East-central Alaska Research Articles

Ordovician geology of Alaska

Abstract Ordovician rocks, found in northern, east-central, interior and southern Alaska, formed in a variety of depositional and palaeogeographic settings. Shallow- and deep-water strata deposited along the northwestern Laurentian margin occur in east-central Alaska (Yukon River area) and probably correlative rocks crop out to the north in the Porcupine River area. Ordovician strata elsewhere in Alaska are parts of continental or island arc fragments that, as indicated by faunal and detrital zircon data, have been variously displaced. In northern Alaska, Ordovician rocks are included in the Arctic Alaska–Chukotka Microplate (AACM), a composite tectonic entity with a complex history. Some Ordovician strata in the AACM (parts of the North Slope subterrane) represent displaced fragments of the northern Laurentian margin. Coeval strata in southwestern parts of the AACM (York and Seward terranes, Hammond subterrane) share distinctive lithologic and biotic features with Ordovician rocks in interior Alaska (Farewell and related terranes). Ordovician strata in southeastern Alaska (Alexander terrane) also likely compose a composite crustal fragment that accumulated in a complex arc system. Shared features between many of these units suggest similar origins as part of one or more crustal fragments situated in the palaeo-Arctic between Laurentia, Baltica and Siberia during early Paleozoic time.

Population Characteristics, Morphometry, and Growth of Harvested Gray Wolves and Coyotes in Alaska

Few concurrent studies exist of sympatric gray wolf (Canis lupus) and coyote (C. latrans) harvest at far northern latitudes. Moreover, no studies explicitly examine effects of concurrent harvest on phenotypes of wolves and coyotes. We documented changes in sex and age characteristics and morphology of gray wolves and coyotes harvested by hunters near Ptarmigan Lake, east-central Alaska, USA, between 1998 and 2001. We hypothesized that the harvest would result in larger, heavier canids, reduce densities, and increase young to adult ratios in both wolves and coyotes. We generated von Bertalanffy growth curves indicating that wolves and coyotes of both sexes increased in length or weight until 2 or 3 years old. No significant changes in either mean length or weight or length to weight ratios occurred during the 3-year study, except that coyote mean length was longer over the last winter of study. Catch-per-unit effort (CPUE) for wolves ranged from 0.061 to 0.112 killed/day and for coyotes from 0.552 to 0.11 killed/day over the study. CPUE indicated that coyotes but not wolves declined in abundance. Changes in male to female and young to adult ratios did not differ significantly for either canid. We posit that coyote populations were disproportionately affected by the conflation of the severe Arctic environment and sustained harvest. Our findings will be beneficial for managing sympatric canid populations and for understanding demographic responses to density-dependent processes in wolves and coyotes, especially at far northern latitudes.

Update on the Status and Breeding Phenology of the Timberline Sparrow (Spizella breweri taverneri) in Alaska

In July 2020 we located 10 singing Timberline Sparrows [Spizella (breweri) taverneri] in the region of Gold Hill, Nutzotin Mountains, east-central Alaska. All birds were on southeast-facing slopes in the ecotone between subalpine scrub and alpine tundra, to which habitat breeding Timberline Sparrows seem narrowly confined. The population’s estimated density of 0.77 birds/km2 was similar to that at the time of its discovery in 1994. We located the first active nest of the Timberline Sparrow reported for Alaska, ~0.3 m above the ground in a shrubby resin birch (Betula glandulosa). An observation of young fledged on 11 or 12 July 2020 implies egg laying in the third week of June, later than the beginning of the breeding season of Spizella (breweri) breweri.

Yukon Flats Basin Tectono-Sedimentary Element, East-Central Alaska

Abstract Yukon Flats is a vast and remote lowland that straddles the Arctic Circle, and is located along the Yukon River in east-central Alaska. Seismic reflection and gravity data indicate that low-density sedimentary strata beneath Yukon Flats are as thick as 8 km and are underlain by Jurassic–Devonian oceanic rocks of the Angayucham–Tozitna terrane. On the basis of shallow core-hole penetrations and by analogy with nearby outcrops, the low-density strata are thought to consist of fluvial and lacustrine deposits of Neogene, Paleogene and, possibly, Late Cretaceous age. No deep petroleum exploration wells have been drilled in Yukon Flats, but a coalbed methane test reached a total depth of nearly 700 m and found small amounts of microbial methane. Geochemical data from outcrops and shallow core holes suggest that potential source rocks include non-marine coal, mudstone and shale, whereas potential reservoirs include non-marine sandstone and conglomerate. Numerous potential structural and stratigraphic traps are evident on seismic profiles. The United States Geological Survey estimates that undiscovered, technically recoverable, oil resources in the Yukon Flats Basin Tectono-Sedimentary Element range from 0 to almost 600 MMbo, and that undiscovered, technically recoverable, gas resources range from 0 to almost 15 Tcf.

Glass fission-track ages, composition, and origin of tephra beds in the Upper Cretaceous Kanguk Formation, Banks Island, Arctic Canada

More than 50 conspicuous tephra beds occur in the Kanguk Formation on the southwestern coast of Banks Island. Their glass shards are remarkably well preserved and permit comprehensive characterization, offering the potential for reliable, precise correlation of Upper Cretaceous sedimentary rocks across the three major depocentres of the Arctic Archipelago and adjacent northern continental margin: Sverdrup, Banks, and the Beaufort–Mackenzie basins. Twenty-one tephra beds were analyzed; all have a high-K, peraluminous, rhyolitic composition, with quartz, plagioclase, ilmenite, biotite, and zircon as the dominant minerals. Trace-element concentrations, especially low Nb and Ta, show that the parental magmas formed in a continental-margin subduction environment. Glass fission-track ages range from 100 Ma to younger than 60 Ma, and indicate a very low sedimentation rate giving a very condensed sedimentary sequence on southwestern Banks Island — a sequence that may well contain the K–Pg transition. Source calderas are unknown but most likely are situated in east-central Alaska and the central and northern Kuskokwim volcanic belt, some 1000 to 1500 km distant from southwestern Banks Island. It is also possible that some of the very thin tephra beds come from the Okhotsk–Chukotka volcanogenic belt in northeastern Russia.

Hydrogeochemistry in the Yukon-Tanana upland region of east-central Alaska: Possible exploration tool for porphyry-style deposits

A hydrogeochemical study using high resolution ICP-MS was undertaken at the Taurus and other porphyry Cu–Mo(-Au) occurrences and Ag–Au–Cu (+/− Pb, Zn) occurrences with epithermal-style characteristics in the Yukon-Tanana upland region of eastern Alaska. Surface water samples were collected from 30 sites on creeks that drain known deposits and occurrences and surrounding presumably unmineralized areas. Water samples for the entire ~9 km length of McCord Creek, which drains the Taurus deposit, and those from streams draining the areas at and near the Bluff and Dennison porphyry occurrences have high conductivity values (492–1250 μS/cm) and consistently high concentrations of B (3–250 μg/L), Co (2.3–42 μg/L), Mn (339–4750 μg/L), Re (0.012–0.1 μg/L), and SO42− (>200 mg/L), all of which are well above the median value for this data set and significantly greater than concentrations in water samples from the unmineralized areas. These are the best pathfinder elements specifically for porphyry style deposits because most of them are not anomalous in waters near epithermal occurrences. Copper concentrations are high (up to 115 μg/L) in some low-pH water samples from McCord Creek and drainages around Bluff, and a few near neutral pH waters have high molybdenum (>1 μg/L), but neither element is consistently anomalous in close vicinity to the porphyry occurrences, possibly due to a metal-poor, sulfide-poor leached cap (average of ~50 m) that overlies supergene and hypogene mineralized zones and is the dominant rock at surface. High concentrations of Bi and/or As occur in many waters associated with mineralized areas, particularly the Bluff and Dennison occurrences. In general, the element associations related to porphyry deposits reflect the deposit mineralogy, as well as size of the footprint related to alteration and mineralization.

A 25‐Year History of Volcano Magma Supply in the East Central Aleutian Arc, Alaska

AbstractUsing ~25 years of GPS and InSAR observations and the time‐dependent inversion filter, we invert for the volume change history of three active volcanoes in the east central Alaska Aleutian arc and the secular velocity of the region. The inferred time‐dependent volume change shows (1) two exponentially decaying pulses followed by a nearly linear inflation period at Westdahl volcano, (2) multiple episodic inflation events at Akutan volcano, and (3) transient volcanic inflation and long‐term deflation signals at Makushin volcano. Comparing our regional velocity estimates with recently published block models, we find a small signal that can be explained by slip deficit on the megathrust, either from a shallow (<20 km depth) almost fully locked zone or a weakly locked zone confined to 25–50 km depth.

Late Ordovician brachiopods from east-central Alaska, northwestern margin of Laurentia

AbstractA Late Ordovician brachiopod fauna from the Black River quadrangle (D-1 1:63,360 scale) of east-central Alaska comprises taxa typical of the Late Ordovician brachiopod fauna in the pericratonic epeiric seas of Laurentia, includingHesperorthis pyramidalis(Twenhofel, 1928),Plaesiomys occidentalis(Okulitch, 1943),Eoplectodontasp.,Holtehdalinasp.,Leptaenasp.,Brevilamnulella minutan. sp.,Tcherskidium tenuicostatumn. sp.,Rhynchotrema iowenseWang, 1949, andWhitfieldellasp. The presence ofPlaesiomys occidentalisandTcherskidium tenuicostatan. sp. indicates a latest Katian age by correlation with similar species in the Mackenzie Mountains, southern Manitoba, Anticosti Island, the American midcontinent, Kolyma, and Siberia. Cluster analysis based on 20 well-studied late Katian brachiopod faunas from various regions within Laurentia and elsewhere in other tectonic plates suggests that the small brachiopod faunule from Alaska has the strongest paleobiogeographic affinity with Laurentia, confirming that the Black River quadrangle of Alaska was part of Laurentia during the Late Ordovician.UUID:http://zoobank.org/6b387856-61d5-4685-a592-faf5287c1e0f

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Regional Variation in Interior Alaskan Boreal Forests is Driven by Fire Disturbance, Topography, and Climate

We developed landscape-scale spatial models of tree species occupancy and abundance in Yukon–Charley Rivers National Preserve (YCNP), a remote and little-studied region in east-central Alaska. We compared these results with similar work completed in Denali National Park (see Roland et al. 2013). These two areas are a study in contrasts. Specifically, YCNP is a highly continental region with warmer growing seasons, a topographically diverse lowland landscape, and a considerably greater footprint of recent fire disturbance. Our two study areas thus effectively frame a “natural experiment” that allowed us to examine various hypotheses regarding expected changes in the Alaskan boreal forest due to a rapidly warming climate in Alaska. These photographs illustrate the article “Regional variation in interior Alaskan boreal forests is driven by fire disturbance, topography, and climate” by Carl A. Roland, Joshua H. Schmidt, Samantha G. Winder, Sarah. E. Stehn, and E. Fleur Nicklen published in Ecological Monographs. https://doi.org/10.1002/ecm.1369

Extreme sulfur isotope fractionation in the Late Devonian Dry Creek volcanogenic massive sulfide deposit, central Alaska

The Dry Creek Zn-Pb-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit, in east-central Alaska, occurs in a Late Devonian sequence of peralkaline rhyolite tuff, minor graphitic argillite, and local peralkaline quartz-porphyry rhyolite intrusions. Principal mineralized facies are semi-massive and massive sulfide in variably silicified and graphitic rhyolite tuff, massive sulfide in graphitic argillite, disseminated sulfides in graphitic and non-graphitic rhyolite tuff, and vein-hosted sulfides in a subvolcanic, peralkaline quartz-porphyry intrusion. In situ analysis of the sulfur isotope composition of sulfide minerals from all facies of the deposit shows a total range in δ34S values from −48.0 to 23.1‰. This remarkable 71.1‰ variation is more than twice the largest range known for sulfides in nearly all individual VMS deposits, both modern and ancient, which typically is <25‰. Based on results of geochemical reaction modeling presented here, extreme sulfur isotope systematics in the Dry Creek deposit is attributed to two main processes: low-temperature (<100 °C) kinetically controlled bacterial sulfate reduction (BSR) in anoxic to locally sulfidic bottom waters and pore fluids, and higher temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater sulfate during water-rock reactions. Deposition of organic-rich sediments prior to and during mineralization provided the key environment that promoted microbially mediated fractionations with Δ34Ssulfate-sulfide of up to 70‰. During BSR reaction progress modeling, residual sulfate evolved by Rayleigh distillation in pore water, producing δ34SSO4 values up to 58‰; normal Late Devonian seawater sulfate had δ34SSO4 = 22‰. BSR produced FeS precipitates with δ34S values from −50 to −15‰. Integrated textural and δ34S data also suggest that isotopically light H2S produced in pore fluids by BSR was incorporated into hydrothermal fluids and early disseminated sulfide minerals during the emplacement of peralkaline quartz-porphyry rhyolite intrusions in the shallow subsurface, broadly coeval with VMS mineralization. Modeling of moderate- to high-temperature (100–300 °C) hydrothermal sulfate reduction (HSR) of seawater-rock interaction predicts sulfide minerals with δ34S values from −8 to 19‰.High-precision CA-ID-TIMS U-Pb geochronology of zircons from a synvolcanic peralkaline quartz porphyry intrusion yields a weighted mean 206Pb/238U date of 363.02 ± 0.43 Ma (2σ total uncertainty). This date indicates that the formation of graphitic and sulfidic sediments at Dry Creek, and contemporaneous VMS mineralization, occurred at least 3.6 m.y. before, and hence are unrelated to, widespread black shale deposition during the Hangenberg Event at ca. 359 Ma.

Crustal Structure in Alaska From Receiver Function Analysis

AbstractNew maps of crustal thickness and Vp/Vs in Alaska and western Canada were obtained using P receiver functions recorded at 198 stations from the USArray Transportable Array and the Alaska Regional Network. Our results indicate that topography and Moho depth are correlated as crustal thickness varies from 28 to 43 km across Alaska. A thick crust occurs under the mountains in the south and north with relatively thin crust in central Alaska. The deepest crustal root beneath the Brooks Range may have lost its buoyancy based on Airy isostasy. In addition, a buoyant upper mantle is required to support the high topography in east central Alaska. Vp/Vs is determined between 1.7 and 1.8 beneath most of Alaska except a high average of 1.9 in the south central region. We attribute this high Vp/Vs to the underplating of Yakutat and Pacific oceanic crust.

Denning Ecology of Wolves in East-Central Alaska, 1993–2017

Dens are a focal point in the life history and ecology of gray wolves (Canis lupus), and their location can influence access to key resources, productivity, survivorship, and vulnerability to hunting, trapping, and control efforts. We analyzed the selection of den sites and the phenology of their use inside the Yukon-Charley River National Preserve from 1993 to 2017 to enhance our understanding of this resource. At the landscape scale, we found that wolves in east-central Alaska selected den sites that were lower in elevation, snow free earlier in the spring, exposed to greater solar radiation, and closer to water. Den sites were also associated with areas that had burned less recently and had lower terrain ruggedness at the 1 km scale. These results supported our hypothesis that wolves would den relatively close to essential resources (water and prey) and in areas that are drier (melt earlier) in the spring. At the home range scale, wolves also selected den sites at lower elevations and showed a strong selection for the center of their home range. Furthermore, the average distance between active den sites was 37.3 km, which is slightly greater than the average radius (32.5 km) of a home range of a pack. Our results support our hypothesis that dynamic social factors modulate the selection of environmental factors for den site location. Wolves den away from other packs to reduce competition and exposure to intraspecific conflict. High-quality denning habitat does not currently appear to be a limiting factor for this population. Females, on average, entered their dens on 10 May, stayed inside the den for eight days, and remained less than 1 km from the den for an additional six days after emerging. We found that wolves denning at higher elevations entered their dens later than those at lower elevations, which also supported one of our hypotheses. Lastly, we documented limited evidence of earlier denning over time. Long-term monitoring projects, such as ours, are critical in identifying these types of trends.

Detrital zircon geochronology of quartzose metasedimentary rocks from parautochthonous North America, east-central Alaska

Detrital zircon geochronology of quartzose metasedimentary rocks from parautochthonous North America, east-central Alaska

Age of some Pleistocene interglacial beds and associated fossils in eastern Beringia defined by fission tracks in glass shards of Chester Bluff tephra

AbstractApplication of the glass fission-track dating method to Chester Bluff tephra (CBt), exposed in loess deposits at Chester Bluff along the Yukon River in east-central Alaska, has clarified the age of the immediately underlying fossiliferous interglacial bed. Surprise Creek tephra (SZt), at site CRH47 in the northern Old Crow basin of the Yukon Territory, is a correlative of CBt so that the new age information on CBt can also be applied to the interglacial sediments below SZt. Two independent age determinations were obtained on CBt, 243±28 ka and 249±26 ka, giving a weighted mean age and error of 246±19 ka. Therefore, the closely associated interglacial bed belongs to the early part of Marine Oxygen Isotope Stage (MIS) 7. The stratigraphy and paleoenvironmental setting of SZt show that deposition of the tephra occurred soon after interglacial conditions, when the climate became colder, probably between MIS 7.5 and 7.4, that is, slightly younger than the mean fission-track age, but within the 1σ uncertainty. This result tightly constrains the age of the rich mammalian faunal assemblage found at and just below SZt at the CRH47 site.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Yukon-Tanana terrane, Coast Mountains, southeast Alaska

Rocks of the SE Alaska subterrane of the Yukon-Tanana terrane (YTTs) consist of regionally metamorphosed marine clastic strata and mafic to felsic volcanic-plutonic rocks that have been divided into the pre-Devonian Tracy Arm assemblage, Silurian–Devonian Endicott Arm assemblage, and Mississippian–Pennsylvanian Port Houghton assemblage. U-Pb geochronologic and Hf isotopic analyses were conducted on zircons separated from 23 igneous and detrital samples in an effort to reconstruct the geologic and tectonic evolution of this portion of YTT. Tracy Arm assemblage samples are dominated by Proterozoic (ca. 2.0–1.6, 1.2–0.9 Ga) and Archean (2.7–2.5 Ga) zircons that yield typical cratonal eHf( t ) values. Endicott Arm assemblage samples yield U-Pb ages that range from Late Ordovician to Early Devonian and eHf( t ) values that range from highly juvenile to moderately evolved. Port Houghton assemblage samples yield similar Ordovician–Devonian ages and eHf( t ) values, and also include early Mississippian zircons with highly evolved eHf( t ) signatures. Comparison of these age-Hf patterns with data from nearby assemblages suggests the following: (1) Results from YTTs are similar to (or compatible with) available data from rocks of the Yukon-Tanana terrane in eastern Alaska, Yukon, and northern British Columbia (YTTn) and pericratonic strata in east-central Alaska (NAa). (2) YTTs contains abundant Late Ordovician–Early Devonian magmatism that is not recorded in YTTn and NAa. (3) The eHf( t ) values from YTTs display two excursions from juvenile to evolved eHf( t ) values, which are interpreted to record cycles of crustal thinning and then thickening within a convergent margin system. (4) Available data from both YTTs and YTTn support Neoproterozoic(?)–early Paleozoic positions along the northern Cordilleran margin. (5) The Late Ordovician–Early Devonian magmatic record of the southern Alexander terrane is very similar to that of YTTs, which raises the possibility that these assemblages evolved in the same convergent margin system along the northern (Alexander) and northwestern (YTT) margins of Laurentia. These results support a tectonic model in which: (1) YTTs formed outboard of (or northward along strike of) YTTn and NAa along the northern Cordilleran margin during Neoproterozoic(?)–early Paleozoic time; (2) initial subduction-related magmatism during Late Ordovician to Early Devonian time records a progression from crustal thinning to crustal thickening, and is preserved only in YTTs; (3) a second phase of magmatism records Middle–Late Devonian crustal thinning followed by early Mississippian crustal thickening; (4) YTTs and YTTn evolved as an intra-oceanic arc outboard of the Slide Mountain ocean basin during Carboniferous–Permian time and were accreted to the continental margin during Triassic time; and (5) YTTs is interpreted to have been displaced ∼1000 km southward, from an original position outboard of YTTn/NAa to its present position outboard of the Stikine terrane, along a sinistral fault system of Late Jurassic–Early Cretaceous age.

Apatite fission-track evidence for regional exhumation in the subtropical Eocene, block faulting, and localized fluid flow in east-central Alaska

The origin and antiquity of the subdued topography of the Yukon–Tanana Upland (YTU), the physiographic province between the Denali and Tintina faults, are unresolved questions in the geologic history of interior Alaska and adjacent Yukon. We present apatite fission-track (AFT) results for 33 samples from the 2300 km2 western Fortymile district in the YTU in Alaska and propose an exhumation model that is consistent with preservation of volcanic rocks in valleys that requires base level stability of several drainages since latest Cretaceous–Paleocene time. AFT thermochronology indicates widespread cooling below ∼110 °C at ∼56–47 Ma (early Eocene) and ∼44–36 Ma (middle Eocene). Samples with ∼33–27, ∼19, and ∼10 Ma AFT ages, obtained near a major northeast-trending fault zone, apparently reflect hydrothermal fluid flow. Uplift and erosion following ∼107 Ma magmatism exposed plutonic rocks to different extents in various crustal blocks by latest Cretaceous time. We interpret the Eocene AFT ages to suggest that higher elevations were eroded during the Paleogene subtropical climate of the subarctic, while base level remained essentially stable. Tertiary basins outboard of the YTU contain sediment that may account for the required &gt;2 km of removed overburden that was not carried to the sea by the ancestral Yukon River system. We consider a climate driven explanation for the Eocene AFT ages to be most consistent with geologic constraints in concert with block faulting related to translation on the Denali and Tintina faults resulting from oblique subduction along the southern margin of Alaska.

Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes

The Mesozoic magmatic history of the North American margin records the evolution from a more segmented assemblage of parautochthonous and allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic mineralization in the western Fortymile mining district, east-central Alaska, where parautochthonous and allochthonous Paleozoic tectonic assemblages are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP U-Pb zircon ages and published U-Pb and 40 Ar/ 39 Ar ages indicate four episodes of plutonism in the western Fortymile district: Late Triassic (216–208 Ma), Early Jurassic (199–181 Ma), mid-Cretaceous (112–94 Ma), and Late Cretaceous (70–66 Ma). All age groups have calc-alkalic arc compositions that became more evolved through time. Pb isotope compositions of feldspars from Late Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became more radiogenic with time and are consistent with the magmas being mantle derived but extensively contaminated by upper crustal components with evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous rocks have isotopic ratios that indicate magma derivation from upper crustal sources, probably thickened mid-Paleozoic basement. The origin of the mantle component in Late Cretaceous granitoids suggested by Pb isotopic ratios is uncertain, but we propose that it reflects asthenospheric upwelling following slab breakoff and sinking of an inactive inner subduction zone that delivered the previously accreted Wrangellia composite terrane to the North American continental margin, after the outer Farallon subduction zone was established. Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are spatially associated with splays of the northeast-trending Kechumstuk sinistral-normal fault zone and with ca. 68–66 Ma felsic intrusions and dikes. The similarity between Pb isotope compositions of feldspars from the Late Cretaceous igneous bodies and sulfides from the epithermal prospects suggests a Late Cretaceous age for most of the mineralization. Fluid flow along the faults undoubtedly played a major role in mineralization. We interpret displacement on the northeast-trending faults to be a far-field effect of dextral translation along Late Cretaceous plate-scale boundaries and faults that were roughly parallel to the subsequently developed Denali and Tintina fault systems, which currently bound the region.

Preservation of collagen and bioapatite fractions extracted from bison teeth in permafrost conditions

This research addresses the stability of bioapatite and collagen fractions of AMS dated steppe bison (Bison priscus) teeth. Through the course of other research, 8 prehistoric bison molars were submitted for AMS dating of fractions of collagen extracted from the dentine of each tooth. Because the teeth were well preserved and collagen yields were relatively high during the initial analysis, it provided an opportunity to further research differences between AMS dates produced on collagen from dentine and bioapatite fractions from enamel. The specimens were recovered from late Quaternary sediments of the Lost Chicken Creek drainage in east-central Alaska. All of the samples were very well preserved and gave high enough yield of carbon from both fractions. The 14C/13C ratio was measured using 0.5MV tandem AMS system.The 14C age of the samples varied across age ranges between 17,360±50 and 43,370±300 non-calibrated years BP. Such a wide range of ages allows us estimate the stability of each fraction in subarctic permafrost conditions. The results of analyses have shown that 14C ages of bioapatite fraction are rejuvenated as a result of isotopic exchange with the younger carbon from the soil solutions. The dating of bioapatite from the samples collected in the boreal climate of Alaska is possible only with a certain correction for the isotope fractionation.

The Late Cretaceous Middle Fork caldera, its resurgent intrusion, and enduring landscape stability in east-central Alaska

Dissected caldera structures expose thick intracaldera tuff and, uncommonly, cogenetic shallow plutons, while remnants of correlative outflow tuffs deposited on the pre-eruption ground surface record elements of ancient landscapes. The Middle Fork caldera encompasses a 10 km × 20 km area of rhyolite welded tuff and granite porphyry in east-central Alaska, ∼100 km west of the Yukon border. Intracaldera tuff is at least 850 m thick. The K-feldspar megacrystic granite porphyry is exposed over much of a 7 km × 12 km area having 650 m of relief within the western part of the caldera fill. Sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) analyses of zircon from intracaldera tuff, granite porphyry, and outflow tuff yield U-Pb ages of 70.0 ± 1.2, 69.7 ± 1.2, and 71.1 ± 0.5 Ma (95% confidence), respectively. An aeromagnetic survey indicates that the tuff is reversely magnetized, and, therefore, that the caldera-forming eruption occurred in the C31r geomagnetic polarity chron. The tuff and porphyry have arc geochemical signatures and a limited range in SiO 2 of 69 to 72 wt%. Although their phenocrysts differ in size and abundance, similar quartz + K-feldspar + plagioclase + biotite mineralogy, whole-rock geochemistry, and analytically indistinguishable ages indicate that the tuff and porphyry were comagmatic. Resorption of phenocrysts in tuff and porphyry suggests that these magmas formed by thermal rejuvenation of near-solidus or solidified crystal mush. A rare magmatic enclave (54% SiO 2 , arc geochemical signature) in the porphyry may be similar to parental magma and provides evidence of mafic magma and thermal input. The Middle Fork is a relatively well preserved caldera within a broad region of Paleozoic metamorphic rocks and Mesozoic plutons bounded by northeast-trending faults. In the relatively downdropped and less deeply exhumed crustal blocks, Cretaceous–Early Tertiary silicic volcanic rocks attest to long-term stability of the landscape. Within the Middle Fork caldera, the granite porphyry is interpreted to have been exposed by erosion of thick intracaldera tuff from an asymmetric resurgent dome. The Middle Fork of the North Fork of the Fortymile River incised an arcuate valley into and around the caldera fill on the west and north and may have cut down from within an original caldera moat. The 70 Ma land surface is preserved beneath proximal outflow tuff at the west margin of the caldera structure and beneath welded outflow tuff 16–23 km east-southeast of the caldera in a paleovalley. Within ∼50 km of the Middle Fork caldera are 14 examples of Late Cretaceous (?)–Tertiary felsic volcanic and hypabyssal intrusive rocks that range in area from 2 to ∼100 km 2 . Rhyolite dome clusters north and northwest of the caldera occupy tectonic basins associated with northeast-trending faults and are relatively little eroded. Lava of a latite complex, 12–19 km northeast of the caldera, apparently flowed into the paleovalley of the Middle Fork of the North Fork of the Fortymile River. To the northwest of the Middle Fork caldera, in the Mount Harper crustal block, mid-Cretaceous plutonic rocks are widely exposed, indicating greater total exhumation. To the southeast of the Middle Fork block, the Mount Veta block has been uplifted sufficiently to expose a ca. 68–66 Ma equigranular granitic pluton. Farther to the southeast, in the Kechumstuk block, the flat-lying outflow tuff remnant in Gold Creek and a regionally extensive high terrace indicate that the landscape there has been little modified since 70 Ma other than entrenchment of tributaries in response to post–2.7 Ma lowering of base level of the Yukon River associated with advance of the Cordilleran ice sheet.