Boreas最新文献

The low field magnetic susceptibility (χ_LF) measured in the 1116-m-long Dévaványa core (Pannonian Basin) is a quasi-continuous record of the Plio–Pleistocene Mid-European mountain permafrost development. The continuity of fluvial conditions is confirmed by seismic data, and the detrital origin of magnetite is indicated by frequency-dependent susceptibility measurements, scanning electron microscope, and hysteresis investigations. The χ_LF record is correlated to the δ¹⁸O curve (LR04) supported by palaeomagnetic data. The colour of samples documents precession and obliquity cycles in local facies variations, but the χ_LF indicates the dominance of 100-ka eccentricity cycles in the linked mountainous permafrost events. Comparison with orbital solutions revealed that the long-term development of permafrost occurs as a result of amplitude modulation of the 100-ka eccentricity cycles. Increases in amplitude of the 100-ka cycles inhibits permafrost development due to shortened winters. Thus, if extremes are present, the permafrost regions are limited or disappear, but if the 100-ka eccentricity cycles are attenuated, permanent frost can extend into the temperate zone. This amplitude modulation may also be responsible for the early glaciations during the Pliocene, for the intensification of Northern Hemisphere glaciation, foreshadows cooling in the forthcoming 405-ka term, and allows the change from 41-ka cycles to 100-ka ones in the Mid-Pleistocene Transition to be explained. The 41-ka cycles are the result of obliquity-controlled changes close to the polar cycles, while 100-ka cycles occur when the amplitude attenuation of the 100-ka eccentricity cycles enables extended glaciations that suppress the regular 41-ka cycles. Higher mountains in the catchments enable higher resolution of permafrost records documenting even smaller glaciations. However, the similarities in the overall trends in χ_LF records of catchment areas with 1500-m difference in their altitude is a potential counter-argument when considering the role of tectonic elevations in the expansions of mountainous permafrost.

Southeastern France occupies a key biogeographical position on the lower Rhone corridor, at the interface between southern and northern Europe. It is also at the heart of long-standing ecological debates about the respective roles of natural and human drivers in shaping the Mediterranean landscape. Molluscan analysis may represent a valuable contribution to this issue. In this paper, land snail assemblages from three calcareous tufa deposits in the Luberon mountain were used to reconstruct Lateglacial and Holocene palaeoenvironments. The Lateglacial communities only differ from the Pleniglacial ones by the development of hygrophilic snails. We note a high moisture budget at the end of the Lateglacial Interstadial (LGI). A patchy, steppe landscape is attested. A time lag in recolonization by woodland species during the LGI is conceivable. The postglacial woodland assemblages then trace a laborious reassembly of forest snail communities. It takes place mainly between c. 8000 and c. 6600 cal. a BP with spatial disparities and delayed recruitments. The French Mediterranean region has not benefitted from the macrorefugia that it would have sheltered or with which it would have been close. Woodlands, however, appear progressively more closed and complex c. 8000 cal. a BP. They reached their optimum c. 7500 to 7400 cal. a BP although their canopy seems to have stayed quite open. Anthropization remained weak during the Early Neolithic. A significant woodland opening is observed in the Early–Middle Neolithic. Human impact becomes clear from the Late Neolithic. Nevertheless, there are substantial differences compared with Basse-Provence, where a more marked openness of the landscape from 7000 cal. a BP was accompanied by the development of Mediterranean synanthropic snails. The molluscan successions of the Luberon mountain should be a reference for the development of the Lateglacial and Holocene malacofauna in the SE of France, at the northern boundary of the western Mediterranean domain.

We, the editors, are grateful to all reviewers for their help in processing volume 53 of Boreas.

Jan A. Piotrowski

Nicolaj Krog Larsen

The Vimmerby Moraine is the only significant ice-marginal moraine on the eastern side of southern Sweden, but no detailed studies exist on its formation during the final deglaciation of the Fennoscandian Ice Sheet. Through ground-penetrating radar surveys and detailed sediment logging, we provide evidence for an active, oscillating ice margin during the formation of the Vimmerby Moraine, suggesting that the deglaciation of the South Swedish Uplands was, at least in some regions, dynamic. Ground-penetrating radar surveys enabled imaging of internal sediment and delineation of the bedrock surface. These were complemented by common mid-point surveys and sediment logging, as well as lithofacies analysis at three exposures in agravel pit. This approach revealed multiple subglacial till units partially separated by intercalated glacifluvial deposits. The glacifluvial sediments exhibit evidence of glaciotectonism, suggesting active overriding by the last ice sheet. Further evidence of an active ice margin is provided by the ground-penetrating radar profiles collected perpendicular to the moraine crest. These contain a series of northerly dipping reflectors, which we interpret as evidence of repeated basal freeze-on and melt-out of sediment slabs during ice margin oscillations, as has been observed at contemporary glacier margins in Iceland, Norway, and the Alps. The data presented here demonstrate that the last Fennoscandian Ice Sheet remained active around the time of the Vimmerby Moraine formation. This work highlights the benefits of including detailed sediment logging and near-surface geophysical surveys in the interpretation of deglaciation dynamics.

As in most chalk river valleys in NW Europe, the sedimentary fill of the Somme valley is mainly composed of fluviogenic alkaline peat. The site of Morcourt exhibits a thick and well-preserved fluvial sequence (10 m, including 6 m of peat). This sequence provides the framework for reconstructing fluvial and palaeoenvironmental dynamics from the end of the Upper Weichselian Pleniglacial (~20 000 cal. a BP) to the High Middle Ages (~700 cal. a BP). The palaeoenvironmental reconstruction is based on a high-resolution stratigraphic study of 60 transect cores, 36 radiocarbon dates and sedimentological, geochemical, pollen and plant macrofossil analyses. There are three main phases in the development of the valley floor: (i) after the incision of the abandoned Pleniglacial braided river channels, a first generation of localized peat developed during the Bølling and the Allerød interstadial (<1 m thick); (ii) at the beginning of the Holocene, a peat formation phase began in the deepest parts of the valley and then spread over the valley floor by the end of the Middle Holocene (~4700 cal. a BP), with the limited runoff shifting to a small lateral channel; and (iii) at the transition to the Late Holocene, environmental changes, driven by the intensification of human activities and perhaps accentuated by climatic modifications, caused the incision of the peat system as a result of the formation of a channel. This channel drained the valley, and then mixed a detrital load into the peat. By the Low Middle Ages, the system had been altered to such an extent that the peat was completely covered by organic silty alluvium. The water table was lower, which definitively inhibited peat formation. The Morcourt sedimentary record (thickness and continuity) and the resumption of turfigenesis during the Late Holocene are remarkable in NW Europe, making this site a model of continuous morpho-fluvial evolution since the Lateglacial.

Two major glaciations have been identified on land in England during the Middle Pleistocene. The earliest occurred during the Anglian Stage (= Elsterian, c. Marine Isotope Stage, MIS 12), evidence for which is best developed in lowland Britain, as well as offshore in the southern North Sea and Irish Sea basins. The second took place during the late Middle Pleistocene, with the most compelling evidence found in the West Midlands, intermediate between the Hoxnian (= Holsteinian; broadly MIS 11) and Ipswichian (= Eemian; broadly MIS 5e) interglacial stages during the Late Wolstonian Substage. Until recently this younger glacial episode was less clearly represented in the Pleistocene record and, as a result, had been little studied and weakly defined. Interpreted as the Moreton Stadial glaciation during the Late Wolstonian Substage (= Late Saalian Substage/Drenthe Stadial, c. MIS 6), it was originally recognized in the English Midlands, subsequently being identified in Yorkshire, Lincolnshire and northern East Anglia, and potentially further SW as far as the Bristol Channel. Mapping, in particular by members of the British Geological Survey, however, resulted in the Wolstonian Stage glacial deposits being thought to pre-date the stage. This was particularly so in East Anglia where there was considerable controversy concerning the number and relationships of glacial sequences, during the 1970–1980s. Yet to the west of East Anglia there remained unequivocal evidence for glaciation during the stage, particularly in Fenland and the eastern English Midlands. Recent radiometric dating across lowland Britain on glacial sediments long thought to belong to a glaciation event in the Wolstonian Stage have now placed a geochronological control on the established regional stratigraphy and confirmed that glaciation occurred in two phases between 199 and 147 ka during the Late Wolstonian Substage. The glacial events of the British Middle Pleistocene can clearly be correlated with the European continent.

In this study, the morphotectonic features of the Yenişehir pull-apart basin in Bursa in northwestern Turkey were analysed. The faults in the region are the most important structural elements that characterize the morphology of the Yenişehir pull-apart basin. The aim of this study is to show the deformation effects of these active faults on the geomorphology with some basic morphometric indices. The morphotectonic features were evaluated using the hypsometric curve and hypsometric integral (H_i), drainage basin asymmetry (the asymmetry factor (A_F) and the transverse topographic symmetry factor (T)), river length gradient index, mountain front sinuosity, and valley floor ratio. The morphometric indices that play an important role in the analysis of catchment areas are the H_i, A_F, and T indices, which indicate tilting and erosion activities. According to the results of the hypsometric curve and hypsometric integral, the Yenişehir Basin is a young basin with a uniform topography as expected in an active pull-apart basin formed by faulting in the Quaternary. The relative tectonic activity index (Iat) of the study area was calculated using the values of hypsometric integral, transverse topographic symmetry factor, asymmetry factor, mountain front sinuosity, valley floor ratio and stream length gradient index. The Iat values indicate the tectonic activity of the basin from Pliocene to Recent. Three different tectonic classes were identified in the study area. These are class 1 (high relative tectonic activity), class 2 (moderate relative tectonic activity), and class 3 (low relative tectonic activity). The Iat classification data show that the impact of tectonic activity on the geomorphology is lower in the Iat class 3 indices than in the others. The Iat data show high tectonic activity in the region where active faults of the North Anatolian Fault Zone southern branch are located.

The middle of the first millennium AD in Sweden witnessed significant changes in the archaeological record, prompting various discussions about the underlying causes. This paper reviews the research developments surrounding the global climate cooling, which was caused by severe volcanic eruptions in AD 536/540 and is referred to as the Late Antique Little Ice Age or Fimbulwinter, and its potential impacts, with a specific focus on Swedish Iron Age societies, drawing upon published climatic and archaeological data. Furthermore, it discusses the research gaps that impede a comprehensive understanding of the potential relationship between the AD 536/540 event and socioeconomic changes in Iron Age Sweden. Additionally, it suggests that isotopic analysis will offer additional insights and help bridge the research gap.

The Middle Pleistocene Reinsdorf sequence at the Lower Palaeolithic sites of Schöningen offers the opportunity to reconstruct a rarely well-preserved post-Holsteinian environmental transition from an interglacial into a glacial phase along with its highly dynamic interjacent climatic oscillations. Combining biological proxies, element composition and stable isotope ratios of two lakeshore sequences at excavation site 13 II, we demonstrate repeated variations in climate, hydrology and catchment vegetation cover. New ostracod-based quantitative mean summer and winter air temperature reconstructions with the Mutual Ostracod Temperature Range (MOTR) method provide the first detailed information about the temperature evolution. The interglacial temperature maximum, probably corresponding to Marine Isotope Stage 9e, is followed by a first dry phase and, during the younger part of the Reinsdorf sequence, by a second dry period. Both were marked by lower precipitation/evaporation ratios, reduced vegetation cover in the catchment and increased surface inflows from springs. Temperature reconstructions of these two steppe (open woodland) phases yield very narrow ranges for mean January (−4–0 °C) and July (+17–19 or +17–21 °C) air temperatures, demonstrating that, while summers were similar to those of today, winters were at least 1 °C colder, hinting at a more pronounced continental climate. Precise temperature estimates for the interjacent woodland and steppe (woodland) phase are hindered by generally wider ranges produced by the MOTR method (January mean −4–3 °C, July mean +15–21 °C). The development of a more extensive vegetation cover, reducing surface runoff and erosion in favour of increased river and groundwater discharge, as indicated by a shift in microfossil and stable isotope records, suggests generally more humid climates with higher precipitation/evaporation ratios as well as reduced seasonal temperature variations.