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During the middle Pleistocene Nome River glaciation of northwestern Alaska, glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals. The age of the Nome River glaciation is constrained by laser-fusion 40Ar/39Ar analyses of basaltic lava that overlies Nome River drift at Minnie Creek, central Seward Peninsula, that average 470,000 ± 190,000 yr (±1σ). Milligram-size subsamples of the lava were dated to identify and eliminate extraneous 40Ar enrichments that rendered the mean of conventional K-Ar dates on larger bulk samples of the same flow too old (700,000 ± 570,000 yr). While the 40Ar/39Ar analyses provide a minimum limiting age for the Nome River glaciation, maximum ages are provided by a provisional K-Ar date on a basaltic lava flow that underlies the Nome River drift at nearby Lave Creek, by paleomagnetic determinations of the drift itself at and near the type locality, and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the Anvilian marine transgression that underlie Nome River drift on the coastal plain at Nome. Taken together, the new age data indicate that the glaciation took place between 580,000 and 280,000 yr ago. The altitude of the Anvilian deposits suggests that eustatic sea level during the Anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression; by correlation with the marine oxygen-isotope record, the transgression probably dates to stage 11 at 410,000 yr, and the Nome River glaciation is younger still. Analyses of floor altitudes of presumed Nome River cirques indicate that the Nome River regional snowline depression was at least twice that of the maximum late Wisconsin. The cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern Alaska during the middle Pleistocene.
Using CHIRP subbottom profiling across the Chukchi shelf, offshore NW Alaska, we observed a large incised valley that measures tens of kilometers in width. The valley appears to have been repeatedly excavated during sea level lowering; however, the two most recent incisions appear to have been downcut during the last sea level rise, suggesting an increase in the volume of discharge. Modern drainage from the northwestern Alaskan margin is dominated by small, low-discharge rivers that do not appear to be large enough to have carved the offshore drainage. The renewed downcutting and incision during the deglaciation and consequent base level rise implies there must have been an additional source of discharge. Paleoprecipitation during deglaciation is predicted to be at least 10% less than modern precipitation and thus cannot account for the higher discharge to the shelf. Glacial meltwater is the most likely source for the increased discharge.
The Pekulney Mountains and adjacent Tanyurer River valley are key regions for examining the nature of glaciation across much of northeast Russia. Twelve new cosmogenic isotope ages and 14 new radiocarbon ages in concert with morphometric analyses and terrace stratigraphy constrain the timing of glaciation in this region of central Chukotka. The Sartan Glaciation (Last Glacial Maximum) was limited in extent in the Pekulney Mountains and dates to ∼20,000 yr ago. Cosmogenic isotope ages > 30,000 yr as well as non-finite radiocarbon ages imply an estimated age no younger than the Zyryan Glaciation (early Wisconsinan) for large sets of moraines found in the central Tanyurer Valley. Slope angles on these loess-mantled ridges are less than a few degrees and crest widths are an order of magnitude greater than those found on the younger Sartan moraines. The most extensive moraines in the lower Tanyurer Valley are most subdued implying an even older, probable middle Pleistocene age. This research provides direct field evidence against Grosswald’s Beringian ice-sheet hypothesis.
The last interglacial high sea-level stand, the Pelukian transgression of isotope substage 5e, is recorded along the western and northern coasts of Alaska by discontinuous but clearly traceable marine terraces and coastal landforms up to about 10 m altitude. The stratigraphy indicates that sea level reached this altitude only once during the last interglacial cycle. From the type area at Nome, to St. Lawrence Island in the Bering Sea, to the eastern limit of the Beaufort Sea, Pelukian deposits contain extralimital faunas indicating that coastal waters were warmer than present. Amino acid ratios in molluscs from these deposits decrease to the north toward Barrow, consistent with the modern regional temperature gradient. Fossil assemblages at Nome and St. Lawrence Island suggest that the winter sea-ice limit was north of Bering Strait, at least 800 km north of its present position, and the Bering Sea was perennially ice-free. Microfauna in Pelukian sediments recovered from boreholes indicate that Atlantic water may have been present on the shallow Beaufort Shelf, suggesting that the Arctic Ocean was not stratified and the Arctic sea-ice cover was not perennial for some period. In coastal regions of western Alaska, spruce woodlands extended westward beyond their modern range and in northern Alaska, on the Arctic Coastal Plain, spruce groves may have entered the upper Colville River basin. The Flaxman Member of the Gubik Formation on the Alaskan Arctic Coastal Plain was deposited during marine isotope substage 5a and records the breakup of an intra-stage 5 ice sheet over northwestern Keewatin.
Two previously undocumented Pleistocene marine transgressions on Wrangel Island, northeastern Siberia, question the presence of an East Siberian or Beringian ice sheet during the last glacial maximum (LGM). The Tundrovayan Transgression (459,000–780,000 yr B.P.) is represented by raised marine deposits and landforms 15–41 m asl located up to 18 km inland. The presence of high sea level 64,000–73,000 yr ago (the Krasny Flagian Transgression) is preserved in deposits and landforms 4–7 m asl in the Krasny Flag valley. These deposits and landforms were mapped, dated, and described using amino acid geochronology, radiocarbon, optically stimulated luminescence, electron spin resonance, oxygen isotopes, micropaleontology, paleomagnetism, and grain sizes. The marine deposits are eustatic and not isostatic in origin. All marine deposits on Wrangel Island predate the LGM, indicating that neither Wrangel Island nor the East Siberian or Chukchi Seas experienced extensive glaciation over the last 64,000 yr.
Aspartic acid (Asp) racemization occurs at a significantly higher rate than isoleucine epimerization and consequently provides better temporal resolution of Arctic marine deposits (from Alaska, Spitsbergen, and Baffin Island). Heating experiments (at 100°C) on the bivalves Mya and Hiatella show the Asp racemization rate decreases with increasing D/L values, as is typical for biogenic carbonates. Based on these experimental racemization rates and rates determined from racemization of samples radiocarbon dated to ca. 10,000–12,000 yr B.P., activation energies for Mya and Hiatella are estimated to be 30.6 and 30.0 kcal/mol, respectively, for Asp racemization, and 29.0 and 29.5 for isoleucine epimerization. Analysis of a time series of Plio–Pleistocene Hiatella from the north coast of Alaska shows that last-interglacial mollusks can be readily distinguished from modern samples by Asp but not by isoleucine. D/L Asp values indicate a younger age for the Fishcreekian transgression than does isoleucine epimerization. For Spitsbergen, D/L Asp shows a slight age difference (ca. 12,000 yr) between two units of the “episode B” interstadial and suggests that the age of these units may be closer to 65,000 than to 80,000 yr B.P., two possible ages suggested by other evidence. The age of the Loks Land Interstadial on Baffin Island is likely to be greater than that indicated by radiocarbon ages. Within deposits from each region, D/L Asp values are less variable among individual shells than isoleucine epimerization values. This may indicate better reliability of Asp for geochronology.
Tidewater glaciers deposit sediment at their terminus, thereby reducing the relative water depth. Reduced water depth can lead to increased glacier stability through decreased rates of iceberg calving, glacier thinning and submarine melting. Here we investigate sedimentation processes at the termini of Kronebreen and Kongsvegen, Svalbard. We mapped the fjord floor bathymetry in August 2009 and calculate sedimentation rates based on our bathymetry and that from a similar study in 2005. A grounding-line fan is developing near the current position of the subglacial stream. An older, abandoned grounding-line fan that likely formed between ∼1987 and 2001 is degrading near the middle of the ice front. Our findings indicate that sediment gravity flows reduce the height of the sediment mound forming at the glacier terminus. The future impact of glacimarine sedimentation processes on glacier stability will depend on the net balance between the observed gravity flows and sediment deposition.
Baldwin Peninsula, northwest Alaska, is a middle Pleistocene push-moraine complex composed of marine, fluvial, and glaciogenic sediments. The peninsula was formed by three ice lobes emanating from the De Long and Baird mountains and the Selawik Lowlands in the southwest Brooks Range during the Anaktuvuk River glaciation. This glaciation was nearly an order of magnitude more areally extensive than late Pleistocene glaciations in the same region and occurred c. 500 to 600 ka B.P. based on paleomagnetism, and amino-stratigraphic and morphostratigraphic correlations with other numerically-dated northwest Alaskan deposits.
Extensive deposits of massive and laminated clayey silt with striated, faceted stones indicate that local sea level was high as glacial ice reached its maximum extent. Glacio-isostasy does not seem to have been important in maintaining a high relative sea level; therefore we infer that eustatic sea level remained high during ice advance. For this to occur, high latitude glaciation must have preceded the build-up of ice in lower latitudes. The source of moisture for such massive glaciation may have come from submerged Bering and Chukchi shelves, enhanced flow of North Pacific/southern Bering Sea winter storms, reduced intensity of the winter Arctic High, or a combination thereof.
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