bluuebunny/crossref_metadata_2025_split huggingface dataset:
Upper Quaternary vegetation dynamics and palaeoclimatology of the La Chonta bog area (Cordillera de Talamanca, Costa Rica)
Authors: Hooghiemstra H. , Cleef A. M. , Noldus C. W. , Kappelle M. 
Published: February 2007

AbstractTwo pollen records from La Chonta bog (2310 m altitude) and one pollen record of a soil profile (2430 m altitude) at a short distance from the bog permit the reconstruction of the vegetational history and climatic sequence of probably the last ca. 80 000 yr of the Cordillera de Talamanca, Costa Rica. The three pollen records can be correlated on the basis of three radiocarbon‐dated horizons (> 39 900 yr BP, 34 850 yr BP and 1390 yr BP) and palynostratigraphy. Pollen concentration data provide indirect time control between radiocarbon‐dated horizons of the La Chonta 2 core. During the Early Glacial successive fluctuations of the upper forest line occurred between ca. 2300 m and 2800 m altitude and the bog is alternately situated close to the subalpine rainforest belt and in the uppermontane rainforest belt. During the Pleniglacial (ca. 50 000‐13 000 yr BP) the forest line fell to ca. 2000 m altitude and paramo vegetation surrounded the La Chonta site. During the Late Glacial the forest line shifted rapidly in ca. 270 yr to 2700–2800 m altitude and the lake became surrounded by Quercus‐dominated upper montane rainforest during a period of ca. 1655 yr. A distinct climate cooling (by 2–2.5°C) followed during ca. 580 yr and the upper forest line dropped to ca. 2400 m altitude. This temperature rebound is probably equivalent to the Younger Dryas event. In the depression extensive Alnus carr and swamp vegetation developed. Finally, during the Holocene, the forest line shifted to 3300–3500 m elevation and the lower montane rainforest reached close to ca. 2300 m altitude. In the later part of the Holocene subparamo scrub with Hypericum, Puya, Compositae, Escallonia and Ericaceae largely replaced Alnus carr and the regional and local vegetation closely resembles the presentday composition. During the Last Glacial to Holocene transition a shift of the upper forest line of ca. 1400 m is inferred, corresponding to a glacial temperature depression at 2300 m altitude of ca. 8°C.
The origin of very flinty dry‐valley deposits in the Marlow area, Buckinghamshire, England
Authors: Espejo J. M. Recio , Catt J. A. , Mackney D. 
Published: February 2007

AbstractSome of the soils on the floors of dry chalkland valleys north of the River Thames in the Marlow area have surface horizons of moderately to very flinty, weakly or non‐calcareous, poorly sorted loams, which were deposited in the late Holocene. These overlie typical chalkland gelifluction and periglacial fluvial deposits. Fine calcareous colluvial loams also occur but are less common than in valleys elsewhere on the English chalk. Particle size and mineralogical analyses suggest that the loamy matrix of the flinty surface horizons was derived from loess and early Tnames terrace deposits or Clay‐with‐flints occurring on interfluves between the dry‐valleys. On the evidence of their poorly sorted nature, lack of carbonate, abundance of large flints and irregularly ridged surfaces, it is suggested that they originated mainly as valley‐side debris flows.
Source of a freshwater influx at the last glacial maximum in the Indian Ocean: An alternative interpretation
Authors: Gupta S. K. , Sharma P. , Shah S. K. 
Published: February 2007

AbstractRecent analysis of a sediment core in the eastern Arabian Sea revealed a negative pulse of about 1% in the δ18O value of the planktonic Foraminifera around the last glacial maximum (LGM). This pulse has been attributed to (i) increased runoff into the Bay of Bengal from the east‐flowing south Indian rivers due to enhancement of the northeast winter monsoon, and (ii) an increase in Arabian sea‐surface temperature caused by the weakening of the southwest monsoon at the LGM. We show that the speculation on which the latter hypothesis is based, is not supported by observational data and cannot fully account for the observed magnitude of the spike. With a view to assessing the validity of the first mechanism, we have modelled the mixed layer of the Bay of Bengal as a well‐mixed box. The model calculations show that to account for the pulse requires a change of about 10% in either the annual rate of river input or its isotopic composition. For the northeast monsoon to account for the pulse it would mean that the rainfall should have increased by a factor of five to ten during the LGM. No evidence for such an increase is indicated in the available palaeoclimatic data. We explain the freshwater spike by invoking increased discharge of glacial meltwater from the Tibetan plateau into the Bay of Bengal. We show that the proxy climate data from the Indo‐Tibetan region that has become available recently provides substantial evidence for the occurrence of a warming event around the LGM, which supports our mechanism.
Late Quaternary fluvial activity in the dry valleys (mekgacha) of the Middle and Southern Kalahari, southern Africa
Authors: Shaw Paul A. , Thomas David S. G. , Nash David J. 
Published: February 2007

AbstractKalahari dry ‘fossil’ valleys (mekgacha) have been little used in palaeoenvironmental reconstruction, partly because the origins and functions of the valleys are a subject of debate, and partly because they contain few datable sediments. In the Middle Kalahari the valleys are endoreic, and, following a decline in ground‐water tables owing to human interference over the past 150 years, rarely contain water. Evidence from three sites suggests that the valleys contained standing water between 15000 and 12000 yr BP, and that the main Okwa‐Mmone system flowed into palaeolake Makgadikgadi at 920 m a.s.l. during this period.The Southern Kalahari contains the exoreic Molopo network which is hydrologically more active, being prone to surface flows in response to extreme rainfall events. The spring—fed Kuruman valley has two terraces, the lower composed of a series of flood deposits of late Holocene age. The flood of February 1988 provides a useful analogue for the conditions under which these deposits were formed, and of the temporal regime of the Kuruman River. Comparison with adjacent sites suggests that evidence put forward to indicate cycles of climatic amelioration in the last 4000 yr may, in part, represents the effect of extreme precipitation events. Historical floods show a strong correlation to Southern Oscillation high‐phase (cold event) episodes, suggesting possibilities for extension of the Southern Oscillation record through palaeoflood studies.
Constraints on ice‐sheet thickness over Tibet during the last 40 000 years
Authors: Gupta S. K. , Sharma P. , Shah S. K. 
Published: February 2007

AbstractThe Late Quaternary glaciation of Tibet has received considerable attention in the last few decades due to its influence on the regional climate, especially the Asian summer monsoon. Recently, however, it has been argued that the Tibetan ice sheet also might have played an important role in initiating global‐scale palaeoclimatic changes. Controversy, however, exists on the nature of Late Quaternary ice cover over Tibet due largely to the subjectivity in the interpretation of the sparse and complex geomorphological evidence. We have examined this problem in the light of δ 18O data (a temperature proxy) of ice cores‐from the Dunde ice cap on the northern flank of Tibet. Considering only the gross features in the Dunde ice‐core isotopic data, we have interpreted a temperature decrease of 4°–6°C and consequent lowering of equilibrium line altitude (ELA) in the range 700–850 m during the last glacial stage (LGS). This could have caused depression of the snow line below the mean altitutde of the Tibetan plateau, resulting in an areally extensive but marginally thick ice cover. However, if one also considers the possibility that precipitation on the Tibetan plateau during LGS may have been significantly lower than at present, the ELA depression would be much less than that estimated by considering the temperature effect alone.
Post‐glacial loess deposition in a montane environment: South Thompson River valley, British Columbia, Canada
Authors: Roberts Michael C. , Cunningham Frank F. 
Published: February 2007

AbstractIn the late Wisconsinan, the South Thompson River valley, British Columbia, was occupied by an ice‐dammed lake. After the lake drained, the exposed lacustrine silt became the source material for a Holocene loess. The purpose of this paper is to establish the stratigraphic, depositional and geomorphic framework of loess occurring along the South Thompson River valley immediately east of Kamloops, British Columbia. This montane environment of loess deposition was characterised by active slope and fluvial processes depositing sediments contemporaneously with the accumulation of loess.The loess reaches an average of 4 m in thickness in the central part of the valley and thins towards the valley sides. Two tephras—Mount St Helens Y (3.4 ka) and Mount Mazama (6.8 ka)— occur in the loess and are invaluable stratigraphic markers. Most of the loess was probably deposited between 8.2 ka and 3.4 ka, a period coinciding with mid‐Holocene increased summer temperatures and decreased precipitation in south‐central British Columbia. Debris flows and small streams, originating on the valley sides, flowed out on to the loess depositing sand and gravel beds. These deposits form a definite proximal—distal relation across valley with the slope‐derived sediments decreasing and the loess increasing in thickness towards the centre of the valley.The lactustrine silt particles were mobilised by diurnal mountain and valley, gravity, and canalised winds flowing within the South Thompson valley. An analysis of contemporary wind‐flow data was undertaken to provide a possible analogue for valley wind flows in the mid‐Holocene.
A uranium‐thorium dated late Eemian or early midlandian organic deposit from near Kilfenora between Spa and Fenit, Co. Kerry, Ireland
Authors: Heijnis Henk , Ruddock Julie , Coxon Peter 
Published: February 2007

AbstractThis paper discusses the dating of stratigraphically important Quaternary sequences from a site near Fenit, Co. Kerry, which have been the subject of debate since they were first described by Mitchell in 1970. The overall stratigraphy of the Quaternary deposits have been investigated and detailed analyses of the organic material carried out. Pollen from biogenic sediments have been analysed and samples of peat dated using the uranium‐thorium disequilibrium method. The pollen assemblages match no others previously recorded in Ireland and appear to represent a cool temperate phase following the last interglacial. The uranium‐thorium dates of between 114000 and 123 000 yr BP indicate that the deposit dates from Oxygen Isotope Stage 5, possibly post‐dating the last temperate stage (the Eemian Stage interglacial; Oxygen Isotope Substage 5e). The dating of this deposit and the realisation that it is not penultimate temperate stage (Gortian) in age invalidates much recent speculation on the age of the Gortian interglacial.
‘Little ice age’ variations of outlet glaciers from the jostedalsbreen ice‐cap, Southern Norway: A regional lichenometric‐dating study of ice‐marginal moraine sequences and their climatic significance
Authors: Bickerton Richard W. , Matthews John A. 
Published: February 2007

AbstractComplex moraine‐ridge sequences in front of seven outlet glaciers of the Jostedalsbreen ice‐cap (Austerdalsbreen, Bergsetbreen, Fåbergstølsbreen, Lodalsbreen, Stegaholbreen, Tuftebreen, Bødalsbreen) are dated using families of lichenometric dating curves established previously at an eighth outlet (Nigardsbreen). Applicability of the Nigardsbreen curves at the regional level is tested using independent historical evidence: moraines deposited during the present century are dated to an accuracy of ± 9.4 yr (16.0%), and most of them are dated to an accuracy of ± 5.5 yr (9.4%). Results from the moraine sequences are combined to form a composite ‘Jostedalsbreen’ record. Median predicted dates for moraine ridges cluster around AD 1939 ± 2 yr, 1929 ± 3, 1908 ± 3, 1886 ± 5, 1875 ± 2, 1867 ± 4, 1855 ± 3, 1842 ± 5, 1822 ± 5, 1807 ± 4 and 1785 ± 5. At least four glaciers reached their ‘Little ice age’ maxima prior to AD 1780, two (Nigardsbreen and Bødalsbreen) at ca. 1750, one (Fåbergstølsbreen) at ca. 1705. Stegaholbreen attained its maximum ca. 1863.Since the ‘Little ice age’ maximum, and despite large differences in glacier size, frontal variations of the various outlets have exhibited a high degree of synchroneity, which suggests that the moraine sequences contain a sensitive record of high‐frequency climatic variations over the last ca. 250 yr. During the early twentieth century, measured readvances of the order of 5–150 m over 1–10 yr led to moraine formation. Dendroclimatic evidence indicates that since the late eighteenth century, moraine ridges formed about 5 yr after summer temperature minima and correlate with runs of cool summers (temperature depression of 0.5–1.0°C below the AD 1700–1950 average). Almost simultaneous glacier advances appear to have been caused by reduced ablation. This near‐immediate response to climatic variation, by glacier tongues that descend to relative low altitudes, is superimposed upon the longer‐term dynamic response of the ice cap to climate.
Thermoluminescence dating of the holsteinian marine formation of Herzeele, northern France
Authors: Balescu Sanda , Lamothe Michel 
Published: February 2007

AbstractThermoluminescence dating of potassium feldspar coarse grains from the fossiliferous Holsteinian marine formation of Herzeele, at the southern limit of the North Sea Basin, yields a minimum age of 228 £ 30 ka. A tentative correction for the long‐term fading of the TL signal in potassium feldspar is proposed, assuming a lifetime (τ) of 711 ka. This correction would yield a preliminary age estimate of 271 £ 36 ka. These results support a minimum Oxygen Isotope Stage 7 age, but do not preclude, at this stage of our research, a possible correlation with late Oxygen Isotope Stage 9. These results are in agreement, within the error of the method, with independent absolute dates (U‐Th and ESR) obtained onin situmarine shells, which range from 225 ka to 348 ka. This study provides evidence of the reliability of TL age estimates on potassium feldspar coarse grains from shallow‐marine sediments older than the Last Interglacial.
OSL and TL dating of buried podsols and overlying sands in Ostrobothnia, western Finland
Authors: Hütt G. , Jungner H. , Kujansuu R. , Saarnisto M. 
Published: February 2007

AbstractIn Ostrobothnia, western Finland, buried fossil soils have been recognised in a number of places in the sandy sediments that occur between glaciofluvial deposits and overlying till. Samples from the soil horizons as well as below and above them were taken for optically stimulated luminescence (OSL) dating. The same sites were also sampled for thermoluminescence (TL) dating. Altogether five TL dates and seventeen OSL dates were obtained. The OSL dates can be grouped into two age classes, (i) 120‐163 ka and (ii) 76‐106 ka, whereas all TL dates are of the order 135‐155 ka. A comparison between the results obtained from the two dating methods shows that OSL dates are generally younger than the TL dates for the same sample. The discrepancy may arise partly from problems of setting a correct residual level in TL dating. If this is the case then the TL dates may indicate an upper limit for the true age. The results support the geological interpretation that the fossil soils were formed during the last interglacial, that the soil‐forming processes possibly also continued during the first Early Weichselian stadial and interstadial (Brørupsensu lato), and that in general the till‐covered glaciofluvial sequences, interpreted as eskers in Ostrobothnia, were deposited during the Saalian or Early Weichselian deglaciation.
Late quaternary chronology of the Taitao Peninsula, southern Chile
Authors: Lumley Susie H. , Switsur Roy 
Published: February 2007

AbstractSix new radiocarbon ages and a pollen sequence are provided for Laguna Stibnite, a small lake on the Taitao Peninsula, Chile (latitude 46°25'S, longitude 74°24'W). The sediments record a late‐glacial to present sequence, with a basal age of 14 335 ± 145 yr BP (Q‐2840). The radiocarbon ages provide the basis for a sound chronology in a region of Chile where few palynological studies have been made and where previous radiocarbon ages are ambiguous. The chronology from Laguna Stibnite supports the acceptance of a radiocarbon chronology based on wood fragments (rather than bulk analysis) at a site near Puerto Edén further south in the Chilean Channels (49°08'S). These data from Laguna Stibnite provide evidence for an early deglaciation (before 14 000 yr BP) in this region of southern Chile. The sequence provides no evidence for a climatic reversal between 11 000 yr BP and 10 000 yr BP, the so‐called Younger Dryas chron.
Thaw modification of frost‐fissure wedges, Richards Island, Pleistocene Mackenzie Delta, Western Arctic Canada
Authors: Murton Julian B. , French Hugh M. 
Published: February 2007

AbstractThaw modification is the general process whereby frost‐fissure wedges are modified during thaw, and by which frost‐fissure pseudomorphs may develop. Specific processes of thaw modification are inferred from ice‐wedge pseudomorphs, composite‐wedge pseudomorphs and deformed sand wedges in the Pleistocene Mackenzie Delta: i.e. thermal erosion, collapse, subsidence, refreezing, loading, buoyancy, spreading, folding and shearing.Thaw modification is believed to result in selective preservation of pseudomorphs and wedges. Sand wedges are more likely to be preserved than are ice‐wedge pseudomorphs or compositewedge pseudomorphs, because only those sand wedges that penetrate massive ice or icy sediments are prone to thaw modification. Furthermore, whereas ice wedges preferentially develop in ice‐rich, fine‐grained sediments (thaw‐sensitive), their pseudomorphs appear to be selectively preserved in ice‐poor, coarse‐grained sediments (thaw‐stable).
The impact of the Laacher See Tephra on terrestrial and aquatic ecosystems in the Black Forest, southern germany
Authors: Lotter A. F. , Birks H. J. B. 
Published: February 2007

AbstractLate‐glacial lake sediments containing the Laacher See Tephra (LST, 11000 yr BP) have been analysed for pollen and diatoms at two sites (Hirschenmoor, Rotmeer) in the montane belt of the Black Forest (southern Germany) in order to detect the possible impact of this major volcanic eruption on terrestrial and aquatic biota.The pollen assemblages at both sites show a minor increase in grasses and sedges following the tephra deposition, whereas the pollen of aquatic macrophytes do not show any consistent pattern. Partial redundancy analysis and Monte Carlo permutation tests suggest, however, that the LST had no statistically significant effect either on the terrestrial or aquatic pollen assemblages at either site when the effects of time and climatic change are allowed for statistically.The diatom assemblages at both sites changed after the deposition of the LST and diatominferred pH estimates suggest a short but non‐significant excursion towards lower values. Multivariate analyses of the diatom data indicate a statistically significant impact of the deposition of the LST and associated changes in sediment lithology on the diatom assemblages when the effects of climatic change and time are allowed for statistically. However, owing to the interaction between tephra and lithology it is not possible to conclude if the diatom changes were caused by the deposition of the LST or by lithological changes.
Glacial Lake development and marine inundation, Deer Lake area, Newfoundland, Canada: Topographically controlled deglaciation of an interior Basin
Authors: Batterson Martin J. , Liverman David G. E. , Kirby Gary E. 
Published: February 2007

AbstractA large interior basin in west‐central Newfoundland (covering the area of modern Deer Lake, Grand Lake, Sandy Lake and Birchy Lake) is connected to the sea by a narrow breach of a coastal mountain range. During retreat of Late Wisconsinan glaciers, this basin was occupied by a short‐lived glacial lake impounded by remnant ice in coastal fjords, and drained by a spillway at the western end of the lake. Evidence for this lake is fragmentary, and consists of strandline features that fall on a plane of elevation consistent with the post‐glacial isostatic tilt. Following collapse of the ice dams and subsequent lake drainage at some time prior to 12 220 yr BP, the Deer Lake basin was inundated by the sea to an elevation of about 45 m a.s.l. Deltas were formed at the basin edges, and thick successions of fine‐grained rhythmites blanketed the basin floor. Isostatic rebound resulted in falling relative sea levels, and, following a stillstand marked by a period of deltaic deposition at 33 m a.s.l., isolation of the Deer Lake basin from marine influence in the early Holocene.This style of deglaciation differs from previously accepted models for this part of eastern Canada, which showed progressive retreat from the coast to remnant centres on topographic highs. In the model proposed here, a large, low‐elevation basin was deglaciated early, at a similar time to ice retreat to coastal positions from offshore. This pattern of deglaciation may be found in other areas with similar topographic settings.
Gibbsitic soils on former nunataks: implications for ice sheet reconstruction
Authors: Ballantyne Colin K. 
Published: February 2007

AbstractX‐ray diffraction analyses of soils above and below a periglacial trimline developed across the basalts of the Trotternish Escarpment (Isle of Skye, Scotland) demonstrate that gibbsite is restricted to soils above the trimline. This suggests that the gibbsite is a relict of pre‐Late Devensian weathering, and that the trimline did not develop after the last ice sheet achieved its maximum thickness. The sharpness of the boundary between frost‐weathered regolith and gibbsitic soils upslope and ice‐scoured bedrock associated with gibbsite‐free soils downslope suggests that the trimline represents the altitude of the last ice sheet at its maximum thickness rather than a former boundary between passive cold‐based ice and erosive warm‐based ice. These findings illustrate how identification of high‐level periglacial trimlines and associated contrasts in clay mineralogy provide a means for constraining reconstructions of the form of the last ice sheets.
The last glaciation of Germania Land and adjacent areas, northeast Greenland
Authors: Landvik Jon Y. 
Published: February 2007

AbstractA profile across the unglaciated coast of northeast Greenland at 77°N was studied with regard to the Quaternary stratigraphy and glacial history. The Germania Land peninsula is characterised by till‐covered lower ground which contrasts sharply with the blockfields and extensive gelifluction deposits of its higher altitudes. Two glaciations are distinguished. The older one extended over the entire area and had its margin on the continental shelf. The younger one, of Late Weichselian age, reached the present coastline and several mountains and high plateaus on western Germania Land formed nunataks.The Late Weichselian glaciation was more extensive and occurred later on the Germania Land peninsula than on the coast further south. Radiocarbon dates suggest that the glacier margin rested to the east of the present coastline until ca. 10 000 yr BP. This correlates with the Late Weichselian Milne Land Stage, which is found as a late glacial readvance along the coast of East Greenland. A series of recessional moraines formed during the deglaciation were probably caused by glacier dynamics, as opposed to being of climatic origin.