Boreas最新文献

Northern Eurasia underwent major hydroclimatic changes since the beginning of the Holocene interglacial. A rapid warming period reaching the Holocene Thermal Maximum (HTM), followed by a general cooling trend until recent times, was observed in Eurasian lacustrine diatom oxygen isotope (δ¹⁸O_diatom) records. In this study, we present a new Holocene δ¹⁸O_diatom record from Lake Khamra (59.99°N, 112.98°E, Siberia). Our record aligns with Holocene δ¹⁸O_diatom records across the Northern Hemisphere, showing a general millennial-scale cooling trend following an initial maximum at 11.2 cal. ka BP and a second maximum at 6.7 cal. ka BP. These maxima correspond to the summer insolation maximum and elevated Northern Hemisphere air temperatures, as well as increased bioproductivity. Variability on centennial scales is likely driven by precipitation changes, which coincide with higher sedimentation rates and overlay the general decreasing trend throughout the Holocene. In addition, we compared two multiproxy datasets with decadal resolution from Lake Khamra, including δ¹⁸O_diatom data and biogeochemical proxies such as total organic carbon (TOC), total nitrogen (TN), stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes, and total mercury (THg). The datasets cover a ~210-year period (c. 6.140–6.350 cal. ka BP) at the end of the HTM and a recently published ~220-year record (c. 1790–2015 CE) that embraces the anthropogenic times. The comparison of these two warm phases reveals distinct differences in both the absolute values and the variability of the records. Regarding the δ¹⁸O_diatom data, the recent period shows a nearly threefold increase in range and double the standard deviation, suggesting greater hydroclimatic variability compared to the end of the HTM. Notably, THg levels indicate a sharp increase in recent decades, while δ¹³C declined, contrasting with the observations at the end of the HTM. We attribute these observations partially to far-reaching anthropogenic effects on remote lake systems.

The site of Barmaky currently marks the oldest Epigravettian occupation of north-western Ukraine shortly after the Last Glacial Maximum (LGM), dated to around 19 cal. ka BP. Stable isotope analyses of bone collagen from six terrestrial mammals and two bird species show a comparatively highly structured palaeo-food web. Characteristic are high δ¹⁵N values in mammoth (Mammuthus primigenius) bone collagen compared with other herbivores and δ¹³C values showing niche partitioning between mammoth and hare on the one hand and reindeer on the other. These findings may indicate differentiated herbivore niches, with different diets and habitats between herbivorous groups. Additionally, different mobility patterns could cause isotopic scatter in food webs. To better understand the context of early post-LGM mammoth behaviour, we compare the mammoth δ¹⁵N values of Barmaky (19.0 cal. ka BP; 7.2±0.1‰) with those of Yudinovo (lower layer: 19.1/18.3–15.1 cal. ka BP and upper layer: 14.8–13.8 cal. ka BP; 7.3±1.0‰), Yeliseevichi (17.6 cal. ka BP; 5.7±0.8‰) and Mezhyrich (18.5–17.5 cal. ka BP; 4.3±0.8‰). At all sites, δ¹⁵N values of mammoths are comparatively high, except for Mezhyrich and a single specimen from Barmaky. In comparison with sedentary hares, mammoths from Barmaky suggest a non-locally dominated δ¹⁵N signal, while the mammoths from Mezhyrich show the local isotope signal. We also discuss the potential role of differences in temperature, precipitation and permafrost development for the different locations. Results from climate models indicate a gradient in temperature and precipitation with possible implications for food availability in the mammoth steppe.

This paper presents the results of a dual approach for assessing fossil benthic foraminiferal communities using both traditional morphology-based analyses and sedimentary ancient DNA (sedaDNA) metabarcoding. The main objectives are to test the feasibility of sedaDNA analyses to assess foraminiferal biodiversity in temperate shelf sediments (Le Croisic, France) off a major river system through time (Mid- to Late Holocene), and to point out the similarities and differences between classical and molecular methods. Our results show that, in contrast to the high foraminiferal diversity obtained from classic morphological analysis (over 140 taxa), the sedaDNA analysis yielded only 20 operational taxonomic units (OTUs), which can be considered as equivalent to species. This strongly suggests a bad preservation of foraminiferal DNA downcore, likely due to the relatively ‘high’ temperature of the study site (14 °C) and/or to a methodological bias (e.g. insufficient amount of extracted sediment). In the total sedaDNA, more than 90% of the reads were assigned to monothalamids (organic-shelled foraminifera). In contrast, only a small number of mineralized taxa, highly dominant when identified using the morphological approach, were detected. This could be due to the naturally higher abundance of monothalamids compared to hard-shelled foraminifera. While this abundance is reflected in sedaDNA, it is not preserved in fossil morphological assemblages. In addition, the sedaDNA of monothalamids might be easier to extract and their barcode to amplify than hard-shelled foraminifera. The discrepancies between the microfossil data and sedaDNA also include several species (e.g. Ammonia confertitesta (T6), Elphidium oceanense (S3), Nonionella sp. T4 and Nonionella sp. T6) that were rarely or not at all found in the fossil material which might be an indication of the presence of propagules, morphologically undetected in the >63 μm size fraction used. Finally, the presence of sequences of A. confertitesta and fossil specimens in the deep layers of the study cores suggests that this species, considered until now as recently invasive on the European coast, could have been present in the Atlantic coast several thousand years ago, much before any anthropogenic activity involving international shipping and commercial trades.

Low-relief dry channels are observed across the Bellevue-Castalia Karst Plain of north-central Ohio, USA. The area was repeatedly glaciated, and the channel forms are of interest because the linear pattern may be related to glacial processes. The 3–5 m deep channels are discontinuous, 0.15–0.4 km wide, and ~2–11 km long. Elongated large dolines of collapse/subsidence origin are parallel with ice flow, bedrock strike, fractures and joints, and are associated with the channels. There is no evidence of continued collapse since deglaciation. Smaller suffosion dolines developed within sediment are active and common across the landscape. Electrical resistivity tomography across uplands, channels, and large dolines indicates two units, a low-resistivity till over a high-resistivity limestone bedrock. Low-relief ridges a few hundred metres long are within, and parallel to, some channels, and are composed of bedrock with till veneer. Where till is exposed, it is tightly packed into dissolutionally enlarged fractures and cavities within bedrock, reducing groundwater connectivity into the subsurface. Channel orientations are offset and overlap with bedrock fractures, joints and elongated doline orientations. Three hypotheses are proposed to explain the channels. A subglacial meltwater channel hypothesis is supported by the channels incised into bedrock, covered in till and cross-cut by deglacial shorelines. A linear arrangement of collapse dolines hypothesis is supported by many channels orientated parallel to fractures within the bedrock and a crenulated channel margin. A fluviokarst origin for some channels relies upon preglacial subaerial streams progressively captured within Columbus Limestone bedrock, and subsequent non-selective glacial erosion. Erosion during the last glaciation may have been limited to ~1 m of bedrock, which would support preservation of karst features through successive glaciations. The second and third hypotheses are related, and with all three hypotheses these various previously unrecognized channels can be explained.

The glacial/interglacial cycles have shaped the landscape of temperate Europe for the past 2.5 million years, with open landscapes prevailing during the glacial and forested landscapes during the interglacial periods. However, the survival and recolonization strategies of temperate forest species during glacial phases remain poorly understood and hotly debated. This study investigates the persistence and postglacial dispersal of forest molluscs in the Western Carpathians by analysing molluscs from 126 Last Glacial and Early Holocene sites. Radiocarbon dating was applied to directly date shells of the target forest species, minimizing the risk of contamination from colluvial sediments. Our results confirm the presence of 15 forest snails in 33 sites in the region for the last 44 ka, with evidence of five species surviving the MIS 2. These findings support the hypothesis of localized microrefugia in the Western Carpathians, which allowed forest species to persist during unfavourable climatic conditions and facilitated their recolonization in the Holocene. In addition, our study highlights considerable temporal variation in mollusc successional patterns, with a sharp increase in forest species during the post-Last Glacial Maximum period. Despite these advances, the exact location of glacial (micro)refugia remains unclear for many species, highlighting the need for further research. This study provides new insights into the complex biogeographical history of forest molluscs in temperate Europe, emphasizing the need for high-resolution dating techniques and extensive sampling to accurately reconstruct past environmental changes.

Palaeoecological studies reporting long-term development histories of subarctic fens—explicitly, orohemiarctic peatlands—are scarce, and overall, permafrost-free peatlands located in the immediate vicinity of permafrost zones have received little attention in Fennoscandia. Here, we use a multiproxy approach to study the millennial-scale dynamics of two neighbouring peatlands located in Finnish Lapland (Katsapuli and Maader). In addition to studying autogenic succession and external forcing, we aimed to resolve the impact of surrounding landscape changes, potentially related to reindeer herding. The pollen data did not indicate major changes in regional vegetation, except for an increase in the proportion of sedges towards modern times and a decrease in the proportion of tree pollen. This, together with an increase in regional fire (microcharcoal) and erosion rates (measured as the mineral component in the sediments) in the area, coincided with both colder temperatures (Little Ice Age) and the emergence of reindeer-based pastoralism. The macrofossil peat plant data of the two profiles suggested a clear and relatively simultaneous local regime shift from sedge-dominated local habitat conditions to a Sphagnum community, where S. lindbergii became increasingly dominant towards the present day, suggesting a prevalence of relatively moist conditions. In both sites, the regime shift was coeval with the onset of Medieval Climate Anomaly climate conditions and is sustained thereafter. Vegetation changes in Sphagnum mosses induced high peat growth rates, and most of the peat stock is relatively young. Based on these data, we suggest that Sphagnum moss communities are resilient to climate fluctuations and might continue to act as effective carbon accumulation systems under warmer climates.

Portable optically stimulated luminescence (pOSL) analysis allows rapid relative dating of sediments that potentially can be calibrated against known OSL ages to provide an age estimate for landforms across a large area. This study uses 186 pOSL measurements and 20 OSL ages to investigate the postglacial landscape evolution inside the Younger Dryas moraines in NE Finnmark and Finnish Lapland. Aeolian sediments are widespread in the study area, with activity initiating in Finnmark around 10.2 ka. Increases in aeolian activity and dune formation occur from 6.5 ka across the study area, with a peak in the last 1500 years. Glacial sediments and landforms provided more challenging targets for both OSL and pOSL analyses and dating, but properties determined from pOSL proxy data show potential for determining sediment history of mobilization and deposition. Calibration of pOSL data against OSL age estimates, a developing method of luminescence dating, lets us build a chronology of activity from a larger data set, and in a much less time- and cost-consuming way than using only OSL would allow.

Marine-based ice sheets are susceptible to collapse and faster retreat than their land-terminating counterparts. One of the key factors contributing to the relative stability of marine-terminating ice sheets is the presence of shallow bathymetric banks and islands acting as pinning points for the ice sheet. This study examines the seabed geomorphology of Storbanken, a shallow bank in the central Barents Sea. The preserved geomorphology/bedforms visible in the bathymetry provide new insights into the dynamics of the palaeo-Svalbard Barents Sea Ice Sheet (SBSIS). Contrary to previous assumptions that ice rises are transient features within ice shelves, the findings suggest that the Storbakken marine ice dome was a persistent ice accumulation centre that probably detached from the SBSIS after the collapse of its divide at ∼14–15 ka and before the ice cap was confined mainly to Svalbard area ∼12 ka. The geomorphic imprint left on the seabed reveals evidence for active deglaciation and the reorganization of multiple small ice streams, responding to shifts in driving stress within the marine ice dome. This challenges traditional models of gradual ice margin retreat and suggests that smaller marine ice domes and rises may have been more common during the deglaciation than previously recognized. Additionally, the study highlights discrepancies between empirical observations and existing ice-sheet reconstructions, which typically depict a stepwise ice-margin retreat without considering the formation of independent ice rises. The findings underscore the need to reevaluate the role of shallow bathymetric banks in influencing the stability and decay of marine-based ice sheets, with concomitant implications for understanding ice-sheet behaviour under future climate scenarios, which is particularly relevant for the chiefly marine-based West Antarctic Ice Sheet.

The Mekong River is a mega-river in East and Southeast Asia, and precipitation in its watershed influences more than 300 million people. However, the controlling factor in precipitation and weathering in the basin remains controversial. Here we present a high-resolution record of weathering of the Mekong River basin over the past 3200 years by magnesium isotopes (δ²⁶Mg) of silicate components in a sediment core from an area off the Vietnam coast in the western South China Sea. The results show that the δ²⁶Mg values of silicates in marine sediments range from −0.40 to 0.036‰, and display evident change patterns. Our data support a theory that weathering in the Mekong River basin is controlled by three monsoon systems including the East Asian summer monsoon (EASM), the Indian summer monsoon (ISM), and the Western North Pacific summer monsoon (WNPSM), but the ISM may have played a more important role in the past century.