Boreas最新文献

This study presents a detailed investigation of the Velika Vrbica loess-palaeosol sequence, situated in the Wallachian Basin of northeastern Serbia, with the aim to reconstruct palaeoenvironmental changes spanning Marine Isotope Stages (MIS) 3 to 1. Using optically stimulated luminescence (OSL) dating, low field magnetic susceptibility (χ_lf), and mass accumulation rates (MARs), we have developed a robust geochronological framework and analysed sedimentary and environmental processes over the last glacial–interglacial cycle. The OSL chronology reveals consistent loess deposition from ~41 to 3 ka, with peak accumulation rates recorded during MIS 3 and late MIS 2. The MAR data challenge conventional models of loess formation, as higher deposition rates are observed during the interstadial MIS 3 compared to the Last Glacial Maximum. This finding suggests regional variations in aeolian activity and climate dynamics that differ from the widely accepted pattern of intensified dust deposition during colder glacial periods. The environmental magnetic record of the Velika Vrbica loess sequence presented here shows more similarities with equivalent sections in the Wallachian Basin than in the Vojvodina region. These findings contribute to the broader understanding of loess formation processes and palaeoenvironmental variability on a continental scale, reinforcing the importance of high-resolution chronological and sedimentological studies in disentangling global vs. regional influences on past dust deposition.

Divergent views debated over the past 20 years on the Wolstonian depositional record of Fenland and the Peterborough area have centred on whether there is evidence of a single (middle or late) glaciation or of both a middle and a late glaciation. A recent review promoted a single late Wolstonian glaciation, despite there being incontrovertible evidence of extensive cold-stage gravels associated with glacially related drainage diversion both pre-dating and post-dating late Wolstonian interglacial deposits.

The archaeological sediment successions from the Wanbei site provide an ideal case for reconstructing both natural and human-induced environmental changes in the middle and lower reaches of the Huai River valley located in the transitional zone of eastern China during the Middle Holocene. The findings of this work, in terms of phytoliths, pollen and spores, and chemical elemental analyses suggest that the climatic conditions that were recorded in the Wanbei profile responded well to broader climatic changes, with an overall warm and humid climate during 5700–5300 a BP, albeit with a cooling trend between 5600 and 5400 a BP. Rice (Oryza sativa) was always the main cereal crop in mixed rice-millet farming owing to warm and humid conditions, although the proportions of broomcorn millet (Panicum miliaceum) and foxtail millet (Setaria italica) significantly increased during the cooling period. The structural characteristics and adjustments of mixed farming over time show an adaptation strategy in response to the climatic changes at Wanbei. Furthermore, multiple indicators reveal that human activities at the Wanbei site would increase rather than decrease during the cooling period between 5600 and 5400 a BP. This study not only highlights the dynamic nature of early agricultural societies, but also enhances our understanding of their adaptation strategies in response to climatic changes in the middle and lower Huai River valley during the Middle Holocene.

Based on a regional 3D seismic data set and a small high-resolution 3D seismic data set (~40 km²) we have mapped a buried glacially eroded trough on the North Sea Plateau, west of the Norwegian Channel (latitude 59°N, longitude 3°E). The trough, which we informally name Marstein Trough, is 60 km long, 30 km wide, 120 m deep, and trends NE–SW. Marstein Trough contains an extensive pattern of glacial lineations at its base, which follow the trough axis, and is infilled by two seismic units interpreted as tills. From its stratigraphical position, we infer that the trough was eroded by an ice stream that flowed from western Norway and crossed the Norwegian Channel in a southwesterly direction, probably during the penultimate, Saalian glaciation. Marstein Trough, and its diagnostic landforms, provide detailed evidence of complex, switching ice flow across the North Sea during the Quaternary. Westward ice flow from Norway took place during early Scandinavian Ice-Sheet build-up prior to the activation of the Norwegian Channel Ice Stream. In contrast, ice-flow patterns during full-glacial conditions caused ice flow to reorientate to a S–N direction when the Norwegian Channel Ice Stream with a huge catchment that included the Baltic was established. Our results highlight the complex patterns of ice flow experienced over this region of the North Sea, with implications for reconstructions of Quaternary history, modern renewable energy infrastructure installation, and glacial processes during the build-up phase of ice sheets.

Crater lakes in core regions of former ice sheets have the potential to preserve long-term sedimentary archives that are otherwise rare in glaciated landscapes due to pervasive glacial erosion. Lake Wiyâshâkimî, an impact crater lake located in the inner core of the Québec-Labrador Dome of the Laurentide Ice Sheet, provides a rare example of such a setting. The lake was therefore investigated along the region surrounding it through remotely sensed mapping and acoustic sub-bottom profiling to reconstruct its glacial to postglacial environmental evolution from its morphosedimentary record. Mapping of ice-flow landforms suggests three distinct glacial flowsets in the region, indicating a transition from external ice-stream influences to local topographical controls during deglaciation. Contrary to previous hypotheses, mapping evidence suggests that the Tyrrell Sea deglacial marine transgression did not reach Lake Wiyâshâkimî, with surrounding strandlines likely of glaciolacustrine origin. Five acoustic facies were identified from the analysis and interpretation of the acoustic sub-bottom profiles: AF1, bedrock, till and/or glacifluvial sands forming the acoustic basement; AF2, an acoustically transparent deposit filling the deeper basins of the lake, a fine-grained diamicton either associated with mass-movements or full glacial conditions (till); AF3A and AF3B, glaciolacustrine sediments deposited proximally and distally from the ice margin, respectively; and AF4, paraglacial and postglacial deposits that have been partly, at least in the upper tens of metres of the water column, remobilized by bottom currents induced by strong winds prevailing in the large lake. While our analysis shows it is unlikely that sediments pre-dating the last glaciation were preserved in the lake basin, the findings provide new insights into the lateglacial and deglacial history of the central Québec-Labrador region and highlight the role of postglacial processes in shaping the lake-bottom morphosedimentary archives.

We designed an automated tool to delineate and analyse the shape of subglacial bedforms using a recently defined land surface parameter, the volumetric obscurance. The tool is based on the assumption that the diversity of subglacial bedform shapes reflects a continuum; therefore, unlike traditional methods, no pre- or post-mapping classification of bedforms is performed. It uses digital elevation models and optical satellite images to generate regional morphological maps (bedform outlines and crestlines) and regional morphometric maps (spatialized statistical analysis of bedform morphometrics). We tested the tool on the ArcticDEM, over a portion of the former Laurentide Ice Sheet bed that displays a wide diversity of bedform shapes (Keewatin Ice Dome, northern Canada). The produced morphological maps are consistent, with a correspondence of approximately 75% on individual bedform outlines, with two reference maps digitized manually by two different glacial geomorphologists. Despite the 25% difference between individual bedform outlines generated automatically and manually, the derived morphometric maps are similar. They can be interpreted in the context of subglacial deformation and hydrology, providing a new potential for palaeoglaciological reconstructions at the ice-sheet scale. The tool was developed in Python and is freely accessible.

Geological reconstructions of relative sea level (RSL) from southern Norway show falling RSL during the last 7000 cal. a BP, but tide gauge measurements document a slow RSL rise since at least 1960 CE. With an age gap of c. 1400 years between the youngest geologically reconstructed sea-level index point (SLIP) and the installation of the Tregde tide gauge in southernmost Norway, the exact nature and timing of the onset of RSL rise in southern Norway remain unknown. To fill this gap, we collected peat cores from a salt marsh to reconstruct RSL trends over the past 1000 years using a multiproxy approach, including ²¹⁰Pb and ¹⁴C dating, grain-size analysis, loss-on-ignition (LOI), geochemistry (stable carbon isotopes, carbon to nitrogen ratios and XRF) and diatoms. Our data suggest decreasing tidal current strength and salinity over most of the last millennium, suggesting falling RSL. Sediment geochemistry also appears to vary with wetter and drier climatic periods. An increase in marine-brackish diatoms in combination with an acceleration in sedimentation rates after 1930 CE (1899–1954 CE) suggest that the onset of RSL rise began around this time in southernmost Norway. While most of the proxy data appear to have delayed sensitivity to RSL changes and may be linked to other causal processes, they, nonetheless, provide valuable insight into the environmental response of high-latitude temperate salt marshes to slow rates of RSL change.

Basal conditions that facilitate fast ice flow are still poorly understood and their parameterization in ice-flow models results in high uncertainties in ice-flow and consequent sea-level rise projections. Direct observations of basal conditions beneath modern ice streams are limited due to the inaccessibility of the bed. One approach to understanding basal conditions is through investigating the basal landscape of ice streams and glaciers, which has been shaped by ice flow over the underlying substrate. Bedform variation together with observations of ice-flow properties can reveal glaciological and geological conditions present during bedform formation. Here we map the subglacial landscape and identify basal conditions of Rutford Ice Stream (West Antarctica) using different visualization techniques on novel high-resolution 3D radar data. This novel approach highlights small-scale features and details of bedforms that would otherwise be invisible in conventional radar grids. Our data reveal bedforms of <300 m in length, surrounded by bedforms of >10 km in length. We correlate variations in bedform dimensions and spacing to different glaciological and geological factors. We find no significant correlation between local (<3 × 3 km) variations in bedform dimensions and variations in ice-flow speed and (surface or basal) topography. We present a new model of subglacial sediment discharge, which proposes that variations in bedform dimensions are primarily driven by spatial variation in sediment properties and effective pressure. This work highlights the small-scale spatial variability of basal conditions and its implications for basal slip. This is critical for more reliable parameterization of basal friction of ice streams in numerical models.