Showing papers in "Boreas in 2010"
TL;DR: In this article, the authors assume that ice-sheet basal conditions during Marine Isotope Stage 3 (MIS 3) are the same as those for the LGM, and then extrapolate to the earlier period using observationally constrained locations of the ice margins.
Abstract: Glacial rebound modelling, to establish constraints on past ice sheets from the observational evidence of palaeo-shoreline elevations, is well established for the post- Last Glacial Maximum (post-LGM) period, for which the observational evidence is relatively abundant and well distributed spatially and in time This is particularly the case for Scandinavia For the earlier part of the glacial cycle this evidence becomes increasingly sparse and uncertain such that, with the exception of the Eemian period, there are very few, if any, direct sea-level indicators that constrain any part of the Scandinavian Ice Sheet evolution before the LGM Instead, we assume that ice-sheet basal conditions during Marine Isotope Stage 3 (MIS 3) are the same as those for the LGM, focus on establishing these conditions from the rebound analysis for the LGM and Lateglacial period, and then extrapolate to the earlier period using observationally constrained locations of the ice margins The glacial rebound modelling and inversion follow previously established formulations, with the exception that the effects of water loading from proglacial lakes that form within the Baltic Basin and elsewhere have been included The data set for the inversion of the sea- and lake-level data has been extended to include marine-limit data in order to extend the observational record further back in time The result is a sequence of time slices for the Scandinavian Ice Sheet from the time of MIS 4 to the Lateglacial that are characterized by frozen basal conditions until late in the LGM interval when rapid thinning occurred in the eastern and southern sectors of the ice sheet The primary function of these models is as an interpolator between the fragmentary observational constraints and to produce quantitative models for the glaciation history with predictive capabilities, for example regarding the evolution of the Baltic Basin
206 citations
TL;DR: Wen et al. as discussed by the authors used the weighted averaging partial least squares method to reconstruct the Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia.
Abstract: Wen, R. L., Xiao, J. L., Chang, Z. G., Zhai, D. Y., Xu, Q. H., Li, Y. C. & Itoh, S. 2009: Holocene precipitation and temperature variations in the East Asian monsoonal margin from pollen data from Hulun Lake in northeastern Inner Mongolia, China. Boreas, 10.1111/j.1502-3885.2009.00125.x. ISSN 0300-9483.
Quantitative palaeoclimatic reconstruction with the weighted averaging partial least squares method was applied to the pollen profile from Hulun Lake in northeastern Inner Mongolia. The data provide a detailed history of variations in precipitation and temperature over the northeastern margin of the East Asian summer monsoon during the Holocene. A warm and dry climate prevailed over the lake region until c. 8000 cal. BP. During the period c. 8000–4400 cal. BP, precipitation increased markedly and temperature gradually declined. The interval between c. 4400 and 3350 cal. BP was marked by extremely dry and relatively cold conditions. Precipitation recovered from c. 3350 to 1000 cal. BP, with temperatures rising c. 3350–2050 cal. BP and dropping c. 2050–1000 cal. BP. During the last 500 years, the climate of the lake region displayed a general trend of warming and wetting. While Holocene temperature variations in the mid-high latitude monsoonal margin were controlled by changes in summer solar radiation in the Northern Hemisphere, they could also be related to the strength of the East Asian summer monsoon. The lack of precipitation during the early Holocene could be attributed to the weakened summer monsoon resulting from the existence of remnant ice sheets in the Northern Hemisphere. Changes in the monsoonal precipitation during the middle to late Holocene would have been associated with the ocean–atmosphere interacting processes occurring in the western tropical Pacific.
168 citations
TL;DR: Houmark-Nielsen et al. as discussed by the authors investigated the extent, age and dynamics of Marine Isotope Stage 3 glaciations in the southwestern Baltic Basin and found that the ice-sheet dynamics during the MIS 3 glacier expansion were similar to those for the post-LGM advances.
Abstract: Houmark-Nielsen, M. 2010: Extent, age and dynamics of Marine Isotope Stage 3 glaciations in the southwestern Baltic Basin. Boreas, 10.1111/j.1502-3885.2009.00136.x. ISSN 0300-9483
The southwestern Baltic region is known as a major crossroad for the expansion of Pleistocene glaciers from the Scandinavian Ice Sheet (SIS). At the peak of the Last Glacial Maximum (LGM, 25–20 kyr BP), steady-flowing inter-stream glaciers expanded radially from the major ice divide over central Scandinavia. During the subsequent deglaciation phase (20–15 kyr BP), streaming ice was flowing through the Baltic gateway onto the North European lowland. The lithology and directional ice-flow properties of pre-LGM till formations of Baltic provenance in Denmark (the Ristinge till and Klintholm till) suggest that the ice-sheet dynamics during the Marine Isotope Stage (MIS) 3 glacier expansion were similar to those for the post-LGM advances. Increasing geological evidence indicates that glaciers extended onto the Circum-Baltic lowlands during MIS 3. Reconstructions of flow paths and estimates of the basal ice-sheet coupling in Denmark suggest that southward flow of the SIS through the Baltic was probably the result of ice streaming. Despite methodological uncertainties, available OSL and 14C dates indicate that glaciers advanced at least twice during the mild second half of the Middle Weichselian (c. 75–25 kyr BP), most probably in connection with Dansgaard-Oeschger (D-O) events 14–13 (54–46 kyr BP) and 8–5 (35–30 kyr BP). The chronology and dynamics of glacier expansion in the southwestern Baltic in response to long-term cooling trends, the contemporary presence of a low Arctic biota in large parts of Scandinavia and of possible leads or lags in relation to North Atlantic climate changes during MIS 3 are discussed.
104 citations
TL;DR: Frenzel et al. as discussed by the authors presented an illustrated key for brackish water Ostracoda (Crustacea) from the Baltic Sea, based exclusively on valve morphology, providing a taxonomical base for geoscience and biological studies.
Abstract: Frenzel, P., Keyser, D. & Viehberg, F.A. 2010: An illustrated key and (pala6e6o)ecological primer for Postglacial to Recent Ostracoda (Crustacea) of the Baltic Sea. Boreas, Vol. 39, pp. 567–575. 10.1111/j.1502-3885.2009.00135.x. ISSN 0300-9483
This synopsis of Baltic Sea brackish water Ostracoda gives an overview of all known Recent species for the first time. It also includes Holocene taxa now extinct in the area. There are 131 species, two of which are recorded only from Yoldia stage (Preboreal) sediments. The illustrated key is based exclusively on valve morphology, providing a taxonomical base for geoscience and biological studies using ostracods from the Baltic Sea area. A list of ecological tolerances and preferences as well as the latitudinal distribution of all species is intended as a reference for palaeoenvironmental analyses. Salinity, temperature and oxygen tolerance values as well as preferences for latitudinal distribution, water depth and energy, habitat and substrate are given. The data are based on quantitative sampling in the southern, central and western Baltic Sea and on information gained from literature.
91 citations
TL;DR: Heikkila et al. as discussed by the authors presented a pollen-based summer temperature (Tsummer) reconstruction revealing the Holocene climate history in southeastern Latvia and contributes to the limited understanding of past climate behaviour in the eastern sector of northern Europe.
Abstract: Heikkila, M. & Seppa, H. 2010: Holocene climate dynamics in Latvia, eastern Baltic region: a pollen-based summer temperature reconstruction and regional comparison. Boreas, Vol. 39, pp. 705–719. 10.1111/j.1502-3885.2010.00164.x. ISSN 0300-9483.
A pollen-based summer temperature (Tsummer) reconstruction reveals the Holocene climate history in southeastern Latvia and contributes to the limited understanding of past climate behaviour in the eastern sector of northern Europe. Notably, steady climate warming of the early Holocene was interrupted c. 8350–8150 cal. yr BP by the well-known 8.2 ka cold event, recorded as a decrease of 0.9 to 1.8 °C in Tsummer. During the Holocene Thermal Maximum, c. 8000–4000 cal. yr BP, the reconstructed summer temperature was ∼2.5–3.5 °C higher than the modern reconstructed value, and subsequently declined towards present-day values. Comparison of the current reconstruction with other pollen-based reconstructions in northern Europe shows that the 8.2 ka event is particularly clearly reflected in the Baltic region, possibly as a result of distinct climatic and ecological gradients and the sensitivity of the vegetation growth pattern to seasonal temperature change. The new reconstruction also reveals that the Holocene Thermal Maximum was warmer in Latvia than in central Europe and Fennoscandia. In fact, a gradient of increasing positive temperature anomalies is detected from northernmost Fennoscandia towards the south and from the Atlantic coast in Norway towards the continental East European Plain. The dynamics of the temperate broadleaved tree species Tilia and Quercus in Latvia and adjacent northern Europe during the mid-Holocene give complementary information on the multifaceted climatic and environmental changes in the region.
81 citations
TL;DR: Zhu et al. as discussed by the authors used magnetic and loss-on-ignition (LOI) measurements, along with a 3-m long peat sequence recovered from the Dajiuhu Basin, the Shennongjia Mountains in Central China.
Abstract: Zhu, C., Ma, C., Yu, S.-Y., Tang, L., Zhang, W. & Lu, X. 2009: A detailed pollen record of vegetation and climate changes in Central China during the past 16 000 years. Boreas, 10.1111/j.1502-3885.2009.00098.x. ISSN 0300-9483.
Detailed pollen analyses, along with magnetic and loss-on-ignition (LOI) measurements, were conducted on a 3 m long peat sequence recovered from the Dajiuhu Basin, the Shennongjia Mountains in Central China. Ten AMS 14C dates provide a firm age control on this pollen record in terms of vegetation changes governed essentially by the rise and fall of the Asian summer monsoon during the past 16 000 years. Between 16 000 and 12 700 cal. yr BP, pollen assemblages were dominated by coniferous and broad-leaved trees, indicating a mixed forest landscape corresponding to the initial establishment of the monsoonal climate after the Last Glaciation. The progressive increases in percentages of evergreen tree pollen after 12 700 cal. yr BP point to a steady enhancement of the summer monsoon, which was episodically weakened during the Younger Dryas stadial. From 11 000 to 6000 cal. yr BP, values of coniferous and deciduous tree pollen decreased, while evergreen broad-leaved tree pollen increased substantially, implying a stronger than normal monsoonal climate condition corresponding to the Holocene Hypsithermal Interval. A great reduction in the values of evergreen tree pollen at about 4000 cal. yr BP indicates a sudden retreat of the summer monsoon from this area.
75 citations
TL;DR: The Tianshan mountain range has been extensively and repeatedly glaciated during the late Quaternary. Multiple moraines and their ages are described in three sub-regions as discussed by the authors.
Abstract: The Tianshan mountain range has been extensively and repeatedly glaciated during the late Quaternary. Multiple moraines in this region record the extent and timing of late Quaternary glacier fluctuations. The moraines and their ages are described in three sub-regions: eastern, central and western Tianshan. Notable glacial advances occurred during marine oxygen isotope stages (MIS) 6, 4, 3, 2, the Neoglacial and the Little Ice Age (LIA) in these sub-regions. Glaciers in western Tianshan advanced significantly also during MIS 5, but not in eastern and central Tianshan. The local last glacial maximum (llgm) of the three sub-regions pre-dated the Last Glacial Maximum (LGM) and occurred during MIS 4 in eastern and central Tianshan, but during MIS 3 in western Tianshan. The spatial and temporal distribution of the glaciers suggests that precipitation (as snow at high altitude) is the main factor controlling glacial advance in the Tianshan. The late Quaternary climate in the Tianshan has been generally cold-dry during glacial times and warm-humid during interglacial times. Between neighbouring glacial times, the climate has had a more arid tendency in eastern and central Tianshan. These palaeoclimatic conditions inferred from glacial landforms indicate important relationships between the mid-latitude westerly, the Siberian High and the Asian monsoon.
74 citations
TL;DR: Wohlfarth et al. as mentioned in this paper evaluated and evaluated 14C dates for Sweden older than the Last Glacial Maximum ice advance and found that central and northern Sweden was ice free during the early and middle part of Marine Oxygen Isotope Stage 3 and that southern Sweden remained ice-free until c. 25 cal. kyr BP.
Abstract: Wohlfarth, B. 2010: Ice-free conditions in Sweden during Marine Oxygen Isotope Stage 3? Boreas, 10.1111/j.1502-3885.2009.00137.x. ISSN 0300-9483
Published and unpublished 14C dates for Sweden older than the Last Glacial Maximum ice advance were evaluated. Acceptable 14C dates indicate that age ranges for interstadial organic material in northern and central Sweden are between c. 60 and c. 35 cal. kyr BP and for similar deposits in southern Sweden are between c. 40 and c. 25 cal. kyr BP, which is in good agreement with recently derived Optical Stimulated Luminescence ages. 14C dates on mammoth remains show a larger scatter, possibly as a result of incomplete laboratory pretreatment. A possible scenario based on calibrated 14C dates from interstadial deposits is that central and northern Sweden was ice-free during the early and middle part of Marine Oxygen Isotope Stage 3 and that southern Sweden remained ice-free until c. 25 cal. kyr BP. A first ice advance into northern and central Sweden might have occurred as late as around 35 cal. kyr BP, more or less coeval with the Last Glacial Maximum ice advance onto the Norwegian shelf. To test the conclusions drawn here, new multi-proxy and high-resolution investigations of several key sites in north, central and south Sweden are required.
67 citations
TL;DR: Matthews et al. as mentioned in this paper developed linear age-calibration curves using Holocene bedrock surfaces from the Jotunheimen-Jostedalsbreen regions of southern Norway Boreas.
Abstract: Matthews, J A & Owen, G 2009: Schmidt hammer exposure-age dating: developing linear age-calibration curves using Holocene bedrock surfaces from the Jotunheimen–Jostedalsbreen regions of southern Norway Boreas, 101111/j1502-3885200900107x ISSN 0300-9483
The approach to calibrated-age dating of rock surfaces using Schmidt hammer R-values is developed, potential errors in dating Holocene rock surfaces are estimated and limitations are assessed Multiple sites from glacially abraded bedrock outcrops of two ages (glacier forelands deglaciated for c 100 years and adjacent late-Preboreal terrain deglaciated for c 9700 years) are used to analyse the variability of mean R-values and to construct linear age-calibration curves for three sub-regions in the Jotunheimen–Jostedalsbreen regions of southern Norway Conservative potential dating errors of 246–632 years are estimated using 95% confidence intervals associated with two control points, the width of the error limits being significantly greater for the Preboreal surfaces than for the younger Little Ice Age surfaces Substantial improvements over previous age calibrations are largely attributable to the use of multiple sites as part of a research design that has effectively controlled for geological differences between the three sub-regions In the context of the Holocene time scale, the technique is seen as complementary to cosmogenic-nuclide dating (which currently has lower precision) and lichenometric dating (which has a lower temporal range)
67 citations
TL;DR: Kuzmin et al. as mentioned in this paper suggested that extinction of these large herbivores in Eurasia was closely related to landscape changes near the Pleistocene-Holocene boundary (c. 12,000-9000 uncalibrated radiocarbon years ago, yr BP), mainly involving the widespread forest formations in the temperate and arctic regions of northern Eurasia and the loss of grasslands crucial to the existence of woolly mammoth and rhinoceros.
Abstract: Kuzmin, Y. V. 2009: Extinction of the woolly mammoth (Mammuthus primigenius) and woolly rhinoceros (Coelodonta antiquitatis) in Eurasia: Review of chronological and environmental issues. Boreas, 10.1111/j.1502-3885.2009.00122.x. ISSN 0300-9483.
The current evidence for date and environmental preferences of the extinction of two middle–late Pleistocene megafaunal species, the woolly mammoth (Mammuthus primigenius Blum.) and woolly rhinoceros (Coelodonta antiquitatis Blum.), is presented in this review. It is suggested that extinction of these large herbivores in Eurasia was closely related to landscape changes near the Pleistocene–Holocene boundary (c. 12 000–9000 uncalibrated radiocarbon years ago, yr BP), mainly involving the widespread forest formations in the temperate and arctic regions of northern Eurasia and the loss of grasslands crucial to the existence of woolly mammoth and rhinoceros. However, some woolly mammoth populations survived well into the Holocene (up to c. 3700 yr BP), showing that the process of final extinction was fairly complex, with delays in some regions of up to several millennia. The possible role of Palaeolithic humans in the extinction of Late Pleistocene megafauna is also considered.
65 citations
TL;DR: Alkane biomarker and pollen data were obtained from a 15m-high and probably c 240kyr-old loess-like permafrost palaeosol sequence (Tumara Palaeosols Sequence, TPS) in northeast Siberia as mentioned in this paper.
Abstract: Alkane biomarker and pollen data were obtained from a 15-m-high and probably c 240-kyr-old loess-like permafrost palaeosol sequence (‘Tumara Palaeosol Sequence’, TPS) in northeast Siberia The alkane results were corrected for degradation effects by applying an end-member model and were evaluated by comparing them with the palynological results The two data sets are generally in good agreement and suggest that the lower part of the TPS developed mainly under larch forests, whereas the upper part of the sequence reflects the expansion of mammoth steppes during the Weichselian glaciation and finally reforestation during the Lateglacial and the early Holocene For the lower part of the TPS, the palaeoclimatic interpretation according to modern analogue methods would indicate warm, interglacial conditions, but this is at odds with the climate chronostratigraphy based on a multi-proxy palaeopedological approach and numeric dating Provided that the correlation of the discussed stratigraphic unit with the Late Saalian glaciation and the Marine Oxygen Isotope Stage 6 is correct, our results suggest that temperature was not a limiting factor for tree growth at that time Furthermore, it seems very likely that it was not mainly temperature changes but rather increasing aridity and continentality during the course of the last glacial that favoured the expansion of the mammoth steppe
TL;DR: Helmens et al. as discussed by the authors compared three sediment sequences from western, eastern and northeast Finland with respect to their chronology, vegetation reconstruction and climatic inferences, and the combined results suggest ice-free and warm conditions in major parts of eastern Fennoscandia in early MIS 3, possibly during Greenland Interstadial (GIS) 14 around 53kyr BP ago.
Abstract: Helmens, K. F. & Engels, S. 2010: Ice-free conditions in eastern Fennoscandia during early Marine Isotope Stage 3: lacustrine records. Boreas, 10.1111/j.1502-3885.2010.00142.x. ISSN 0300-9483.
The traditional notion that Fennoscandia was glaciated throughout Marine Isotope Stages (MIS) 4–2, from c. 70 kyr BP to the deglaciation 15–10 kyr BP ago, has been challenged during the last decade. Recent studies have shown that climate and environmental settings during MIS 3 were more dynamic than previously assumed, and lacustrine sediment bodies indicate open-water conditions for several sites in eastern Fennoscandia. In this study, three sediment sequences from western, eastern and northeast Finland are compared in detail with respect to their chronology, vegetation reconstruction and climatic inferences. OSL-dating places the sediments in early MIS 3. Pollen evidence suggests the presence of isolated birch trees and open birch forest close to the retreating ice margin, in contrast to vegetation reconstructions from central Europe, which indicate tree-less vegetation. Furthermore, reconstructions of climate using transfer functions have yielded surprising results, indicating present-day summer temperatures in northeast Finland. The combined results suggest ice-free and warm conditions in major parts of eastern Fennoscandia in early MIS 3, possibly during Greenland Interstadial (GIS) 14 around 53 kyr BP ago.
TL;DR: Zhou et al. as discussed by the authors used radiocarbon dates from the Hongyuan peatland in the Zoige Basin to reveal the long-term dynamics of an alpine wetland ecosystem on the eastern margin of the Tibetan Plateau over the last 13,500 years.
Abstract: Zhou, W., Yu, S.-Y., Burr, G. S., Kukla, G. J., Jull, A. J. T., Xian, F., Xiao, J., Colman, S. M., Yu, H., Liu, Z. & Kong, X. 2010: Postglacial changes in the Asian summer monsoon system: a pollen record from the eastern margin of the Tibetan Plateau. Boreas, Vol. 39, pp. 528–539. 10.1111/j.1502-3885.2010.00150.x. ISSN 0300-9483.
A new pollen record constrained by 32 AMS radiocarbon dates from the Hongyuan peatland in the Zoige Basin reveals the long-term dynamics of an alpine wetland ecosystem on the eastern margin of the Tibetan Plateau over the last 13 500 years. Changes in pollen assemblages and influxes suggest that local vegetation has experienced three distinct stages, from alpine coniferous forest–meadow (13 500–11 500 cal. a BP), through alpine coniferous forest (11 500–3000 cal. a BP), back to alpine coniferous forest–meadow (3000 cal. a BP–present). This record reflects an ecosystem response along a transition zone where the South Asian and East Asian monsoon systems may have had different palaeoclimatic influences. A comparison of this record with other pollen records across the Tibetan Plateau shows common features with regard to large-scale Holocene climatic changes, but highlights a pattern of regional temporal and spatial variability that depends on the topography and position relative to the South Asian and East Asian monsoon fronts.
TL;DR: Weckstrom et al. as discussed by the authors studied the initiation and lateral expansion patterns of five small sub-arctic peatlands in the Fennoscandian tree-line region.
Abstract: Weckstrom, J., Seppa, H. & Korhola, A. 2010: Climatic influence on peatland formation and lateral expansion in sub-arctic Fennoscandia. Boreas, Vol. 39, pp. 761–769. 10.1111/j.1502-3885.2010.00168.x. ISSN 0300-9843.
The initiation and lateral expansion patterns of five small sub-arctic peatlands in the Fennoscandian tree-line region were studied by 21 accelerator mass spectrometry (AMS) 14C-dated basal-peat samples representing three to six dates per site. The radiocarbon dates were converted to calendar years and are based on the median probability. When combined with earlier basal-peat dates from the region, four distinctive periods can be observed in the cumulative record of the dates. The early Holocene, from c. 10 000 to 8000 cal. yr BP, was characterized by the fast initiation and rapid expansion of peatlands, whereas at 8000–4000 cal. yr BP lateral expansion was modest. The most intensive period of peatland expansion occurred at the beginning of the late Holocene at c. 4000 to 3000 cal. yr BP, after which it slowed down towards the present. All these periods are in rough agreement with the main Holocene climatic periods in the area, namely the relatively warm and moist early Holocene, the warm and dry Holocene thermal maximum (HTM) at 8000–4000 cal. yr BP, and the start of the cooler and moister trend (neoglacial cooling) from c. 4000 cal. yr BP to the present, indicating a broad-scale climatic control on the lateral growth of sub-arctic peatlands in Fennoscandia. In order to study the lateral expansion of peatlands and to evaluate their Holocene succession patterns, more studies based on multiple dates from the same peatland are needed.
TL;DR: Mangerud et al. as discussed by the authors used accelerator mass spectrometry (AMS) 14C dates obtained on well-preserved bones from caves in western Norway to demonstrate that the coast was ice-free during the so-called Alesund Interstadial.
Abstract: Mangerud, J., Gulliksen, S. & Larsen, E. 2009: 14C-dated fluctuations of the western flank of the Scandinavian Ice Sheet 45–25 kyr BP compared with Bolling–Younger Dryas fluctuations and Dansgaard–Oeschger events in Greenland. Boreas, 10.1111/j.1502-3885.2009.00127.x. ISSN 0300-9483.
We present 32 accelerator mass spectrometry (AMS) 14C dates obtained on well-preserved bones from caves in western Norway. The resulting ages of 34–28 14C kyr BP demonstrate that the coast was ice-free during the so-called Alesund Interstadial. New AMS 14C dates on shells aged 41–38 14C kyr BP are evidence of an earlier (Austnes) ice-free period. The Alesund Interstadial correlates with Greenland interstadials 8–7 and the Austnes Interstadial with Greenland interstadials 12–11. Between and after the two interstadials, the ice margin reached onto the continental shelf west of Norway. These events can be closely correlated with the Greenland ice core stratigraphy, partly based on identification of the Laschamp and Mono Lake palaeomagnetic excursions. We found that the pattern of the NGRIP δ18O curves for the two periods Greenland Interstadial (GI) 8 to Greenland Stadial (GS) 8 and GI 1–GS 1 (Bolling–Younger Dryas) were strikingly similar, which leads us to suggest that the underlying causes of these climate shifts could have been the same. We therefore discuss some aspects of glacial fluctuations during the Bolling–Younger Dryas in order to elucidate processes during Dansgaard–Oeschger events.
TL;DR: In this paper, an improved method of the plant functional type modern analogues technique (PFT-MAT) was presented, in which environmental proxies and a moisture index (alpha, i.e. ratio of actual evapo-ranspiration to potential evapotranspiration) were used to constrain the selection of modern analogue.
Abstract: This study presents an improved method of the plant functional type modern analogues technique (PFT-MAT) in which environmental proxies and a moisture index (alpha, i.e. ratio of actual evapotranspiration to potential evapotranspiration) are used to constrain the selection of modern analogues. The method is tested using high-resolution, precisely dated palaeorecords (pollen, Pediastrum and delta 18O of authigenic carbonate) from Lake Bayanchagan, northern China. The unconstrained and constrained PFT-MAT produces general agreement for Holocene climate changes, with a wet period between 11 000 and 5500 cal. yr BP and a warm interval between 11 000 and 8000 cal. yr BP. However, there are significant differences in the details of their reconstruction. The constrained PFT-MAT generally yields smaller error bars for the reconstructed climate parameters than the unconstrained PFT-MAT. In addition, three prominent climatic events are identified from the constrained reconstructions; namely, a cold event around 8400 cal. yr BP and two warm events around 6000 and 2000 cal. yr BP, which is consistent with other regional palaeoclimatic records. Our data show that changes in tree components correlate well with alpha variations during the entire Holocene, with the highest tree components and highest alpha values between 8000 and 5500 cal. yr BP, indicating the dominant role of alpha in the growth of trees in northern China rather than single temperature or precipitation. The improved PFT-MAT is therefore an efficient method for quantitative reconstructions of palaeoclimate in arid and semi-arid regions.
TL;DR: Birks et al. as mentioned in this paper present a review of the history of palaeoclimatic research and the development of current ideas about climatic changes with particular reference to the Lateglacial and its rapid climatic fluctuations, such as the Younger Dryas period, and to the Holocene, with its thermal maximum and shifts in precipitation.
Abstract: Birks, H. J. B. & Seppa, H. 2010: Late-Quaternary palaeoclimatic research in Fennoscandia – A historical review. Boreas, Vol. 39, pp. 655–673. 10.1111/j.1502-3885.2010.00160.x. ISSN 0300-9483.
Ideas and understanding of Late-Quaternary climatic history, especially in the Lateglacial and the postglacial, were initiated in Fennoscandia in the 19th century with pioneering studies on peat stratigraphy, megafossils and macrofossils. This review traces the history of palaeoclimatic research and the development of current ideas about climatic changes, with particular reference to the Lateglacial and its rapid climatic fluctuations, such as the Younger Dryas period, and to the Holocene, with its thermal maximum and shifts in precipitation.
TL;DR: In this paper, state-of-the-art climate models were used to simulate climate conditions in Europe during Greenland Stadial (GS) 12 at 44 ka BP, and the results showed a cold climate on the global scale, with global annual mean surface temperatures 51C colder than the modern climate.
Abstract: State-of-the-art climate models were used to simulate climate conditions in Europe during Greenland Stadial (GS) 12 at 44 ka BP. The models employed for these simulations were: (i) a fully coupled atmosphere–ocean global climate model (AOGCM), and (ii) a regional atmospheric climate model (RCM) to dynamically downscale results from the global model for a more detailed investigation of European climate conditions. The vegetation was simulated off-line by a dynamic vegetation model forced by the climate from the RCM. The resulting vegetation was then compared with the a priori vegetation used in the first simulation. In a subsequent step, the RCM was rerun to yield a new climate more consistent with the simulated vegetation. Forcing conditions included orbital forcing, land–sea distribution, ice-sheet configuration, and atmospheric greenhouse gas concentrations representative for 44 ka BP. The results show a cold climate on the global scale, with global annual mean surface temperatures 51C colder than the modern climate. This is still significantly warmer than temperatures derived from the same model system for the Last Glacial Maximum (LGM). Regional, northern European climate is much colder than today, but still significantly warmer than during the LGM. Comparisons between the simulated climate and proxy-based sea-surface temperature reconstructions show that the results are in broad agreement, albeit with a possible cold bias in parts of the North Atlantic in summer. Given a prescribed restricted Marine Isotope Stage 3 ice-sheet configuration, with large ice-free regions in Sweden and Finland, the AOGCM and RCM model simulations produce a cold and dry climate in line with the restricted ice-sheet configuration during GS 12. The simulated temperature climate, with prescribed ice-free conditions in south-central Fennoscandia, is favourable for the development of permafrost, but does not allow local ice-sheet formation as all snow melts during summer.
TL;DR: Pilgrimstad in central Sweden is an important locality for reconstructing environmental changes during the last glacial period (the Weichselian) and its central location has implications for the Scand...
Abstract: Pilgrimstad in central Sweden is an important locality for reconstructing environmental changes during the last glacial period (the Weichselian). Its central location has implications for the Scand ...
TL;DR: In this article, a 30-metre long borehole in the central North Sea was used to study the history of the marine Late Pleistocene British-Fennoscandian Ice Sheet.
Abstract: Geological constraints on ice-sheet deglaciation are essential for improving the modelling of ice masses and understanding their potential for future change. Here we present a detailed interpretation of depositional environments from a new 30-metre long borehole in the central North Sea, with the aim of improving constraints on the history of the marine Late Pleistocene British–Fennoscandian Ice Sheet. Seven units characterise a sequence of compacted and distorted glaciomarine diamictons, which are overlain by interbedded glaciomarine diamictons and soft, bedded to homogeneous marine muds. Through correlation of borehole and 2D/3D seismic observations, we identify three palaeo-regimes. These are: a period of advance and ice-sheet overriding; a phase of deglaciation; and a phase of postglacial glaciomarine-to-marine sedimentation. Deformed subglacial sediments correlate with a buried suite of streamlined subglacial bedforms, and indicate overridding by the SE–NW-flowing Witch Ground ice-stream. AMS 14C dating confirms ice-stream activity and extensive glaciation of the North Sea during the Last Glacial Maximum, between c. 30 and 16.2 14C ka BP. Sediments overlying the ice-compacted deposits have been reworked, but can be used to constrain initial deglaciation to no later than 16.2 14C ka BP. A re-advance of British ice during the last deglaciation, dated at 13.9 14C ka BP, delivered ice-proximal deposits to the core site and deposited glaciomarine sediments rapidly during the subsequent retreat. A transition to more temperate marine conditions is clear in lithostratigraphic and seismic records, marked by a regionally-pervasive iceberg-ploughmarked erosion surface. The iceberg discharges that formed this horizon are dated to between 13.9 and 12 14C ka BP, and may correspond to oscillating ice-sheet margins during final, dynamic ice-sheet decay.
TL;DR: Wang et al. as mentioned in this paper presented rice phytolith evidence from six archaeological sites in the Guanzhong Basin, central North China, dating from c. 5500 to 2100 cal. a BP.
Abstract: The history of rice (Oryza sativa) cultivation in North China is ambiguous owing to a lack of evidence from rice remains with precise ages in archaeological sites. In this paper, we present rice phytolith evidence from six archaeological sites in the Guanzhong Basin, central North China, dating from c. 5500 to 2100 cal. a BP (calibrated/ calendar ages) based on 19 AMS-dates. The phytoliths found in the three archaeological sites located on the second river terrace (Quanhu, Yangguanzhai and Anban) include three types of phytoliths from rice, namely bulliform, parallel-bilobe and double-peaked. These findings suggest that the earliest cultivated rice in central North China occurred not later than c. 5690 cal. a BP. After c. 5500 cal. a BP, the farming pattern in the Guanzhong Basin was characterized by dominant dry crops (e.g. millets) and locally cultivated rice. A likely spread route of rice from the lower reaches of the Huanghe (Yellow) River towards the Guanzhong Basin in central North China is speculated to have happened at c. 5690 cal. a BP. The findings of this study help us to understand the farming pattern in the area and how rice spread across the semi-arid regions of East Asia.
TL;DR: In this article, a multi-dating approach was applied to proglacial deltaic ‘Hohenschotter’ deposits in locations outside the ice extent of the Last Glacial Maximum, and results of U/Th and luminescence dating suggest a correlation of the investigated deposits with MIS 6 and hence with the Riss Glaciation.
Abstract: The number and the timing of Quaternary glaciations of the Alps are poorly constrained and, in particular, the age of the Most Extensive Glaciation (MEG) in Switzerland remains controversial. This ice advance has previously been tentatively correlated with the Riss Glaciation of the classical alpine stratigraphy and with Marine Isotope Stage (MIS) 6 (186–127 ka). An alternative interpretation, based on pollen analysis and stratigraphic correlations, places the MEG further back in the Quaternary, with an age equivalent to MIS 12 (474–427 ka), or even older. To re-evaluate this issue in the Swiss glaciation history, a multi-dating approach was applied to proglacial deltaic ‘Hohenschotter’ deposits in locations outside the ice extent of the Last Glacial Maximum. Results of U/Th and luminescence dating suggest a correlation of the investigated deposits with MIS 6 and hence with the Riss Glaciation. Cosmogenic burial dating suffered from large measurement uncertainties and unusually high 26Al/10Be ratios and did not provide robust age estimates.
TL;DR: Dortch et al. as mentioned in this paper used geomorphic and 10Be terrestrial cosmogenic nuclide surface exposure dating (SED) methods to assess the timing and nature of late Quaternary glaciation and moraine stabilization.
Abstract: Dortch, J. M., Owen, L. A., Caffee, M. W. & Brease, P. 2009: Late Quaternary glaciation and equilibrium line altitude variations of the McKinley River region, central Alaska Range. Boreas, 10.1111/j.1502-3885.2009.00121.x. ISSN 0300-9483
Glacial deposits and landforms produced by the Muldrow and Peters glaciers in the McKinley River region of Alaska were examined using geomorphic and 10Be terrestrial cosmogenic nuclide (TCN) surface exposure dating (SED) methods to assess the timing and nature of late Quaternary glaciation and moraine stabilization. In addition to the oldest glacial deposits (McLeod Creek Drift), a group of four late Pleistocene moraines (MP-I, II, III and IV) and three late Holocene till deposits (‘X’, ‘Y’ and ‘Z’ drifts) are present in the region, representing at least eight glacial advances. The 10Be TCN ages for the MP-I moraine ranged from 2.5 kyr to 146 kyr, which highlights the problems of defining the ages of late Quaternary moraines using SED methods in central Alaska. The Muldrow ‘X’ drift has a 10Be TCN age of ∼0.54 kyr, which is ∼1.3 kyr younger than the independent minimum lichen age of ∼1.8 kyr. This age difference probably represents the minimum time between formation and early stabilization of the moraine. Contemporary and former equilibrium line altitudes (ELAs) were determined. The ELA depressions for the Muldrow glacial system were 560, 400, 350 and 190 m and for the Peters glacial system 560, 360, 150 and 10 m, based on MP-I through MP-IV moraines, respectively. The difference between ELA depressions for the Muldrow and Peters glaciers likely reflects differences in supraglacial debris-cover, glacier hypsometry and topographic controls on glacier mass balance.
TL;DR: In this article, a re-analysis of one of the main cores used to generate the precursor concept, OMEX-2K from the Goban Spur covering the last 30 ka, is presented to identify whether the British-Irish Ice Sheet (BIIS) IRD fluxes occur only as precursors before H layers.
Abstract: Increased fluxes of ice-rafted detritus (IRD) from European ice sheets have been documented some 1000–1500 years before the arrival of Laurentide Ice Sheet (LIS)-sourced IRD during Heinrich (H) events. These early fluxes have become known as ‘precursor events’, and it has been suggested that they have mechanistic significance in the propagation of H events. Here we present a re-analysis of one of the main cores used to generate the precursor concept, OMEX-2K from the Goban Spur covering the last 30 ka, in order to identify whether the British–Irish Ice Sheet (BIIS) IRD fluxes occur only as precursors before H layers. IRD characterization and planktonic foraminiferal δ18O measurements constrained by a new age model have enabled the generation of a continuous record of IRD sources, timing, frequency and flux, and of local contemporary hydrographic conditions. The evidence indicates that BIIS IRD precursors are not uniquely, or mechanistically, linked to H events, but are part of the pervasive millennial-scale cyclicity. Our results support an LIS source for the IRD comprising H layers, but the ambient glacial sections are dominated by assemblages typical of the Irish Sea Ice Stream. Light isotope excursions associated with H events are interpreted as resulting from the melting of the BIIS, with ice-sheet destabilization attributed to eustatic jumps generated by LIS discharge during H events. This positive-feedback mechanism probably caused similar responses in all circum-Atlantic ice-sheet margins, and the resulting gross freshwater flux contributed to the perturbation of the Atlantic Meridional Overturning Circulation during H events.
TL;DR: Changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice-sheet reconstructions, according to an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin.
Abstract: Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice-stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice-sheet reconstructions.
TL;DR: The Store Koldewey Formation as mentioned in this paper was found at 120m above sea level in northeast Greenland and was referred to the Olduvai normal polarity subchron at 1.95-1.78Ma BP based on palaeomagnetic studies, amino acid epimerization ratios and evidence from marine and non-marine fossils.
Abstract: Marine Quaternary deposits, here named the Store Koldewey Formation, are found at ~120m above sea level in northeast Greenland (76°N). The sequence is referred to the Olduvai normal polarity subchron at 1.95-1.78Ma BP based on palaeomagnetic studies (palaeomagnetically reversed), amino acid epimerization ratios and evidence from marine and non-marine fossils. The sediments and the fauna show that the sequence was deposited on a mid or inner shelf, and some elements of the marine mollusc and foraminiferal assemblages indicate water temperatures between -1 and +1°C and seasonal sea ice cover during deposition. Mean summer air temperatures were around 6°C higher than at present, as demonstrated by the occurrence of southern extralimital terrestrial species. Well-preserved remains of land plants indicate that the adjacent land area was dominated by sub-arctic forest-tundra with the trees Larix and Betula, shrubs, herbs and mosses. Most of the species represented as fossils have recent circumpolar geographical ranges. An extinct brachiopod species and an extinct gastropod species have been found, but the other macrofossils are referred to extant species. The brachiopod is erected as a new genus and species, Laugekochiana groenlandica. Correlation of the Koldewey Formation with the ile de France Formation farther to the north is suggested. Member A of the Kap Kobenhavn Formation in North Greenland is referred to the Late Pliocene, whereas Member B of the Kap Kobenhavn Formation is suggested to be slightly older than the Store Koldewey Formation. © 2010 The Authors, Journal compilation © 2010 The Boreas Collegium.
TL;DR: Romundset et al. as discussed by the authors presented the first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway, covering the entire Holocene.
Abstract: Romundset, A., Lohne, O. S., Mangerud, J. & Svendsen, J. I. 2009: The first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway. Boreas, 10.1111/j.1502-3885.2009.00108.x. ISSN 0300-9483.
The first relative sea-level (RSL) curve from the mid-Hardangerfjorden area covering the entire Holocene is presented. The curve is based on a series of AMS 14C dates on terrestrial plant macrofossils across the isolation level in each of five lakes located between 3.5 and 74.5 m a.s.l. During the first 1200 years, the RSL fell very rapidly from the marine limit at 98 m a.s.l. to 33 m a.s.l., i.e. at a rate of 5.4 cm yr−1. The emergence rate then slowed considerably and was close to standstill 8000–6500 cal. yr BP. However, an emergence of 16.5 m has taken place during the past 6000 years. Radiocarbon dates of terrestrial plant macrofossils from the basal strata in a lake above the marine limit and mollusc shells from glaciomarine silt in the isolation basins yielded a mean age for the local ice-margin retreat of 11 300 cal. yr BP. This verifies that Hardangerfjorden was glaciated during the Younger Dryas – an interpretation that has recently been disputed. The ice margin retreated at a rate of about 300 m yr−1 from the position of the Younger Dryas moraine to this site some 60 km further into the fjord.
TL;DR: Bjune et al. as discussed by the authors developed a modern pollen-climate calibration data set for Norway based on 191 lakes and 321 lakes, which was used to reconstruct mean July temperature and mean annual precipitation for a Holocene site in northwest Norway and a Lateglacial site in west-central Norway.
Abstract: Bjune, A. E., Birks, H. J. B., Peglar, S. M. & Odland, A. 2010: Developing a modern pollen–climate calibration data set for Norway. Boreas, Vol. 39, pp. 674–688. 10.1111/j.1502-3885.2010.00158.x. ISSN 0300-9483.
Modern pollen–climate data sets consisting of modern pollen assemblages and modern climate data (mean July temperature and mean annual precipitation) have been developed for Norway based on 191 lakes and 321 lakes. The original 191-lake data set was designed to optimize the distribution of the lakes sampled along the mean July temperature gradient, thereby fulfilling one of the most critical assumptions of weighted-averaging regression and calibration and its relative, weighted-averaging partial least-squares regression. A further 130 surface samples of comparable taphonomy, taxonomic detail and analyst became available as a result of other projects. These 130 samples, all from new lakes, were added to the 191-lake data set to create the 321-lake data set. The collection and construction of these data sets are outlined. Numerical analyses involving generalized linear modelling, constrained ordination techniques, weighted-averaging partial least-squares regression, and two different cross-validation procedures are used to asses the effects of increasing the size of the calibration data set from 191 to 321 lakes. The two data sets are used to reconstruct mean July temperature and mean annual precipitation for a Holocene site in northwest Norway and a Lateglacial site in west-central Norway. Overall, little is to be gained by increasing the modern data set beyond about 200 lakes in terms of modern model performance statistics, but the down-core reconstructions show less between-sample variability and are thus potentially more plausible and realistic when based on the 321-lake data set.
TL;DR: Engels et al. as discussed by the authors used chironomid remains, botanical and zoological macroremains as well as sediment lithology to reconstruct past changes in the environment, including climate.
Abstract: Engels, S., Helmens, K. F., Valiranta, M., Brooks, S. J. & Birks, H. J. B. 2010: Early Weichselian (MIS 5d and 5c) temperatures and environmental changes in northern Fennoscandia as recorded by chironomids and macroremains at Sokli, northeast Finland. Boreas, Vol. 39, pp. 689–704. 10.1111/j.1502-3885.2010.00163.x. ISSN 0300-9483.
A 25-m-long sediment record spanning the time from the Eemian to the Holocene was recovered from Sokli, northeast Finland. This study focuses on a 6-m-long sediment interval that is dated to the Early Weichselian period (MIS 5d and 5c) and consists of lacustrine and fluvial deposits. Using chironomid remains, botanical and zoological macroremains as well as sediment lithology, we were able to reconstruct past changes in the environment, including climate. The results indicate that the site was situated on a flood-plain during the latter stages of MIS 5d (Herning Stadial) and that summer temperatures might have been ∼6 °C lower than at present. Although this value should be treated with caution, as numerical analysis shows that it has a very poor fit-to-temperature, this low reconstructed value concurs with several other reconstructions that are available from western Europe. During MIS 5c (Brorup interstadial), the depositional environment changed into a lake system, initially with stratification of the water and subsequently with complete mixing and a strong influence of streams. Both chironomid-based and macroremain-based temperature inferences indicate past July air temperatures that were significantly higher than at present. This result is in contrast to other (low-resolution) reconstructions from northern Fennoscandia that indicate past temperatures 6–7 °C lower than present using fossil coleopteran assemblages. However, several central European sites indicate that there was a phase during the Brorup interstadial that was characterized by high summer temperatures, and a comparison between the high-resolution reconstructions from western Europe and the results presented in this study suggests that the north–south July air temperature gradient between the mid- and high-latitudes was much weaker during the Brorup interstadial than it is at present. High solar insolation values (particularly the obliquity) during the Brorup interstadial might explain the low summer temperature gradient over the European continent. A return to fluvial conditions occurred in the upper parts of the sediment sequence, and, after a brief interval of gyttja deposition under cooling conditions, the site became glaciated during MIS 5b.
TL;DR: Weichselian ice free intervals at Riipiharju, northern Sweden as discussed by the authors, interpretations of vegetation and climate from fossil and modern pollen records from modern and ancient pollen records.
Abstract: Weichselian ice free intervals at Riipiharju, northern Sweden : interpretations of vegetation and climate from fossil and modern pollen records