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Palaeoclimatic Record (palaeoclimatic + record)
Selected AbstractsA POTENTIAL CENTURY,SCALE RHYTHM IN SIX MAJOR PALAEOCLIMATIC RECORDS IN THE NORTHERN HEMISPHEREGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 2 2007MAXIM OGURTSOV ABSTRACT. Six millennial proxy records of temperature in the northern hemisphere were analysed using both the Fourier and wavelet approaches. We found that the analysed temperature proxies have appreciable synchrony at multidecadal and centennial time scales. These data also show evidence for the presence of a roughly regular large-scale rhythm with a periodicity of 50,130 years in the climate of the northern hemisphere over the last millennium. It is shown that the amplitude of this variation might reach 0.20,0.28°C and contribute appreciably to the rise of global temperature over the first part of the 20th century. Possible origins of the global centennial climatic cycles are discussed. [source] Pollen,plant,climate relationships in sub-Saharan AfricaJOURNAL OF BIOGEOGRAPHY, Issue 3 2007Julie Watrin Abstract Aim, To demonstrate that incorporating the bioclimatic range of possible contributor plants leads to improved accuracy in interpreting the palaeoclimatic record of taxonomically complex pollen types. Location, North Tropical Africa. Methods, The geographical ranges of selected African plants were extracted from the literature and geo-referenced. These plant ranges were compared with the pollen percentages obtained from a network of surface sediments. Climate-response surfaces were graphed for each pollen taxon and each corresponding plant species. Results, Several patterns can be identified, including taxa for which the pollen and plant distributions coincide, and others where the range limits diverge. Some pollen types display a reduced climate range compared with that of the corresponding plant species, due to low pollen production and/or dispersal. For other taxa, corresponding to high pollen producers such as pioneer taxa, pollen types display a larger climatic envelope than that of the corresponding plants. The number of species contained in a pollen taxon is an important factor, as the botanical species included in a taxon may have different geographical and climate distributions. Main conclusions, The comparison between pollen and plant distributions is an essential step towards more precise vegetation and climate reconstructions in Africa, as it identifies taxa that have a high correspondence between pollen and plant distribution patterns. Our method is a useful tool to reassess biome reconstructions in Africa and to characterize accurately the vegetation and climate conditions at a regional scale, from pollen data. [source] A multi-proxy palaeoecological and palaeoclimatic record within full glacial lacustrine deposits, western Tennessee, USA,JOURNAL OF QUATERNARY SCIENCE, Issue 8 2009David A. Grimley Abstract The Fulton Section, along the Mississippi River in western Tennessee, USA, is a 1,km continuous exposure (,20,m vertically) of Quaternary fluvial and lacustrine deposits, inset within Eocene sediments and buried by thick loess. Fossiliferous slackwater lake sediments record maximum aggradation during the last two major glaciations, with deposition between ca. 190,140,ka and 24, 18 14C ka BP, based on amino acid and radiocarbon chronology, respectively. During the onset of full glacial conditions (ca. 24,22 14C ka BP), a relatively permanent shallow lake environment is indicated by ostracods, aquatic molluscs, and both pollen and macrofossils of aquatic plants. By 21.8 14C ka BP, increasing emergent plants, amphibious gastropods (Pomatiopsis) and heavier ,18O compositions suggest marsh-like conditions in a periodically drying lake. The surrounding uplands consisted of Picea,Pinus woodlands mixed with cool-temperate hardwoods (e.g. Quercus, Populus, Carya), grasses and herbs. More open conditions ensued ca. 20 14C ka BP, with loess and slopewash gradually infilling the former lake by 18 14C ka BP. Modern analogue analyses of ostracods and palaeontological evidence imply a full glacial climate similar to today's mixed-boreal zone in central Minnesota, USA, about 9°C cooler in mean annual temperature than present-day western Tennessee. Copyright © 2009 John Wiley & Sons, Ltd. [source] A stable isotope record from freshwater lake shells of the eastern Tibetan Plateau, China, during the past two centuriesBOREAS, Issue 1 2007JINGLU WU Wu, J. L., Schleser, G. H., Lücke, A. & Li, S. 2007 (January): A stable isotope record from freshwater lake shells of the eastern Tibetan Plateau, China, during the past two centuries. Boreas, Vol. 36, pp. 38,46. Oslo. ISSN 0300,9483. Lake Xingcuo is a small, closed, hardwater lake situated in the eastern Tibetan Plateau. Stable isotope data (,18O and ,13C) from the freshwater snail Gyraulus sibirica (Dunker) in a 34-cm-long, radioactive isotope-dated sediment core represent the past 200 years of Lake Xingcuo environmental history. Carbon and oxygen isotope ratios in the shells of the snail yield information on the isotopic composition of the water in which the shell was formed, which in turn relates to climatic conditions prevailing during the snail's life-span. Living and fossil shells from Lake Xingcuo were collected. ,18O values in the living shells from Lake Xingcuo are in equilibrium with ambient waters, while ,13C values may trace snail dietary carbon. On comparing ,18O and ,13C in the shell of Gyraulus sibirica with monitored data for the period 1954,1995, we found that the ,18O composition in the shell is an efficient proxy revealing air temperature during the warmer months from April to September. There is a positive correlation between the ,18O in the shells of Gyraulus sibirica and the running average temperature of the warmer months. Climatic variability in the eastern Tibetan Plateau for the past two centuries has been inferred from the ,18O record from the freshwater snails in the sediments of Lake Xingcuo. As such, the last 200 years' palaeoclimatic record for this region can be separated into three periods representing oscillations between warm and cool conditions consistent with the Guliya ice record in the Tibetan Plateau. [source] Threshold response of Madagascar's littoral forest to sea-level riseGLOBAL ECOLOGY, Issue 1 2009Malika Virah-Sawmy ABSTRACT Aim, Coastal biodiversity hotspots are globally threatened by sea-level rise. As such it is important to understand how ecosystems resist, respond and adapt to sea-level rise. Using pollen, geochemistry, charcoal and diatom records in conjunction with previously published palaeoclimatic records, we investigated the mechanism, interactions and ecosystem response and resilience of Madagascar's littoral forest to late Holocene sea-level rise. Location, Sediment sequences were collected along the south-east coast of Madagascar in two adjacent habitats in Mandena; the highly diverse littoral forest fragment and species-poor Erica -matrix. Methods, We used a multi-proxy approach to investigate the relative influence of environmental changes on the littoral ecosystem. We reconstructed past vegetation and fire dynamics over the past 6500 years at two sites in the littoral forest using fossil pollen and macrofossil charcoal contained in sedimentary sequences. Alongside these records we reconstructed past marine transgressions from the same sedimentary sequences using geochemical analyses, and a salinity and drought index through the analysis of fossil diatoms. Results, Our findings indicated that it was the synergistic effect of sea-level rise coupled with rainfall deficits that triggered a threshold event with a switch from two types of littoral forest (an open Uapaca forest and a closed littoral forest fragment) to an Erica,Myrica heath/grassland occurring in approximately less than 100 years. Resilience to sea-level rise differed in the two adjacent habitats, suggesting that the littoral forest fragment was more resilient to the impacts of sea-level change and aridity than the open Uapaca woodland. Conclusions, We demonstrated that the littoral ecosystem was influenced by late Holocene sea-level rise and climatic desiccation. While climate change-integrated conservation strategies address the effects of climate change on species distribution and dispersal, our work suggests that more attention should be paid to the impacts of interactive climatic variables that affect ecosystem thresholds. [source] Climatic oscillations in central Italy during the Last Glacial,Holocene transition: the record from Lake Accesa,JOURNAL OF QUATERNARY SCIENCE, Issue 4 2006Michel Magny Abstract This paper presents an event stratigraphy based on data documenting the history of vegetation cover, lake-level changes and fire frequency, as well as volcanic eruptions, over the Last Glacial,early Holocene transition from a terrestrial sediment sequence recovered at Lake Accesa in Tuscany (north-central Italy). On the basis of an age,depth model inferred from 13 radiocarbon dates and six tephra horizons, the Oldest Dryas,Bølling warming event was dated to ca. 14,560,cal.,yr,BP and the Younger Dryas event to ca. 12,700,11,650,cal.,yr,BP. Four sub-millennial scale cooling phases were recognised from pollen data at ca. 14,300,14,200, 13,900,13,700, 13,400,13,100 and 11,350,11,150,cal.,yr,BP. The last three may be Mediterranean equivalents to the Older Dryas (GI-1d), Intra-Allerød (GI-1b) and Preboreal Oscillation (PBO) cooling events defined from the GRIP ice-core and indicate strong climatic linkages between the North Atlantic and Mediterranean areas during the last Termination. The first may correspond to Intra-Bølling cold oscillations registered by various palaeoclimatic records in the North Atlantic region. The lake-level record shows that the sub-millennial scale climatic oscillations which punctuated the last deglaciation were associated in central Italy with different successive patterns of hydrological changes from the Bølling warming to the 8.2,ka cold reversal. Copyright © 2006 John Wiley & Sons, Ltd. [source] An improved methodology of the modern analogues technique for palaeoclimate reconstruction in arid and semi-arid regionsBOREAS, Issue 1 2010WENYING JIANG Jiang, W., Guiot, J., Chu, G., Wu, H., Yuan, B., Hatté, C. & Guo, Z. 2009: An improved methodology of the modern analogues technique for palaeoclimate reconstruction in arid and semi-arid regions. Boreas, 10.1111/j.1502-3885.2009.00115.x. ISSN 0300-9483. This study presents an improved method of the plant functional type modern analogues technique (PFT-MAT) in which environmental proxies and a moisture index (,, 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 ,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 , variations during the entire Holocene, with the highest tree components and highest , values between 8000 and 5500 cal. yr BP, indicating the dominant role of , 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. [source] | ||||||||||