Boreas最新文献

Investigations of the geomorphological fingerprints of palaeo-ice streams are essential for enhancing our understanding of ice-stream behaviour. Cross-cutting flowsets of palaeo-ice streams, during and following the Last Glacial Maximum, have been suggested in northeast Iceland based on the mapping of streamlined subglacial bedforms (SSBs). To increase our understanding of the dynamics of the Icelandic Ice Sheet (IIS) during deglaciation, we investigate transverse ridges, together with glacifluvial and ice-marginal landforms, within the largest flowset. The main emphasis is on the transverse ridges, which are primarily interpreted as ribbed moraines – the first to be described in Iceland. Morphological data are combined with sedimentological analyses of the ribbed moraines. Our results show that the ribbed moraines are composed of pre-existing material, often exhibiting a base of stratified glacifluvial sediments draped with subglacial till. Deformation and thrust structures within both units suggest compressional flow during the formation of the ridges. Our results suggest the ribbed moraines develop due to velocity gradients under the lateral shear margin of an ice stream based on their distribution and often oblique orientation in relation to SSBs. The ribbed moraines superimpose and thus post-date the SSBs, signifying the waning stage of ice streaming. We suggest that the ridges formed during ice-stream shutdown following the Younger Dryas and/or Preboreal re-advances. Eskers and ice-marginal landforms imply channelized water drainage during the final deglaciation interrupted by short-lived re-advances. This study has implications for the style of deglaciation associated with ice streaming in the northeastern part of the IIS. We characterize the role of ribbed moraines within the ice sheet and identify directions for future work.

De Geer moraines (DGMs) may act as valuable ice margin indicators; however, to date, their variable mode of formation has presented challenges for this utility. Morphometric investigations provide useful insights into formation processes, which can be developed using sedimentological and geophysical methods. Here we present sedimentological and ground penetrating radar (GPR) data of DGMs located in southwest Finland. Individual lithofacies were identified and interpreted using sediment architectural elements. These were correlated with neighbouring GPR radargrams and extrapolated across the wider study area. Generally, internal architecture presents a multi-phase structure with lower units representing subglacial traction till and ice margin infill deposits, truncated by a larger prominent push unit, which is then successively deformed via the overriding of active ice. Significantly, there are notable differences between proximal and distal structures, with proximal sides characterized by silts, clays, and diamicton with laminae, stratification and thrust planes, and distal sides characterized by poorly consolidated diamicton and proglacial water current reworkings. Internal architecture of both prominent and intermediate ridges is very similar, reflecting similar formation processes, however, slight differences also reflect inter-seasonal variations. Based on our findings, we present an integrated conceptual model for the genesis of DGMs whereby inter-seasonal ridge forming processes occur within a sub-aqueous ice-marginal environment. Our model highlights that DGMs can be subcategorized as: (i) sediment deposition at an unstable margin during summer calving, and/or (ii) sediment pushing at a stabilized margin during a winter re-advance. We do not find evidence of crevasse filling as a mechanism for DGM formation. We propose a landform assemblage classification whereby ‘De Geer terrain’ is used to describe series of parallel ridges arranged in a typical washboard-like configuration. This classification identifies all DGMs derived within a sub-aqueous ice-marginal environment, whilst also capturing the equifinal characteristics between individual landforms.

Wind activity is a powerful force that shapes the landscapes of deserts, coastal areas, and regions adjacent to ice sheets, and it has significant implications for human settlement. In southern Greenland, it has been proposed that the increased wind and soil erosion observed around Norse settlements (~985–1450 CE) were caused by overgrazing by animals, which ultimately contributed to the decline of the Norse culture. Alternatively, some studies have linked the observed intensification of aeolian activity to changes in large-scale atmospheric circulation patterns in the North Atlantic. However, the timing and impact of this increased aeolian activity in southern Greenland remain uncertain due to a lack of well-dated records. In this study, we use a lake record and optically stimulated luminescence (OSL) dating of adjacent dunes to reconstruct the Holocene history of aeolian activity at Igaliku Kujalleq (Søndre Igaliku) in southern Greenland. Our findings indicate two periods of intensified aeolian activity over the past 10 000 years: from ~500 to 1200 CE and ~1450 CE. Importantly, the peak aeolian activity observed in the Igaliku Kujalleq records was unrelated to Norse activities and their decline. Instead, we suggest that changes in the North Atlantic atmospheric circulation pattern combined with Neoglacial glacier advances led to increased katabatic wind activity and triggered increased aeolian activity from large outwash plains.

Arctic ecotones contain dynamic freshwater ecosystems where aquatic biota vary across these transitions and as such can be especially susceptible to environmental change. Here, we examine the palaeoecology of two ponds in the ecotonal Hudson Bay Lowlands, subarctic Canada, to understand how aquatic biota have responded in an increasingly climate-stressed Anthropocene, and to better anticipate future changes. Using a multi-proxy palaeolimnological approach, we reconstruct past environmental conditions through the examination of subfossil chironomids (Diptera: Chironomidae) and compare these records to organic carbon and nitrogen elemental and isotope composition, and previously published cellulose-inferred lake water oxygen isotope records. Despite their close proximity, we found different chironomid assemblages in each pond that reflected differences in hydrological trajectories since 1940; an isolated pond exposed to evaporative stress showed an increasingly littoral chironomid assemblage, while a nearby basin that began receiving waters from a channel fen lost semi-terrestrial taxa associated with flooded grassy margins that became more permanently submerged. Even though large catchment-mediated changes resulted in a shift in some chironomids of both ponds, chironomid-based palaeo-temperature reconstructions demonstrated similar warming trends. Shifts in the ecology of subarctic lakes and ponds are expected to increase as the effects of climate change become more severe.

Subglacial processes exert a major control on ice streaming. Constraining subglacial conditions thus allows for more accurate predictions of ice mass loss. Due to the difficulty in observing large-scale conditions of the modern subglacial environment, we turn to geological records of ice streaming in deglaciated environments. Morphometric values of streamlined subglacial bedforms provide valuable information about the relative speed, direction, and maturity of past ice streams and the relationship between ice streaming and subglacial erosion and deposition. However, manually identifying streamlined subglacial bedforms across deglaciated landscapes, sometimes in clusters of several thousand, is an arduous task with difficult-to-control sources of variability and human-biased errors. This paper presents a new tool that utilizes a machine learning approach to automatically identify glacially derived streamlined features. Slope variations across a landscape, identified by topographic position index, undergo analysis from a series of supervised machine learning models trained from over 600 000 data points identified across the deglaciated Northern Hemisphere. A filtered data set produced through the combination of scientifically driven preprocessing and statistical downsampling improved the robustness of our approach. After cross-validation, we found that Random Forest detected the most true positives, up to 94.5% on a withheld test set, and an ensemble average of machine learning models provided the highest stability when applied within the range of applicable data sets, performing at up to 79% identification of true positives on an out of distribution area of interest. We build these models into an open-source Python package, bedfinder, and apply it to new data in the Green Bay Lobe region, USA, finding the general ice-flow direction and average streamlined subglacial bedform elongation with minimal effort. This type of open, reproducible machine learning analysis is at the leading edge of glacial geomorphology research and will continue to improve with integration of newly acquired and previously collected data.

Jan Mayen is a small volcanic island situated in the Norwegian–Greenland Sea. The entire island was covered by a contiguous ice cap during the Last Glacial Maximum. The deglaciation of the ice cap was interrupted by a glacier advance in the southern part of the island in the Early Holocene. Today, there are no glaciers in this area, and until now it has been unknown whether any glaciers survived there into the Middle–Late Holocene. We show here that glaciers existed at several sites in the mountain areas of southern Jan Mayen. The investigations were triggered by the discovery of a relict glacier completely covered by tephra and impacted by a lava flow. Samples of ice from the glacier have ¹⁸O values that are isotopically indistinguishable from modern precipitation values and fall along the local meteoric water line trend. The lava flow in the glacier catchment and sculpted forms along the base of dry meltwater channels in bedrock show that glacier melting was abrupt and marked by sudden meltwater outbursts (jökulhlaups). Three more sites in southern Jan Mayen have meltwater channels with sculpted beds, gorges and/or sediments associated with lava flows and can be attributed to jökulhlaups caused by rapidly melting glaciers. Radiocarbon dates associated with glacial outwash sediments, cosmogenic dates of meltwater channel incisions, and cosmogenic and K-Ar dates of lava flows associated with former periods of rapid glacier melting show that the four glaciers collapsed at different times in the Holocene. None of the glaciers reformed after their collapses despite subsequent cooling event(s). Likely, the glaciers were on the brink of existence before their sudden demise.

The present-day landscape of the northern Alpine foreland is marked by the cumulated impact of weathering during interglacial, and of erosion and deposition during glacial periods of the Quaternary. Direct traces of the earliest phases of ice advance, as well as thorough studies thereof, exist only sporadically. Here, a succession of diamictic deposits, which has been interpreted as the infill of an Early Pleistocene overdeepened basin, is investigated with a combined sedimentological-geotechnical approach including analysis via μCT scans, and standard tests of the water uptake, consistency, and compaction properties. The diamicts are exposed along a 4.5-m-deep profile, and are subdivided into a yellowish-brown lower unit with a variable, carbonaceous, silty to sandy matrix, and a reddish-brown upper unit that is free from carbonate and appears largely homogeneous. Although the lower unit is rather loose and surficially bioturbated, it contains microstructures indicative of subglacial deformation, which are lacking in the compact and clay-rich upper unit. The lower part is interpreted as a secondary glacial deposit (i.e. it has been affected by limited sorting in water) that was overridden and sheared by a glacier briefly after deposition, and recently bioturbated. The upper part is less sorted, more massive and compact, and thus likely of a primary glacial nature. It is further characterized by a strong pedogenetic overprint typical of prolonged warm periods. This suggests that it is separated from the overlying glacifluvial gravel, which has an equivalent petrographic composition, by a full interglacial at least. Thus, the combination of geotechnical testing and CT-based micromorphology offers a new, practical and cost-effective approach to the characterization of glacially derived sediments.

The Greenland Ice Sheet is highly sensitive to climate change, leading to significant retreat along its edges. This rapid ice loss contributes to rising sea levels and impacts the Earth's climate stability. Understanding the extent of recent glacier retreat is crucial in order to determine if it is unprecedented or within ranges of natural variability. Palaeoenvironmental studies aim to identify past glacial phases and landscape changes using advanced dating methods such as cosmic ray exposure (CRE) dating. In NE Greenland, CRE dating has helped establish the timing of glacial oscillations, yet a comprehensive understanding of glacial fluctuations during specific periods still needs to be developed. This study aims to chronologically constrain the postglacial landscape evolution of two NE Greenland valleys from the Young Sund–Tyrolerfjord area (74°N, 20–25°E) from the onset of deglaciation and throughout the Holocene to better understand glacial and postglacial changes. The chronological framework relies on 27 ¹⁰Be cosmic-ray exposure ages that constrain our interpretation of the geomorphological features in both valleys. Inconsistencies were observed in the ages dataset, highlighting potential bias associated with nuclide inheritance and post-glacial dynamics. Despite limitations, the CRE results confirm the general pattern observed in NE Greenland: (i) major deglaciation and disconnection of glaciers from the main glacial systems during the Lateglacial and Early Holocene with a rapid but not homogeneous deglaciation within the range from ~14.3 to 11.9 ka; (ii) no evidence of glacial activity during the Middle Holocene, probably associated with the withdrawn position of the ice masses’ fronts; and (iii) glacier expansion during the Late Holocene, with a Little Ice Age advance as the last significant period of glacial regrowth.

We investigate how luminescence signals imprinted on fluvial sediments vary depending on the depositional environment and vary through time in the same river. We collected sediment samples from four geomorphically distinct locations on the modern floodplain and modern point bar on the Buffalo River in northwest Arkansas, USA, in order to determine if different depositional environments are associated with distinct bleaching characteristics in the sediments. Our analysis revealed that all samples from different depositional environments yielded ages consistent with modern deposition. The samples collected from the floodplain and bar head contained a higher proportion of grains with residual doses, indicative of incomplete bleaching during transport, while samples from the mid-bar and bar tail appeared well bleached. Our results are particularly intriguing for two significant reasons. First, they highlight distinct equivalent dose distributions in different depositional environments. Second, they shed light on an intriguing relationship: despite generally well-bleached modern floodplain samples, ancient sediments from corresponding terraces displayed equivalent dose (D_e) distributions that suggest partial bleaching in some cases. This research contributes to the growing body of work that seeks to establish a relationship between luminescence properties and sediment transport processes and offers valuable insight into how luminescence signals vary locally in modern fluvial deposits, which can help guide the interpretation of older fluvial deposits.

A high-resolution palynological record from northwest Belgium is presented. The record encompasses the second part of the Younger Dryas and the Early Holocene. The basal part of the pollen record reflects a relatively open landscape, characteristic for the Younger Dryas. The transition to the Holocene is marked by an expansion of birch woodland, followed by an expansion of pine in the Late Preboreal (~11.2–10.7 cal. ka BP). Subsequently, the record shows an expansion of mixed deciduous forest with hazel, elm and oak, characteristic for the Boreal (~10.7–8.6 cal. ka BP). Early Holocene forest expansion was interrupted by a number of short-lived fluctuations, presumably driven by climatic changes. The most distinct event is the Rammelbeek phase (~11.4–11.2 cal. ka BP), during which forest development was temporarily interrupted in favour of grasses, while wildfires increased. Following this climatic event, hunter-gatherers returned to the area after a period of almost 1.5 millennia of low population density. They most likely were attracted by the increased temperatures and abundance of edible plants present in the birch-pine forests and on the banks of the river Kale/Durme. A temporary expansion of pine during the Boreal (~10.7–8.6 cal. ka BP) may correspond with the 9.3-ka event. At this time, superimposed on a trend of gradual infilling of the channel, a temporary change to drier conditions is observed. The significant drop in the number of prehistoric sites in the Moervaart region clearly cannot be attributed to this short-term climatic event alone but was most likely caused by a combination of environmental changes, such as the decreasing availability of hazelnuts as well as freshwater and edible (semi-)aquatic plants as the Kale/Durme river gradually turned dry. The study provides insight into, partly climate-driven, Early Holocene environmental changes and the effect this may have had on human occupation.