Late Quaternary Environments, Denali National Park and Preserve, Alaska

Late Quaternary pollen, plant macrofossils, and insect fossils were studied from sites along three rivers in the foothills north of the Alaska Range in Denali National Park and Preserve. The aim was to carry out a reconnaissance of late Quaternary organic sediments in the region, emphasizing the mid-Wisconsin, or Boutellier interstadial interval. Samples of probable early-to mid-Boutellier age (ca. 60 000 to 40 000 B.P.) from Unit 2 at the Toklat High Bluffs site indicate open boreal woodland with dense alder shrub vegetation. Organic Unit 1 at the Foraker River Slump site indicates open taiga with shrubs of probable Boutellier age. Fossil evidence from the youngest horizon in this unit indicates graminoid tundra environments, marking the transition from interstadial to late Wisconsin glacial environments. Early Holocene samples from the Foraker exposures suggest birch shrub tundra; coniferous forest apparently became established only after 6500 B.P. Local variations in forest composition at the Foraker and Sushana sites were probably the result of disturbances, such as fire.Les grains de pollen et les pièces macrofossiles de plantes et d'insectes, caractérisant le quaternaire tardif, ont été étudiés dans des sites localisés le long de trois rivières coulant à proximité du versant nord de l'Alaska Range, dans le parc national et la réserve Denali. Cette étude porte essentiellement sur les sédiments organiques de la région, plus particulièrement ceux caractérisant l'intervalle interstadiaire Boutellier (milieu de la période du Wisconsin). Les échantillons récoltés dans l'unité numéro deux du site Toklat High Bluffs et datant probablement du début ou du milieu de l'intervalle Boutellier (60 000 à 40 000 BP) témoignent de la présence d'une forêt boréale ouverte avec couvert arbustif dense composé essentiellement d'aulnes. Les macrorestes et le pollen de l'unité organique numéro un du site Foraker River Slump témoignent, pour leur part, de la présence d'une taïga ouverte parsemée d'arbustes. Selon toute vraisemblance, cette taïga daterait de l'intervalle Boutellier. Les macrorestes et le pollen contenus dans l'horizon le plus jeune de cette unité indiquent que le paysage était constitué d'une toundra herbacée, marquant ainsi une transition entre la végétation caractérisant l'interstade Boutellier et celle de la fin de la période wisconsinienne. Les échantillons datant du début de la période holocène du site Foraker suggèrent la présence d'une toundra arbustive composée surtout de bouleaux. La forêt coniférienne ne se serait établie qu'après 6500 BP. Les différences observées au niveau de la composition forestière des sites Foraker et Sushana résultent probablement de l'impact de perturbations, tel le feu

Fossil Pollen and Insect Evidence for Postglacial Environmental Conditions, Nushagak and Holitna Lowland Regions, Southwest Alaska

This paper discusses the results of pollen and insect analyses of postglacial samples from the Nushagak and Holitna lowlands, southwest Alaska. Although radiocarbon dating control is poor, the samples can be arranged in a relative-age sequence based on stratigraphic occurrence. The fossil pollen data record the regional transition from a late-glacial dry graminoid tundra through the postglacial Birch, Alder, and Spruce zones. The lack of xeric insect species in the early postglacial suggests that the lowlands of southwest Alaska experienced maritime climatic conditions, in contrast to the interior. Rapid climatic warming is subsequently indicated by the fossil insect data, although the arrival of alder in the region postdates 8500 yr BP. There is no evidence for coniferous forest in the Nushagak lowland at any time in the postglacial, although spruce arrived in the Holitna lowland in the mid-postglacial.Key words: pollen analysis, fossil insects, paleoenvironments, postglacial, southwest AlaskaRÉSUMÉ. Cet article traite des résultats d’analyses polliniques et d’insectes d’échantillons postglaciaires venant des basses-terres de Nushagak et d’Holitna, dans le sud-ouest de l’Alaska. Bien que le contrôle de la datation par le radiocarbone soit médiocre, les échantillons peuvent être classés en ordre d’âger elatif, d’après leur occurrence stratigraphique. Les données de pollens fossiles traduisent le passage de la région d’une toundra sèche de graminées datant de la fin de l’époque glaciaire, à des zones postglaciaires de bouleaux, d’aulnes et d’épicéas. L’absence d’espèces d’insectes xérophiles au début du postglaciaire donne à penser que les basses-terres du sud-ouest de l’Alaska ont connu des conditions climatiques maritimes, contrairement à l’intérieur. Les données sur les insectes fossiles permettent donc d’établir qu’il y a eu un réchauffement climatique rapide, bien que l’arrivée de l’aulne dans la région soit postérieure à 8500 ans avant le présent. On n’a pas de preuve de l’existence d’une forêt de coniferes dans la basse-terre de Nushagak à un moment quelconque du postglaciaire, bien que l’épicéa fasse son arrivée dans la basse-terre d’Holitna au milieu du postglaciaire.Mots clés: analyse pollinique, insectes fossiles, paléoenvironnements, postglaciaire, sud-ouest de l’Alask

The impact of the Kasatochi eruption on the Moon's illumination during the August 2008 lunar eclipse

The Moon's changeable aspect during a lunar eclipse is largely attributable to variations in the refracted unscattered sunlight absorbed by the terrestrial atmosphere that occur as the satellite crosses the Earth's shadow. The contribution to the Moon's aspect from sunlight scattered at the Earth's terminator is generally deemed minor. However, our analysis of a published spectrum of the 16 August 2008 lunar eclipse shows that diffuse sunlight is a major component of the measured spectrum at wavelengths shorter than 600 nm. The conclusion is supported by two distinct features, namely the spectrum's tail at short wavelengths and the unequal absorption by an oxygen collisional complex at two nearby bands. Our findings are consistent with the presence of the volcanic cloud reported at high northern latitudes following the 7-8 August 2008 eruption in Alaska of the Kasatochi volcano. The cloud both attenuates the unscattered sunlight and enhances moderately the scattered component, thus modifying the contrast between the two contributions.Comment: Accepted for publication in Geophysical Research Letter

Pressurized groundwater outflow experiments and numerical modeling for outflow channels on Mars

The landscape of Mars shows incised channels that often appear abruptly in the landscape, suggesting a groundwater source. However, groundwater outflow processes are unable to explain the reconstructed peak discharges of the largest outflow channels based on their morphology. Therefore, there is a disconnect between groundwater outflow processes and the resulting morphology. Using a combined approach with experiments and numerical modeling, we examine outflow processes that result from pressurized groundwater. We use a large sandbox flume, where we apply a range of groundwater pressures at the base of a layer of sediment. Our experiments show that different pressures result in distinct outflow processes and resulting morphologies. Low groundwater pressure results in seepage, forming a shallow surface lake and a channel when the lake overflows. At intermediate groundwater pressures, fissures form and groundwater flows out more rapidly. At even higher pressures, the groundwater initially collects in a subsurface reservoir that grows due to flexural deformation of the surface. When this reservoir collapses, a large volume of water is released to the surface. We numerically model the ability of these processes to produce floods on Mars and compare the results to discharge estimates based on previous morphological studies. We show that groundwater seepage and fissure outflow are insufficient to explain the formation of large outflow channels from a single event. Instead, formation of a flexure-induced subsurface reservoir and subsequent collapse generates large floods that can explain the observed morphologies of the largest outflow channels on Mars and their source areas