James E. Beget

Paleoclimatic forcing of magnetic susceptibility variations in Alaskan loess during the late Quaternary

Visual matches and statistical tests suggest correlations between marine isotope curves, retrodictive solar insolation at lat 65°N, and magnetic susceptibility profiles through late Quaternary age Alaskan loess sections. The susceptibility changes largely appear to reflect variability in magnetite content due to climatically controlled changes in wind intensity and competence. Magnetic susceptibility profiles through massive loess can provide stratigraphic context for intercalated paleosols and tephras. A prominent paleosol correlated with marine isotope stage 5 occurs several metres above the Old Crow ash in loess sections, indicating that this important tephra is older than suggested by thermoluminescence dates, and may have been deposited ca. 215 ±25 ka.

Stratigraphy and palaeoclimatic significance of Late Quaternary loess-palaeosol sequences of the Last Interglacial-Glacial cycle in central Alaska

Loess is one of the most widespread subaerial deposits in Alaska and adjacent Yukon Territory and may have a history that goes back 3 Ma. Based on mineralogy and major and trace element chemistry, central Alaskan loess has a composition that is distinctive from other loess bodies of the world, although it is quartz-dominated. Central Alaskan loess was probably derived from a variety of rock types, including granites, metabasalts and schists. Detailed stratigraphic data and pedologic criteria indicate that, contrary to early studies, many palaeosols are present in central Alaskan loess sections. The buried soils indicate that loess sedimentation was episodic, or at least rates of deposition decreased to the point where pedogenesis could keep ahead of aeolian input. As in China, loess deposition and pedogenesis are likely competing processes and neither stops completely during either phase of the loess/soil formation cycle. Loess deposition in central Alaska took place before, and probably during the last interglacial period, during stadials of the mid-Wisconsin period, during the last glacial period and during the Holocene. An unexpected result of our geochronological studies is that only moderate loess deposition took place during the last glacial period. Our studies lead us to conclude that vegetation plays a key role in loess accumulation in Alaska. Factors favouring loess production are enhanced during glacial periods but factors that favour loess accumulation are diminished during glacial periods. The most important of these is vegetation; boreal forest serves as an effective loess trap, but sparsely distributed herb tundra does not. Thus, thick accumulations of loess should not be expected where tundra vegetation was dominant and this is borne out by modern studies near the treeline in central Alaska. Much of the stratigraphic diversity of North American loess, including that found in the Central Lowlands, the Great Plains, and Alaska is explained by a new model that emphasizes the relative importance of loess production factors versus loess accumulation factors.

Continuous late Quaternary proxy climate records from loess in Beringia

Abstract Loess deposits in eastern Beringia contain continuous proxy records of the effects of past climatic change on terrestrial landscapes at high latitudes. Variations of environmental magnetism and sedimentology of high-latitude loess deposits indicate that the timing and pattern of responses to local variations in wind intensity, storminess, and pedogenesis in eastern Beringia closely resemble the pattern of global climate change during the Late Quaternary deduced from studies of marine and ice core records. The age of paleoclimatic fluctuations, permafrost features, volcanic ash horizons, buried forest layers and paleosols, and other features of the eastern Beringian loess record can be determined using a variety of Quaternary dating methods. Tephrochronologic correlations between the loess record and the glacial history of eastern Beringia indicate the Delta Glaciation occurred during marine isotope stage 6. Several other middle and Late Quaternary glaciations across eastern Beringia can be tephrochronologically tied to the loess record, and appear to have been in phase with episodes of global cooling recorded in deep-sea records.

Properties of magnetic mineralogy of Alaskan loess: evidence for pedogenesis

Abstract The in situ pedogenic enhancement of ferrimagnetic content provides the well-established patterns of magnetic susceptibility variation within mid- to low-latitude loess deposits such as those of China and Central Europe. However, this pattern of high magnetic susceptibility in palaeosols, and lower values in unweathered loess, is not replicated in the higher-latitude loess deposits of Alaska and Siberia. In these localities the relationship is inverted, with high values in loess, and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. However, the results of the magnetic study presented in this paper suggest that there are differences in magnetic properties between Alaskan loess and palaeosols, not only in magnetic grain size and concentration but also in magnetic mineralogy. This complicates the simple hypothesis of a `wind velocity’ signal by introducing an additional factor into the climatic signal. In contrast to the enhancement of susceptibility observed in palaeosols of the Loess Plateau, China, we suggest that the low magnetic susceptibility values in the Alaskan palaeosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes associated with waterlogging (i.e. gleying) of the soils.

The 1883 and late-prehistoric eruptions of Augustine volcano, Alaska

Abstract The eruptive history of Augustine volcano has been characterized by cycles of growth and destruction of the volcano. Repeated failure of 5 – 10% of the edifice has produced mobile debris avalanches that reached the sea on all sides. High lava extrusion rates rapidly restore the volcano to its pre-failure configuration. This equilibrium between constructive and destructive processes has resulted in a relatively low lava-dome complex surrounded by an apron of volcaniclastic debris three times the volume of the dome complex. The most recent edifice collapse occurred in 1883, producing the 0.3 km3 Burr Point debris-avalanche deposit. Three major slide blocks extended the shoreline up to 2 km and produced a tsunami that swept across Cook Inlet. Hummock morphologies change from a proximal radial orientation to a dominantly transverse alignment reflecting deceleration and compression of the avalanches as they enter the sea. The breached crater formed by collapse was then largely filled by a 0.09 km3 lava dome and a 0.04 km3 lava flow travelled down the north flank. Lithic block-and-ash flows and pyroclastic surges reached the coast. The Burr Point avalanche deposit partially overlaps the Rocky Point debris-avalanche deposit to the west that was probably emplaced 200–400 years B.P. A major collapse event at ca. 1540 ± 110 A.D. produced the West Island debris avalanche, ending a period of expansion of the western side of the island. An accompanying lateral blast overtopped the avalanche, covering a 40 ° sector of the west flank. The plinian tephra layer B may also have been erupted at the time of the West Island eruption. Today Augustine volcano has rebuilt itself to a size similar to that which preceded the last edifice failure in 1883. The frequency of past collapses (three in the last 500 years) suggests that summit collapse is a possibility during any future eruption. The next major collapse is expected to involve 0.1 – 0.5 km3 of the summit; the ensuing debris avalanche would likely reach the coast, producing a tsunami that could impact populated areas of the Kenai Peninsula. The largest tsunami would result from collapse in directions other than to the north or west. Tsunami magnitude is contingent on failure volume, direction and timing with respect to tides.

Age, Extent and Climatic Significance of the c. 3400 BP Aniakchak Tephra, Western Alaska, USA

Tephra deposits at Whitefish Lake and Cape Espenberg on the northernmost Seward Peninsula of western Alaska were derived from the Aniakchak Caldera on the Alaska Peninsula more than 1500 km to the south. Radiocarbon dates on the climactic, caldera-forming Aniakchak eruption and on proximal and distal tephra indicate the Aniakchak tephra was deposited about 3435 ± 40 BP (3614-3815 cal. BP), and forms an isochronous marker horizon across much of western Alaska. The Aniakchak tephra eruption and several other targe eruptions during the seventeenth century BC may have had widespread effects on climate.

Tephrochronology and paleoclimatology of the Last Interglacial-Glacial cycle recorded in Alaskan loess deposits

Abstract Alaskan loess deposits contain long paleoclimatic records analogous to those found in marine and lacustrine sediments and in ice sheets. The sedimentology and geophysical properties of Alaskan loess were influenced by climatically forced changes in wind intensity, storminess, temperature, and precipitation. Loess deposits also contain evidence of episodic permafrost and paleosol formation, explosive volcanic eruptions, paleoecologic changes in high latitude regions, Quaternary fossils, and early human sites and artifacts. The Old Crow tephra (ca. 140,000 ± 10,000 BP) dates the transition between full-glacial conditions and the last interglaciation in central Alaska. Climatic amelioration in Alaska apparently began prior to astronomically forced increases in insolation, a finding consistent with the well-dated Devils Hole record. However, time-series analysis of loess proxy climate data also demonstrates the effects of orbital precession and obliquity on Alaskan climate change. Numerous short-term climate events are recorded in Alaskan loess deposits, including a climate reversal corresponding to the Younger Dryas.

Trace-element geochemistry of individual glass shards of the Old Crow tephra and the age of the Delta glaciation, central Alaska

Abstract Two widespread tephra deposits constrain the age of the Delta Glaciation in central Alaska. The Old Crow tephra (ca. 140,000 ± 10,000 yr), identified by electron microprobe and ion microprobe analyses of individual glass shards, overlies an outwash terrace coeval with the Delta glaciation. The Sheep Creek tephra (ca. 190,000 yr) is reworked in alluvium of Delta age. The upper and lower limiting tephra dates indicate that the Delta glaciation occurred during marine oxygen isotope stage 6. We hypothesize that glaciers in the Delta River Valley reached their maximum Pleistocene extent during this cold interval because of significant mid-Pleistocene tectonic uplift of the east-central Alaska Range.

Latest Pleistocene increase in wind intensity recorded in eolian sediments from central Alaska

Abstract A brief increase in wind intensity between ca. 11,100 and 10,700 yr B.P. is recorded by a sharp increase in sediment grain size at eolian sections along the Nenana River in central Alaska. This occurred at the same time as the Younger Dryas climatic reversal in northern Europe and an increase in the vigor of atmospheric circulation recorded by Greenland ice cores. Climatic fluctuations in high latitude areas during Younger Dryas time may reflect variations in the CO2 content of the atmosphere.

Late Pliocene to late Pleistocene environments preserved at the Palisades Site, central Yukon River, Alaska

Abstract The Palisades Site is an extensive silt-loam bluff complex on the central Yukon River preserving a nearly continuous record of the last 2 myr. Volcanic ash deposits present include the Old Crow (OCt; 140,000 yr), Sheep Creek (SCt; 190,000 yr), PA (2.02 myr), EC (ca. 2 myr), and Mining Camp (ca. 2 myr) tephras. Two new tephras, PAL and PAU, are geochemically similar to the PA and EC tephras and appear to be comagmatic. The PA tephra occurs in ice-wedge casts and solifluction deposits, marking the oldest occurrence of permafrost in central Alaska. Three buried forest horizons are present in association with dated tephras. The uppermost forest bed occurs immediately above the OCt; the middle forest horizon occurs below the SCt. The lowest forest bed occurs between the EC and the PA tephras, and correlates with the Dawson Cut Forest Bed. Plant taxa in all three peats are common elements of moist taiga forest found in lowlands of central Alaska today. Large mammal fossils are all from common late Pleistocene taxa. Those recovered in situ came from a single horizon radiocarbon dated to ca. 27,000 14 C yr B.P. The incongruous small mammal assemblage in that horizon reflects a diverse landscape with both wet and mesic environments.