a b s t r a c tLate Quaternary palaeoenvironments of the Siberian Arctic were reconstructed by combining data from several fossil bioindicators (pollen, plant macro-fossils, ostracods, insects, and mammal bones) with sedimentological and cryolithological data from permafrost deposits. The record mirrors the environmental history of Beringia and covers glacial/interglacial and stadial/interstadial climate variations with a focus on the Middle Weichselian interstadial (50-32 kyr BP). The late Pleistocene to Holocene sequence on Kurungnakh Island reflects the development of periglacial landscapes under changing sedimentation regimes which were meandering fluvial during the Early Weichselian, colluvial or proluvial on gently inclined plaines during the Middle and Late Weichselian, and thermokarstaffected during the Holocene. Palaeoecological records indicate the existence of tundra-steppe vegetation under cold continental climate conditions during the Middle Weichselian interstadial. Due to sedimentation gaps in the sequence between 32 and 17 kyr BP and 17 and 8 kyr BP, the Late Weichselian stadial is incompletely represented in the studied outcrops. Nevertheless, by several palaeoecological indications arctic tundra-steppe vegetation under extremely cold-arid conditions prevailed during the late Pleistocene. The tundra-steppe disappeared completely due to lasting paludification during the Holocene. Initially subarctic shrub tundra formed, which later retreated in course of the late Holocene cooling.

a b s t r a c tPerennially-frozen deposits are considered as excellent paleoenvironmental archives similar to lacustrine, deep marine, and glacier records because of the long-term and good preservation of fossil records under stable permafrost conditions. A permafrost tunnel in the Vault Creek Valley (Chatanika River Valley, near Fairbanks) exposes a sequence of frozen deposits and ground ice that provides a comprehensive set of proxies to reconstruct the late Quaternary environmental history of Interior Alaska. The multi-proxy approach includes different dating techniques (radiocarbon-accelerator mass spectrometry [AMS 14 The studied sequence consists of 36-m-thick late Quaternary deposits above schistose bedrock. Main portions of the sequence accumulated during the early and middle Wisconsin periods. The lowermost unit A consists of about 9-m-thick ice-bonded fluvial gravels with sand and peat lenses. A late Sangamon (MIS 5a) age of unit A is assumed. Spruce forest with birch, larch, and some shrubby alder dominated the vegetation. High presence of Sphagnum spores and Cyperaceae pollen points to mires in the Vault Creek Valley. The overlying unit B consists of 10-m-thick alternating fluvial gravels, loess-like silt, and sand layers, penetrated by small ice wedges. OSL dates support a stadial early Wisconsin (MIS 4) age of unit B. Pollen and plant macrofossil data point to spruce forests with some birch interspersed with wetlands around the site. The following unit C is composed of 15-m-thick ice-rich loess-like and organic-rich silt with fossil bones and large ice wedges. Unit C formed during the interstadial mid-Wisconsin (MIS 3) and stadial late Wisconsin (MIS 2) as indicated by radiocarbon ages. Post-depositional slope processes significantly deformed both, ground ice and sediments of unit C. Pollen data show that spruce forests and wetlands dominated the area. The macrofossil remains of Picea, Larix, and Alnus incana ssp. tenuifolia also prove the existence of boreal coniferous forests during the mid-Wisconsin interstadial, which were replaced by treeless tundra-steppe vegetation during the late Wisconsin stadial. Unit C is discordantly overlain by the 2-m-thick late Holocene deposits of unit D. The pollen record of unit D indicates boreal forest vegetation similar to the modern one.The permafrost record from the Vault Creek tunnel reflects more than 90 ka of periglacial landscape dynamics triggered by fluvial and eolian accumulation, and formation of ice-wedge polygons and postdepositional deformation by slope processes. The record represents a typical Wisconsin valley-bottom facies in Central Alaska.

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The bulk diffusivity of dissolved carbon dioxide (CO 2 and CO 3 2-) in NaAlSi 3 O 8 + nNa 2 O (n = 0-6.87 wt%) and in NaAlSi 3 O 8 + nH 2 O (n = 0-2 wt%) melts was investigated at 1523 K and 0.5 GPa using the diffusion couple technique. CO 2 contents of the starting glass pairs varied between 0 and 0.2 wt%. Symmetrical concentration-distance profiles of bulk CO 2 were determined by infrared spectroscopy. An error function was fitted to the profiles to obtain apparent chemical diffusion coefficients of bulk CO 2 . In the investigated compositional range, the diffusivity of bulk CO 2 increases exponentially with Na 2 O and H 2 O content and thus exponentially with the ratio of nonbridging oxygen atoms per tetrahedral cations (NBO/ T). The bulk CO 2 diffusivity increases from logD CO 2 = -11.38 (D CO 2 in m 2 /s) in NaAlSi 3 O 8 melt to logD CO 2 = -10.92 in NaAlSi 3 O 8 melts containing 6.87 wt% Na 2 O excess, and to logD CO 2 = -10.91 in NaAlSi 3 O 8 melts containing 2 wt% H 2 O. These data imply that either: (1) the diffusivities of the CO 2 species (molecular CO 2 and CO 3 2-) are very similar, or (2) the speciation of CO 2 in the quenched glasses is very different from the speciation in the melt.

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