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1.
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   

2.
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   

3.
Western Ireland, located adjacent to the North Atlantic, and with a strongly oceanic climate, is potentially sensitive to rapid and extreme climate change. We present the first high‐resolution chironomid‐inferred mean July temperature reconstruction for Ireland, spanning the late‐glacial and early Holocene (LGIT, 15–10 ka BP). The reconstruction suggests an initial rapid warming followed by a short cool phase early in the interstadial. During the interstadial there are oscillations in the inferred temperatures which may relate to Greenland Interstadial events GI‐1a–e. The temperature decrease into the stadial occurs in two stages. This two‐stage drop can also be seen in other late‐glacial chironomid‐inferred temperature records from the British Isles. A stepped rise in temperatures into the Holocene, consistent with present‐day temperatures in Donegal, is inferred. The results show strong similarities with previously published LGIT chironomid‐inferred temperature reconstructions, and with the NGRIP oxygen‐isotope curve, which indicates that the oscillations observed in the NGRIP record are of hemispherical significance. The results also highlight the influence of the North Atlantic on the Irish climate throughout the LGIT. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

4.
This paper presents the results of a high‐resolution Late‐glacial chironomid stratigraphy from Hawes Water, a small carbonate lake in northern Lancashire. The samples were from a core taken from the terrestrialised margin of the present lake, which represents an intermediate depth between the true littoral and the profundal. The chironomid assemblage showed a high degree of sensitivity to both broad‐scale and short‐term temperature changes. Comparison with an existing proxy temperature record (δ18O) for the site confirmed the presence of four temperature inversions within the Late‐glacial Interstadial. A mean July air temperature inference model, derived from acid, soft‐water lakes in Norway and Svalbard, was applied to the data. Despite the absence of carbonate lakes within the Norwegian training set, there was a close similarity between trends in estimated July air temperature and the δ18O trace, with a particularly strong correspondence in the periods of clay deposition. This suggests that this model is highly robust. The inferred maximum Interstadial temperature was 13.4°C, dropping initially to 7.5°C in the Loch Lomond Stadial. Temperatures reach a maximum of nearly 10°C in this period, cool for a short period before rising rapidly to 13.2°C at the start of the Holocene. These temperatures are similar to but slightly higher than those estimated for Whitrig Bog, southeast Scotland, and lower than those inferred from coleopteran‐based models for sites in South Wales. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

5.
A detailed shoreline displacement curve documents the Younger Dryas transgression in western Norway. The relative sea‐level rise was more than 9 m in an area which subsequently experienced an emergence of almost 60 m. The sea‐level curve is based on the stratigraphy of six isolation basins with bedrock thresholds. Effort has been made to establish an accurate chronology using a calendar year time‐scale by 14C wiggle matching and the use of time synchronic markers (the Vedde Ash Bed and the post‐glacial rise in Betula (birch) pollen). The sea‐level curve demonstrates that the Younger Dryas transgression started close to the Allerød–Younger Dryas transition and that the high stand was reached only 200 yr before the Younger Dryas–Holocene boundary. The sea level remained at the high stand for about 300 yr and 100 yr into Holocene it started to fall rapidly. The peak of the Younger Dryas transgression occurred simultaneously with the maximum extent of the ice‐sheet readvance in the area. Our results support earlier geophysical modelling concluding a causal relationship between the Younger Dryas glacier advance and Younger Dryas transgression in western Norway. We argue that the sea‐level curve indicates that the Younger Dryas glacial advance started in the late Allerød or close to the Allerød–Younger Dryas transition. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

6.
《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   

7.
Climatic and environmental changes during the Younger Dryas stadial (GS‐1) and preceding and following transitions are inferred from stable carbon and oxygen isotope records obtained from the sediments of ancient Lake Torreberga, southern Sweden. Event GS‐1 is represented in the sediment sequence by 3.5 m of clay containing lacustrine carbonates of various origins. Comparison of isotopic records obtained on mollusc shells, ostracod valves, and Chara encrustations precipitated during specific seasons of the year supports estimates of relative changes in both lake water and mean annual air temperatures. Variations in soil erosion rates can also be estimated from a simple isotope–mass‐balance model to separate allochthonous and autochthonous carbonate contributions to the bulk carbonate content of the sediments. The well‐known, rapid climatic shifts characterising the Last Termination in the North Atlantic region are clearly reflected in the isotopic data, as well as longer‐term changes within GS‐1. Following maximum cooling shortly after the Allerød–Younger Dryas (GI‐1–GS‐1) transition, a progressive warming and a slight increase in aquatic productivity is indicated. At the Younger Dryas–Preboreal (GS‐1–PB) transition mean annual air temperature rapidly increased by more than 5°C and summer lake‐water temperature increased by ca. 12°C. The subsequent Preboreal oscillation is characterised by an increase in soil erosion and a slight decrease in mean annual air temperature. These results are in harmony with recent findings about large‐scale climate dynamics during the Last Termination. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

8.
A lake sediment record from the Friedländer Groβe Wiese in northeast Germany was studied to reconstruct summer temperature changes associated with changes in vegetation development during the Weichselian Lateglacial. The record was analysed for pollen, chironomids, and oxygen and carbon isotopes of lake marl. The combination of radiocarbon dates, the presence of the Laacher See Tephra and correlation of lithological and palynological changes with other records from the region indicated that the record encompassed the Allerød to the early Holocene. Pollen assemblages reflect development of birch and later pine‐dominated forests during the Allerød, comparable to other sites in the region. Chironomid‐inferred mean July air temperatures (C‐IT) for this period range between ～14.0 and 14.8°C. A temporary decrease in C‐IT of ～1°C, a negative shift in the isotope records, and a minor decline of birch may correspond to Greenland Interstadial 1b. Even though the transition to the Younger Dryas appears to be affected by reworking and redeposition processes, a drop in C‐IT to ～11.1°C is reconstructed for the later part of the Younger Dryas, while it appears that pine locally persisted in the region. Comparison with a nearby pollen record further indicates a local expansion of wetland grasses during this period. At the transition to the Holocene, C‐IT increased to ～15.7°C, while birch and pine forests re‐expanded. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

9.
Lake sediment records from the Weerterbos region, in the southern Netherlands, were studied to reconstruct summer temperature and environmental changes during the Weichselian Lateglacial Interstadial. A sediment core obtained from a small lacustrine basin was analysed for multiple proxies, including lithological changes, oxygen isotopes of bulk carbonates, pollen and chironomids. It was found that the oxygen isotope record differed strongly from the other proxies. Based on a comparison with three additional lake sediment records from the same region, it emerged that the oxygen isotope records were strongly affected by local environmental conditions, impeding the distinction of a regional palaeoclimate signal. The chironomid‐inferred July air temperature reconstruction produced inferred interstadial temperatures ranging between ～15° and 18°C, largely consistent with previously published results from the northern part of the Netherlands. A temporary regressive phase in the pollen record, which can be tentatively correlated with the Older Dryas, preceded the expansion of birch woodland. Despite differences between the four pollen records from the Weerterbos region, a comparable regressive vegetation phase that was possibly the result of a shift to drier conditions could be discerned in all of the profiles. In addition, a temporary temperature decline of ～1.5°C was inferred from the chironomid record during this regressive phase. The multi‐proxy approach used here enabled a direct comparison of inferred changes in temperature, vegetation and environmental conditions at an individual site, while the multi‐site approach provided insight into the factors influencing the pollen and isotope records from these small‐scale depressions.  相似文献   

10.
Late glacial and early Holocene summer temperatures were reconstructed based on fossil chironomid assemblages at Lake Brazi (Retezat Mountains) with a joint Norwegian–Swiss transfer function, providing an important addition to the late glacial quantitative climate reconstructions from Europe. The pattern of the late glacial temperature changes in Lake Brazi show both similarities and some differences from the NGRIP δ18O record and other European chironomid-based reconstructions. Our reconstruction indicates that at Lake Brazi (1740 m a.s.l.) summer air temperature increased by ~ 2.8°C at the Oldest Dryas/Bølling transition (GS-2/GI-1) and reached 8.1–8.7°C during the late glacial interstade. The onset of the Younger Dryas (GS-1) was characterized by a weak (< 1°C) decrease in chironomid-inferred temperatures. Similarly, at the GS-1/Holocene transition no major changes in summer temperature were recorded. In the early Holocene, summer temperature increased in two steps and reached ~ 12.0–13.3°C during the Preboreal. Two short-term cold events were detected during the early Holocene between 11,480–11,390 and 10,350–10,190 cal yr BP. The first cooling coincides with the Preboreal oscillation and shows a weak (0.7°C) temperature decrease, while the second is characterized by 1°C cooling. Both cold events coincide with cooling events in the Greenland ice core records and other European temperature reconstructions.  相似文献   

11.
Cosmogenic 36Cl was measured in bedrock and moraine boulders in the Za Mnichem Valley (High Tatra Mountains). The post‐LGM deglaciation of the study area occurred about 15.9 ka ago. The northernmost part of the valley slopes was ice‐free around 15 ka ago. The terminal moraine on the valley threshold was finally stabilized 12.5 ka ago during the Younger Dryas cold event (Greenland Stadial 1). At that time, the Za Mnichem glacier was 1.3 km long and had an area of 0.57 km2. The AAR equilibrium line of the glacier was located at 1990 m a.s.l., which corresponds to an ELA depression of ～500 m compared to today. The mean summer temperature was colder by 4°–4.5°C than the present‐day temperature. The mean annual temperature was colder by 6°C than today. Such conditions suggest a decrease of the annual precipitation by ～15–25% compared with the present‐day annual average. These data indicate a probable uniform temperature change across central and western Europe, with the precipitation being the most significant factor affecting the mass balance of mountain glaciers. The spatial distribution of balance data suggests increasing continentality towards the east during the Younger Dryas.  相似文献   

12.
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   

13.
By using heavy coring equipment in two high-altitudinal lakes (1253 and 1316 m a.s.l.) at Dovre, Central Norway, 1–1.5 m of unsorted coarsely minerogenic sediments were retrieved below the Holocene organic sediments. The minerogenic sequence contained well-preserved pollen and chironomid remains, revealing new and detailed palaeoenvironmental knowledge of the mountains in Central Norway during the last 5–6000 years of the Lateglacial (LG) period. However, the LG chronology is based on biostratigraphical correlations and not on 14C-dates, due to low organic content in the minerogenic sediments. The emerging LG nunataks, probably indicating a thin and multi-domed Scandinavian ice-sheet, was rapidly inhabited by immigrating species which could explain the present centric distributions of certain arctic-alpine plants. The LG vegetation development included a pre-interstadial dominated by mineral-soil pioneers, an interstadial dominated by shrubs and dwarf-shrubs, and the Younger Dryas cold period with recurring dominance of pioneers. Pollen and stomata of Pinus and Picea indicate their local LG presence at Dovre. LG climate oscillations are indicated by pollen stratigraphy and for the later part of LG also by chironomids. These oscillations could correspond to Heinrich event 1, GI-1d, GI-1b, and the Younger Dryas cold events. The LG interstadial reached July mean temperatures of more than 7–8 °C, similar to the present. Chironomids colonized the lake already during the onset of the interstadial, albeit at very low richness and abundances. Starting from YD, there are sufficient chironomid head capsules to perform a temperature reconstruction. The Holocene warming of about 2 °C initiated a vegetation closure from snow beds and dwarf-shrub tundra to shrubs and forests. Birch-forests established about 10 ka cal BP, slightly earlier than pine forests. Alnus expanded ca 9.2 ka cal BP and a thinning of the local forests occurred from ca 7 ka cal BP. Two short-lasting climate deteriorations found in the pollen record and the chironomid record may represent the Preboreal Oscillation and the 8.2 event. The Holocene Thermal Maximum is indicated around ca 7.8–7.3 ka cal BP showing a chironomid-inferred July mean of at least 11 °C. This is ca 3 °C warmer than today.  相似文献   

14.
A south‐east Australian speleothem stable isotope record displaying an apparent cooling synchronous with the northern hemisphere Younger Dryas climate event (12.9–11.7 ka) has significantly influenced scientific thinking on the climatic response of the southern hemisphere following the Last Glacial Maximum. This is one of very few records displaying such a response, and yet the cooling was inferred from substantial extrapolation between just three uranium‐series ages. Technological advances since then have produced major improvements in both the spatial resolution and the accuracy of uranium‐series geochronologies. Re‐analysis of this sample has yielded ages of 7.96 ± 0.36 to 7.69 ± 0.33 ka for the interval previously inferred to span the Younger Dryas, and reveals a substantial hiatus in deposition from 6.93 ± 0.64 to 1.83 ± 0.16 ka. These data not only refute the original evidence for an inter‐hemispheric synchroneity of the Younger Dryas but also reject any evidence for neoglacial conditions at 3 ka. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

15.
An excavation primarily intended to investigate the Bronze Age deposits at Hautrive‐Champréveyres, Neuchâtel, Switzerland, encountered beneath the Bronze Age levels a sequence of Late‐glacial sediments that were deposited between about 13000 yr BP and 11800 yr BP. Within these deposits Upper Palaeolithic hearths, bones and flint implements were found in a context that left no doubt that they accumulated on the actual living floors. Two separate cultures were involved; an earlier Magdalenian one overlain by a rather later Azilian assemblage. Coleoptera from the associated organic silts and sands provide detailed ecological and climatic information about the time when these people lived in the area. Radiocarbon dates indicate that the Magdalenians lived in the area at about 13000 yr BP. The Coleoptera show that the mean July temperature at this time was about 9°C and mean temperature of the coldest month was about −25°C. The landscape was bare of trees with an open patchy vegetation. Shortly after the area was abandoned by the Magdalenian hunters, the climate became suddenly warmer and mean July temperatures rose abruptly to at least 16°C and winter temperatures rose to levels not much different from those of the present day. There is evidence that at this time, intense slope instability and mud flows may have rendered the locality unsuitable for human occupation. About seven centuries after the episode of sudden climatic warming, namely at about 12300 yr BP, palaeolithic Azilian hunters occupied the area at a time when the climate was thoroughly temperate and the landscape was clothed in birch and willow woodland. This was gradually replaced by pine forest at the top of the sequence and Late‐glacial deposition ceased by about 11800 yr BP. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

16.
Annually resolved June–July–August (JJA) temperatures from ca. 570 BC to AD 120 (±100 a; approximately 690 varve years) were quantified from biogenic silica and chironomids (Type II regression; Standard Major Axis calibration‐in‐time) preserved in the varved sediments of Lake Silvaplana, Switzerland. Using 30 a (climatology) moving averages and detrended standard deviations (mean–variability change, MVC), moving linear trends, change points and wavelets, reconstructed temperatures were partitioned into a warmer (+0.3°C; ca. 570–351 BC), cooler (?0.2°C; ca. 350–16 BC) and moderate period (+0.1°C; ca. 15 BC to AD 120) relative to the reconstruction average (10.9°C; reference AD 1950–2000 = 9.8°C). Warm and variable JJA temperatures at the Late Iron Age–Roman Period transition (approximately 50 BC to AD 100 in this region) and a cold anomaly around 470 BC (Early–Late Iron Age) were inferred. Inter‐annual and decadal temperature variability was greater from ca. 570 BC to AD 120 than the last millennium, whereas multi‐decadal and lower‐frequency temperature variability were comparable, as evident in wavelet plots. Using MVC plots of reconstructed JJA temperatures from ca. 570 BC to AD 120, we verified current trends and European climate model outputs for the 21st century, which suggest increased inter‐annual summer temperature variability and extremes in a generally warmer climate (heteroscedasticity; hotspot of variability). We compared these results to MVC plots of instrumental and reconstructed temperatures (from the same sediment core and proxies but a different study) from AD 1177 to AD 2000. Our reconstructed JJA temperatures from ca. 570 BC to AD 120 showed that inter‐annual JJA temperature variability increased rapidly above a threshold of ～10°C mean JJA temperature. This increase accelerated with continued warming up to >11.5°C. We suggest that the Roman Period serves with respect to inter‐annual variability as an analogue for warmer 21st‐century JJA temperatures in the Alps. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

17.
Sedimentological, micropalaeontological (benthic foraminifers and dinoflagellate cysts), stable isotope data and AMS 14C datings on cores and surface samples, in addition to acoustic data, have been obtained from Voldafjorden, western Norway. Based on these data the late glacial and Holocene sedimentological processes and variability in circulation and fjord environments are outlined. Glacial marine sedimentation prevailed in the Voldafjorden between 11.0 kyr and 9.2 kyr BP (radiocarbon years). In the later part of the Allerød period, and for the rest of the Holocene, there was deposition of fine‐grained normal marine sediments in the fjord basin. Turbidite layers, recorded in core material and on acoustic profiles, dated to ca. 2.1, 6.9–7.6, ca. 9.6 and ca. 11.0 kyr BP, interrupted the marine sedimentation. The event dated to between 6.9 and 7.6 kyr BP probably corresponds to a tsunami resulting from large‐scale sliding on the continental margin off Norway (the Storegga Tsunami). During the later part of the Allerød period, Voldafjorden had a strongly stratified water column with cold bottom water and warm surface water, reaching interglacial temperatures during the summer seasons. During the Younger Dryas cold event there was a return to arctic sea‐surface summer temperatures, possibly with year‐round sea‐ice cover, the entire benthic fauna being composed of arctic species. The first strong Holocene warming, observed simultaneously in bottom and sea‐surface temperature proxies, occurred at ca. 10.1 kyr BP. Bottom water proxies indicate two cold periods, possibly with 2°C lowering of temperatures, at ca. 10.0 (PBO 1) and at 9.8 kyr BP (PBO 2). These events may both result from catastrophic outbursts of Baltic glacial lake water. The remainder of the Holocene experienced variability in basin water temperature, indicated by oxygen isotope measurements with an amplitude of ca. 2°C, with cooler periods at ca. 8.4–9.0, 5.6, 5.2, 4.6, 4.2, 3.5, 2.2, 1.2 and 0.4–0.8 kyr BP. Changes in the fjord hydrology through the past 11.3 kyr show a close correspondence, both in amplitude and timing of events, recorded in cores from the Norwegian Sea region and the North Atlantic. These data suggest a close relationship between fjord environments and variability in large‐scale oceanic circulation. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

18.
High‐resolution multi‐proxy analyses of a sediment core section from Lake Jeserzersee (Saissersee) in the piedmont lobe of the Würmian Drau glacier (Carinthia, Austria) reveal pronounced climatic oscillations during the early late glacial (ca. 18.5–16.0k cal a BP). Diatom‐inferred epilimnetic summer water temperatures show a close correspondence with temperature reconstructions from the adjacent Lake Längsee record and, on a hemispheric scale, with fluctuations of ice‐rafted debris in the North Atlantic. This suggests that North Atlantic climate triggered summer climate variability in the Alps during the early late glacial. The expansion of pine (mainly dwarf pine) between ca. 18.5 and 18.1k cal a BP indicates warming during the so‐called ‘Längsee oscillation’. The subsequent stepwise climate deterioration between ca. 18.1 and 17.6k cal a BP culminated in a tripartite cold period between ca. 17.6 and 16.9k cal a BP with diatom‐inferred summer water temperatures 8.5–10 °C below modern values and a shift from wet to dry conditions. This period probably coincides with a major Alpine glacier advance termed the Gschnitz stadial. A warmer interval between ca. 16.9 and 16.4k cal a BP separates this cold phase from a second, shorter and less pronounced cold phase between ca. 16.4 and 16.0k cal a BP, which is thought to correlate with the Clavadel/Senders glacier advance in the Alps. The following temperature increase, coupled with wet (probably snow‐rich) conditions, caused the expansion of birch during the transition period to the late glacial interstadial. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

19.
The sedimentary succession of piston core RC26-16, dated by 14C accelerator mass spectrometry, provides a nearly continuous palaeoceanographic record of the northeastern South China Sea for the last 15000 yr. Planktic foraminiferal assemblages indicate that winter sea-surface temperatures (SSTs) rose from 18°C to about 24°C from the last glacial to the Holocene. A short-lived cooling of 1°C in winter temperature centred at about 11000 14C yr ago may reflect the Younger Dryas cooling event in this area. Summer SSTs have remained between 27°C and 29°C throughout the record. The temperature difference between summer and winter was about ca. 9°C during the last glacial, much higher than the Holocene value of ca. 5°C. During the late Holocene a short-lived cooling event occurred at about 4000 14C yr ago. Oxygen and carbon isotopic gradients between surface (0–50 m) and subsurface (50–100 m) waters were smaller during the last glacial than those in the Holocene. The fluctuation in the isotopic gradients are caused most likely by changes in upwelling intensity. Smaller gradients indicate stronger upwelling during the glacial winter monsoon. The fauna-derived estimates of nutrient content of the surface waters indicate that the upwelling induced higher fertility and biological productivity during the glacial. The winter monsoon became weaker during the Holocene. The carbonate compensation depth and foraminiferal lysocline were shallower during the Holocene, except for a short-lived deepening at about 5000 14C yr ago. A preservation peak of planktic foraminifera and calcium carbonate occurred between 13400 and 12000 14C yr ago, synchronous to the global preservation event of Termination I.  相似文献   

20.
Numerical techniques were used to study chironomid distribution and abundance in lakes from a 1000 km transect in Finland, with special interest on the effect of local summer air temperatures on chironomid assemblages. The final aim of the study was to develop a chironomid‐based palaeotemperature inference model. The dataset consisted of 82 lakes (of which 77 were used in the model after deletion of outliers), with catchments spanning from boreal coniferous forests to mountain birch woodland and tundra vegetation. Numerical analysis showed that the mean July air temperature was the most significant variable explaining the distribution and abundance of chironomids in Finnish lakes. Weighted‐averaging partial least squares techniques were used to develop a palaeotemperature inference model for mean July air temperature reconstructions. The model performance statistics were favourable, with cross‐validated coefficient of determination (r2) of 0.78, root mean squared error of prediction of 0.721°C and maximum bias of 0.794°C. Based on these values, the transfer function is a valid means of performing quantitative palaeotemperature estimates in downcore studies. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

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