Flora, vegetation and climate at Sokli, northeastern Fennoscandia, during the Weichselian Middle Pleniglacial

A 2 m thick laminated lacustrine deposit of silt and clay recovered from the high-latitudinal site at Sokli (northern Finland) provides a unique mid-Weichselian fossil record for Fennoscandia. High-resolution botanical and zoological analyses of the lacustrine deposit allow detailed reconstruction of the regional vegetational development and of the history of the lake and the wetland ecosystem within the Sokli basin during the early part of the Weichselian Middle Pleniglacial (=equivalent to Marine Isotope Stage (MIS) 3). The inferred terrestrial vegetation represented by the Sokli MIS 3 sequence (so-called Tulppio Interstadial) was probably low-arctic tundra, treeless but with shrub elements including juniper, willow, dwarf birch, ericoids, lycopods and a rich herb flora with a variety of arctic–alpine taxa and heliophilous, pioneer elements. The presence of herbs such as Rubus chamaemorus, Epilobium palustre, Potentilla palustris and Sphagnum, Drepanocladus and other mosses suggests that the lake was fringed by wet meadows and peatlands or peaty telmatic communities. The distributional ranges of pine and tree birch were probably only a few hundred kilometres south or southeast of Sokli. This is concordant with evidence for the presence of boreal tree taxa during the MIS 3 in the Baltic countries and further east in Europe, but contradicts with the commonly inferred treeless tundra or grass-dominated steppe conditions in central Europe.     

Pleistocene explosions and pyroclastic currents in west-central Anatolia

The discovery of unexpectedly young tephra, with K-Ar dates of 1.50 ± 0.18 and 1.38 ± 0.13 Ma BP, in the Lake Eirdir area sheds new light on the time-span and magnitude of explosive volcanism in west-central Anatolia and on the physical behaviour of pyroclastic currents. The Gölcük maar near Isparta, widely thought to have ceased its activity in Pliocene time, was apparently still erupting with strong explosions in the Early Pleistocene. The Eirdir tephra, 34 km NE from the volcano, indicate deposition from powerful pyroclastic surges directed by the topography. The depositing currents were fully turbulent and carried unusually large lithic clasts, up to 2–3 cm in diameter, in suspension, which is incompatible with the existing physical model for pyroclastic currents. Based on the tephra characteristics, a conceptual model of the pyroclastic current's segmentation, or lateral dynamic partition, is suggested and used further to explain the current's specific response to an encountered topographic barrier. The widely held concept of a pyroclastic current's vertical separation, or sharp rheological decoupling, is thought to describe a common secondary phenomenon.     

River sections at the Byzovaya Palaeolithic site – keyholes into the late Quaternary of northern European Russia

Heggen, H. P., Svendsen, J. I. & Mangerud, J. 2009: River sections at the Byzovaya Palaeolithic site – keyholes into the late Quaternary of northern European Russia. Boreas, 10.1111/j.1502‐3885.2009.00109.x. ISSN 0300‐9483. The geological history of northern European Russia over the past two glacial cycles is reconstructed from the stratigraphy in river bluffs along the upper reaches of the Pechora River. From a till bed near the base of the sections it is inferred that the Barents–Kara Ice Sheet covered the area during the late Saalian (MIS 6). After deglaciation, and prior to the last interglacial, the area was flooded by an ice‐dammed lake, suggesting that the Pechora Basin was blocked by a subsequent ice advance at the very end of the Saalian. Ice‐wedge casts and periglacial sediments reflect a pronounced cooling with formation of permafrost during the Early Weichselian (MIS 5d). An overlying thick sequence of shallow lacustrine sediments accumulated in the ice‐dammed Lake Komi, formed by the advancing Barents–Kara Ice Sheet 80–100 kyr BP (MIS 5b?). Following drainage of the lake, many of the older formations were eroded by fluvial activity. Animal remains found together with palaeolithic artefacts within debrisflow sediments at the base of one of the incised gullies yielded radiocarbon ages around 28 000–30 000 ¹⁴C yr BP (33–34 cal. kyr BP). The surface with traces of human activities was subsequently covered by aeolian sediments representing the northern extension of the European belt of periglacial coversand that accumulated in the cold and dry climate during the late Weichselian (MIS 2). The results of this work confirm the assumption that the last shelf‐centred ice sheet that covered this part of Russia occurred during the late Saalian (MIS 6), but that this glaciation was followed by a younger and less extensive ice advance that has not been described before. There are no indications that local glaciers originating in the Ural Mountains reached the Pechora River valley throughout the last two glacial cycles.     

Surface form of the south-western sector of the last Kara Sea Ice Sheet

Recent results concerning the extent of the last Weichselian (Valdaian) Kara Sea Ice Sheet in the area around the Polar Urals and the north-eastern Russian Plain allow reconstruction of the surface form of this part of the ice sheet by using a combination of moraine-ridge elevation data and ice-flow indicators. The resulting reconstruction suggests a thin ice sheet with a pronounced lowering of surface gradient at the transition from bedrock substrate around the Urals to a substrate consisting of unconsolidated sediments in the Pechora Basin. Comparison with similar reconstructions from along the southern and north-western parts of the Laurentide Ice Sheet margin, for which a deformable-bed model of glacier dynamics has been proposed, shows strong similarities in surface gradients and ice thicknesses as well in overall sedimentological and morphological characteristics of the associated basal till-deposits. This suggests comparable styles of glacier dynamics for the two ice sheets. If this first approximation of the Kara Sea Ice Sheet surface form is correct, it can be postulated that at least the south-western part of the ice sheet was much more mobile and dynamic than previously expected.     

Numerical simulations of Rossby–Haurwitz waves

Simulations of Rossby–Haurwitz waves have been carried out using four different high‐resolution numerical shallow water models: a spectral model, two semi‐Langrangian models predicting wind components and potential vorticity respectively, and a finite‐volume model on a hexagonal–icosahedral grid. The simulations show that (i) unlike the nondivergent case, the shallow water Rossby–Haurwitz wave locally generates small‐scale features and so has a potential enstrophy cascade, and (ii) contrary to common belief, the zonal wavenumber 4 Rossby–Haurwitz wave is dynamically unstable and will eventually break down if initially perturbed. Implications of these results for the use of the Rossby–Haurwitz wave as a numerical model test case are discussed. The four models tested give very similar results, giving confidence in the accuracy and robustness of the results. The most noticeable difference between the models is that truncation errors in the hexagonal–icosahedral grid model excite the Rossby–Haurwitz wave instability, causing the wave to break down quickly, whereas for the other models in the configurations tested the instability is excited only by roundoff error at worst, and the Rossby–Haurwitz wave breaks down much more slowly or not at all.     

Synoptic climatology of Northern Hemisphere available potential energy collapses

Three recurring regional patterns of extratropical baroclinic development associated with synoptic‐scale collapses of Northern Hemisphere available potential energy (APE) are identified using a 1979–95 time series derived from the National Centers for Environmental Prediction (NCEP) reanalysis. A time series of the intraseasonal signal (from 1.6 to 180 days) of APE is used to discern an average cycle of approximately 3 days in the APE generation rate d A /d t (referred to as APE depletion rate if negative). An APE depletion event is defined as a fall and subsequent rise in the time series of d A /d t associated with this cycle. We define synoptic‐scale APE collapses as APE depletion events with maximum depletion rates (d A /d t _min) and maximum APE falls (Δ A _min) of less than −0.145× 10⁶ J m⁻² day⁻¹ and −0.280×10⁶ J m⁻², respectively. All are cold season (15 October–15 April) events. APE collapses were classified based on the evolution of regional synoptic patterns during the 2 days centered at the time of d A /d t _min . All are accompanied by deep tropospheric warming. The west Pacific warm surge (Type A) is driven by cyclogenesis over Japan and anticyclogenesis over the west‐central North Pacific. The Bering warm surge (Type B) is associated with an intense southerly flow across the Bering Strait brought on by cyclogenesis near the Kamchatka Peninsula and an intense anticyclone over Alaska. The Atlantic Canada warm surge (Type C) is characterized by an onshore flow of warm air ahead of a continental storm track over eastern North America.