排序方式： 共有32条查询结果，搜索用时 234 毫秒
11.
Abrupt climate change and early lake development – the Lateglacial diatom flora at Hässeldala Port,southeastern Sweden 
下载免费PDF全文
下载免费PDF全文 Linda Ampel Malin E. Kylander Margret Steinthorsdottir Barbara Wohlfarth 《Boreas: An International Journal of Quaternary Research》2015,44(1):94-102
The fossil diatom record from the Hässeldala Port palaeolake, southeastern Sweden, offers an excellent opportunity to investigate how past climatic shifts influenced catchment conditions and early lake development. The record, dating to between 13 900 and 11 200 cal. a BP, covers a climatically dynamic period, starting with deglaciation followed by oscillations between warmer and colder climate states. The stratigraphical changes in the fossil diatom assemblages show a trend of less open‐water taxa and a successively more complex periphytic community as the lake shallows and the aquatic habitat structure develops. A diatom‐based reconstruction of lake water pH indicates a natural acidification trend early in the record from 13 900 to 12 500 cal. a BP. From 12 500 cal. a BP, coincident with the start of climate cooling, to 11 300 cal. a BP this trend is disrupted and lake waters become more alkaline. A cooler and drier climate most likely resulted in reduced soil organic matter build‐up as well as more frozen ground that impeded hydrological flow and decreased the input of dissolved organic matter and organic acids into the lake system. This study demonstrates the importance of the hydrological system as a link between terrestrial and aquatic ecosystems during early lake ontogeny. 相似文献
12.
Barbara Wohlfarth Svante Bjoerck Svend Funder Michael Houmark-Nielsen Olafurlngolfsson Juha-Pekka Lunkka Jan Mangerud Matti Saarnisto Tore Vorren 《《幕》》2008,31(1):73-81
The Nordic countries have experienced multiple glaciations and intervening interglacials during the last ca. 2.5-3 million years. Although evidence from Greenland and Iceland shows that ice sheets started to expand some time before 3 Ma, little is known about the glaciations and intervening interglacials older than the last Glacial Maximum due to repeated phases of glacial erosion and reworking. The extensive Saalian glaciation （c. 140 ka BP） contributed to high sea levels in Greenland and in the Baltic area during the early part of the last interglacial （Eemian）. Temperatures were about 5 ℃ higher during the Eemian than they are today and the Greenland ice sheet was reduced to about half of its present size, causing globally higher sea levels than we have today. Ice extent in Fennoscandia was restricted during early Weichselian stadials, but middle Weichselian ice advances in Scandinavia reached as far as Denmark. During the Last Glacial Maximum, large ice sheets were present in all Nordic countries and coalesced with neighboring ice sheets. Deglaciation commenced around 17-15 ka BP in most areas and was promoted by rapidly rising global sea level and glacial isostasy. The Younger Dryas cold event（c. 12.6-11.5 ka BP） is seen as a short-term re-advance, still-stand or fluctuation of land-based ice sheet margins. Around 7-9 ka BP ice sheets had disappeared or had attained their present size. While uplift is still going on in some regions, others are subject to submergence. The different stages of development of the Baltic Sea are an example of how the intricare interplay between glacial eustasy and isostasy influences sedimentation, basin size and drainage patterns. 相似文献
13.
14.
C.J. Van Meerbeeck H. Renssen D.M. Roche B. Wohlfarth S.J.P. Bohncke J.A.A. Bos S. Engels K.F. Helmens M.F. Sánchez-Goñi A. Svensson J. Vandenberghe 《Quaternary Science Reviews》2011,30(25-26):3618-3637
15 abrupt warming transitions perturbed glacial climate in Greenland during Marine Isotope Stage 3 (MIS 3, 60–27 ka BP). One hypothesis states that the 8–16 °C warming between Greenland Stadials (GS) and Interstadials (GI) was caused by enhanced heat transport to the North Atlantic region after a resumption of the Atlantic Meridional Overturning Circulation (AMOC) from a weak or shutdown stadial mode. This hypothesis also predicts warming over Europe, a prediction poorly constrained by data due to the paucity of well-dated quantitative temperature records. We therefore use a new evidence from biotic proxies and a climate model simulation to study the characteristics of a GS–GI transition in continental Europe and the link to enhanced AMOC strength. We compare reconstructed climatic and vegetation changes between a stadial and subsequent interstadial – correlated to GS15 and GI14 (～55 ka BP) – with a simulated AMOC resumption using a three-dimensional earth system model setup with early-MIS 3 boundary conditions. Over western Europe (12°W–15°E), we simulate twice the annual precipitation, a 17 °C warmer coldest month, a 8 °C warmer warmest month, 1300 °C-day more growing degree days with baseline 5 °C (GDD5) and potential vegetation allowing tree cover after the transition. However, the combined effect of frequent killing frosts, <20 mm summer precipitation and too few GDD5 after the transition suggest a northern tree limit lying at ～50°N during GI14. With these 3 climatic limiting factors we provide a possible explanation for the absence of forests north of 48°N during MIS 3 interstadials with mild summers. Finally, apart from a large model bias in warmest month surface air temperatures, our simulation is in reasonable agreement with reconstructed climatic and vegetation changes in Europe, thus further supporting the hypothesis. 相似文献
15.
16.
Barbara Wohlfarth Francesco Muschitiello Sarah L. Greenwood August Andersson Malin Kylander Rienk H. Smittenberg Margret Steinthorsdottir Jenny Watson Nicola J. Whitehouse 《Boreas: An International Journal of Quaternary Research》2017,46(2):143-161
The Last Termination (19 000–11 000 a BP) with its rapid and distinct climate shifts provides a perfect laboratory to study the nature and regional impact of climate variability. The sedimentary succession from the ancient lake at Hässeldala Port in southern Sweden with its distinct Lateglacial/early Holocene stratigraphy (>14.1–9.5 cal. ka BP) is one of the few chronologically well‐constrained, multi‐proxy sites in Europe that capture a variety of local and regional climatic and environmental signals. Here we present Hässeldala's multi‐proxy records (lithology, geochemistry, pollen, diatoms, chironomids, biomarkers, hydrogen isotopes) in a refined age model and place the observed changes in lake status, catchment vegetation, summer temperatures and hydroclimate in a wider regional context. Reconstructed mean July temperatures increased between c. 14.1 and c. 13.1 cal. ka BP and subsequently declined. This latter cooling coincided with drier hydroclimatic conditions that were probably associated with a freshening of the Nordic Seas and started a few hundred years before the onset of Greenland Stadial 1 (c. 12.9 cal. ka BP). Our proxies suggest a further shift towards colder and drier conditions as late as c. 12.7 cal. ka BP, which was followed by the establishment of a stadial climate regime (c. 12.5–11.8 cal. ka BP). The onset of warmer and wetter conditions preceded the Holocene warming over Greenland by c. 200 years. Hässeldala's proxies thus highlight the complexity of environmental and hydrological responses across abrupt climate transitions in northern Europe. 相似文献
17.
Linda Ampel Barbara Wohlfarth Jan Risberg Daniel Veres Melanie J. Leng Päivi Kaislahti Tillman 《Journal of Paleolimnology》2010,44(2):397-404
The sedimentary record from the paleolake at Les Echets in eastern France allowed a reconstruction of the lacustrine response
to several abrupt climate shifts during the last glacial period referred to as Dansgaard–Oeschger (DO) cycles. The high-resolution
diatom stratigraphy has revealed distinct species turnover events and large fluctuations in stable oxygen isotope values in
diatom frustules, as a response to DO climate variability. More or less identical species compositions became re-established
during each DO stadial and interstadial phases, respectively. However, the relative abundance of the most dominant species
within these assemblages varies and might indicate differences in climatic conditions. Interstadial phases are characterized
by identical species successions. Transitions from stadial to interstadial conditions show a distinct Fragilaria–Cyclotella succession, which resembles the diatom regime shifts that have been recognized in some lakes in the Northern Hemisphere since
the mid-nineteenth century. 相似文献
18.
15,000–13,600 cal yr BP) Barbara Wohlfarth Gina Hannon Angelica Feurdean Lucretia Ghergari Bogdan P. Onac Gran Possnert 《Quaternary Science Reviews》2001,20(18):3
High resolution pollen, plant macrofossil, charcoal, mineral magnetic and sedimentary analyses, combined with AMS 14C measurements, were performed on multiple sediment sequences along a transect through the former crater lake Preluca
iganului in northwestern Romania in order to reconstruct the climatic and environmental changes during the early part of the Last Termination. Lake sediments started to accumulate at 14,700 cal yr BP. Initially the upland vegetation consisted of an open forest with mainly Betula and Salix and few Pinus sp., but from 14,500 cal yr BP onwards, Pinus mugo, P. sylvestris and Populus and later on also Larix became established around the lake. Between 14,150 and 13,950 cal yr BP, Pinus cembra seems to have replaced P. mugo and P. sylvestris. At 13,950 cal yr BP the tree cover increased and Picea appeared for the first time, together with Pinus cembra, P. mugo and Larix. From 13,750 cal yr BP onwards, a Picea forest developed around the site. Based on the combined proxy data the following climatic development may be inferred: 14,700–14,500 cal yr BP, cooler and wet/humid; 14,500–14,400 cal yr BP: gradually warmer temperatures, wet/humid with dry summers; 14,400–14,320 cal yr BP: warm and dry; 14,320–14,150 cal yr BP: cooler and wet/humid; 14,150–14,100 cal yr BP: warm and dry; 14,100–13,850 cal yr BP: warmer and wet/humid; <13,850 cal yr BP: warm and dry. The tentative correlation of this development with the North Atlantic region assumes that the period >14,700 cal yr could correspond to GS-2a, the time span between 14,700 and 14,320 to GI-1e, the phase between 14,320 and 14,150 cal yr BP to GI-1d and the time frame between 14,150 and 13,600 cal yr BP to the lower part of GI-1c. 相似文献
19.
Analyses of two infilled lakes in Blekinge, southeast Sweden, indicate the presence of at least three tephra horizons of Termination 1 and early Holocene age. Geochemical analyses confirm the presence of the Borrobol Tephra, the Askja Tephra (10,000 14C yr B.P.), and one previously unreported tephra of Icelandic origin. Extending the limits of the Borrobol Tephra to Scandinavia illustrates that this ash is far more widespread than previously realized and is therefore, an important marker horizon for determining the rate and timing of the initial warming at the start of Greenland Interstade 1 (GI-1) within Europe. The relatively unknown Askja Tephra and the newly discovered Hässeldalen Tephra are stratigraphically placed at the Younger Dryas/Preboreal transition. This paper demonstrates the suitability and success associated with the extraction techniques for tracing microtephra horizons in areas distal to volcanic sources. 相似文献
20.
Highly variable atmospheric radiocarbon concentrations are a distinct feature during the last deglaciation. The synchronisation of two high-resolution AMS 14C-dated records, Lake Go?ci??, and a floating Late Weichselian glacial varve chronology at the Allerød-Younger Dryas transition allowed us to assess in detail atmospheric Δ14C changes between late Allerød and early Preboreal. The combined data set shows a drastic rise in Δ14C during the first 200 years or so of Younger Dryas and the two following about 500 year-long 14C plateaux. Model experiments which included variations in the geomagnetic field, atmospheric CO2 variations and a drastic reduction in North Atlantic Deep Water flux at the onset of Younger Dryas allowed to reproduce the distinct rise in Δ14C during the first 200 years of Younger Dryas fairly well. Also the drop in Δ14C at the Younger Dryas/Holocene boundary seems reasonably explained by changes in North Atlantic Deep Water circulation. However, the reason behind the anomalous behaviour of the Δ14C signal in the middle of Younger Dryas remains an open question. 相似文献