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1.
Using glacial rebound models we have inverted observations of crustal rebound and shoreline locations to estimate the ice thickness for the major glaciations over northern Eurasia and to predict the palaeo-topography from late MIS-6 (the Late Saalian at c. 140 kyr BP) to MIS-4e (early Middle Weichselian at c. 64 kyr BP). During the Late Saalian, the ice extended across northern Europe and Russia with a broad dome centred from the Kara Sea to Karelia that reached a maximum thickness of c. 4500 m and ice surface elevation of c. 3500 m above sea level. A secondary dome occurred over Finland with ice thickness and surface elevation of 4000 m and 3000 m, respectively. When ice retreat commenced, and before the onset of the warm phase of the early Eemian, extensive marine flooding occurred from the Atlantic to the Urals and, once the ice retreated from the Urals, to the Taymyr Peninsula. The Baltic-White Sea connection is predicted to have closed at about 129 kyr BP, although large areas of arctic Russia remained submerged until the end of the Eemian. During the stadials (MIS-5d, 5b, 4) the maximum ice was centred over the Kara-Barents Seas with a thickness not exceeding c. 1200 m. Ice-dammed lakes and the elevations of sills are predicted for the major glacial phases and used to test the ice models. Large lakes are predicted for west Siberia at the end of the Saalian and during MIS-5d, 5b and 4, with the lake levels, margin locations and outlets depending inter alia on ice thickness and isostatic adjustment. During the Saalian and MIS-5d, 5b these lakes overflowed through the Turgay pass into the Aral Sea, but during MIS-4 the overflow is predicted to have occurred north of the Urals. West of the Urals the palaeo-lake predictions are strongly controlled by whether the Kara Ice Sheet dammed the White Sea. If it did, then the lake levels are controlled by the topography of the Dvina basin with overflow directed into the Kama-Volga river system. Comparisons of predicted with observed MIS-5b lake levels of Komi Lake favour models in which the White Sea was in contact with the Barents Sea.  相似文献   

2.
In North-West Europe no evidence has been found for glaciations older than the Elsterian. The Elsterian seems to consist of at least two separate ice advances with a minor interval in between. During the Elsterian in the area south of the Baltic Sea over 400 m deep channels were cut into the underlying substratum by a combination of glacial erosion and meltwater activity. The channels were active until the Late Elsterian and were subsequently filled with meltwater deposits and glaciolacustrine silt and clay. During the Saalian no comparative channels were being formed. Three different ice advances can be distinguished within the Saalian, the stratigraphical correlation of which is to be discussed. No convincing evidence was found for any interglacial within the Saalian sequence. There are good reasons to assume that the ice advances were only separated from each other by minor ice-free intervals.  相似文献   

3.
Passchier, S., Laban, C., Mesdag, C.S. & Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483. Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions.  相似文献   

4.
Based on a large number of new boreholes in northern Denmark, and on the existing data, a revised event‐stratigraphy is presented for southwestern Scandinavia. Five significant Late Saalian to Late Weichselian glacial events, each separated by periods of interglacial or interstadial marine or glaciolacustrine conditions, are identified in northern Denmark. The first glacial event is attributed to the Late Saalian c. 160–140 kyr BP, when the Warthe Ice Sheet advanced from easterly and southeasterly directions through the Baltic depression into Germany and Denmark. This Baltic ice extended as far as northern Denmark, where it probably merged with the Norwegian Channel Ice Stream (NCIS) and contributed to a large discharge of icebergs into the Norwegian Sea. Following the break up, marine conditions were established that persisted from the Late Saalian until the end of the Early Weichselian. The next glaciation occurred c. 65–60 kyr BP, when the Sundsøre ice advanced from the north into Denmark and the North Sea, where the Scandinavian and British Ice Sheets merged. During the subsequent deglaciation, large ice‐dammed lakes formed before the ice disintegrated in the Norwegian Channel, and marine conditions were re‐established. The following Ristinge advance from the Baltic, initiated c. 55 kyr BP, also reached northern Denmark, where it probably merged with the NCIS. The deglaciation, c. 50 kyr BP, was followed by a long period of marine arctic conditions. Around 30 kyr BP, the Scandinavian Ice Sheet expanded from the north into the Norwegian Channel, where it dammed the Kattegat ice lake. Shortly after, c. 29 kyr BP, the Kattegat advance began, and once again the Scandinavian and British Ice Sheets merged in the North Sea. The subsequent retreat to the Norwegian Channel led to the formation of Ribjerg ice lake, which persisted from 27 to 23 kyr BP. The expansion of the last ice sheet started c. 23 kyr BP, when the main advance occurred from north–northeasterly directions into Denmark. An ice‐dammed lake was formed during deglaciation, while the NCIS was still active. During a re‐advance and subsequent retreat c. 19 kyr BP, a number of tunnel‐valley systems were formed in association with ice‐marginal positions. The NCIS finally began to break up in the Norwegian Sea 18.8 kyr BP, and the Younger Yoldia Sea inundated northern Denmark around 18 kyr BP. The extensive amount of new and existing data applied to this synthesis has provided a better understanding of the timing and dynamics of the Scandinavian Ice Sheet (SIS) during the last c. 160 kyr. Furthermore, our model contributes to the understanding of the timing of the occasional release of large quantities of meltwater from the southwestern part of the SIS that are likely to enter the North Atlantic and possibly affect the thermohaline circulation.  相似文献   

5.
Mapping of glacial meltwater channels along the length of the 25-km Mid-Cheshire Ridge reveals evidence for four distinctive channel morphologies, which are used to establish the pattern of meltwater flow during the Late Devensian glaciation. A key characteristic of all channels is an abrupt change in morphology between inception on the Mid-Cheshire Ridge and the downstream continuation on the surrounding Cheshire Plain, with large reductions in channel cross-sectional area at this point. The interpretation of this evidence is that meltwater flowing off the bedrock ridge was absorbed into a layer of permeable sediment beneath the Late Devensian ice sheet. This permeable sediment is significant because it would have acted as a deforming layer beneath the former ice sheet in this area. Reconstruction of the Late Devensian ice sheet based on information from the meltwater channels and using values of shear stresses typical of ice sheets resting on deformable beds (ca. 20 kPa) suggests an ice surface elevation over the Irish Sea of ca. 700 m. This value is considerably less than previous estimates of the vertical extent of the ice sheet of ca. 1000–1200 m and has important implications for the rapidity and mode of deglaciation during the Late Devensian. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

6.
As the majority of the data on Quaternary sediments from the North Sea Basin are seismostratigraphical, we analysed the Elsterian Swarte Bank Formation, the Late Saalian Fisher Formation and the Late Weichselian (Dimlington Stadial) Bolders Bank Formation in order to determine genesis and provenance. The Swarte Bank Formation is a subglacial till containing palynomorphs from the Moray Forth and the northeastern North Sea, and metamorphic heavy minerals from the Scottish Highlands. The Fisher Formation was sampled from the northern and central North Sea. In the north, it is interpreted as a subglacial till, with glaciomarine sediments cropping out further south. These sediments exhibit a provenance signature consistent with the Midland Valley of Scotland, the Eocene of the North Sea Basin, the Grampian Highlands and northeast Scotland. The Bolders Bank Formation is a subglacial till containing palynomorphs from the Midland Valley of Scotland, northern Britain, and a metamorphic heavy‐mineral suite indicative of the Grampian Highlands, Southern Uplands and northeast Scotland. These data demonstrate that there was repeated glaciation of the North Sea Basin during the Middle and Late Pleistocene, with ice sheets originating in northern Scotland. There was no evidence for a Scandinavian ice sheet in the western North Sea basin. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

7.
Based on c. 1500 km reflection seismic profiles, the Quaternary formations and their pre-Quaternary substratum in the southeastern Kattegat are described and a geological interpretation is suggested. The major volume of Quaternary deposits is found in a broad north-northwest south-southeast trending topographic depression. The substratum consists of Upper Cretaceous limestone in the region north of the Sorgenfrei–Tornquist Zone, and inside this zone older Mesozoic sedimentary rocks and Precambrian crystalline rocks are found. The Quaternary is divided into four seismic units. No direct stratigraphic control is available, but the units are assumed to represent a period ranging from Late Saalian to Holocene. The oldest unit (unit 3) is composed of deposits of supposed Late Saalian to Middle Weichselian age. This unit was severely eroded probably by the Late Weichselian ice sheets in a zone extending 40–50 km from the Swedish coast. Unit 2 represents the Late Weichselian till deposits. North and east of the island of Anholt unit 3 is cut by a system of channels eroded by glacial meltwater. By the erosion a relief up to c. 100 m was formed. After the recession of the Late Weichselian ice, an up to 100 m thick sequence of water-lain sediments (unit 1) was deposited in the erosional basin and channels. Holocene deposits (unit 0) of considerable thickness have only been identified in the channels in the northern part of the area.  相似文献   

8.
A considerable discussion concerning the extent of the last Scandinavian and Scottish ice sheets has continued for several years. In contrast to earlier models based on an ice sheet extending to the edge of the continental shelf, recent proposals favor a limited geographical and vertical extent and imply that the Scandinavian and British ice sheets did not coalesce in the North Sea. These models indicate an ice-free, open embayment in the northern North Sea and areas of dry land in the southern North Sea region during the Late Weichselian/Devensian glacial maximum. Late Weichselian ice-sheet profiles from the North Sea to the adjacent land areas of southern Norway have been tentatively reconstructed. Low-gradient profiles in the present shelf areas are explained by unconsolidated, deformable sediments on the continental shelf inducing subglacial water pressure and low basal shear stress beneath marginal parts of the Scandinavian ice sheet. Combined with higher basal shear stress conditions in the present mainland areas, this explains the slightly concave and convex shape of the reconstructed ice-sheet profiles in the present coastal and inland areas of western Norway, respectively.  相似文献   

9.
Improved multiparameter records from the northern Barents Sea margin show two prominent freshwater pulses into the Arctic Ocean during MIS 5 that significantly disturbed the regional oceanic regime and probably affected global climate. Both pulses are associated with major iceberg-rafted debris (IRD) events, revealing intensive iceberg/sea ice melting. The older meltwater pulse occurred near the MIS 5/6 boundary (∼131,000 yr ago); its ∼2000 year duration and high IRD input accompanied by high illite content suggest a collapse of large-scale Saalian Glaciation in the Arctic Ocean. Movement of this meltwater with the Transpolar Drift current into the Fram Strait probably promoted freshening of Nordic Seas surface water, which may have increased sea-ice formation and significantly reduced deep-water formation. A second pulse of freshwater occurred within MIS 5a (∼77,000 yr ago); its high smectite content and relatively short duration is possibly consistent with sudden discharge of Early Weichselian ice-dammed lakes in northern Siberia as suggested by terrestrial glacial geologic data. The influence of this MIS 5a meltwater pulse has been observed at a number of sites along the Transpolar Drift, through Fram Strait, and into the Nordic Seas; it may well have been a trigger for the North Atlantic cooling event C20.  相似文献   

10.
High-resolution 2D seismic data from the western side of Dogger Bank (North Sea) has revealed that the glacigenic sediments of the Dogger Bank Formation record a complex history of sedimentation and penecontemporaneous, large-scale, ice-marginal to proglacial glacitectonism. The resulting complex assemblage of glacial landforms and sediments record the interplay between two separate ice masses revealing that Late Devensian ice sheet dynamics across Dogger Bank were far more complex than previously thought, involving the North Sea lobe of the British and Irish Ice Sheet, advancing from the west, interacting with the Dogger Bank lobe which expanded from the north. The active northward retreat of the Dogger Bank lobe resulted in the development of a complex assemblage of arcuate thrust-block moraines (≤ 15 km wide, > 30 km long) composed of highly folded and thrust sediments, separated by sedimentary basins and meltwater channels filled by outwash. The impact of the North Sea lobe was restricted to the western margin of Dogger Bank and led to deep-seated (100–150 m thick) glacitectonism in response to ice-push from the west. During the earlier expansion of the North Sea lobe, this thrust and fold complex initially occupied a frontal marginal position changing to a more lateral ice-marginal position as the ice sheet continued to expand to the south. The complex structural relationships between the two glacitectonic complexes indicates that these ice masses interacted along the western side of Dogger Bank, with the inundation of this area by ice probably occurring during the last glaciation when the ice sheets attained their maximum extents.  相似文献   

11.
The Northern Hemisphere ice sheets decayed rapidly during deglacial phases of the ice-age cycle, producing meltwater fluxes that may have been of sufficient magnitude to perturb oceanic circulation. The continental record of ice-sheet history is more obscured during the growth and advance of the last great ice sheets, ca. 120,000–20,000 yr B.P., but ice cores tell of high-amplitude, millennial-scale climate fluctuations that prevailed throughout this period. These climatic excursions would have provoked significant fluctuation of ice-sheet margins and runoff variability whenever ice sheets extended to mid-latitudes, giving a complex pattern of freshwater delivery to the oceans. A model of continental surface hydrology is coupled with an ice-dynamics model simulating the last glacial cycle in North America. Meltwater discharged from ice sheets is either channeled down continental drainage pathways or stored temporarily in large systems of proglacial lakes that border the retreating ice-sheet margin. The coupled treatment provides quantitative estimates of the spatial and temporal patterns of freshwater flux to the continental margins. Results imply an intensified surface hydrological environment when ice sheets are present, despite a net decrease in precipitation during glacial periods. Diminished continental evaporation and high levels of meltwater production combine to give mid-latitude runoff values that are highly variable through the glacial cycle, but are two to three times in excess of modern river fluxes; drainage to the North Atlantic via the St. Lawrence, Hudson, and Mississippi River catchments averages 0.356 Sv for the period 60,000–10,000 yr B.P., compared to 0.122 Sv for the past 10,000 yr. High-amplitude meltwater pulses to the Gulf of Mexico, North Atlantic, and North Pacific occur throughout the glacial period, with ice-sheet geometry controlling intricate patterns of freshwater routing variability. Runoff from North America is staged in the final deglaciation, with a stepped sequence of pulses through the Mississippi, St. Lawrence, Arctic, and Hudson Strait drainages.  相似文献   

12.
Based on field investigations in northern Russia and interpretation of offshore seismic data, we have made a preliminary reconstruction of the maximum ice-sheet extent in the Barents and Kara Sea region during the Early/Middle Weichselian and the Late Weichselian. Our investigations indicate that the Barents and Kara ice sheets attained their maximum Weichselian positions in northern Russia prior to 50 000 yr BP, whereas the northeastern flank of the Scandinavian Ice Sheet advanced to a maximum position shortly after 17 000 calendar years ago. During the Late Weichselian (25 000-10 000 yr BP), much of the Russian Arctic remained ice-free. According to our reconstruction, the extent of the ice sheets in the Barents and Kara Sea region during the Late Weichselian glacial maximum was less than half that of the maximum model which, up to now, has been widely used as a boundary condition for testing and refining General Circulation Models (GCMs). Preliminary numerical-modelling experiments predict Late Weichselian ice sheets which are larger than the ice extent implied for the Kara Sea region from dated geological evidence, suggesting very low precipitation.  相似文献   

13.
Studies of a deep high-resolution reflection seismic profile through the eastern North Sea basin show that at least four erosional phases have affected the area during the Saalian, Weichselian and Holocene. Foraminiferal investigations of five boreholes make it possible to date the erosional events. When looking at the restricted area of this study, the deep incised valleys appear to have developed during sea-level fall and lowstand as the Quaternary ice sheets were established. Further erosion took place during the deglaciation of the area and the valleys were further deepened when used as drainage paths. The oldest erosional phase recognized from the seismic profiles is interpreted to be of Saalian age. Two later erosive phases were associated with intra-Weichselian glacial advances. The uppermost erosive surface represents river valleys at the transition from the Weichselian glacial to the Holocene.  相似文献   

14.
Despite a long history of investigation, critical issues regarding the last glacial cycle in northwest Europe remain unresolved. One of these refers to the extent, timing and dynamics of Late Devensian/Weichselian glaciation of the North Sea Basin, and whether the British and Scandinavian ice sheets were confluent at any time during this period. This has been the result of the lack of the detailed sedimentological data required to reconstruct processes and environment of sediments recovered through coring. This study presents the results of seismic, sedimentological and micromorphological evidence used to reconstruct the depositional processes of regionally extensive seismic units across the North Sea Basin. Thin section micromorphology is used here to provide an effective means of discriminating between subglacial and glacimarine sediments from cored samples and deriving process‐based interpretations from sediment cores. On the basis of micromorphology, critical formations from the basin have been reinterpreted, with consequent stratigraphic implications. Within the current stratigraphic understanding of the North Sea Basin, a complex reconstruction is suggested, with a minimum of three major glacial episodes inferred. On at least two occasions during the Weichselian/Devensian, the British and Scandinavian ice sheets were confluent in the central North Sea. Whilst micromorphology can provide much greater confidence in the interpretation of Late Quaternary offshore stratigraphic sequences, it is noted that a much better geochronology is required to resolve key stratigraphic issues between the onshore and offshore stratigraphic records. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

15.
The exact number, extent and chronology of the Middle Pleistocene Elsterian and Saalian glaciations in northern Central Europe are still controversial. This study presents new luminescence data from Middle Pleistocene ice‐marginal deposits in northern Germany, giving evidence for repeated glaciations during the Middle Pleistocene (MIS 12 to MIS 6). The study area is located in the Leine valley south of the North German Lowlands. The data set includes digital elevation models, high‐resolution shear wave seismic profiles, outcrop and borehole data integrated into a 3D subsurface model to reconstruct the bedrock relief surface. For numerical age determination, we performed luminescence dating on 12 ice‐marginal and two fluvial samples. Luminescence ages of ice‐marginal deposits point to at least two ice advances during MIS 12 and MIS 10 with ages ranging from 461±34 to 421±25 ka and from 376±27 to 337±21 ka. The bedrock relief model and different generations of striations indicate that the older ice advance came from the north and the younger one from the northeast. During rapid ice‐margin retreat, subglacial overdeepenings were filled with glaciolacustrine deposits, partly rich in re‐worked Tertiary lignite and amber. During MIS 8 and MIS 6, the study area may have been affected by two ice advances. Luminescence ages of glaciolacustrine delta deposits point to a deposition during MIS 8 or early MIS 6, and late MIS 6 (250±20 to 161±10 ka). The maximum extent of both the Elsterian (MIS 12 and MIS 10) and Saalian glaciations (MIS 8? and MIS 6) approximately reached the same position in the Leine valley and was probably controlled by the formation of deep proglacial lakes in front of the ice sheets, preventing a further southward advance.  相似文献   

16.
Three‐dimensional (3D) seismic datasets, 2D seismic reflection profiles and shallow cores provide insights into the geometry and composition of glacial features on the continental shelf, offshore eastern Scotland (58° N, 1–2° W). The relic features are related to the activity of the last British Ice Sheet (BIS) in the Outer Moray Firth. A landsystem assemblage consisting of four types of subglacial and ice marginal morphology is mapped at the seafloor. The assemblage comprises: (i) large seabed banks (interpreted as end moraines), coeval with the Bosies Bank moraine; (ii) morainic ridges (hummocky, push and end moraine) formed beneath, and at the margins of the ice sheet; (iii) an incised valley (a subglacial meltwater channel), recording meltwater drainage beneath former ice sheets; and (iv) elongate ridges and grooves (subglacial bedforms) overprinted by transverse ridges (grounding line moraines). The bedforms suggest that fast‐flowing grounded ice advanced eastward of the previously proposed terminus of the offshore Late Weichselian BIS, increasing the size and extent of the ice sheet beyond traditional limits. Complex moraine formation at the margins of less active ice characterised subsequent retreat, with periodic stillstands and readvances. Observations are consistent with interpretations of a dynamic and oscillating ice margin during BIS deglaciation, and with an extensive ice sheet in the North Sea basin at the Last Glacial Maximum. Final ice margin retreat was rapid, manifested in stagnant ice topography, which aided preservation of the landsystem record. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

17.
The sediment–landform associations of the northern Taymyr Peninsula in Arctic Siberia tell a tale of ice sheets advancing from the Kara Sea shelf and inundating the peninsula, probably three times during the Weichselian. In each case the ice sheet had a margin frozen to its bed and an interior moving over a deforming bed. The North Taymyr ice‐marginal zone (NTZ) comprises ice‐marginal and supraglacial landsystems dominated by thrust‐block moraines 2–3 km wide and large‐scale deformation of sediments and ice. Large areas are still underlain by remnant glacier ice and a supraglacial landscape with numerous ice‐walled lakes and kames is forming even today. The proglacial landsystem is characterised by subaqueous (e.g. deltas) or terrestrial (e.g. sandar) environments, depending on location/altitude and time of formation. Dating results (OSL, 14C) indicate that the NTZ was initiated ca. 80 kyr BP during the retreat of the Early Weichselian ice sheet and that it records the maximum limit of a Middle Weichselian glaciation (ca. 65 kyr BP). During both these events, proglacial lakes were dammed by the ice sheets. Part of the NTZ was occupied by a thin Late Weichselian ice sheet (20–12 kyr BP), resulting in subaerial proglacial drainage. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

18.
The coastal cliff section at Kås Hoved in northern Denmark represents one of the largest exposures of marine interglacial deposits in Europe. High‐resolution analyses of sediments, foraminifera, ostracods, and stable isotopes (oxygen and carbon) in glacial‐interglacial marine sediments from this section, as well as from two adjacent boreholes, are the basis for an interpretation of marine environmental and climatic change through the Late Elsterian‐Holsteinian glacial‐interglacial cycle. The overlying glacial deposits show two ice advances during the Saalian and Weichselian glaciations. The assemblages in the initial glacier‐proximal part of the marine Late Elsterian succession reveal fluctuations in the inflow of sediment‐loaded meltwater to the area. This is followed by faunal indication of glacier‐distal, open marine conditions, coinciding with a gradual climatic change from arctic to subarctic environments. Continuous marine sedimentation during the glacial‐interglacial transition is presumably a result of a large‐scale isostatic subsidence caused by the preceding extended Elsterian glaciation. The similarity of the climatic signature of the interglacial Holsteinian and Holocene assemblages in this region indicates that the Atlantic Ocean circulation was similar during these two interglacials, whereas Eemian interglacial assemblages indicate a comparatively high water temperature associated with an enhanced North Atlantic Current. The foraminiferal zones are correlated with other Elsterian‐Holsteinian sites in Denmark, as well as those in the type area for the Holsteinian interglacial in northern Germany and the southern North Sea. Correlation of the NW European Holsteinian succession with the marine isotope stages MIS 7, 9 or 11 is still unresolved.  相似文献   

19.
Changes in ocean temperature, carbonate productivity, and ice-rafted detritus in the North Atlantic suggest that half of the Northern Hemisphere ice volume at the last glacial maximum had disappeared by 13,000 yr B.P., despite the still-extensive limits of the ice sheets. This early thinning of the ice sheets occurred during a time when summer insolation values were slowly rising but when pollen evidence south of the ice margins indicates cold, dry air masses. We infer that this rapid early ice disintegration (16,000–13,000 yr B.P.) was caused by oceanic mechanisms: (1) rising sea level, causing increased calving along ice margins; (2) the chilling of the sea-surface by icebergs and meltwater, reducing moisture extraction by the atmosphere and transport to the ice sheets; and (3) winter freezing of the low-salinity meltwater layer, suppressing local moisture extraction and the regional influx of moisture-bearing storms from lower latitudes in winter and hence starving the ice sheets. These oceanic feedback mechanisms were strongest from 16,000 to 13,000 yr B.P., and weaker but still active from that date until the end of deglaciation at 6000 yr B.P.  相似文献   

20.
Continuous high-resolution mass accumulation rates (MAR) and X-ray fluorescence (XRF) measurements from marine sediment records in the Bay of Biscay (NE Atlantic) have allowed the determination of the timing and the amplitude of the ‘Fleuve Manche’ (Channel River) discharges during glacial stages MIS 10, MIS 8, MIS 6 and MIS 4–2. These results have yielded detailed insight into the Middle and Late Pleistocene glaciations in Europe and the drainage network of the western and central European rivers over the last 350 kyr. This study provides clear evidence that the ‘Fleuve Manche’ connected the southern North Sea basin with the Bay of Biscay during each glacial period and reveals that ‘Fleuve Manche’ activity during the glaciations MIS 10 and MIS 8 was significantly less than during MIS 6 and MIS 2. We correlate the significant ‘Fleuve Manche’ activity, detected during MIS 6 and MIS 2, with the extensive Saalian (Drenthe Substage) and the Weichselian glaciations, respectively, confirming that the major Elsterian glaciation precedes the glacial MIS 10. In detail, massive ‘Fleuve Manche’ discharges occurred at ca 155 ka (mid-MIS 6) and during Termination I, while no significant discharges are found during Termination II. It is assumed that a substantial retreat of the European ice sheet at ca 155 kyr, followed by the formation of ice-free conditions between the British Isles and Scandinavia until Termination II, allowed meltwater to flow northwards through the North Sea basin during the second part of the MIS 6. We assume that this glacial pattern corresponds to the Warthe Substage glacial maximum, therefore indicating that the data presented here equates to the Drenthe and the Warthe glacial advances at ca 175–160 ka and ca 150–140 ka, respectively. Finally, the correlation of our records with ODP site 980 reveals that massive ‘Fleuve Manche’ discharges, related to partial or complete melting of the European ice masses, were synchronous with strong decreases in both the rate of deep-water formation and the strength of the Atlantic thermohaline circulation. ‘Fleuve Manche’ discharges over the last 350 kyr probably participated, with other meltwater sources, in the collapse of the thermohaline circulation by freshening the northern Atlantic surface water.  相似文献   

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