correlate spatiotemporally to published NW Cordillera near-trench magmatism, even considering possible terrane translation. We attribute these gaps to thermal erosion related to ridge subduction and model mid-ocean ridges within these reconstruction gap mid-points. Our reconstructions show two coeval ridge-trench intersections that bound an additional “Resurrection”-like plate along the NW Cordillera prior to 40 Ma. In this model, the Yukon slab represents a thermally eroded remnant of the Resurrection plate. Our reconstructions support a “northern option” Farallon ridge geometry and allow up to ∼1200 km Chugach terrane translation since Paleocene time, providing a new “tomographic piercing point” for the Baja-British Columbia debate.

Revisiting the Timing of the Northern Lobe of the White River Ash Volcanic Event in Eastern Alaska and Western Yukon

ABSTRACTThe northern lobe of the White River Ash (WRAn) is part of a bilobate distribution of tephras that originated from the Wrangell Volcanic Field near the border of Alaska, USA, and Yukon, Canada. It is distributed across northeastern Alaska and the northwestern portion of the Yukon. The timing of this eruption has seen little critical analysis relative to the younger and more extensive eastern lobe eruption of the White River Ash. We compiled 38 radiocarbon (14C) dates from above and below the WRAn, and employed several statistical approaches to identify and eliminate or down-weight outliers, combine dates, and different Bayesian models, to provide a revised age estimate for the timing of the WRAn tephra deposition. Our results indicate that the most accurate modeled age estimate for the northern lobe of the White River Ash deposition is between 1689 and 1560 cal BP, with a mean and median of 1625 and 1623 cal BP, respectively. This age range is 90 to 200 years younger than previous age estimates.

Estimating regional-scale methane flux and budgets using CARVE aircraft measurements over Alaska

Abstract. Methane (CH4) is the second most important greenhouse gas but its emissions from northern regions are still poorly constrained. In this study, we analyze a subset of in situ CH4 aircraft observations made over Alaska during the growing seasons of 2012–2014 as part of the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE). Net surface CH4 fluxes are estimated using a Lagrangian particle dispersion model which quantitatively links surface emissions from Alaska and the western Yukon with observations of enhanced CH4 in the mixed layer. We estimate that between May and September, net CH4 emissions from the region of interest were 2.2 ± 0.5 Tg, 1.9 ± 0.4 Tg, and 2.3 ± 0.6 Tg of CH4 for 2012, 2013, and 2014, respectively. If emissions are only attributed to two biogenic eco-regions within our domain, then tundra regions were the predominant source, accounting for over half of the overall budget despite only representing 18 % of the total surface area. Boreal regions, which cover a large part of the study region, accounted for the remainder of the emissions. Simple multiple linear regression analysis revealed that, overall, CH4 fluxes were largely driven by soil temperature and elevation. In regions specifically dominated by wetlands, soil temperature and moisture at 10 cm depth were important explanatory variables while in regions that were not wetlands, soil temperature and moisture at 40 cm depth were more important, suggesting deeper methanogenesis in drier soils. Although similar environmental drivers have been found in the past to control CH4 emissions at local scales, this study shows that they can be used to generate a statistical model to estimate the regional-scale net CH4 budget.

Geophysical investigation and reconstruction of lithospheric structure and its control on geology, structure, and mineralization in the Cordillera of northern Canada and eastern Alaska

A reconstruction of the Tintina fault is applied to regional geophysical and topographic data, facilitating the definition of west trending lineaments within the lower crust and/or mantle lithosphere, oblique to the NW trending structure of the Cordilleran terranes. The lineaments, which exhibit a range of geophysical and geological signatures, are interpreted to be related to the Liard transfer zone, continuous to the Denali fault, that divided lower and upper plates during late Proterozoic-Cambrian rifting of the Laurentian margin. Three-dimensional gravity models show a density increase in the lower crust and mantle lithosphere to the north. The transfer zone also divides bimodal mantle xenolith suites to the south from unimodal suites to the north. These conclusions suggest that extended North American basement, related to Laurentian margin rifting that would have brought mantle lithosphere rocks to a shallow depth, continuously underlies a thin carapace of accreted terranes in western Yukon and eastern Alaska. The interpreted continuity of North American basement reaffirms that if oroclinal bending of the Intermontane terranes occurred, then it was prior to its emplacement upon the rifted basement. Examination of the spatial relationships between mineral occurrences and postaccretionary, Cretaceous lithospheric lineaments, from their manifestation in geophysical, geological, and topographic data, suggest that the late Proterozoic lineaments influenced Mesozoic mineralization through influence on the development of the shallow crustal structure, intrusion, and exhumation and erosion.

Contrasting geochemistry of orogenic gold deposits in Yukon, Canada and Otago, New Zealand

The Yukon-Tanana Terrane (YTT) of western Yukon Territory in NW Canada and Otago Schist belt (OSB) of South Island, New Zealand share similar geological evolutionary histories as convergent orogenic belts. Both belts host orogenic gold deposits of mainly Jurassic to Early Cretaceous age. Jurassic mineralization in the YTT occurred during convergent orogenesis and stacking of previously-metamorphosed (Palaeozoic) greenschist-amphibolite facies metasediments, metavolcanic rocks, and metagranitoids. Early Cretaceous OSB mineralization occurred in the latter stages of terrane accretion of un-metamorphosed turbidites with minor basaltic rocks. Metamorphism of the OSB turbidites mobilised background levels of Au (0.6–1.3 ppb), As (2–20 ppm), Sb (0.1–1 ppm), and W (< 10 ppm), primarily under greenschist to lower amphibolite facies conditions when diagenetic pyrite (Au c . 0.5–2 ppm; As c . 500–10 000 ppm) transformed to pyrrhotite on a regional scale. In contrast, the previously-metamorphosed YTT rocks had generally low background As contents (1–2 ppm) apart from some As-rich quartzites (up to 100 ppm As). Consequently, there was less As available for orogenic mobilisation, and YTT Au deposits generally have lower concentrations of this pathfinder element compared to the OSB. YTT host rocks, especially metagranitoids, have anomalous levels of Mo (10–300 ppm), and many orogenic deposits contain elevated Mo, locally including molybdenite. OSB turbidites have elevated Mo (2–200 ppm), along with elevated Au and As, in diagenetic pyrite, but this Mo became largely dispersed through the metamorphic pile as metamorphic grade increased and pyrite transformed to pyrrhotite. OSB orogenic deposits have only marginally elevated Mo ( c . 1 ppm), no molybdenite, and accessory scheelite in these deposits is distinctly Mo-poor. Only minor mobilisation of base metals occurred in these orogenic belts, and orogenic Au deposits contain sparse base metal sulphides. Orogenic deposits in the YTT and OSB differ in that Au (and other associated elements) in many of the orogenic deposits in the YTT was remobilised from relatively local sources (e.g. pre-existing Cu-Mo-Au porphyry or volcanogenic sulphide mineralization) whereas Au in the OSB was mobilised from larger volumes of homogeneous rock at depth.

Extracting ore-deposit-controlling structures from aeromagnetic, gravimetric, topographic, and regional geologic data in western Yukon and eastern Alaska

Aeromagnetic lineaments interpreted from reduced-to-pole (RTP) magnetic grids were compared with gravity, topography, and field-based geologic maps to infer regional structural controls on hydrothermal mineral occurrences in a poorly exposed portion of the North American Cordillera in western Yukon and eastern Alaska. High-frequency and variable-intensity aeromagnetic lineaments corresponding to discontinuities with an aeromagnetic domain change were interpreted as steep-dipping and either magnetite-destructive or magnetite-additive faults. These structures were interpreted to be predominantly Cretaceous in age and to have formed after the collision of the Intermontane terranes with the ancient Pacific margin of North America. To demonstrate the reliability of the aeromagnetic interpretation, we developed a multidata set stacking methodology that assigns numeric values to individual lineaments depending on whether they can be traced in residuals and first vertical derivative of RTP aeromagnetic grids, isostatic residual gravity grids, digital topography, and regional geologic maps. The sum of all numeric values was used to estimate the likelihood of the aeromagnetic lineament as a true geologic fault. Fault systems were interpreted from zones of lineaments with high spatial density. Using this procedure, 10 major northwest-trending fault systems were recognized. These were oriented subparallel to the regional Cordilleran deformation fabric, the mid-Cretaceous Dawson Range magmatic arc, and well-established crustal-scale dextral strike-slip fault systems in the area. These orogen-parallel fault systems were interpreted to play a structural role in the emplacement of known porphyry Cu-Au and epithermal Au systems of mid-Cretaceous (115–98 Ma) and Late Cretaceous (79–72 Ma) age. The procedure also identified seven northeast-trending, orogen-perpendicular fault-fracture systems that are prominent in eastern Alaska and exhibit sinistral-to-oblique extensional kinematics. These structures were interpreted to govern the emplacement of Late Cretaceous (72–67 Ma) porphyry Mo- and Ag-rich polymetallic vein and carbonate replacement systems in the region.

Ancient DNA Reveals Genetic Continuity in Mountain Woodland Caribou of the Mackenzie and Selwyn Mountains, Northwest Territories, Canada

We examine the mitochondrial genetic stability of mountain woodland caribou (Rangifer tarandus caribou) in the Mackenzie and Selwyn Mountains, Northwest Territories, over the last 4000 years. Unlike caribou populations in the Yukon, populations in the Northwest Territories show no evidence for mitochondrial genetic turnover during that period, which indicates that they were not adversely affected by the widespread deposition of the White River tephra around 1200 years ago. We detect moderate genetic differentiation between mountain woodland and barren-ground caribou in both territories, lending support to the current subspecies designations. In addition, we identify moderate genetic differentiation between Northwest Territories and western Yukon mountain woodland caribou, suggesting that there has been minimal mixing of matrilines between these herds.

Vowels of Upper Tanana Athabascan

This paper provides a detailed phonetic, synchronic description of the oral vowels of Upper Tanana, an Athabascan language spoken in eastern interior Alaska and the western Yukon Territory. Upper Tanana has, depending on the dialect, six or seven oral monophthongs that can be modified for (low) tone, nasalization, and length. Previous accounts are auditory. They disagree regarding the articulation of two of the vowels (which we represent as /ɘ/ and /ʌ/), and raise the question of whether ‘length’ indicates a quantity or a quality distinction. In contrast to earlier accounts, we provide acoustic measurements of F1, F2, and the duration of the vowels of two Alaskan dialects of Upper Tanana: Northway and Tetlin. We find that the phonetic realization of one of the vowels is quite different from the earlier auditory descriptions. For the other vowels and for the quantity distinction, our findings support earlier descriptions while supplying new details. Several co-articulation effects, previously undescribed, are also identified.

The timing and provenance record of the Late Permian Klondike orogeny in northwestern Canada and arc‐continent collision along western North America

The northern Canadian Cordillera exhibits coeval accreted arc, subduction zone, ocean basin, and continental margin assemblages that make the region an exceptional place to understand tectonic processes involved in arc‐continent collision. In this study, we use U‐Pb zircon and monazite geochronology to define the timing and provenance record of Late Permian collisional orogeny related to the accretion of the Yukon‐Tanana terrane onto the ancestral North American continental margin of northwestern Canada. New U‐Pb crystallization ages of Permian intrusive rocks in the Klondike District of western Yukon bracket the timing of collision‐related ductile deformation and greenschist‐ to amphibolite‐facies metamorphism on the Yukon‐Tanana terrane between 260 and 252.5 Ma. This tectonothermal event is herein named the Klondike orogeny. Detrital zircon U‐Pb geochronology of Triassic strata provides the sedimentary record of arc‐continent collision and crustal reworking along the Cordilleran margin. Arc‐derived detrital zircons in Early to Middle Triassic (251–235 Ma) strata overlying the ancestral North American continental margin in Yukon suggest that a foreland‐style basin developed adjacent to the Klondike orogen. Regionally extensive Late Triassic (235–200 Ma) strata containing primarily North American detrital zircons form an overlap assemblage that covered the accreted terranes and western North America. The timing of the Klondike orogeny is roughly synchronous with other contractional events along the ∼5000 km strike length of the Cordillera, including the Late Permian‐Early Triassic Sonoman orogeny in Nevada. Global plate reorganization linked to assembly of Pangaea may have been the tectonic engine for late Paleozoic‐early Mesozoic development of the North American Cordillera.

Clonal and nonclonal genetic structure of subarctic black spruce (Picea mariana) populations in Yukon territory

Black spruce (Picea mariana) reproduces sexually from seeds and asexually by layering. There is a prevalent concept that clonal reproduction maintains populations of this species in the subarctic and arctic regions. We used microsatellite DNA markers of the nuclear genome to investigate the genetic structure of montane and subalpine black spruce populations from the Western Yukon Plateau in relation to this concept. Sixty individual trees at a minimum distance of 4 m from each other were sampled from each of four populations and individual trees were genotyped for eight microsatellite loci. Each of the 60 individuals from three montane pure black spruce populations growing on flat terrain at relatively low elevations had unique multilocus genotypes, indicating an absence of clonal structure in those populations. However, in an anthropologically undisturbed climax white spruce-dominated subalpine black spruce population on a northwest slope near Mount Nansen, the majority of the sampled individuals belonged to eight genetically distinct clones (genets). Clone size differed by altitude, the dominant genet being nearest the timberline–tundra ecotone. The results indicate that black spruce reproduction is variable and adaptive, being primarily sexual in flat-terrain montane populations previously subjected to fire disturbance, but mixed vegetative–sexual in the anthropogenically undisturbed subalpine population. This study is the first to employ molecular markers a priori to examine the mode of reproduction in natural black spruce populations.

Chapter 35 The Tatonduk inlier, Alaska–Yukon border

Abstract Glaciogenic deposits of the Rapitan and Hay Creek Groups are exposed in the Tatonduk inlier of east-central Alaska and the western Yukon. The Rapitan Group ranges in thickness from c. 50 to 700 m with Fe-formation common in the upper 10 m. In the most distal settings, the Rapitan Group is separated from the diamictite of the Hay Creek Group by over 100 m of sandstone and siltstone; however, the Hay Creek Group contains large erosive surfaces and cannibalizing breccia, and rarely preserves strata between the two glaciogenic deposits. The diamictite of the Hay Creek Group is capped by a white- to buff-coloured dolostone with pseudo-teepee structures, bed-parallel, isopachous sheet-crack cements, and a depleted C-isotope signature. Late Neoproterozoic glacial deposits in the Tatonduk inlier were formerly assigned to the Tindir Group. To simplify the nomenclature in the northwestern Canadian Cordillera, the Tindir Group was abandoned and replaced with nomenclature consistent with that of the Windermere Supergroup in the Mackenzie Mountains. The mixed lithology and anchizone-grade metamorphism distinguish the Rapitan and Hay Creek Groups in the Tatonduk inlier as attractive future targets for integrated micropalaeontology, geochemistry, palaeomagnetism and geochronology.

Early Neoproterozoic scale microfossils in the Lower Tindir Group of Alaska and the Yukon Territory

The Tindir Group is a <4-km-thick Neoproterozoic succession exposed in the Tatonduk inlier of east-central Alaska and the western Yukon Territory. The Tindir Group is informally divided into the Lower Tindir Group, which consists of <2 km of mixed carbonate and clastic rocks, and the overlying Upper Tindir Group, which contains two Cryogenian glacial deposits and an additional Ediacaran succession of mixed carbonate and clastic strata. Unique mineralized scale microfossils have been recovered from sections previously correlated with the Upper Tindir Group, and interpreted variously as Cryogenian to early Cambrian in age. Our remapping of the area indicates that these sections are stratigraphically below an early Cryogenian glacial diamictite, unit 2 of the Upper Tindir Group, and are actually part of the Lower Tindir Group. Carbon and strontium isotope correlations further suggest that the fossiliferous Lower Tindir Group is correlative with early Neoproterozoic strata of the northwestern Canadian Cordillera. This new age model is consistent with the accompanying microfossil assemblage and indicates that the diverse microfossils in the Lower Tindir Group can be added to the early Neoproterozoic record of eukaryotic evolution.

The architecture of oceanic plateaus revealed by the volcanic stratigraphy of the accreted Wrangellia oceanic plateau

The accreted Wrangellia flood basalts and associated sedimentary rocks that compose the prevolcanic and postvolcanic stratigraphy provide an unparalleled view of the architecture, eruptive environment, and accumulation and subsidence history of an oceanic plateau. This Triassic large igneous province extends for ∼2300 km in the Pacific Northwest of North America, from central Alaska and western Yukon (Nikolai Formation) to Vancouver Island (Karmutsen Formation), and contains exposures of submarine and subaerial volcanic rocks representing composite stratigraphic thicknesses of 3.5–6 km. Here we provide a model for the construction of the Wrangellia oceanic plateau using the following information and visualization tools: (1) stratigraphic summaries for different areas of Wrangellia; (2) new 40Ar/39Ar geochronology results; (3) compilation and assessment of geochronology and biostratigraphy for Wrangellia; (4) compiled digital geologic maps; (5) an online photographic archive of field relationships; and (6) a Google Earth file showing the mapped extent of Wrangellia flood basalts and linked field photographs.

The architecture of oceanic plateaus revealed by the volcanic stratigraphy of the accreted Wrangellia oceanic plateau

The accreted Wrangellia flood basalts and associated sedimentary rocks that compose the prevolcanic and postvolcanic stratigraphy provide an unparalleled view of the architecture, eruptive environment, and accumulation and subsidence history of an oceanic plateau. This Triassic large igneous province extends for ∼2300 km in the Pacific Northwest of North America, from central Alaska and western Yukon (Nikolai Formation) to Vancouver Island (Karmutsen Formation), and contains exposures of submarine and subaerial volcanic rocks representing composite stratigraphic thicknesses of 3.5–6 km. Here we provide a model for the construction of the Wrangellia oceanic plateau using the following information and visualization tools: (1) stratigraphic summaries for different areas of Wrangellia; (2) new 40Ar/39Ar geochronology results; (3) compilation and assessment of geochronology and biostratigraphy for Wrangellia; (4) compiled digital geologic maps; (5) an online photographic archive of field relationships; and (6) a Google Earth file showing the mapped extent of Wrangellia flood basalts and linked field photographs.

The architecture of oceanic plateaus revealed by the volcanic stratigraphy of the accreted Wrangellia oceanic plateau

The accreted Wrangellia flood basalts and associated sedimentary rocks that compose the prevolcanic and postvolcanic stratigraphy provide an unparalleled view of the architecture, eruptive environment, and accumulation and subsidence history of an oceanic plateau. This Triassic large igneous province extends for ∼2300 km in the Pacific Northwest of North America, from central Alaska and western Yukon (Nikolai Formation) to Vancouver Island (Karmutsen Formation), and contains exposures of submarine and subaerial volcanic rocks representing composite stratigraphic thicknesses of 3.5–6 km. Here we provide a model for the construction of the Wrangellia oceanic plateau using the following information and visualization tools: (1) stratigraphic summaries for different areas of Wrangellia; (2) new 40Ar/39Ar geochronology results; (3) compilation and assessment of geochronology and biostratigraphy for Wrangellia; (4) compiled digital geologic maps; (5) an online photographic archive of field relationships; and (6) a Google Earth file showing the mapped extent of Wrangellia flood basalts and linked field photographs. Based on combined radiometric (U-Pb, 40Ar/39Ar, K-Ar), paleontological, and magnetostratigraphic age constraints, the Wrangellia flood basalts were emplaced during a single phase of tholeiitic volcanism ca. 230–225 Ma, and possibly within as few as 2 Myr, onto preexisting submerged arc crust. There are distinct differences in volcanic stratigraphy and basement composition between Northern and Southern Wrangellia. On Vancouver Island, ∼6 km of high-Ti basalts, with minor amounts of picrites, record an emergent sequence of pillow basalt, pillow breccia and hyaloclastite, and subaerial flows that overlie Devonian–Mississippian (ca. 380–355 Ma) island arc rocks and Mississippian–Permian marine sedimentary strata. In contrast, Alaska and Yukon contain 1–3.5-km-thick sequences of mostly subaerial high-Ti basalt flows, with low-Ti basalt and submarine pillow basalts in the lowest parts of the stratigraphy, that overlie Pennsylvanian–Permian (312–280 Ma) volcanic and sedimentary rocks. Subsidence of the entire plateau occurred during and after volcanism, based on late-stage interflow sedimentary lenses in the upper stratigraphic levels and the presence of hundreds of meters to >1000 m of overlying marine sedimentary rocks, predominantly limestone. The main factors that controlled the resulting volcanic architecture of the Wrangellia oceanic plateau include high effusion rates and the formation of extensive compound flow fields from low-viscosity, high-temperature tholeiitic basalts, sill-dominated feeder systems, limited repose time between flows (absence of weathering, erosion, sedimentation), submarine versus subaerial emplacement, and relative water depth (e.g., pillow basalt–volcaniclastic transition).

An ImageJ plugin for the rapid morphological characterization of separated particles and an initial application to placer gold analysis

A new method for the fast, quantitative, automated morphological analysis of large numbers of discrete particles is reported. The method is built around a Java plugin coded for the open source program ImageJ, and utilizes automated image analysis to record morphological parameters. Conventional measurements (e.g., area, perimeter) as well as Fourier shape analysis parameters are produced. Preliminary results from the study of morphological changes of placer gold during alluvial transport using samples from the Klondike placer gold district in the western Yukon Territory, Canada show that the data produced by this model should allow for a quantitative transport distance:shape relationship to be defined. The large data set generated is also used to make a first examination of the utility of a number of previously unevaluated morphological parameters. Quantitative morphological characterization has numerous applications including the study of placer and tracer minerals, sedimentology, descriptive biology and archeological artifact examination.

Ordovician sponges from west-central and east-central Alaska and western Yukon Territory, Canada

Ordovician sponges from west-central and east-central Alaska and western Yukon Territory, Canada

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

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

Granulite facies xenoliths from Prindle volcano, Alaska: Implications for the northern Cordilleran crustal lithosphere

Xenoliths collected from Prindle volcano, Alaska (Lat. 63.72°N; Long. 141.82°W) provide a unique opportunity to examine the lower crust of the northern Canadian Cordillera. The cone's pyroclastic deposits contain crustal and mantle-derived xenoliths. The crustal xenoliths include granulite facies metamorphic rocks and charnockites, comprising orthopyroxene (opx)–plagioclase (pl)–quartz (qtz) ± mesoperthite (msp) and clinopyroxene (cpx). Opx–cpx geothermometry yields equilibrium temperatures ( T) from 770 to 1015 °C at 10 kbar. Pl–cpx–qtz geobarometry yields pressures ( P) of ∼ 6.6–8.0 kbar. Integrated mesoperthite compositions suggest minimum temperatures of 1020–1140 °C at 10 kbar using solvus geothermometry. The absence of garnet in these rocks indicates a range of maximum pressure of 5–11.3 kbar, and calculated solidi constrain upper temperature limits. We conclude that the granulite facies assemblages represent relatively dry metamorphism at pressures indicative of crustal thicknesses similar to present day (∼ 36 km). Zircon separates from a single crustal xenolith yield mainly Early Tertiary (48–63 Ma) U–Pb ages which are considerably younger than the cooling ages of the high-pressure amphibolites exposed at the surface. The distribution of zircon ages is interpreted as indicating zircon growth coincident with at least two different thermal events as expressed at surface: (i) the eruption of the Late Cretaceous Carmacks Group volcanic rocks in western Yukon and adjacent parts of Alaska, and (ii) emplacement of strongly bimodal high level intrusions across much of western Yukon and eastern Alaska possibly in an extensional tectonic regime. The distributions of zircon growth ages and the preservation of higher-than-present-day (> 25 ± 3 °C km − 1 ) geothermal gradients in the granulite facies rocks demonstrate the use of crustal xenoliths for recovering records of past, lithospheric-scale thermal–tectonic events.