Boreas最新文献

The analysis of faecal biomarkers in lake sediments has been used to reconstruct human population densities and animal husbandry practices in an increasing number of studies in recent years. However, terrigenous biomarkers can decompose in soils, can be stored and redeposited in colluvium and on flood-plains prior to their ultimate deposition in lakes. These and other effects can blur and distort biomarker signals. Therefore, we analysed sediments from two maars in Westeifel to evaluate whether signals of the faecal biomarkers (5β-stanols, bile acids) demonstrate statistically significant differences between contrasting periods in land-use intensity. In Holzmaar, palaeoenvironmental data showed evidence for agriculture including cereal cultivation and grassland during the pre-Roman Iron Age and Middle Ages compared with those from periods that were less influenced by land use and showed a higher abundance of broadleaf forest. However, the specific domesticated taxa of livestock in the locale from these periods remain speculative. We found statistically significantly different faecal biomarker signals, which we interpret to be related to an enhanced deposition of faeces of horses, pigs and ruminants in the core sections that represented periods of amplified land use. The analyses of grass- and broadleaf-tree characteristic n-alkanes supported the applicability of biomarkers for land-use reconstruction. Stanol data from a core section dating to the Mesolithic showed no clear results. Analyses of two core sections from Ulmener Maar, which covered periods before and after the decline of elm in the Neolithic, indicated input of pig faeces in the younger section. This study provides important evidence that faecal biomarkers can be used for land-use reconstruction in central European lakes with small catchment areas for time periods from the Neolithic onwards. The results underscore the importance of bile acid analyses in addition to stanol analyses for an identification of faeces inputs from different animals.

The physical, sedimentological, mineralogical and elemental geochemical properties of sediment cores AMD1803-02BC and 01PC from the Cape Norton Shaw Inlet were investigated to reconstruct glacial sediment discharges from southeastern Manson Icefield and document the impact of ice–ocean interactions on the sediment dynamics and opening of the North Water Polynya (NOW) in northwestern Baffin Bay since the last deglaciation. Laminated glaciomarine sediments rich in quartz and feldspar are observed prior to 11 cal. ka BP and were probably deposited by hyperpycnal currents triggered by the local retreat of the southern margin of the Innuitian Ice. Detrital proxies suggest that Early Holocene sediment dynamics were mainly influenced by sea ice and iceberg rafting and meltwater discharges related to the deglaciation of eastern Smith (~11 to 10.65 cal. ka BP) and Jones (~10.7 cal. ka BP) sounds. This also provides an upper limit to the timing of formation of the NOW. The high detrital carbonate contents during 8.8 to 6.6 cal. ka BP confirm that enhanced carbonate-rich sediment export from Nares Strait to northern Baffin Bay occurred during and after the deglaciation of Kennedy Channel (8.8 to 8.2 cal. ka BP). Canadian Shield sediment inputs have dominated since 6.6 cal. ka BP, indicating that sedimentation is mainly influenced by Cape Norton Shaw glacier discharges. The lower level of sedimentation recorded in core 01PC during the Middle to Late Holocene suggests an accelerated landward retreat of the Cape Norton Shaw glaciers in response to warmer marine conditions. During the Neoglacial period, higher sedimentation rates and detrital proxies in the cores suggest increased glacial erosional processes, probably associated with the long-term declines in boreal summer insolation and glacier growth. Finally, mineralogical and grain-size data in core 02BC support the idea that increased Arctic atmospheric temperatures have had an important influence on the glacial dynamics during the industrial period.

Palaeoclimatic evidence is necessary to place the current warming and drying trends of the Mediterranean region in a long-term perspective of pre-industrial variability. Annually resolved and absolutely dated climate proxies that extend back into medieval times are, however, limited to a few sites only. Here we present a network of long ring width chronologies from Pinus nigra tree-line sites in northern Corsica (France) that cohere exceptionally well over centuries and support the development of a single high-elevation pine chronology extending back to 974 CE. We apply various detrending methods to these data to retain high-to-low frequency ring width variability and scale the resulting chronologies against instrumental precipitation and drought observations to produce hydroclimate reconstructions for the last millennium. Proxy calibration and transfer are challenged by a lack of high-elevation meteorological data, however, limiting our understanding of precipitation changes in sub-alpine tree-line environments. Our new reconstructions extend beyond existing records and provide evidence for low-frequency precipitation variability in the central-western Mediterranean from 974–2016 CE. Comparison with a European scale drought reconstruction network shows that regional predictor chronologies are needed to accurately estimate long-term hydroclimate variability on Corsica.

Arid central Asia is a key region in the study of global climate change; however, the characteristics and mechanisms of regional hydroclimate changes during the late Quaternary remain poorly known. Here we present a new hydroclimate record from Lake Ebinur in arid northwestern China based on a comprehensive multiproxy analysis. The results show that Lake Ebinur formed at ~33.5 ka under relatively warm and humid conditions that continued to 26.7 ka. The following Last Glacial Maximum was cold and dry from 26.7 to 18.5 ka, most notably during the interval between 21.3 and 18.5 ka, suggesting a two-step hydroclimate change. The moisture conditions started to improve at 15 ka and reached their highest level during the Middle Holocene (8.7–4.4 ka). A comparison of our record with other records in the Northern Hemisphere indicated that the moisture changes in central Asia followed similar variability trends to those of the east Asia monsoon region during the last glacial period, suggesting common driving forces, such as the boreal solar insolation and its associated Northern Hemisphere ice-sheet volume. The Holocene moisture optimum in central Asia was delayed by 2000–3000 years relative to that in east Asia, which demonstrated a large influence of the remnant Northern Hemisphere ice-sheet forcings on the mid-latitude Asia atmospheric circulation (e.g. mid-latitude westerlies and Siberian High) in the Early Holocene.

Increased hydrological connectivity due to permafrost degradation is likely to have substantial implications for shallow aquatic systems common to sub-arctic landscapes due to changes to overland and subsurface flow of water and transport of sediments and dissolved nutrients. Here, we explore the influence of increased connectivity on aquatic productivity based on multi-parameter palaeolimnological analysis of two lakes located near Inuvik (Northwest Territories, Canada). We contrast a lake with little evidence of permafrost degradation in the surrounding area (Lake PG03) to one that has multiple connections to the terrestrial landscape through a network of thaw polygons in the lake catchment (Lake PG09). Comparisons of biological indicators (chironomids) and organic carbon and nitrogen elemental and isotope composition reveal recent divergent lake histories. The chironomid assemblage of Lake PG03 followed an expected temperature gradient, with a warming signal evident since ˜1970 CE, whereas the chironomid assemblage of Lake PG09 was found to primarily respond to nutrient availability and changes in habitat, likely as a result of increasing hydrological connectivity to the landscape. Rapid assemblage and habitat change along with a prominent increase in chironomid abundance were observed at Lake PG09 after ˜1960 CE, following a shift to greater inputs from the terrestrial environment as indicated by high C:N ratios (>15) and low δ¹³C_org (−30‰). Increased aquatic productivity following high allochthonous additions (˜1960–2014 CE) is supported by decreased C:N and rapidly increasing organic matter (C_org, N). These results demonstrate that increased connectivity along the terrestrial–aquatic interface for lakes is likely to foster elevated productivity in the future. Likewise, increased production poses a challenge to chironomid-inferred July air temperature reconstructions in lakes that are less resilient to secondary gradients, where analogue mismatches can occur due to shifts in dominance of indicators that are orthogonal to the temperature gradient.

Brown bears are one of the few large carnivore species that survived the final Pleistocene wave of extinctions, perhaps in part owing to their wide ecological plasticity, variety of forms and polyphagia. Although the brown bear has become a well-studied system, many questions remain regarding the ecological, trophic and genetic diversity throughout their distribution. For example, knowledge about Asiatic Russian brown bears from the Late Pleistocene arctic tundra steppe, an ecosystem with no analogue in modern times, is sparse. Here we compared diets, morphometry and genetic affinities of Late Pleistocene bears based on broadly sampled subfossil remains from Asiatic Russia. Collecting sites included the Ural Mountains, the lower reaches of the Irtysh River, the upper reaches of the Ob River, the Altai Mountains of western Siberia, the Indigirka–Kolyma Lowlands and northwestern Chukotka. An extremely large bear specimen from the middle Indigirka (41 090 ¹⁴C a BP) that lived in landscapes of treeless shrubs and wet meadows had a diet composed principally of large herbivorous mammals. A bear from western Chukotka (25 880 ¹⁴C a BP), much smaller in size, had a diet close to that of modern brown bears. These two Late Pleistocene NE Russian brown bears may comprise a previously undiscovered, but extinct, genetic lineage. At the end of the Pleistocene (MIS 3 and MIS 2), the brown bears from the Ob River Valley and Urals lived in periglacial forest-steppes and those from the southern Urals in conditions of periglacial steppe. Brown bears from the Ob River valley and Urals, as well as ancient Altai bears, were characterized by a varied diet, from polyphagia to vegetarianism. In living brown bears, the proportions of different dietary foods are primarily related to food availability, which depends on the geographical zone and climatic conditions. We conclude that the same was true for Late Pleistocene brown bears of NE Siberia.

Reconstruction of deglacial ice margins provides insights into the demise of past ice sheets and ice-marginal lakes and helps to understand how former ice sheets responded to climate change. Here, we reconstruct deglacial Laurentide Ice Sheet margins across Manitoba (Canada), a dynamic region that in MIS 2 spanned from an inner core region of the Keewatin dome to the periphery of the ice sheet (~900 km north of the Last Glacial Maximum limit). The area was also overrun by ice flowing from both the Quebec-Labrador dome and the Hudson Bay Ice Saddle. The surficial landscape of Manitoba contains inherited relict and palimpsest glacial landscapes, which need to be separated from deglacial features. Ice-impounded glacial Lake Hind was present in southwest Manitoba at 13.0 cal. ka BP, meaning most of Manitoba was covered by ice at the start of the Younger Dryas. Northwest drainage of glacial Lake Agassiz in front of the Highrock Lake–Cree Lake moraine could have occurred near the end of the Younger Dryas, prior to 11.5 cal. ka BP, though the volume of the lake varies greatly depending on ice-margin reconstructions. Our interpretation is incompatible with the hypothesis that Lake Agassiz drainage to the Arctic Ocean triggered the Younger Dryas climatic cooling. Numerous ice streams developed across central and southern Manitoba during deglaciation, including the Souris, Red River, The Pas, Hayes and Quinn Lake. The dominant ice source was from the north early in deglaciation, switching to the northeast with growth of the Hudson Bay Ice Saddle and then back to the north again with demise of the saddle. The ice-margin ages are largely unconstrained, and thus we are unable to accurately assign climatic drivers to various ice stream events. Nonetheless, we record the development and demise of terrestrial ice streams over both hard-bed and soft-bed substrates.

Mammal remains preserved in the permafrost zone often bear traces of postmortem transformations, reflecting aspects of the palaeoenvironment and the processes that took place during the accumulation of host sediments. Multidisciplinary studies including radiocarbon dating, infrared spectroscopy, and microfossil analyses and grain size of infilling sediments from remains allow recognition of their stratigraphical and palaeogeographical origins and facilitate reconstructions of taphonomic pathways and the pre-burial environments. Here, as exemplified by skulls of woolly rhinoceros, cave lion, and ancient bison, some distinct features of postmortem changes such as nonpyrogenic charring and vivianite encrustation indicate that the remains have undergone a complex range of burial processes in aerobic and anaerobic conditions in the Pleistocene landscapes of Arctic northeastern Russia. We hypothesize that these processes were mainly confined to the warmer intervals in the Late Pleistocene.

Lake-level reconstructions are a key tool in hydro-climate reconstructions, based on the assumption that lake-level changes primarily reflect climatic changes. Although it is known that land cover changes can affect evapotranspiration and groundwater formation, this factor commonly receives little attention in the interpretation of past lake-level changes. To address this issue in more detail, we explore the effects of land cover change on Holocene lake-level fluctuations in Lake Tiefer See in the lowlands of northeastern Germany. We reconstruct lake-level changes based on the analysis of 28 sediment records from different water depths and from the shore. We compare the results with land cover changes inferred from pollen data. We also apply hydrological modelling to quantify effects of land cover change on evapotranspiration and the lake level. Our reconstruction shows an overall lake-level amplitude of about 10 m during the Holocene, with the highest fluctuations during the Early and Late Holocene. Only smaller fluctuations during the Middle Holocene can unambiguously be attributed to climatic fluctuations because the land cover was stable during that period. Fluctuations during the Early and Late Holocene are at least partly related to changes in natural and anthropogenic land cover. For several intervals the reconstructed lake-level changes agree well with variations in modelled groundwater recharge inferred from land cover changes. In general, the observed amplitudes of lake-level fluctuations are larger than expected from climatic changes alone and thus underline that land cover changes in lake catchments must be considered in climatic interpretations of past lake-level fluctuations.

Understanding past responses of ice sheets to climate change provides an important long-term context for observations of present day, and projected future, ice-sheet change. In this work, we reconstruct the deglaciation of the marine-terminating western margin of the Scandinavian Ice Sheet in the outer Hardangerfjorden area of southwestern Norway, following the Last Glacial Maximum (LGM) until the start of the Holocene. We base our interpretations on a combination of geomorphological mapping using high-resolution (LiDAR) terrain models, 68 new cosmogenic nuclide ¹⁰Be exposure ages and radiocarbon-dated lake sediment cores, supported by the stratigraphic position of the 12.1 ka Vedde Ash. We show that even the highest mountain summits in the area (~1200–1400 m a.s.l.) were ice-covered during the LGM, thus settling debates concerning the Scandinavian Ice Sheet thickness in this region. These summits emerged as nunataqs through the ice sheet about 22–18 ka, potentially owing to upstream ice thinning caused by the break-up and retreat of the Norwegian Channel Ice Stream. Following the break-up of the Norwegian Channel Ice Stream, the ice margin seemingly stabilized at the outermost coast for 3500–5500 years before the mouth of Hardangerfjorden became ice free at c. 14.5 ka. Subsequently, during the Bølling and Allerød periods, the ice sheet retreated rapidly into the inner parts of Hardangerfjorden before a major ice sheet re-advance during the Younger Dryas. We identify and reconstruct a sizeable, independent ice cap on the Ulvanosa mountain massif during the Younger Dryas (YD), a massif that earlier was mapped as covered by the Scandinavian Ice Sheet during the YD. We also document ice-free areas that are more extensive than previously thought between Hardangerfjorden and Matersfjorden during the YD.