摘要:

AbstractRegional pollen assemblage zones for the late‐glacial period of the Swiss Plateau are introduced and defined. They include four major zones (Artemisia, Juniperus—Hippophaë, Betula, PinusPAZ) with several subzones. Pollen and oxygen‐isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000‐9500 yr BP.The first oscillation, termed the Aegelsee fluctuation, occurs shortly before 12 000 yr BP and varve counts suggest its duration was ca. 100 yr. It is characterised by a short decrease in the oxygen isotopes as well as a short increase in NAP associated with a depression in birch pollen values. The second oscillation, which occurs in the δ18O record shortly before the deposition of the Laacher See Tephra (ca. 11 000 yr BP), is termed the Gerzensee fluctuation. It occurs during a pine‐dominated phase and its vegetational effects cannot be determined palynologically. The most prominent regressive phase is the Younger Dryas biozone (ca. 10 700‐10 000 yr BP) characterised by an increase in heliophilous NAP and low δ18O values. The Younger Dryas biozone can often be subdivided palynologically into two parts: a first part rich in grasses and juniper and a second part with higherFilipendulaand birch values. During the Preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios.Comparison of the Swiss oxygen isotope profiles with the Greenland Dye 3 record suggests that not only the three major shifts in the δ18O curves but also the minor ones are closely comparable, suggesting some common c

ISSN:0267-8179    

DOI:10.1002/jqs.3390070302

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

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 byQuercus‐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 extensiveAlnuscarr 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 withHypericum, Puya, Compositae,Escalloniaand Ericaceae largely replacedAlnuscarr 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 altit

ISSN:0267-8179    

DOI:10.1002/jqs.3390070303

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

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 deb

ISSN:0267-8179    

DOI:10.1002/jqs.3390070304

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA synthesis of pollen evidence from Etang du Pourra, a small pond in Rhodanian Provence, France, complemented by new sedimentological, mineralogical and palynological data allow a reassessment of the Lateglacial‐Holocene vegetation and climatic history of the area. The Etang du Pourra succession is shown to consist of two parts; the lower part dates from the Lateglacial and the upper reveals the Holocene pollen‐stratigraphic succession typical of the region. In the Lateglacial, a humid and cooler phase is recorded before the Younger Dryas conditions progressively set in. In the Holocene succession, the consequences of human modifications on the environment are clearly visible in changes in mineralogy and pollen stratigra

ISSN:0267-8179    

DOI:10.1002/jqs.3390070305

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

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 m

ISSN:0267-8179    

DOI:10.1002/jqs.3390070306

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractMineral magnetic analyses are applied to a series of diamict samples taken from the Quaternary sequence in the Isle of Man in order to assess the relative magnitude of the intra‐and inter‐unit variation of the magnetic properties. The results are analysed both qualitatively and quantitatively and show that the level of intra‐unit variation is significantly less than the level of inter‐unit variation. The intra‐unit variation detected appears to show some relationship to the basic stratigraphy. Previous correlations based upon field evidence of diamict units exposed upon the east and west coasts of the Isle of Man are supported by their mineral magnetic properties. The statistically significant differences in the magnetic mineralogy of the stratigraphically lower Wyllin and Shellag Tills and the upper Orrisdale, Ballavarkish and Ballaquark Tills suggests distinct provenances for these two suites of sediments, allowing potential source areas to be suggested. These source areas are consistent with published data on ice flow directions in the Irish Sea Basin during the

ISSN:0267-8179    

DOI:10.1002/jqs.3390070307

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390070308

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

ISSN:0267-8179    

DOI:10.1002/jqs.3390070301

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY