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Kyr BP (kyr + bp)
Kinds of Kyr BP Selected AbstractsComplex population history of two Anopheles dirus mosquito species in Southeast Asia suggests the influence of Pleistocene climate change rather than human-mediated effectsJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 6 2008S. M. O'LOUGHLIN Abstract Anopheles dirus and Anopheles baimaii are closely related species which feed on primates, particularly humans, and transmit malaria in the tropical forests of mainland Southeast Asia. Here, we report an in-depth phylogeographic picture based on 269 individuals from 21 populations from mainland Southeast Asia. Analysis of 1537 bp of mtDNA sequence revealed that the population history of A. baimaii is far more complex than previously thought. An old expansion (pre-300 kyr BP) was inferred in northern India/Bangladesh with a wave of south-eastwards expansion arriving at the Thai border (ca 135,173 kyr BP) followed by leptokurtic dispersal very recently (ca 16 kyr BP) into peninsular Thailand. The long and complex population history of these anthropophilic species suggests their expansions are not in response to the relatively recent (ca 40 kyr BP) human expansions in mainland Southeast Asia but, rather, fit well with our understanding of Pleistocene climatic change there. [source] 55 Ice age kelp forests: climate-driven changes in kelp forest distribution since the last glacial maximumJOURNAL OF PHYCOLOGY, Issue 2003M. H. Graham Kelp forest distributions are constrained by the availability of rocky substrate within the depth range tolerable for growth and reproduction, which can vary over relatively short geological timescales (millennia) due to interactions between coastal bathymetry and climate-driven changes in eustatic sea level. Using GIS, a digital bathymetric map, sea level curves, and published kelp depth tolerances, I reconstructed changes in the size and distribution of giant kelp (Macrocystis pyrifera) forests in the Southern California Bight since the last glacial maximum. Reconstructions predicted that the total area of available kelp forest habitat for the California Channel Islands during the last glacial maximum (18.5 kyr BP; 628 square km) was greater than at present (382 square km) but less than at 16.5 kyr BP (1130 square km). Available kelp forest habitat along the southern California mainland also increased rapidly from 18.5 to 16.5 kyr BP but continued to increase with sea level rise. Differences in the effects of sea level rise on coastal geomorphology between the islands and mainland further constrained the extent of rocky substrate available to kelps. Given biomass and productivity estimates from present-day kelp forests, these reconstructions suggest more productive and spatially extensive island kelp forests near the last glacial maximum than at present, but the opposite pattern for the mainland. These climate-driven changes in kelp forest distribution and productivity likely had important historical impacts on the ecology and evolution of the present-day kelp ecosystem including kelp forest exploitation by early human inhabitants of southern California. [source] Palaeovegetational and palaeoenvironmental trends in the summit of the Guaiquinima massif (Venezuelan Guayana) during the Holocene,JOURNAL OF QUATERNARY SCIENCE, Issue 2 2005Valentí Rull Abstract The summits of the table mountains (tepuis) from the Neotropical Guayana region are remote environments suitable for palaeoecological studies with evolutionary, biogeographical and palaeoclimatic implications. Here, using palynological analyses of two radiocarbon-dated peat bogs from a tepui summit, the Holocene palaeovegetational trends are reconstructed, and related to possible forcing factors. Because of the pristine character of the Guaiquinima summit, the recorded palaeoenvironmental changes are probably due to natural causes, which makes them valuable archives of the natural component of climatic change at a millennial time scale. The sequence begins with pioneer communities or meadows similar to present-day ones, between about 8.4 and 4.5,ky BP. After this date, and until about 2,kyr BP the expansion of gallery forests suggests an increase in precipitation, documented also at regional (Neotropical) level. Between ca. 2,kyr BP and the last century, gallery forests are replaced by forests characteristic of the upper Guaiquinima altitudes, coinciding with a regional phase of reduced moisture. The present-day meadows, established relatively quickly during the last century, substituted the former upland forests. In the locality studied, the main controlling factor of the vegetation during the Holocene seems to have been the moisture balance. In contrast to other tepui summits, there is no clear evidence for changes linked to temperature oscillations. This could be due to the elevation of the site, far from any characteristic ecological boundary, that makes it insensitive to this parameter. Copyright © 2005 John Wiley & Sons, Ltd. [source] Chronology of the last recession of the Greenland Ice SheetJOURNAL OF QUATERNARY SCIENCE, Issue 3 2002Ole Bennike Abstract A new deglaciation chronology for the ice-free parts of Greenland, the continental shelf and eastern Ellesmere Island (Canada) is proposed. The chronology is based on a new compilation of all published radiocarbon dates from Greenland, and includes crucial new material from southern, northeastern and northwestern Greenland. Although each date provides only a minimum age for the local deglaciation, some of the dates come from species that indicate ice-proximal glaciomarine conditions, and thus may be connected with the actual ice recession. In addition to shell dates, dates from marine algae, lake sediments, peat, terrestrial plants and driftwood also are included. Only offshore and in the far south have secure late-glacial sediments been found. Other previous reports of late-glacial sediments (older than 11.5 cal. kyr BP) from onshore parts of Greenland need to be confirmed. Most of the present ice-free parts of Greenland and Nares Strait between Greenland and Ellesmere Island were not deglaciated until the early Holocene. Copyright © 2002 John Wiley & Sons, Ltd. [source] Integrated marine and terrestrial evidence for abrupt Congo River palaeodischarge fluctuations during the last deglaciationJOURNAL OF QUATERNARY SCIENCE, Issue 8 2001Fabienne Marret Abstract We present a high-resolution reconstruction of tropical palaeoenvironmental changes for the last deglacial transition (18 to 9 cal. kyr BP) based on integrated oceanic and terrestrial proxies from a Congo fan core. Pollen, grass cuticle, Pediastrum and dinoflagellate cyst fluxes, sedimentation rates and planktonic foraminiferal ,18O ratios, u37K, sea-surface temperature and alkane/alkenone ratio data highlight a series of abrupt changes in Congo River palaeodischarge. A major discharge pulse is registered at around 13.0 cal. kyr BP which we attribute to latitudinal migration of the Intertropical Convergence Zone (ITCZ) during deglaciation. The data indicate abrupt and short-lived changes in the equatorial precipitation regime within a system of monsoonal dynamics forced by precessional cycles. The phases of enhanced Congo discharge stimulated river-induced upwelling and enhanced productivity in the adjacent ocean. Copyright © 2001 John Wiley & Sons, Ltd. [source] Late Pleistocene and Holocene whale remains (Cetacea) from Denmark and adjacent countries: Species, distribution, chronology, and trace element concentrationsMARINE MAMMAL SCIENCE, Issue 2 2010Kim Aaris-Sørensen Abstract We describe and review the subfossil whale bones (mammalian order Cetacea) material from the southern Scandinavian area, that is, Skagerrak, Kattegat, the inner Danish waters and the southwestern Baltic Sea. Fifteen species were identified from the subfossil records of which all, except for the bowhead whale (Balaena mysticetus), have also been encountered in the modern times. Fifty-one specimens were radiocarbon dated covering 12 of the subfossil species. The dates fell in three distinct clusters with a few specimens before the last glacial maximum (LGM), a large group between LGM and the Pleistocene/Holocene boundary (ca. 17.0,11.7 cal. kyr BP), and another large group from ca. 8.0 cal. kyr BP onward. Seventeen of the radiocarbon dated specimens have been subjected to trace element analysis by Instrumental Neutron Activation Analysis. Cross plots of the concentrations of Fe and Zn, and Fe and Co show that it is possible to distinguish crayfish eaters from fish/squid eaters. This can be used as a novel and independent method for the determination to species of whale remains of otherwise uncertain speciation. [source] Observations on the relationship between lake formation, permafrost activity and lithalsa development during the last 20,000 years in the Tso Kar basin, Ladakh, IndiaPERMAFROST AND PERIGLACIAL PROCESSES, Issue 4 2008Bernd Wünnemann Abstract A close relationship has existed between high mountain permafrost and lake history in the Tso Kar basin, Ladakh, India throughout the last 20,kyr BP. Analyses of sediment cores indicate phases of permafrost growth during low lake levels between >20,10,cal. kyr BP and after 5,cal. kyr BP. Palaeo-shorelines and lake carbonate outcrops indicate a maximum lake size at ca. 8,5,cal. kyr BP when permafrost is believed to have been absent in the basin. Regression of the lake with accompanying salinisation since about 4,cal. kyr BP allowed permafrost aggradation. Permafrost mounds and thermokarst features are inferred to have continued to develop in the last 60 years. Sections in two 6,10,m high unvegetated mounds revealed thick ice lenses and reticulate cryostructures leading to their classification as lithalsas. Mound collapse is induced by surface erosion and slumping, while sediment reworking by wave action and other processes appears to prevent rampart formation. If lake levels continue to fall, further permafrost aggradation is expected in this high elevation enclosed basin. Copyright © 2008 John Wiley & Sons, Ltd. [source] Edge-roundness of boulders of Torridonian Sandstone (northwest Scotland): applications for relative dating and implications for warm and cold climate weathering ratesBOREAS, Issue 2 2010MARTIN P. KIRKBRIDE Kirkbride, M.P. & Bell, C.M. 2009: Edge-roundness of boulders of Torridonian Sandstone (northwest Scotland): applications for relative dating and implications for warm and cold climate weathering rates. Boreas, 10.1111/j. 1502-3885.2009.00131.x. ISSN 0300-9483. The relative ages of late Quaternary morainic and rock avalanche deposits on Late Precambrian Torridonian Sandstone are determined from the characteristic edge-roundness of constituent boulders. Because weathering of sandstone is manifest as edge-rounding by granular disintegration, a relative chronology can be derived by measuring the effective radii of curvature of a sample of boulder edges. Thirteen samples totalling 597 individual boulder edges fall into two statistically distinct groups. Moraines of inferred Younger Dryas age (12.9,11.5 kyr BP) are distinguished from moraines of the Wester Ross Re-advance (,14.0 kyr BP). One moraine previously assumed to be of Younger Dryas age is reassigned to the older group. The method allows spatial extrapolation of deposit ages from dated sites where lithological and sampling criteria are met. Calculated rates of edge-rounding imply that granular disintegration was several times more rapid during cold stadial climates than during the Holocene. Used as a proxy for boulder ,erosion rate', this indicates that surface loss of grains in glacial climates exceeds that during interglacials by a factor of 2,5, with implications for the calculation of exposure ages from cosmogenic nuclides. [source] 14C-dated fluctuations of the western flank of the Scandinavian Ice Sheet 45,25 kyr BP compared with Bølling,Younger Dryas fluctuations and Dansgaard,Oeschger events in GreenlandBOREAS, Issue 2 2010JAN MANGERUD 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 Bølling,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 Ålesund Interstadial. New AMS 14C dates on shells aged 41,38 14C kyr BP are evidence of an earlier (Austnes) ice-free period. The Ålesund 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 ,18O curves for the two periods Greenland Interstadial (GI) 8 to Greenland Stadial (GS) 8 and GI 1,GS 1 (Bølling,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 Bølling,Younger Dryas in order to elucidate processes during Dansgaard,Oeschger events. [source] Rapid tunnel-valley formation beneath the receding Late Weichselian ice sheet in Vendsyssel, DenmarkBOREAS, Issue 4 2009PETER B. E. SANDERSEN Interpretation of Transient ElectroMagnetic (TEM) data and wire-line logs has led to the delineation of an intricate pattern of buried tunnel valleys, along with new evidence of glaciotectonically dislocated layers in recessional moraines in the central part of Vendsyssel, Denmark. The TEM data have been compared with recent results of stratigraphical investigations based on lithological and biostratigraphical analyses of borehole samples and dating with Optically Stimulated Luminescence (OSL) and radiocarbon. This has provided an overview of the spatial distribution of the late Quaternary lithostratigraphical formations, and the age of the tunnel valleys has been estimated. The tunnel valleys are typically 5,10 km long, 1 km wide and are locally eroded to depths of more than 180 m b.s.l. The valleys are interpreted to have been formed by subglacial meltwater erosion beneath the outermost part of the ice sheet during temporary standstills and minor re-advances during the overall Late Weichselian recession of the Scandinavian Ice Sheet. The formation of the tunnel valleys occurred after the retreat of the Main ice advance c. 20 kyr BP and before the Lateglacial marine inundation c. 18 kyr BP. Based on the occurrence of the tunnel valleys and the topography, four ice-marginal positions related to the recession of the northeastern Main advance and seven ice-marginal positions related to the recession from the following eastern re-advance across Vendsyssel are delineated. All the tunnel valleys were formed within a time interval of a few thousand years, giving only a few hundred years or less for the formation of the tunnel valleys at each ice-marginal position. [source] Palaeo-ice streams and the subglacial landscape mosaic of the North American mid-continental prairiesBOREAS, Issue 3 2009MARTIN ROSS The analysis of the glacial landscape of southern Saskatchewan (Canada) through multiple data sets (e.g. digital elevation model, till compositional data) has revealed previously unrecognized subglacial sediment,landform assemblages. A southwest-trending corridor of mega-scale till lineations (Maskwa corridor) bounded on each side by hummocky terrain extends from the Canadian Shield to southwestern Saskatchewan. This landform assemblage is clearly cross-cut by a broad south to southeast trending corridor (Buffalo corridor) consisting of subparallel curvilinear till ridges. The carbonate content of the surface till is spatially consistent within these assemblages, suggesting a strong sediment,landform relationship. The two corridors are interpreted as the product of palaeo-ice streams. The Maskwa palaeo-ice stream flowed up the regional slope and across preglacial valleys, indicating it was thick and stable. Narrow dispersal trains extending across as well as down-glacier from the Athabasca Basin suggest that the Maskwa palaeo-ice stream extended far into the ice sheet across contrasting shield and platform terrains. In comparison, the Buffalo palaeo-ice stream was thinner and largely controlled by subglacial geology and topography. Its catchments were located at the Canadian Shield boundary and the system was oriented along-slope. It experienced lateral shifts and it was fed by a network of tributaries. The glacial dynamics shift from the Maskwa to the Buffalo system occurred at about 13.5 14C kyr BP. The Buffalo system later evolved into thin outlet lobes until final deglaciation of the area. The proposed model has implications for ice-sheet reconstruction and the assessment of till properties in the prairies and in similar terrains. [source] Intriguing climatic shifts in a 90 kyr old lake record from northern RussiaBOREAS, Issue 1 2008MONA HENRIKSEN A 22 m long sediment core from Lake Yamozero on the Timan Ridge in northern Russia has provided evidence of intriguing climatic shifts during the last glacial cycle. An overall shallowing of the lake is reflected in the lower part of the cores, where pollen indicates a transition from glacial steppe vegetation to interstadial shrub-tundra. These beds are capped by a well-defined layer of compact clay deposited in relatively deep water, where pollen shows surrounding spruce forests and warmer-than-present summer temperatures. The most conservative interpretation is that this unit represents the last interglacial period. However, a series of Optical Stimulated Luminescence (OSL) dates suggests that it corresponds with the Early Weichselian Odderade interstadial (MIS 5a). This would imply that the Odderade interstadial was just as warm as a normal interglacial in this continental part of northern Europe. If correct, then pollen analysis, as a correlation tool, is less straightforward and the definition of an interglacial is more complex than previously thought. We discuss the validity and possible systematic errors of the OSL dates on which this age model is based, but conclude they really indicate a MIS 5a age for the warm period. Above the clay is an unconformity, most likely reflecting a period of subaerial exposure implying dry conditions. Deposition of silt under fluctuating cold climates in the Middle Weichselian continued until a second gap in the record at c. 40 kyr BP. The lake basin started to fill up again around 18 kyr BP. [source] Late Pleistocene glacial and lake history of northwestern RussiaBOREAS, Issue 3 2006EILIV LARSEN Five regionally significant Weichselian glacial events, each separated by terrestrial and marine interstadial conditions, are described from northwestern Russia. The first glacial event took place in the Early Weichselian. An ice sheet centred in the Kara Sea area dammed up a large lake in the Pechora lowland. Water was discharged across a threshold on the Timan Ridge and via an ice-free corridor between the Scandinavian Ice Sheet and the Kara Sea Ice Sheet to the west and north into the Barents Sea. The next glaciation occurred around 75,70 kyr BP after an interstadial episode that lasted c. 15 kyr. A local ice cap developed over the Timan Ridge at the transition to the Middle Weichselian. Shortly after deglaciation of the Timan ice cap, an ice sheet centred in the Barents Sea reached the area. The configuration of this ice sheet suggests that it was confluent with the Scandinavian Ice Sheet. Consequently, around 70,65 kyr BP a huge ice-dammed lake formed in the White Sea basin (the ,White Sea Lake'), only now the outlet across the Timan Ridge discharged water eastward into the Pechora area. The Barents Sea Ice Sheet likely suffered marine down-draw that led to its rapid collapse. The White Sea Lake drained into the Barents Sea, and marine inundation and interstadial conditions followed between 65 and 55 kyr BP. The glaciation that followed was centred in the Kara Sea area around 55,45 kyr BP. Northward directed fluvial runoff in the Arkhangelsk region indicates that the Kara Sea Ice Sheet was independent of the Scandinavian Ice Sheet and that the Barents Sea remained ice free. This glaciation was succeeded by a c. 20-kyr-long ice-free and periglacial period before the Scandinavian Ice Sheet invaded from the west, and joined with the Barents Sea Ice Sheet in the northernmost areas of northwestern Russia. The study area seems to be the only region that was invaded by all three ice sheets during the Weichselian. A general increase in ice-sheet size and the westwards migrating ice-sheet dominance with time was reversed in Middle Weichselian time to an easterly dominated ice-sheet configuration. This sequence of events resulted in a complex lake history with spillways being re-used and ice-dammed lakes appearing at different places along the ice margins at different times. [source] From ice age to modern: a record of landscape change in an Andean cloud forestJOURNAL OF BIOGEOGRAPHY, Issue 9 2010B. G. Valencia Abstract Aim, To investigate the palaeoecological changes associated with the last ice age, subsequent deglaciation and human occupation of the central Andes. Location, Lake Pacucha, Peruvian Andes (13°36,26, S, 73°19,42, W; 3095 m elevation). Methods, Vegetation assemblages were reconstructed for the last 24 cal. kyr bp (thousand calibrated 14C years before present), based on pollen analysis of sediments from Lake Pacucha. An age model was established using 14C accelerator mass spectrometry dates on bulk sediment. Fossil pollen and sedimentological analyses followed standard methodologies. Results, Puna brava replaced the Andean forest at the elevation of Lake Pacucha at the Last Glacial Maximum (LGM). Deglaciation proceeded rapidly after 16 cal. kyr bp, and near-modern vegetation was established by c. 14 cal. kyr bp. The deglacial was marked by the range expansion of forest taxa as grassland taxa receded in importance. The mid-Holocene was marked by a lowered lake level but relatively unchanged vegetation. Quinoa and maize pollen were found in the latter half of the Holocene. Main conclusions, Temperatures were about 7,8 °C colder than present at this site during the LGM. The pattern of vegetation change was suggestive of microrefugial expansion rather than simple upslope migration. The mid-Holocene droughts were interrupted by rainfall events sufficiently frequent to allow vegetation to survive largely unchanged, despite lowering of the lake level. Human activity at the lake included a 5500-year history of quinoa cultivation and 3000 years of maize cultivation. [source] New approaches to understanding late Quaternary climate fluctuations and refugial dynamics in Australian wet tropical rain forestsJOURNAL OF BIOGEOGRAPHY, Issue 2 2009Jeremy VanDerWal Abstract Aim, We created spatially explicit models of palaeovegetation stability for the rain forests of the Australia Wet Tropics. We accounted for the climatic fluctuations of the late Quaternary, improving upon previous palaeovegetation modelling for the region in terms of data, approach and coverage of predictions. Location, Australian Wet Tropics. Methods, We generated climate-based distribution models for broad rain forest vegetation types using contemporary and reconstructed ,pre-clearing' vegetation data. Models were projected onto previously published palaeoclimate scenarios dating to c. 18 kyr bp. Vegetation stability was estimated as the average likelihood that a location was suitable for rain forest through all climate scenarios. Uncertainty associated with model projections onto novel environmental conditions was also tracked. Results, Upland rain forest was found to be the most stable of the wet forest vegetation types examined. We provide evidence that the lowland rain forests were largely extirpated from the region during the last glacial maximum, with only small, marginally suitable fragments persisting in two areas. Models generated using contemporary vegetation data underestimated the area of environmental space suitable for rain forest in historical time periods. Model uncertainty resulting from projection onto novel environmental conditions was low, but generally increased with the number of years before present being modelled. Main conclusions, Climate fluctuations of the late Quaternary probably resulted in dramatic change in the extent of rain forest in the region. Pockets of high-stability upland rain forest were identified, but extreme bottlenecks of area were predicted for lowland rain forest. These factors are expected to have had a dramatic impact on the historical dynamics of population connectivity and patterns of extinction and recolonization of dependent fauna. Finally, we found that models trained on contemporary vegetation data can be problematic for reconstructing vegetation patterns under novel environmental conditions. Climatic tolerances and the historical extent of vegetation may be underestimated when artificial vegetation boundaries imposed by land clearing are not taken into account. [source] | |||||||||||||