Boreas最新文献

Lakes play a vital role in numerous human activities, yet extensive economic development has led to significant pollution and nutrient enrichment, resulting in widespread eutrophication and ecological disruption. To accurately assess the current state of lakes, it is essential to understand their historical, pre-impact conditions. Palaeolimnological methods, including the analysis of subfossil Cladocera, offer a valuable tool for studying lake ecosystems over extended timeframes that go beyond modern instrumental records. Our research aims to identify the primary environmental factors that influence the distribution of individual Cladocera species and evaluate the systematic patterns within species assemblages to enhance their use as ecological indicators. This study presents an analysis of Cladocera assemblages based on environmental surveys conducted in 64 lakes across northeastern Poland. Principal component analysis revealed that the two dominant factors driving variability in the dataset are lake trophic state, indicated by total phosphorus (TP) levels, and lake depth. Species distributions across these gradients showed a turnover, reflecting distinct environmental preferences. Further, end member (EM) analysis identified five distinct assemblages of Cladocera, each associated with specific combinations of trophic status and lake size. EM1 is associated with higher trophic states, while EM2, EM3 and EM4 are indicative of medium to low trophic states. EM5 is independent of trophic status, associated with large and deep-water bodies.

Reconstructions of the maximum extent and dynamics of the Laurentide Ice Sheet (LIS) during the Last Glacial Maximum (LGM) are needed for understanding how marine-based ice sheets will respond to modern climate change. The partitioning of ice masses and the location of dominant ice flows within the LIS are broadly known at regional scales, but their influence on the marine-based maximum extent of the ice sheet remains poorly constrained. Here, we provide evidence for the maximal extent of the LIS margin during the LGM in the Broughton and Merchants cross-shelf troughs off southeastern Baffin Island based on newly acquired sea floor geomorphology data. Grounding-zone wedges (GZWs), moraines and ice-stream bedforms within the Merchants cross-shelf trough delimit the maximum extent of the LIS in this sector to near the mouth of the modern fjords. Two lateral moraines, mega-scale glacial lineations (MSGLs), and iceberg scours also indicate the presence of an ice shelf extending beyond the former grounding line. In Broughton Trough, the maximum extent of the LIS is interpreted from the distribution of MSGLs and iceberg scours. The presence of sticky spots near the shelf edge may have played a role in subglacial flow and thus glacier behaviour. The difference in ice extent between the two nearby troughs is attributed to partitioning of the LIS between ice flowing from the Penny Ice Cap into Broughton Trough and ice from local alpine glaciers flowing into Merchants Trough. In addition, the presence of a dominant ice flow south of the Cumberland Peninsula may have drained ice masses of Cumberland Peninsula towards the south and caused minimal expansion in Merchants Trough to the north. These results provide a clearer understanding of the relationship between ice-sheet extent along continental margins and inland ice dynamics.

Geological information contained in tills is important for deciphering processes that operated under past ice sheets and helps to constrain the fundamentals of glacial erosion, transport and deposition. Despite the progress in understanding glacial systems, interpretation of ancient tills and related deposits remains fragmentary, mostly because of the inaccessibility of ice–beds under present glaciers as modern analogues of the ancient systems. Here we report on the properties of a glacial sediment succession including various till facies and subglacial meltwater deposits from a site in central Poland close to the outermost extent of the Scandinavian Ice Sheet during the last glaciation. We use outcrop-scale and micromorphological proxies to interpret the processes of till formation and deformation, and to infer the ice movement mechanisms. A combination of macro-scale sedimentological and micromorphological data (till fabric signatures, grain-size patterns, glaciotectonic structures, features of channelized and distributed subglacial drainage) indicates time-transgressive succession of ice-marginal deposition, subglacial deposition and deformation, pressurized meltwater flows at the ice–bed interface, and material release from stagnant ice. The investigated tills are to a large extent hybrid products of multiple superposed processes involving lodgement, deformation, and material cannibalization that mostly fall into the category of subglacial traction tills. Ample field and micromorphological evidence of basal decoupling suggests that the basal water pressure fluctuated around the ice flotation point. Ice movement was by a combination of enhanced basal sliding and very shallow (millimetre-thick) bed deformation. Drainage of basal meltwater was facilitated by channelized flows in conduits subsequently filled by glacifluvial outwash. Our data indicate a mosaic of (sub)glacial processes that generated a complex deposit bearing evidence of overlapping accretion and deformation modulated by hydrological conditions at the ice–bed interface.

Jan Mayen is a volcanic island situated 550 km NE of Iceland. It has been shown previously that the island and probably also shallow shelf areas around it were covered by a contiguous ice cap during the last glaciation. Today, active glaciers exist only in the northern part of the island around the Beerenberg volcano. Until now, information on Lateglacial and Holocene glacier and environmental variations has been sparse. We focus on the northern part (Nord-Jan), discussing glacier evolution and environmental changes from the Lateglacial to the Holocene. Widespread volcanic activity during the postglacial era resulted in lava flows and volcanic craters amidst glaciogenic deposits, complicating glacial and environmental reconstructions. We suggest that the ice cap covering Jan Mayen separated into two (northern and southern) approximately 20 ka ago, based on observations from glacial striae and terrestrial cosmogenic nuclide exposure dating. Deglaciation of the southern flank of the Beerenberg ice cap was accelerated by initial subglacial volcanic activity, which probably rapidly evolved to protrude above the glacier surface. Subsequently, around 18.9 ka, vertical melting and ice-marginal recession intensified in Nord-Jan. A relatively warm period followed thereafter, evidenced by organic material giving a calibrated age of 17 to 16 cal. ka BP. The extent of deglaciation during this period is unknown as the dated material was found redeposited in lake sediments. Several cosmogenic exposure ages from bedrock and glacial erratics correspond to the Lateglacial period. The interpretation of these ages is not obvious, but we favour a hypothesis involving a Lateglacial readvance of the glaciers when lateral moraines were also deposited. Mild conditions, characterized by plant growth and soil formation, prevailed during the Early Holocene. During this period, the lateral moraines formed during the Lateglacial probably disintegrated, leaving only the remnants visible today.

We analyse the effect of Quaternary glaciations on the complex tectonic pattern within the southwestern Baltic Sea, a sector of the transition zone from the East European Craton to the West European Platform. This area comprises the Caledonian Trans–European Suture Zone in the south and the Tornquist Zone in the north. Multiple fault zones in between, with different strike and dip angles, and characters (normal, thrust/reverse, strike-slip), document like scars the alternately transpressional and transtensional stress activities since the Palaeozoic. We determine the strike directions and dip angles of more than 40 potential glacially reactivated faults identified in 2D marine reflection seismic data. Finite element simulations of different glacial isostatic adjustment models provide glacially induced Coulomb failure stress changes (ΔCFS) at the faults over time, starting 200 000 years ago (200 ka, Saalian phase) up to 1000 years into the future. Assuming strike-slip or thrust/reverse background stresses, a potential reactivation of each fault is analysed. The detected reactivation phases are related to the waxing and waning ice masses (Late Saalian ice advances: c. 170–135 ka ago; Weichselian ice advances: 70–60, 45–38, 26–14 ka ago) and point to an activation in front of the ice margin. Comparing the ΔCFS results of the individual faults laterally and over time, we found that the location of the fault, depending on its position during a glacial maximum, has an important effect on its reactivation potential. The closer a fault was located to the former ice margin, the higher was the glacially induced stress during the ice retreat. Based on earlier findings in Germany and Denmark, glacially triggered faults are a typical consequence of the Fennoscandian glaciation throughout northern central Europe, and this also applies to future glaciation phases.

The Middle Danube (Carpathian) Basin is considered to encompass some of the thickest and most complete records of aeolian dust deposition in Europe, covering the last two glacial–interglacial cycles. In this study, independent, published chronologies based on absolute dating methods were compiled for 34 loess-palaeosol sequences and modelled using Bayesian statistics. Mass accumulation rates (MARs) calculated from the Bayesian age models range from 50 to 1922 g m⁻² a⁻¹ with a mean value of 317±35 g m⁻² a⁻¹. The glacial–interglacial changes of the MAR estimates only partially agree with the accepted dust deposition model, where high accumulation rates are observed during the cold glacial stages, while low values are typical for the warmer interglacial periods. There is a rapid increase of MAR values during Marine Isotope Stage (MIS) 2, with the highest deposition in most cases observed ~25–18 ka. During MIS 3, the dust deposition was 1.8 times higher than during MIS 4. Regionally, the MARs appear to be highest in the northern part of the Middle Danube Basin, showing decreasing values towards the south. However, due to a variety of factors such as low dating resolution, age distribution, and biases in the dating method, it is debatable whether these results are representative estimates of atmospheric dust activity in the Middle Danube Basin during MIS 5–MIS 1.

This paper investigates the time-transgressive evolution of the Fennoscandian Ice Sheet (FIS) during the Late Weichselian glaciation. Recent models of the Last Glacial Maximum (LGM) strongly challenge a synchronous development of the FIS in the SW Baltic Sea area. Especially in northern Germany, a time-transgressive LGM is controversial. We present a new set of optically stimulated luminescence (OSL) ages, from sediments deposited at locations previously ascribed to the LGM main stationary line, from which numerical age data have not been published so far. The aim of this study was to contribute to the research on the maximum Weichselian ice extent in northern Germany, and to close the gap between Denmark in the north and the area in Germany and Poland in the SE, for which numerical age data of the LGM extent of the FIS are available. Samples were taken from sandur deposits proximal to the former ice margin. In total nine OSL samples were taken and analysed on quasi-single-grain level, taking into account possible effects of insufficient bleaching of sample material. With an average age of ~23±1 ka, the samples indicate LGM sandur formation during Marine Isotope Stage (MIS) 2. Our dating provides new insights into Late Weichselian (MIS 2, 27–11.7 ka) ice-sheet dynamics and strongly challenge a synchronous development of the FIS in the SW Baltic Sea area. The integration of our results into regional models argues for a small-scale asynchronous extent of the last FIS in northern Germany.

Subglacial overdeepenings are of interest for a number of applied and academic reasons, for example as groundwater reservoirs or as sedimentary archives of environmental change. Thus, efforts have been made to compile and morphometrically analyse such features on a large scale. While this includes several prominent studies focusing on the western part of the European Alps and their foreland, their eastern part lacks information. We present an inventory of >100 individual overdeepened basins in southern Germany and Austria compiled from heterogeneous sources. These basins are typically incised ~200 m (max. ~1000 m) below ground surface and ~20 km (max. ~100 km) long, and thus of similar dimensions as overdeepenings in the western (Swiss) Alps and foreland. We observe a positive correlation of overdeepening area and depth with catchment size and both peak and average elevations, although large and deep basins also occur in small catchments. Generally, large overdeepenings are restricted to inner-Alpine fault zones or the primarily weak foreland Molasse. Especially in inner-Alpine settings, longitudinal profiles are highly irregular. In the eastern Alpine foreland, individual basins are frequently arranged in a radial pattern, a morphology that is not typical of the western foreland, and is a result of less constrained ice flow. On the valley scale, the depth of overdeepened basins correlates with modelled ice-flow parameters, thickness and basal velocity at peak glacial conditions. Together with their occurrence in low-elevation reaches, this suggests a rapid formation over a few millennia at most. On a smaller, local scale, basin depth is highly irregular. Finally, the existence of several generations of (similarly) large foreland overdeepenings, sometimes superimposed, indicates that these basins do not grow progressively over consecutive glacial cycles but that a negative feedback, likely related to the freezing of supercooled basal water, limits their dimensions.

Tunnel valleys are impressive subglacial erosional landforms and may attain depths of almost 600 m. Climatic and glaciological factors exert the primary control on tunnel-valley formation. Furthermore, regional geological features, e.g. faults and salt structures, have been suggested as controlling factors for tunnel-valley formation. To improve the understanding of the impact of these geological factors on tunnel-valley incision, we compare the distribution and orientations of buried Pleistocene tunnel valleys in the North German Basin with regional geological features, such as basin geometry and infill, faults and salt structures. Our analysis shows that deep tunnel valleys are restricted to areas with thick erodible Cenozoic deposits. The correlation between the trends of tunnel valleys, faults and salt structures varies between the analysed regions. The orientations of tunnel valleys commonly follow the trends of faults and salt structures in regions where the structural trend is NNW–SSE to east–west and ice-flow directions were approximately parallel to this trend. However, correlations are rarely observed if the regional structural trend is NW–SE to WNW–ESE and ice advances occurred thus normal or oblique to the regional fault trend. Potential future tunnel-valley incision is regarded as a major challenge for the long-term safety of radioactive waste repositories because incision may reach depths under consideration for repositories. The presence and orientations of faults and salt structures, however, do not provide consistent indicators for future tunnel-valley incision.

In the context of climate change, a long-term perspective is essential to understand future trajectories of mountain vegetation. We analysed the sediment record of Golzerensee (1411 m a.s.l.), a mountain lake in the transitional zone between the montane and subalpine vegetation belt in Uri, Central Switzerland. We reconstructed past vegetation responses to anthropogenic impact, fire activity and climatic changes dating back to c. 14 550 cal. a BP using pollen, stomata, macrofossil and microcharcoal analyses. The tree line, composed of Betula and Pinus sylvestris, reached Golzerensee during the Allerød c. 13 200 years ago, but tundra vegetation re-expanded during the Younger Dryas cooling (12 800–11 700 cal. a BP). With the rapid temperature increase at the onset of the Holocene, a closed forest established within a few decades around the study site. Concurrently, temperate taxa (Ulmus, Tilia, Acer, Corylus avellana) increased in the lowlands, likely reaching the elevation of the lake. Abies alba established locally during the Holocene Thermal Maximum (HTM) at 9600 cal. a BP and dominated closed, montane forests, delaying the westward expansion of Picea abies that established only from c. 7700 cal. a BP onwards. The concomitant occurrence of cultural indicator taxa, as well as an increase in charcoal influx and fungal dung spore concentrations suggest that human impact has facilitated the expansion of Picea abies by negatively affecting important competitors such as Abies alba through fire and browsing disturbance. With rising temperatures due to current climate change, Abies alba and other temperate species may regain their former relevance, leading to diverse mixed forests that were abundant during the HTM c. 10 000–5000 cal. a BP. However, future forest composition and landscape diversity will also be influenced by browsing disturbance and land use management.