European climates

Frenzel, B., Pfister, C. & Gläser, B. (eds.) 1992: European Climate Reconstructed from Documentary Data: Methods and Results.     

Intriguing climatic shifts in a 90 kyr old lake record from northern Russia

A 22 m long sediment core from Lake Yamozero on the Timan Ridge in northern Russia has provided evidence of intriguing climatic shifts during the last glacial cycle. An overall shallowing of the lake is reflected in the lower part of the cores, where pollen indicates a transition from glacial steppe vegetation to interstadial shrub-tundra. These beds are capped by a well-defined layer of compact clay deposited in relatively deep water, where pollen shows surrounding spruce forests and warmer-than-present summer temperatures. The most conservative interpretation is that this unit represents the last interglacial period. However, a series of Optical Stimulated Luminescence (OSL) dates suggests that it corresponds with the Early Weichselian Odderade interstadial (MIS 5a). This would imply that the Odderade interstadial was just as warm as a normal interglacial in this continental part of northern Europe. If correct, then pollen analysis, as a correlation tool, is less straightforward and the definition of an interglacial is more complex than previously thought. We discuss the validity and possible systematic errors of the OSL dates on which this age model is based, but conclude they really indicate a MIS 5a age for the warm period. Above the clay is an unconformity, most likely reflecting a period of subaerial exposure implying dry conditions. Deposition of silt under fluctuating cold climates in the Middle Weichselian continued until a second gap in the record at c . 40 kyr BP. The lake basin started to fill up again around 18 kyr BP.     

The Skaergaard Layered Series. Part VI. Excluded Trace Elements

In contrast to the smooth trends of major elements and mineralcompositions, the excluded trace elements in the SkaergaardLayered Series have an irregular distribution that does notconform to the normal trends of Rayleigh-type fractionation.Their concentrations are about constant or even decline throughthe Lower and Middle Zones before increasing sharply to reachmaximum concentrations 100–200 m above the Sandwich Horizon.As in the case of included elements, the relative concentrationsof excluded elements in coexisting phases deviate widely fromthose predicted by experimentally determined partition coefficientsunder presumed magmatic conditions. This is seen most clearlyin the immiscible melanogranophyres and conjugate ferrogabbros.Although the major elements conform to the experimentally determinedrelations for immiscible liquids, the trace elements do not;they follow a totally independent trend. The abrupt increasein the concentrations of excluded elements in the upper partof the intrusion could plausibly be attributed to an additionof new magma or to a density inversion that resulted in upwardmigration of a late liquid or fluid, but these possibilitiesare inconsistent with the compositional and spatial relationsof the upper parts of the intrusion. Although a late residualliquid certainly migrated upward, the most likely explanationfor the observed distribution of excluded elements is that thepartition coefficients were altered by volatile components,which gradually increased during the early stages of crystallizationthen began to exsolve near the top of the Middle Zone. KEY WORDS: igneous differentiation; Skaergaard intrusion     

高浓度黏性泥石流堆积层理结构中筛积层和粗化层形成机理的研究

根据高浓度黏性泥石流的观测资料,应用不同类型黏性泥石流的流变特性和流体结构的分类指标-流核比,对高浓度黏性泥石流堆积中的混杂层、筛积层和粗化层理结构进行了成因差异分析。侧重分析了砾石质点在高浓度黏性泥石流蠕动流场中作向上垂直运动的魏森伯效应,并在表面富集形成了筛积层的成因,及其它与粗化层结构的区别。     

The retreat of the Barents Sea Ice Sheet on the western Svalbard margin

The deglaciation of the continental shelf to the west of Spitsbergen and the main fjord, Isfjorden. is discussed based on sub-bottom seismic records and scdirncnt cores. The sea lloor on the shelf to the west of Isfjorden is underlain by less than 2 m of glaciomarine sediments over a firm diamicton interpreted as till. In central Isfjordcn up to 10 m of deglaciation sediments were recorded, whereas in cores from the innermost tributary, Billefjorden, less than a meter of ice proximal sediments was recognized between the till and the 'normal' Holocene marine sediments. We conclude that the Barents Sea Ice Sheet terminated along the shelf break during the Late Weichselian glacial maximum. Radiocarbon dates from thc glaciomarine sediments above the till indicate a stepwise deglaciation. Apparently the ice front rctrcatcd from the outermost shelf around 14. 8 ka A dramatic increase in the flux of line-grained glaciomarine sediments around 13 ka is assumed to reflect increased melting and/or current activity due to a climatic warming. This second stage of deglaciation was intcrruptcd by a glacial readvance culminating on the mid-shelf area shortly after 12.4 ka. The glacial readvance, which is correlated with a simultaneous readvance of the Fennoscundian ice sheet along the western coast of Norway, is attributed to the so-called 'Older Dryas' cooling event in the North Atlantic region. Following this glacial readvance the outer part of Isljorden became rapidly deglaciated around 12.3 ka. During the Younger Dryas the inner fjord branches were occupied by large outlet glaciers and possibly the ice liont terminated far out in the main fjord. The remnants of the Harcnts Sea Ice Shcet melted quickly away as a response to the Holocene warming around 10 ka.     

Experimental turbidity currents entering density-stratified water: analogues for turbidites in Mediterranean hypersaline basins

Experimental turbidity currents entering two-layer density-stratified water behave differently from similar currents flowing over the same topography into non-stratified water. Experiments were designed as analogues for flows entering Mediterranean hypersaline pools. In both the hypersaline pools and the experiments, the water density changes abruptly across a pycnocline. Turbidity currents generated on a platform at the level of the pycnocline behaved in one of three ways as they flowed from the platform into deeper stratified water. (1) When the bulk density of the current was less than the dense water layer, the current spread at the pycnocline. The head of the current advanced rapidly when it lost contact with the bed. Grains settling out of the current fell through the dense water layer forming an extensive deposit. In nature this behaviour will lead to ‘turbidites’ with sharp but non-erosive bases, strongly developed grading and no traction features. (2) When the bulk density of the current was greater than the dense water layer, the current continued as an underflow, plunging into the deeper water. Sedimentation lowered the bulk density of the current and the low-density interstitial fluid caused the head to loft. Low-density interstitial fluid convected from the body of the current, lofting particles into the water column. These particles were hydraulically sorted during upward transport and subsequent settling to the floor. The resulting turbidites had a more limited extent than the deposits of either non-lofting underflows or interflows. By inference from the experiments, natural deposits of this type may have local (proximal) erosion and traction features at the base and strongly graded tops. (3) In some of the currents with high bulk density, the rising turbid water reached the pycnocline and spread at that level as a secondary interflow. The tail of the turbidity current, which was less dense than the head and body of the current, flowed above the pycnocline adding momentum to the secondary interflow. The thin non-erosive graded deposit from the secondary interflow may extend beyond the deposits of the primary underflow. In all three cases (but more pronounced in cases 2 and 3) the interaction of the current with the pycnocline displaced that surface and generated a wave that was reflected back and forth from each end of the pool. The waves remobilized sediment on the ramp.