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1.
Mutual Climatic Range (MCR) analysis was applied to 20 fossil beetle assemblages from 11 sites dating from 14,500 to 400 yr B.P. The fossil sites represent a transect of the Rocky Mountain region from northern Montana to central Colorado. The analyses yielded estimates of mean July and mean January temperatures. The oldest assemblage (14,500 yr B.P.) yielded mean July values of 10–11°C colder than present and mean January values 26–30°C colder than present. Postglacial summer warming was rapid, as indicated by an assemblage dating 13,200 yr B.P., with mean July values only 3–4°C cooler than modern. By 10,000 yr B.P., several assemblages indicate warmer-than-modern mean summer and winter values. By 9000 yr B.P., MCR reconstructions indicate that both summer and winter temperatures were already declining from an early Holocene peak. Mean July values remained above modern levels and mean January values remained below modern levels until 3000 yr B.P. A series of small-scale oscillations followed.  相似文献   

2.
Mutual climatic range (MCR) analysis was applied to 15 North American beetle assemblages spanning the interval from > 52 000 to 17 200 yr BP, bracketing a Mid-Wisconsin interstadial interval. The analyses yielded estimates of mean July (TMAX) and mean January (TMIN) temperatures. The oldest assemblage (> 52 ka) yielded TMAX values 7.5–8°C lower than present and TMIN values 15–18°C lower than present. A Mid-Wisconsin interstadial warming dating from 43.5–39 ka was rapid and intense. At the peak of the warming event, about 42 ka, TMAX values were only 1–2°C lower than modern. This level of amelioration apparently lasted only about 2000–3000 yr. By 23.7 ka, TMAX values declined to 11.5–10°C lower than modern, but another, small-scale amelioration is indicated by assemblages dating from 20.5 to 19.7 ka. The interstadial event recorded from the site at Titusville, Pennsylvania closely matches the timing and intensity of the climate change estimated from British beetle faunas in the Upton Warren interstadial. Another warm interval (ca. 31–32.5 ka) has been documented from fossil beetle assemblages in Europe and North America. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

3.
Cryoturbated organic beds and channel fills, intercalated with sandy and gravelly fluvial units, have been studied in an opencast brown‐coal mine near Nochten (Niederlausitz), eastern Germany. The fluvial–aeolian sequence covers parts of the Early, Pleni‐ and Late‐glacial. The detailed chronology is based on 11 radiocarbon and 12 OSL dates, covering the period between ca. 100 kyr and 11 kyr BP. Basal peat deposits are correlated with an Early Weichselian interstadial. During this period boreal forests were present and minimum mean summer temperatures were > 13°C. Early Pleniglacial deposits are absent. The Middle and Late Pleniglacial environments were treeless and different types of tundra vegetation can be recognised. Minimum mean summer temperatures varied between 10 and 15°C. Vegetation and climate is reconstructed in detail for the periods around 34–38 kyr BP and 24–25 kyr BP. Around 34–38 ka, a mixture between a low shrub tundra and a cottongrass tussock–subshrub tundra was present. The botanical and sedimentological data suggest that from the Middle to the Late Pleniglacial, the climate became more continental, aridity and wind strength increased, and the role of a protecting winter snow cover decreased. A sedge–grass–moss tundra dominated around 24 and 25 kyr BP. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

4.
Macroscopic plant remains from middle Devensian organic lenses, exposed in a gravel pit near Earith in the valley of the River Great Ouse in Cambridgeshire, were the object of an extensive study by Frances Bell in the late 1960s, which has been much referred to in the subsequent literature. The floras from each sampling site have been taken to be characteristic of Full‐glacial conditions in general. Coleopteran assemblages taken at the same time from exactly the same sampling sites show that at least two periods of markedly different climatic regimes were involved. The first of these (? the older) was characterised by a cold and continental climate with mean July temperatures at or below 10d°C and mean January/February temperatures about −23°C. The second climatic regime was characterised by warm summers with mean July temperatures about 16°C and mean January/February temperatures close to −5°C. In spite of this temperate climate, pollen analysis shows that no trees grew in Britain at that time. This treeless but temperate episode in the middle of the Devensian (Weichselian) is discussed in the context of coleopteran assemblages from other sites in southern England that probably date from the same period. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

5.
Cryoplanation terraces are bedrock steps or terraces on ridge crests and hilltops. The tread or “flat” area is 10 to several hundred meters wide and long and slopes from 1 to 5° parallel to the ridge crests. Terrace scarps may be from 1 to 75 m high. Terraces are cut into all bedrock types and are best developed on closely jointed, fine-grained bedrock. The scarps and treads are covered with frost-rived rubble 1 to 2 m thick. The rubble on treads is perennially frozen at a depth of 1 to 2 m or less on sharp but inactive terraces in Alaska.Cryoplanation terraces exist in many parts of the world in present or past periglacial environments. They occur chiefly in nonglaciated regions and near the general altitude of snowline. Cryoplanation terraces form by scarp retreat as the result of nivation. Surficial debris is removed across the terrace tread by mass-wasting. Terrace morphology depends mainly upon climate, bedrock type, and terrace orientation.No climatic data are available from active terraces. Indirect evidence indicates that climatic requirements include low snowfall and cold summer temperatures. Shallow permafrost is necessary to provide moisture and a base for mass movement as well as a base for nivation.Hundreds of sharp but inactive terraces occur in some areas in Alaska where the summer temperature is colder than 10°C. When these terraces were active, temperatures were colder. Recent work in Alaska indicates that terraces were active in some areas when the mean July temperature was about 4°C. The mean annual air temperature probably was in the neighborhood of ′12°C or colder.  相似文献   

6.
The palynology of stratigraphic sections from road-cut and gravel-pit exposures and from a fen and sphagnum bogs in the southern part of the Chilean lake district (40° 53′ S, 72°37′ W-41°24′ S, 72°53′ W) is the basis for interpreting vegetation and climate during the last interglaciation and glaciation (named Llanquihue Glaciation) and during the post-glacial. To help interpretation, modern pollen rain was studied in relation to vegetation and altitude along a transect on the west slope of the Andes, and average January (summer) temperatures were interpreted. The upper limit of closed Andean forest, where wind is a determinant, appears to be close to the 12°C January isotherm; parkland in southern Chile does not exceed the January isotherm of 9°C.Grassland and later southern beech forest are evident during the interglaciation that is dated at more than 39,900 radiocarbon yr. Climate of the grassland was relatively dry; during the forest phase, it was wet, cool, and approximately the same as at present. During Llanquihue Glaciation, average January temperature is estimated to have been about 8°C colder than today at 19,450 BP, some 5° colder shortly before 36,300 BP, and around 4° colder at 10,000 BP. Antarctic-alpine tundra or parkland, under colder, drier climate, is mostly in evidence in the vicinity of the study sites before about 12,000 BP. During the postglacial, forest communities occupied the lake district, and temperatures there were probably 1–2°C above (by 6500 BP) and as much as 2° below (4500-0 BP) the present-day average of about 16°.This pattern of climatic changes finds accord, in general terms, in other parts of the Southern Hemisphere where palynological, chronological, and glacial geological studies are reported. Postulated as a cause of these changes are shifts in the intensity of air mass circulation in antarctic latitudes.  相似文献   

7.
Chironomids and pollen were studied in a radiocarbon-dated sediment sequence obtained from a former lake near the Maloja Pass in the Central Swiss Alps (1865 m a.s.l.) to reconstruct the Lateglacial environment. Pollen assemblages imply a vegetation development around the Maloja Pass from shrub tundra at the beginning of the Allerød to coniferous forest during the early Holocene with a lowering of the timberline during the Younger Dryas. Chironomid assemblages are characterized by several abrupt shifts in dominant taxa through the Lateglacial. The occurrence of taxa able to survive hypoxia in the second part of the Allerød and during the Preboreal, and their disappearance at the onset of the Younger Dryas cold phase suggest summer thermal stratification and unfavourable hypolimnetic oxygen conditions in the palaeo-lake during the warmer periods of the Lateglacial interstadial and early Holocene. Mean July air temperatures were reconstructed using a chironomid-temperature transfer function from the Alpine region. The pattern of reconstructed temperature changes agrees well with the Greenland δ18O record and other Lateglacial temperature inferences from Central Europe. The inferred July temperatures of ca 10.0 °C during most of the Allerød were slightly lower than modern values (10.8 °C) and increased up to ca 11.7 °C (i.e., above present-day values) at the end of the Allerød. The first part of the Younger Dryas was colder (ca 8.8 °C) than the second part (ca 9.8 °C). During most of the Preboreal, the temperatures persisted within the limits of 13.5–14.5 °C (i.e., ca 3 °C above present-day values). The amplitudes of temperature changes at the Allerød–Younger Dryas–Preboreal transitions were ca 3.5–4.0 °C. The temperature reconstruction also shows three short-lived cooling events of ca 1.5–2.0 °C, which may be attributed to the centennial-scale Greenland Interstadial events GI-1d and GI-1b, and the Preboreal Oscillation.  相似文献   

8.
Palaeoecological studies carried out in the Chilean Lake District and Chilotan Archipelago (41°–43°S) record full-glacial and late-glacial pollen assemblages beginning just after 21000 and beetle assemblages after 18000, both sets extending until 10000 14C yr BP. Pollen records indicate that Subantarctic Parkland, the vegetation of the early millennia of record, changed after about 14000 yr BP to become open woodland and later North Patagonian Evergreen Forest. Assemblages of plants and beetles, responding more or less in unison to a strong rise in temperature (≥ 6°C), behaved in accord at around 14000 until 13000–12500 yr BP, the beetle fauna displaying a marked increase in obligate forest types. During full-glacial conditions (17400–16100 and 15300 and 14400 yr BP) and in the late-glacial interval (after about 13000 yr BP), however, climate evidently coerced populations dissimilarly, the pollen sequence showing an increase in plant taxa indicative of colder climate, whereas the beetle fauna underwent little or no variation. Contrasting climate modes implied by plants and beetles may be attributed to differential responses to apparent low-order temperature changes (≤ 2–3°C).  相似文献   

9.
Cryolithological, ground ice and fossil bioindicator (pollen, diatoms, plant macrofossils, rhizopods, insects, mammal bones) records from Bol'shoy Lyakhovsky Island permafrost sequences (73°20′N, 141°30′E) document the environmental history in the region for the past c. 115 kyr. Vegetation similar to modern subarctic tundra communities prevailed during the Eemian/Early Weichselian transition with a climate warmer than the present. Sparse tundra‐like vegetation and harsher climate conditions were predominant during the Early Weichselian. The Middle Weichselian deposits contain peat and peaty soil horizons with bioindicators documenting climate amelioration. Although dwarf willows grew in more protected places, tundra and steppe vegetation prevailed. Climate conditions became colder and drier c. 30 kyr BP. No sediments dated between c. 28.5 and 12.05 14C kyr BP were found, which may reflect active erosion during that time. Herb and shrubby vegetation were predominant 11.6–11.3 14C kyr BP. Summer temperatures were c. 4 °C higher than today. Typical arctic environments prevailed around 10.5 14C kyr BP. Shrub alder and dwarf birch tundra were predominant between c. 9 and 7.6 kyr BP. Reconstructed summer temperatures were at least 4 °C higher than present. However, insect remains reflect that steppe‐like habitats existed until c. 8 kyr BP. After 7.6 kyr BP, shrubs gradually disappeared and the vegetation cover became similar to that of modern tundra. Pollen and beetles indicate a severe arctic environment c. 3.7 kyr BP. However, Betula nana, absent on the island today, was still present. Together with our previous study on Bol'shoy Lyakhovsky Island covering the period between about 200 and 115 kyr, a comprehensive terrestrial palaeoenvironmental data set from this area in western Beringia is now available for the past two glacial–interglacial cycles.  相似文献   

10.
Mean July and January temperatures are reconstructed from radiocarbon-dated fossil beetle assemblages from late-glacial sites in the Maritimes Region of eastern Canada. Fossil-bearing sediments date from 12 700 14 C yr BP (14 950 cal yr BP) to younger than 10 800 14 C yr BP (12 730 cal yr BP), spanning a period which includes stratigraphic, palynological, chironomid and coleopteran evidence for a climatic deterioration during the Younger Dryas in North America. Mutual Climatic Range data suggest several 'events' in the coleopteran record from the Maritimes that appear similar to climate events recorded in the GRIP ice-core record, including the (Younger Dryas) cooling event from GI-1a to GS-1 beginning c. 12 650 GRIP yr BP Some of the major temperature oscillations of Greenland Interstadial 1 may also be reflected in the coleopteran record of the Maritimes.  相似文献   

11.
We present a Lateglacial and early Holocene chironomid‐based July air temperature reconstruction from Foppe (1470 m a.s.l.) in the Swiss Southern Alps. Our analysis suggests that chironomid assemblages have responded to major and minor climatic fluctuations during the past 17 000 years, such as the Oldest Dryas, the Younger Dryas and the Bølling/Allerød events in the Lateglacial and the Preboreal Oscillation at the beginning of the Holocene. Quantitative July air temperature estimates were produced by applying a combined Norwegian and Swiss temperature inference model consisting of 274 lakes to the fossil chironomid assemblages. The Foppe record infers average July air temperatures of ca. 9.9 °C during the Oldest Dryas, 12.2 °C during most of the Bølling/Allerød and 11.1 °C for the Younger Dryas. Mean July air temperatures during the Preboreal were 14 °C. Major temperature changes were observed at the Oldest Dryas/Bølling (+2.7 °C), the Allerød/Younger Dryas (?2 °C) and the Younger Dryas/Holocene transitions (+3.9 °C). The temperature reconstruction also shows centennial‐scale coolings of ca. 0.8–1.4 °C, which may be synchronous with the Aegelsee (Greenland Interstadial 1d) and the Preboreal Oscillations. A comparison of our results with other palaeoclimate records suggests noticeable temperature gradients across the Alps during the Lateglacial and early Holocene. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

12.
This paper presents the first chironomid‐inferred mean July air temperature reconstruction for the Late‐glacial in Britain. The reconstruction suggests that the thermal maximum occurred early in the interstadial, with temperatures reaching about 12°C. There was then a gradual downward trend to about 11°C, punctuated by four distinct cold oscillations of varying intensity. At the beginning of the Younger Dryas, mean July temperatures fell to about 7.5°C but gradually increased to about 9°C before a rapid rise at the onset of the Holocene. The chironomid‐inferred temperature curve agrees closely, both in general trends and in detail, with the GRIP ice‐core oxygen‐isotope curve. The reconstructed temperatures are 2–4°C lower than coleopteran‐inferred temperatures but are closer to those inferred from plant macrofossils and glacial equilibrium‐line altitudes during the Younger Dryas. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

13.
《Quaternary Science Reviews》2007,26(19-21):2420-2437
Lateglacial environments at Hijkermeer, northwest Netherlands, were reconstructed by means of chironomid, diatom and pollen analyses. Diatom assemblages indicate that Hijkermeer was a shallow, oligo- to mesotrophic lake during this period. Pollen assemblages reflect the typical northwest European Lateglacial vegetation development and provide an age assessment for the record from the beginning of the Older Dryas (ca 14 000 calibrated 14C yr BP) into the early Holocene (to ca 10 700 calibrated 14C yr BP). The chironomid record is characterized by several abrupt shifts between assemblages typically found in mid-latitude subalpine to alpine lakes and assemblages typical for lowland environments. Based on the chironomid record, July air temperatures were reconstructed using a chironomid-temperature transfer-function from central Europe. Mean July air temperatures of ca 14.0–16.0 °C are inferred before the Older Dryas, of ca 16.0–16.5 °C during most of the Allerød, of ca 13.5–14.0 °C during the Younger Dryas, and of ca 15.5–16.0 °C during the early Holocene. Two centennial-scale decreases in July air temperature were reconstructed during the Lateglacial interstadial which are correlated with Greenland Interstadial events (GI)-1d and -1b. The results suggest that vegetation changes in the Netherlands may have been promoted by the cooler climate during GI-1d, immediately preceding the Older Dryas biozone, and GI-1b. The Hijkermeer chironomid-inferred temperature record shows a similar temperature development as the Greenland ice core oxygen isotope records for most of the Lateglacial and a good agreement with other temperature reconstructions available from the Netherlands. This suggests that chironomid-based temperature reconstruction can be successfully implemented in the Northwest European lowlands and that chironomids may provide a useful alternative to oxygen isotopes for correlating European lake sediment records during the Lateglacial.  相似文献   

14.
A peat bed found under solifluction deposits on Godøya island, western Norway, accumulated during a few decades around 11 000 yr BP, at the end of the Allerød period of the Late Weichselian. Palaeoecological investigations showed a local vegetation succession on wet sand culminating in a mire community dominated by Carex nigra. Periodic flooding brought in sand and silt, which decreased as drainage was impeded sufficiently for standing water to develop. The surrounding terrestrial vegetation was dominated by Salix scrub, with some open heath and alpine habitats nearby. Apart from two aquatic species, the 29 insect taxa recorded are characteristic of alpine heaths, plant litter (under Salix scrub) and stream-sides. Their remains, together with the terrestrial plant macrofossils, were washed into the mire from nearby. Because the fossils are locally derived, the environmental reconstructions are of the actual conditions at Godøy at ca. 11 000 yr BP. Palaeotemperature estimates from beetles and plants are in agreement. The coleopteran estimates (Mutual Climatic Range Method) suggest mean July temperatures of 10–13°C, slightly cooler than today (13.5°), and January temperatures between +1 and ?10°C, similar to or much colder than today. Summer temperature estimates from individual plant taxa indicate that temperatures during the Allerød period were similar to today's, but estimates from the reconstructed vegetation and timber-line positions give estimates up to 3.5° cooler. Temperatures fell 2.5–7.5°C at the Younger Dryas. This abrupt and severe cooling initiated the solifluction processes on Godøya that buried the peat. The Godøy peat bed and its contained fossils provide a rare glimpse of Allerød biota and environments at the local (site) scale.  相似文献   

15.
The oxygen isotope records of both benthic and planktonic Foraminifera in five piston cores, collected from the region between the Oyashio and Kuroshio Currents near Japan, clearly show the marked latitudinal shifts of these two currents during the past 25 kyr. Under the present hydrographic condition, a clear relationship between the sea‐surface temperature (SST) and oxygen isotope differences from benthic to planktonic Foraminifera is observed in this region. Using this relationship, we find decreased SSTs of 12–13°C (maximum 15°C) in the southernmost core site at the Last Glacial Maximum (LGM), indicating the Oyashio Current shifted southward. The SSTs at the southern two core sites abruptly increased more than 10°C at 10–11 ka, suggesting the Kuroshio Current shifted northward over these sites at 10–11 ka. In contrast, the northern two core sites have remained under the influence of the cold Oyashio Current for the past 25 kyr. With the reasonable estimate of bottom‐water temperature decrease of 2.5°C at the LGM, the SSTs estimated by this new method give exactly the same SST values calculated from Mg/Ca ratio of planktonic Foraminifera, allowing palaeosea‐surface salinities to be reconstructed. The result suggests that the ice volume effect was 1.0 ± 0.1‰ at the LGM. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

16.
Paleoenvironmental records from a number of permafrost sections and lacustrine cores from the Laptev Sea region dated by several methods (14C-AMS, TL, IRSL, OSL and 230Th/U) were analyzed for pollen and palynomorphs. The records reveal the environmental history for the last ca 200 kyr. For interglacial pollen spectra, quantitative temperature values were estimated using the best modern analogue method. Sparse grass-sedge vegetation indicating arctic desert environmental conditions existed prior to 200 kyr ago. Dense, wet grass-sedge tundra habitats dominated during an interstadial ca 200–190 kyr ago, reflecting warmer and wetter summers than before. Sparser vegetation communities point to much more severe stadial conditions ca 190–130 kyr ago. Open grass and Artemisia communities with shrub stands (Alnus fruticosa, Salix, Betula nana) in more protected and moister places characterized the beginning of the Last Interglacial indicate climate conditions similar to present. Shrub tundra (Alnus fruticosa and Betula nana) dominated during the middle Eemian climatic optimum, when summer temperatures were 4–5 °C higher than today. Early-Weichselian sparse grass-sedge dominated vegetation indicates climate conditions colder and dryer than in the previous interval. Middle Weichselian Interstadial records indicate moister and warmer climate conditions, for example, in the interval 40–32 kyr BP Salix was present within dense, grass-sedge dominated vegetation. Sedge-grass-Artemisia-communities indicate that climate became cooler and drier after 30 kyr BP, and cold, dry conditions characterized the Late Weichselian, ca 26–16 kyr BP, when grass-dominated communities with Caryophyllaceae, Asteraceae, Cichoriaceae, Selaginella rupestris were present. From 16 to 12 kyr BP, grass-sedge communities with Caryophyllaceae, Asteraceae, and Cichoriaceae indicate climate was significantly warmer and moister than during the previous interval. The presence of Salix and Betula reflect temperatures about 4 °C higher than present at about 12–11 kyr BP, during the Allerød interval, but shrubs were absent in the Younger Dryas interval, pointing to a deterioration of climate conditions. Alnus fruticosa, Betula nana, Poaceae, and Cyperaceae dominate early Holocene spectra. Reconstructed absolute temperature values were substantially warmer than present (up to 12 °C). Shrubs gradually disappeared from coastal areas after 7.6 kyr BP when vegetation cover became similar to modern. A comparison of proxy-based paleoenvironmental reconstructions with the simulations performed by an Earth system model of intermediate complexity (CLIMBER-2) show good accordance between the regional paleodata and model simulations, especially for the warmer intervals.  相似文献   

17.
We have compiled what we believe is the longest coherent coastal sea surface temperature record in North America. Near-surface water temperature measurements have been made almost daily at Great Harbon, Woods Hole, Massachusetts, since 1886 with remarkably few gaps. The record shows that there was no significant trend in water temperature at this site for the first 60 yr of observation. There was some cooling during the 1960s that was followed by a significant warming from 1970–2002 at a rate of 0.04°C yr?1. During the 1990s annual mean temperatures averaged approximately 1.2°C warmer than they had been on average between 1890 and 1970; winter (December, January, and February) temperatures were 1.7°C warmer and summer (June, July, and August) temperatures were 1.0°C warmer. There has not been a statistically significant decrease in the annual number of winter days below 1°C or an increase in the annual number of winter days above 5°C. The number of summer days each year with water temperature above 21°C has not increased significantly. The dates of first observations of 10°C and 20°C water in the spring have not changed sufficiently to be statistically significant. There is a weak positive correlation between annual and winter water temperature and the annual and winter North Atlantic Oscillation index, respectively, during the period of record.  相似文献   

18.
A chironomid–July air temperature inference model based on chironomid assemblages in the surface sediments of 81 Swiss lakes was used to reconstruct Late Glacial July air temperatures at Lac Lautrey (Jura, Eastern France). The transfer‐function was based on weighted averaging–partial least squares (WA‐PLS) regression and featured a leave‐one‐out cross‐validated coefficient of determination (r2) of 0.80, a root mean square error of prediction (RMSEP) of 1.53 ° C, and was applied to a chironomid record consisting of 154 samples covering the Late Glacial period back to the Oldest Dryas. The model reconstructed July air temperatures of 11–12 ° C during the Oldest Dryas, increasing temperatures between 14 and 16.5 ° C during the Bølling, temperatures around 16.5–17.0 ° C for most of the Allerød, temperatures of 14–15 ° C during the Younger Dryas and temperatures of ca. 16.5 ° C during the Preboreal. The Lac Lautrey record features a two‐step July air temperature increase after the Oldest Dryas, with an abrupt temperature increase of ca. 3–3.5 ° C at the Oldest Dryas/Bølling transition followed by a more gradual warming between ca. 14 200 and 13 700 BP. The transfer‐function reconstructs a less rapid cooling at the Allerød/Younger Dryas transition than other published records, possibly an artefact caused by the poor analogue situation during the earliest Younger Dryas, and an abrupt warming at the Younger Dryas/Holocene transition. During the Allerød, two centennial‐scale 1.5–2.0 ° C coolings are apparent in the record. Although chronologically not well constrained, the first of these cold events may be synchronous with the beginning of the Gerzensee Oscillation. The second is inferred just before deposition of the Laachersee tephra at Lac Lautrey and is therefore coeval with the end of the Gerzensee Oscillation. In contrast to the Greenland oxygen isotope records, the Lac Lautrey palaeotemperature reconstruction lacks a clearly defined Greenland Interstadial (GI) event 1d and the decreasing temperature trend during the Bølling/Allerød Interstadial. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

19.
In south-central Chile, the Río Ignao nonglacial deposit with a minimal radiocarbon age of 56,000 +2000, ?1700 yr shows the following sequence of pollen assemblages, starting with the oldest: Gramineae (zone 1), Gramineae-Corynabutilon-Lomatia-Ovidia (zone 2), Nothofagus-Embothrium-Gramineae (zone 3A), Nothofagus-Gramineae-Tubuliflorae (zone 3B and D), and Nothofagus-Drimys-Myrtaceae (zone 3A). This sequence implies an initial dry and rather cold climate that later became wetter and somewhat warmer, reached a warm peak, and ultimately became colder and drier. At the warmest time, the Andean tree line is estimated to have been about 900 m lower and average January (summer) temperature about 3–4°C colder than at present. The Río Ignao deposit is believed to represent an early interstade of the last glaciation, known in southern Chile as the Llanquihue Glaciation.  相似文献   

20.
Stratigraphy, mineralogy, major and trace elements, organic carbon, carbonate, sulfate and AMS 14C dates are used to infer Late Quaternary depositional environments and paleo-hydrological conditions in the paleo-lake San Felipe located in the western part of Sonora Desert. Sediment stratigraphy divides the depth profile into aeolian and pluvio-lacustrine regimes. Aeolian regime is constrained to >44.5 14C kyr BP. The pluvio-lacustrine regime consists of two stratigraphic units with characteristic geochemical proxies indicating changing chemical weathering, clastic input, salinity and provenance and provides a measure of varying climatic conditions between ca. 37 and 3 14C kyr BP. Lower catchment erosion and inflow into the basin, higher lake productivity, precipitation of Na-sulfate mineral and higher clastic input from the proximal aeolian deposits during ca. 37–14 14C kyr BP are comparable to the regional registers of dominant winter rainfall related to westerly storm tracks and colder conditions. In the last 12 14C kyr BP, higher sedimentation and inflow and lower productivity are comparable to dominant summer rainfall. Higher humidity and lake productivity during ca. 37–29 14C kyr BP is possibly due to the position of westerly storm tracks at 31°N and the gradually reducing humidity till ca. 14 14C kyr BP is related to northerly migration of westerly storm tracks. Regional arid conditions during ca. 11 14C kyr BP and ca. 6 14C kyr BP are characterized by influx of coarser quartz and feldspars into the basin.  相似文献   

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