Britta J.L. Jensen

Transatlantic distribution of the Alaskan White River Ash

Volcanic ash layers preserved within the geologic record represent precise time markers that correlate disparate depositional environments and enable the investigation of synchronous and/or asynchronous behaviors in Earth system and archaeological sciences. However, it is generally assumed that only exceptionally powerful events, such as supereruptions (≥450 km3 of ejecta as dense-rock equivalent; recurrence interval of ∼105 yr), distribute ash broadly enough to have an impact on human society, or allow us to address geologic, climatic, and cultural questions on an intercontinental scale. Here we use geochemical, age, and morphological evidence to show that the Alaskan White River Ash (eastern lobe; A.D. 833–850) correlates to the “AD860B” ash (A.D. 846–848) found in Greenland and northern Europe. These occurrences represent the distribution of an ash over 7000 km, linking marine, terrestrial, and ice-core records. Our results indicate that tephra from more moderate-size eruptions, with recurrence intervals of ∼100 yr, can have substantially greater distributions than previously thought, with direct implications for volcanic dispersal studies, correlation of widely distributed proxy records, and volcanic hazard assessment.

Fossil and genomic evidence constrains the timing of bison arrival in North America

Significance The appearance of bison in North America is both ecologically and paleontologically significant. We analyzed mitochondrial DNA from the oldest known North American bison fossils to reveal that bison were present in northern North America by 195–135 thousand y ago, having entered from Asia via the Bering Land Bridge. After their arrival, bison quickly colonized much of the rest of the continent, where they rapidly diversified phenotypically, producing, for example, the giant long-horned morphotype Bison latifrons during the last interglaciation. The arrival of bison in North America marks one of the most successful large-mammal dispersals from Asia within the last million years, yet the timing and nature of this event remain poorly determined. Here, we used a combined paleontological and paleogenomic approach to provide a robust timeline for the entry and subsequent evolution of bison within North America. We characterized two fossil-rich localities in Canada’s Yukon and identified the oldest well-constrained bison fossil in North America, a 130,000-y-old steppe bison, Bison cf. priscus. We extracted and sequenced mitochondrial genomes from both this bison and from the remains of a recently discovered, ∼120,000-y-old giant long-horned bison, Bison latifrons, from Snowmass, Colorado. We analyzed these and 44 other bison mitogenomes with ages that span the Late Pleistocene, and identified two waves of bison dispersal into North America from Asia, the earliest of which occurred ∼195–135 thousand y ago and preceded the morphological diversification of North American bison, and the second of which occurred during the Late Pleistocene, ∼45–21 thousand y ago. This chronological arc establishes that bison first entered North America during the sea level lowstand accompanying marine isotope stage 6, rejecting earlier records of bison in North America. After their invasion, bison rapidly colonized North America during the last interglaciation, spreading from Alaska through continental North America; they have been continuously resident since then.

The Kamikatsura event in the Gold Hill loess, Alaska

[1] As part of a broader geological study of loess/palaesol sequences in Alaska, the paleomagnetism of a section at Gold Hill near Fairbanks has been investigated. Samples were collected at 5 cm intervals through a 5 m loess‐paleosol‐ loess sequence. Detailed alternating field demagnetization of the natural remanent magnetization yields excellent results (average MAD = 2°). The Brunhes‐Matuyama (B‐M) polarity transition is present at the level of the paleosol, which was formedduringOxygenIsotopeStage19.Asystematicperturbation in both declination and inclination occurs ∼1.0–1.5 m belowtheB‐Mboundary.Thecorrespondingvirtualgeomagnetic poles define a track centred on the 60°W meridian from high southerly to equatorial latitudes and back again. This is in very good agreement with results from a sequence of lava flows on the island of Maui that are regarded as a record of the Kamikatsura event. The Alaskan results thus confirm the reality of this geomagnetic feature, and simultaneously provide a firm chronological control point for ongoing geological investigations. Mineral magnetic experiments indicate that the remanence of the Gold Hill loess is dominated by magnetite and/or maghemite, with no subsequent chemical overprinting by hematite and/or goethite. Citation: Evans, M. E., B. J. L. Jensen, V. A. Kravchinsky, and D. G. Froese (2011), The Kamikatsura event in the Gold Hill loess, Alaska, Geophys. Res. Lett., 38, L13302, doi:10.1029/2011GL047793.

Varve formation during the past three centuries in three large proglacial lakes in south-central Alaska

The sediments stored in the large, deep proglacial lakes of south-central Alaska are largely unstudied. We analyzed sediments in 20 cores, up to 160 cm long, from Eklutna, Kenai, and Skilak Lakes, using a combination of repeated lamination counting, radionuclide dating, event stratigraphy, and tephrochronology. We show that the characteristically rhythmic layers were deposited annually. Most of these glacial varves consist of one coarse-grained base and a fine-grained top, but varves composed of multiple coarse-grained turbidite pulses are common too. They are likely related to successive episodes of high sediment discharge during flooding, and they become more frequent in all three lakes, along with increased sedimentation rates, during the nineteenth century late phase of the Little Ice Age. These flood turbidites were generated by rain events and intense melting of snow and ice. Other (mega) turbidites are a result of earthquake-triggered slope collapses (e.g., A.D. 1964). Some event layers are present in all three lakes. In addition, the annual time series of varve thickness (normalized annual sedimentation rate) are significantly correlated among the three lakes (p > 0.27; p < 0.001). Differences between the varve thickness records can be attributed partly to the dam construction at Eklutna Lake and outbursts from an ice-dammed lake at Skilak Lake. Geomorphologic differences among the catchments result in further differences in sedimentation patterns in the three lakes.