Boreas最新文献

The sedimentary record of Icelandic ice-contact environments provides critical insights into past glacier margin dynamics and position, relative sea level, and the geomorphic processes that drive the evolution of proglacial environments. This important archive has been little exploited, however, with most glacier and sea-level reconstructions based on limited sedimentary exposures, coring and surface geomorphic evidence. We report an extensive (42 km of data within a 24-km² study area) and deep (reflections recorded at depths up to 100 m) low-frequency (40 and 100 MHz) ground-penetrating radar (GPR) survey of the Sandgígur moraines, SE Iceland. GPR profiles reveal a much larger (67 m high) and extensive (1.25 km wide) buried moraine ridge than that suggested by surface topography (typically 125 m wide and 7 m high). These data reveal that the Sandgígur moraines was deposited during a major Holocene re-advance of Skeiðarárjökull. The moraine ridge is buried by sediments dominated by glacifluvial deposits with an estimated sediment volume of 1.04 km³. We combine GPR-derived subsurface architecture and the surface morphology to develop a conceptual model detailing the geomorphic evolution of the moraine and surrounding region. These results provide new insights into the Holocene evolution of Skeiðarársandur, identifying the presence of a former major ice-margin position, as well as a past relative sea-level limit. Furthermore, we establish that sediment supply and available terrestrial accommodation space are dominant drivers in the formation and evolution of vast sandar environments.

Precipitation is a key climate parameter of vegetation and ecosystems in the Iberian Peninsula. Here, we use a regional pollen–climate calibration model and fossil pollen data from eight sites from the Atlantic coast to southern Spain to provide quantitative reconstructions of annual precipitation trends and excursions and their regional patterns for the last 11 700 years. The Early Holocene (11 700 to 11 000 cal. a BP) was characterized by high precipitation values followed by a slowly declining trend until about 9000 cal. a BP in the south and about 8000 cal. a BP in the north. From 8000 to 6000 cal. a BP the reconstructed precipitation values are the highest in most records, especially in those located in the Mediterranean climatic region in the southern part of the peninsula, with maximum values nearly 100% higher than the modern reconstructed values. The results suggest a declining precipitation during the Late Holocene in the south, with a positive excursion at around 2500 cal. a BP, while in the north precipitation remained high until 500 cal. a BP. However, the Late Holocene climate reconstructions in the Iberian Peninsula are biased by intensifying human impact on vegetation. The statistical time series analyses using SiZer technique do not indicate any statistically significant high-frequency drought events in the region. In general, our results suggest regional differences in the precipitation patterns between the northern and southern parts of the peninsula, with a more distinct Middle Holocene period of high humidity in the south.

Reconstructions of sea-surface conditions during the Holocene were achieved using three sediment cores from northeastern Baffin Bay (GeoB19948-3 and GeoB19927-3) and the Labrador Sea (GeoB19905-1) along a north–south transect based on sea-ice IP₂₅ and open-water phytoplankton biomarkers (brassicasterol, dinosterol and HBI III). In Baffin Bay, sea-surface conditions in the Early Holocene were characterized by extended (early) spring sea ice cover (SIC) prior to 7.6 ka BP. The conditions in the NE Labrador Sea, however, remained predominantly ice-free in spring/autumn due to the enhanced influx of Atlantic Water (West Greenland Current, WGC) from 11.5 until ~9.1 ka BP, succeeded by a period of continued (spring–autumn) ice-free conditions between 9.1 and 7.6 ka BP corresponding to the onset of Holocene Thermal Maximum (HTM)-like conditions. A transition towards reoccurring ice-edge and significantly reduced SIC conditions in Baffin Bay is evident in the Middle Holocene (~7.6–3 ka BP) probably caused by the variations in the WGC influence associated with the ice melting and can be characterized as HTM-like conditions. These HTM-like conditions are predominantly recorded in the NE Labrador Sea area shown by (spring–autumn) ice-free conditions from 5.9–3 ka BP. In the Late Holocene (last ~3 ka), our combined proxy records from eastern Baffin Bay indicate low in-situ ice algae production; however, enhanced multi-year (drifted) sea ice in this area was possibly attributed to the increased influx of Polar Water mass influx and may correlate with the Neoglacial cooling. The conditions in the NE Labrador Sea during the last 3 ka, however, continued to remain (spring–autumn) ice-free. Our data from the Baffin Bay–Labrador Sea transect suggest a dominant influence of meltwater influx on sea-ice formation throughout the Holocene, in contrast to sea-ice records from the Fram Strait area, which seem to follow predominantly the summer insolation trend.

The Arctic is a climate-sensitive area, responding rapidly to present changes, but for the past changes, the record is still incomplete. For instance, the Weichselian glacial history of the Svalbard–Barents Sea Ice Sheet (SBIS) has largely been reconstructed based on studies of the fragmentary Spitsbergen terrestrial and shelf records. However, the sediments removed from the land and shelf areas during peak glacials were deposited on trough mouth fans located along the continental slope. By studying the stratigraphy and processes of the trough mouth fans, comprising a more complete sediment archive, our new data have allowed gaps in the Weichselian glacial history of the SBIS to be refined and filled. Here we present new lithological and geochronological data from the Kongsfjorden Trough Mouth Fan, closely linked to the advance and decay of the SBIS. High-resolution TOPAS seismic profiles reveal three distinct packages of glacigenic debris flows (GDFs) within its upper stratigraphy, each interpreted to represent an advance of the SBIS to the shelf edge. A radiocarbon dated, 12.6-m-long core from the southern flank of the Kongsfjorden Trough Mouth Fan penetrates trough sediments directly linked to the youngest GDF package and terminates in the second GDF, allowing us to study the last two Kongsfjorden ice-stream advances in greater detail than was previously possible. The age model of core GS10-164-09PC, based on combining ¹⁴C-, ¹⁸O-stable isotope and magnetic susceptibility data, spans the last ~54 ka. An Early Weichselian glacial advance is tentatively dated to have ended at ~90 ka. A second peak glaciation is estimated at ~70 ka, followed by a deglaciation from ~54 ka. An ice rafted debris-rich unit (U7) dated between 38 and 34 ka, followed by a plumite (U6), indicates an advance of unknown extent. The Last Glacial Maximum advance is dated to before 24 ka BP, followed by a rapid deglaciation at ~15 ka. The presence of coarser-grained sorted sediments at the present seafloor is attributed to the influence of the West-Spitsbergen Current, acting on water depths of at least 846 m, and is thought to have worked in the vicinity of the coring site since ~14 ka BP.

For the first time, the Holocene history of the Serp-i-Molot tidewater glacier at the eastern side of the Novaya Zemlya archipelago, Kara Sea, has been reconstructed based on AMS ¹⁴C dating and multiproxy analyses (lithology, mineralogy, geochemistry and microfossils) of glaciomarine sediments from the Tsivol’ki Fjord. Three main phases of the Holocene evolution of the fjord environment are identified. During the early phase, c. 10.6 to 4.6 cal. ka BP, the glacier front was located close to the fjord mouth. The middle phase, c. 4.6 to 0.9 cal. ka BP, was characterized by frontal glacier retreat and shallowing of the fjord owing to glacio-isostatic uplift of the North Island of Novaya Zemlya. The late phase, from 0.9±0.1 cal. ka BP until present, reflects the stabilization of the glacier front in the inner fjord. The absence of major Neoglacial advances of the Serp-i-Molot glacier is explained by the local climate of the Kara Sea and Novaya Zemlya, which prevents the penetration of moist air flow from the North Atlantic.

Boreal peatlands are facing significant changes in response to a warming climate. Sphagnum mosses are key species in these ecosystems and contribute substantially to carbon sequestration. Understanding the factors driving vegetation changes on longer time scales is therefore of high importance, yet challenging since species changes are typically affected by a range of internal and external processes acting simultaneously within the system. This study presents a high-resolution macrofossil analysis of a peat core from Store Mosse bog (south-central Sweden), dating back to nearly 10 000 cal. a BP. The aim is to identify factors driving species changes on multidecadal to millennial timescales considering internal autogenic, internal biotic and external allogenic processes. A set of independent proxy data was used as a comparison framework to estimate changes in the bog and regional effective humidity, nutrient input and cold periods. We found that Store Mosse largely follows the expected successional pathway for a boreal peatland (i.e. lake → fen → bog). However, the system has also been affected by other interlinked factors. Of interest, we note that external nutrient input (originating from dust deposition and climate processes) has had a negative effect on Sphagnum while favouring vascular plants, and increased fire activity (driven by allogenic and autogenic factors) typically caused post-fire, floristic wet shifts. These effects interactively caused a floristic reversal and near disappearance of a once-established Sphagnum community, during which climate acted as an indirect driver. Overall, this study highlights that the factors driving vegetation change within the peatland are multiple and complex. Consideration of the role of interlinked factors on Sphagnum is crucial for an improved understanding of the drivers of species change on short- and long-term scales.

The Mammoth Faunas, the famous cold-adapted mammal assemblages, were distributed widely across northern Eurasia and North America during the Late Pleistocene. The now extinct woolly rhinoceros, Coelodonta antiquitatis, was a major component. Abundant fossil remains of this species with radiocarbon dates have been reported through almost all of northern Eurasia, but the fossil rhinoceroses of Mongolia are poorly known. Here, we describe a rhinocerotid skeleton from Ondorkhaan, eastern Mongolia, and compare it with four Late Pleistocene rhinoceros species of northern Eurasia (Elasmotherium sibiricum, Stephanorhinus hemitoechus, Stephanorhinus kirchbergensis and C. antiquitatis), resulting in its identification as a woolly rhinoceros (C. antiquitatis). Accelerator mass spectrometry radiocarbon dates obtained from two samples (ODK01 and ODK02) of the skeleton essentially agree, 42 160–40 040 cal. a BP and 42 105–39 955 cal. a BP, and the two samples had δ¹³C values of −19.5‰ and −20.2‰ SMMKW, respectively. This find suggests that the Mammoth Faunas were distributed in eastern Mongolia c. 45–40 ka during the period of climatic amelioration between Heinrich events 5 (H5: 46 000 cal. a BP) and 4 (H4: 39 000 cal. a BP).

Predicting the effects of ongoing climate warming on vegetation requires a long-term perspective of past ecosystem dynamics. We therefore analysed the sedimentary record from the mire Gola di Lago (985 m a.s.l.) in southern Switzerland, to better understand the vegetation response to past rapid climate change. We present a high-resolution pollen and plant macrofossil study from the Lateglacial to the Early Holocene (13 400–10 400 cal. a BP), a climatic transition that represents a close analogue to current global warming. The vegetation responses during this time match previously analysed palaeoecological sites south of the Alps. At the end of the Bølling-Allerød interstadial, the site was surrounded by open larch forest. The beginning of the Younger Dryas was characterized by the local presence and rapid expansion of Pinus cembra, whereas the second part was dominated by Pinus sylvestris, Betula and Larix decidua. These vegetation dynamics agree well with independent climate data indicating a cold and dry start and a subsequent shift to slightly warmer climate. The following rapid temperature increase at the beginning of the Holocene resulted in an increase in forest density and the establishment of novel temperate vegetation assemblages, without major changes in species diversity. Noteworthy, during the Younger Dryas–Early Holocene transition, long-lived cold-adapted tree line species such as P. cembra and L. decidua persisted over several centuries, while open boreal forests dominated by P. sylvestris and Betula expanded, indicating high resilience to climatic changes. The results of Gola di Lago suggest that climate warming led to a significant change in vegetation communities through species range shifts. Our study also indicates that some species may potentially be able to locally persist under current global warming, forming novel vegetation assemblages with newly establishing species.

Here we present the first fossil record of a bone assemblage that could have been accumulated by New World vultures (Cathartidae). The bone remains were found in an archaeological rockshelter called Gruta do Presépio (GPR), located in the tropical rainforest environment of Santa Catarina State, southern Brazil, where groups of hunters and gatherers lived from the Early to Middle Holocene (9.3–4.6 cal. ka BP) until the Late Holocene (1.3 cal. ka BP). The results of taphonomic analysis of the archaeological sample are compared with actualistic taphonomic modifications produced by New World vultures. The findings of autopodials (mainly phalanges) of medium and large mammals, with a high degree of digestive corrosion but without tooth marks, could be related to the pellet deposit pattern of New World vultures. Seven mammal taxa associated with a forest environment were identified, four of which were obtained from pellets deposited by Cathartidae; these included some native taxa (Cervidae, Dasypodidae, Felidae and Tayassuidae) that are frequently consumed today by these scavenger birds. The bone and tooth remains contained in the pellets regurgitated by Cathartidae were found together with faunal remains discarded by humans that lived in GPR from the Early to the Late Holocene. This palimpsest highlights the relevance of archaeological and actualistic taphonomic studies.

A digital database for submarine glacial landforms and sediments formed in the Arctic during and since the Last Glacial Maximum was created in order to facilitate and underpin new research on palaeo-ice sheets and tidewater glacier dynamics. The glacimarine database (GlaciDat) documents and standardises evidence of previous glacial activity as visible on the contemporary seafloor of fjords and continental shelves around Svalbard, Greenland, Alaska, northern Russia and north of 66°30′N in Canada and Norway. An extensive literature search was conducted to create GlaciDat, which compiles nearly 60 000 individual submarine landforms, more than 1000 sediment cores and 232 radiocarbon dates. Glacial landforms included are cross-shelf troughs, trough-mouth fans, grounding-zone wedges, lateral moraines, overridden moraines, (mega-scale) glacial lineations, drumlins, crag-and-tails, medial moraines, terminal moraines, debris-flow lobes (including glacier-contact fans), recessional moraines, De Geer moraines, crevasse-fill ridges, eskers, hill-hole pairs, crescentic scours, and submarine channels. They were digitised as point, line and polygon features alongside a list of their individual characteristics. Sediment core locations are attributed with a description of the sampled lithofacies and sedimentation rates where available. Landforms and sediments have been standardised according to predefined nomenclatures to make the glacial evidence as consistent as possible. Marine radiocarbon dates were included when thought to be relevant for constraining the timing of large-scale palaeo-ice dynamics. Outlines of bathymetric data sets, which have previously been used for glacial geomorphological mapping, were also included to give an overview of already investigated research areas. GlaciDat is available for download (https://doi.pangaea.de/10.1594/PANGAEA.937782) and will aid researchers in the reconstruction of past ice dynamics and the interpretation of Arctic glacial landform–sediment assemblages. Moreover, as well as providing a comprehensive bibliography on Arctic glacial geomorphological and sedimentological research, it is intended to serve as a basis for future modelling of Arctic glacier and ice-sheet dynamics.