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1.
The following events have been identified from a complex sequence of Midlandian (Devensian) sediments recently exposed at Aghnadarragh, County Antrim: (1) A lower till interpreted as Early Midlandian was deposited by a major lowland ice sheet which moved south-eastwards from central Ulster across the Lough Neagh depression. (2) Deglacial conditions were followed by a periglacial phase characterised by ice-wedge growth and sedimentation by gravelly debris flows. The latter contain the oldest known remains of Mammuthus primigenius in Ireland. (3) An Early Midlandian interstadial is represented by a woody detritus peat, with evidence of Betula, Pinuos and Picea woodlands and a rich beetle fauna. Wood from this horizon has been shown to be beyond the range of radiocarbon dating (>48 180 BP). (4) Interstadial conditions were succeeded by a cold, non-glacial phase dominated by in-channel gravelly flows and deposition of organic muds which contain plant and insect fossils. These horizons are older than 46 850 BP. (5) An upper, non-drumlinised till was deposited during the Late-Midlandian by a major lowland ice sheet which moved generally eastwards across the Lough Neagh Lowlands from central Ulster. This glaciation probably reached its maximum at ca 20–24 ka. (6) Drumlin formation occurred in the Lough Neagh Lowlands towards the end of the Late-Midlandian. The limiting moraines are dated to 17 ka. The lower till at Aghnadarragh is the first positive record of a major lowland ice sheet in Ireland during the early part of the last cold stage. The Early Midlandian interstadial peats have not been documented elsewhere in Ireland and correlate broadly with the Chelford Interstadial complex of the English Midlands. Related exposures elsewhere in Ulster confirm that the middle part of the last cold stage was free of major lowland ice masses but deposits of this period are absent from Aghnadarragh.  相似文献   

2.
Lake Chungará (18°15′S, 69°09′W, 4520 m above sea‐level) is the largest (22·5 km2) and deepest (40 m) lacustrine ecosystem in the Chilean Altiplano and its location in an active volcanic setting, provides an opportunity to evaluate environmental (volcanic vs. climatic) controls on lacustrine sedimentation. The Late Quaternary depositional history of the lake is reconstructed by means of a multiproxy study of 15 Kullenberg cores and seismic data. The chronological framework is supported by 10 14C AMS dates and one 230Th/234U dates. Lake Chungará was formed prior to 12·8 cal kyr bp as a result of the partial collapse of the Parinacota volcano that impounded the Lauca river. The sedimentary architecture of the lacustrine succession has been controlled by (i) the strong inherited palaeo‐relief and (ii) changes in the accommodation space, caused by lake‐level fluctuations and tectonic subsidence. The first factor determined the location of the depocentre in the NW of the central plain. The second factor caused the area of deposition to extend towards the eastern and southern basin margins with accumulation of high‐stand sediments on the elevated marginal platforms. Synsedimentary normal faulting also increased accommodation and increased the rate of sedimentation in the northern part of the basin. Six sedimentary units were identified and correlated in the basin mainly using tephra keybeds. Unit 1 (Late Pleistocene–Early Holocene) is made up of laminated diatomite with some carbonate‐rich (calcite and aragonite) laminae. Unit 2 (Mid‐Holocene–Recent) is composed of massive to bedded diatomite with abundant tephra (lapilli and ash) layers. Some carbonate‐rich layers (calcite and aragonite) occur. Unit 3 consists of macrophyte‐rich diatomite deposited in nearshore environments. Unit 4 is composed of littoral sediments dominated by alternating charophyte‐rich and other aquatic macrophyte‐rich facies. Littoral carbonate productivity peaked when suitable shallow platforms were available for charophyte colonization. Clastic deposits in the lake are restricted to lake margins (Units 5 and 6). Diatom productivity peaked during a lowstand period (Unit 1 and subunit 2a), and was probably favoured by photic conditions affecting larger areas of the lake bottom. Offshore carbonate precipitation reached its maximum during the Early to Mid‐Holocene (ca 7·8 and 6·4 cal kyr bp ). This may have been favoured by increases in lake solute concentrations resulting from evaporation and calcium input because of the compositional changes in pyroclastic supply. Diatom and pollen data from offshore cores suggest a number of lake‐level fluctuations: a Late Pleistocene deepening episode (ca 12·6 cal kyr BP), four shallowing episodes during the Early to Mid‐Holocene (ca 10·5, 9·8, 7·8 and 6·7 cal kyr BP) and higher lake levels since the Mid‐Holocene (ca 5·7 cal kyr BP) until the present. Explosive activity at Parinacota volcano was very limited between c. >12·8 and 7·8 cal kyr bp . Mafic‐rich explosive eruptions from the Ajata satellite cones increased after ca 5·7 cal kyr bp until the present.  相似文献   

3.
Stratigraphical investigations, geomorphological mapping, and diatom, plant macrofossil and pollen analyses were undertaken in and around two lakes in central Ireland to establish correlations between changes in lake conditions and catchment vegetation throughout the Holocene. Similar investigations of an adjacent mire reveal early Holocene changes in lake level and area. The palaeoecological data show high correlations related to variations in lake depth and area, catchment vegetation type, organic inputs and trophic status. Catchment‐scale deforestation is gradual and occurs through the Bronze and the Iron Ages, and the construction of a crannog in the early Medieval period (seventh century AD ) appears to be associated with a widespread increase in deforestation and mixed agriculture in the catchment. Both pollen and plant macrofossils suggest that one of the crannogs was used for crop storage in addition to domestic and any other activities. In the early to middle Holocene similarities in the proxy‐data appear to be climatically driven through changing lake levels and areal extent whereas the later Holocene record is clearly dominated by anthropogenic changes within the catchment and the construction of crannogs in the lakes. The advantages of combining multi‐proxy indicators of lake hydroecology with the vegetation record are illustrated. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

4.
The late Pleistocene–Holocene ecological and limnological history of Lake Fúquene (2580 m a.s.l.), in the Colombian Andes, is reconstructed on the basis of diatom, pollen and sediment analyses of the upper 7 m of the core Fúquene‐7. Time control is provided by 11 accelerator mass spectrometry (AMS) 14C dates ranging from 19 670 ± 240 to 6040 ± 60 yr BP. In this paper we present the evolution of the lake and its surroundings. Glacial times were cold and dry, lake‐levels were low and the area was surrounded by paramo and subparamo vegetation. Late‐glacial conditions were warm and humid. The El Abra Stadial, a Younger Dryas equivalent, is reflected by a gap in the sedimentary record, a consequence of the cessation of deposition owing to a drop in lake‐level. The early Holocene was warm and humid; at this time the lake reached its maximum extension and was surrounded by Andean forest. The onset of the drier climate prevailing today took place in the middle Holocene, a process that is reflected earlier in the diatom and sediment records than in the pollen records. In the late Holocene human activity reduced the forest and transformed the landscape. Climate patterns from the Late‐glacial and throughout the Holocene, as represented in our record, are similar to other records from Colombia and northern South America (the Caribbean, Venezuela and Panama) and suggest that the changes in lake‐level were the result of precipitation variations driven by latitudinal shifts of the Intertropical Convergence Zone. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

5.
We present early to Mid‐Holocene paleo‐geographic reconstructions for the Ramore Head area (Northern Ireland). This coastal area is characterized by Mesolithic occupation (c. 10–6 ka) and preserved early–Mid‐Holocene peats both on‐ and offshore. This paper improves on previous reconstructions by employing a backstripping methodology, which removes accumulated recent deposits from identified buried paleo‐landsurfaces instead of using modern topography as an analogue to the past landscape. Paleo‐landsurfaces are identified offshore from seismic profiles supplemented by cores, and onshore through legacy borehole records. The paleo‐landsurface can be traced offshore to depths of −2 to −19 m and is buried by <5 m of modern sediment. It extends onshore under the coastal town of Portrush and is buried <2.5–10 m below modern ground level. The identified paleo‐landsurface is combined with sea‐level curves from recent Glacio‐Isostatic‐Adjustment models to reconstruct marine transgression during the early–Mid‐Holocene. Comparison is also made with reconstructions based on modern topography. Together, the identified paleo‐landsurfaces and revised reconstructions can assist future site prospection on‐ and offshore and delimit high‐potential areas for heritage management. Revised reconstructions also allow placement of extant archaeology into a more accurate context of landscape change and help develop insights into local‐scale site location patterns.  相似文献   

6.
We present evidence of a large lake (Glacial Lake Victoria) that existed in Victoria Valley in the dry valleys region of Antarctica between at least 20 000 and 8600 14C yr BP. At its highstands, Glacial Lake Victoria covered 100 km2 and was ca. 200 m deep. The chronology for lake‐level changes comes from 87 AMS radiocarbon dates of lacustrine algae preserved in deltas and glaciolacustrine deposits that extend up to 185 m above present‐day lakes on the valley floor. The existence of Glacial Lake Victoria, as well as other large lakes in the dry valleys, indicates a climate regime significantly different from that of today at the last glacial maximum and in the early Holocene. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

7.
The role and extent of climate as a cause of the expansion and decline of human cultures is still debatable. It is clear, however, that human–environment interactions are enhanced and interplay more closely in climatically sensitive areas such as around hydrologically closed basins. Lago Cardiel is located at 49° S in the very arid rain shadow east of the Andes, providing an exceptionally receptive system to changes in hydrological balance. Results of a geophysical survey combined with sedimentological and geochemical studies provide a continuous Lateglacial–Holocene record of substantial water‐level changes. These variations, combined with archaeological results from the catchment area, offer a unique possibility to explore the pattern of peopling within this remote area of the globe and its possible relation to climate change. Human occupation in Patagonia is well documented towards the Andes throughout the entire Holocene. Archaeological data from the Lago Cardiel basin, however, show an apparent lack of human activity during the first part of this period, which coincides with well‐constrained high lake levels. Our results show an intriguing coincidence between low lake level and increasing human occupation, suggesting that the Lago Cardiel basin has focused human use during intervals with relatively lower effective moisture such as during the Late Pleistocene, but its evidence may have been submerged. This interpretation is confirmed by archaeological remains from Lago Strobel, another perennial lake with a comparable catchment located in the same climatic region and thus sharing the same climatic history as Lago Cardiel. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

8.
The Fehmarn Belt is a key area for the Late Pleistocene and Holocene development of the Baltic Sea as it was a passage for marine and fresh water during its different stages. The pre‐Holocene geological development of this area is presented based on the analysis of seismic profiles and sedimentary gravity cores. Late Pleistocene varve sediments of the initial Baltic Ice Lake were identified. An exceptionally thick varve layer, overlain by a section of thinner varves with convolute bedding in turn covered by undisturbed varves with decreasing thicknesses is found in the Fehmarn Belt. This succession, along with a change in varve geochemistry, represents a rapid ice‐sheet withdrawal and increasingly distal sedimentation in front of the ice margin. Two erosional unconformities are observed in the eastern Mecklenburg Bight, one marking the top of the initial Baltic Ice Lake deposits and the second one indicating the end of the final Baltic Ice Lake. These unconformities join in Fehmarn Belt, where deposits of the final Baltic Ice Lake are missing due to an erosional hiatus related to a lake‐level drop during its final drainage. After this lake‐level drop, a lowstand environment represented by river deposits developed. These deposits are covered by lake marls of Yoldia age. Tilting of the early glacial lake sediments indicates a period of vertical movements prior to the onset of the Holocene. Deposits of the earliest stages of the Baltic Sea have been exposed by ongoing erosion in the Fehmarn Belt at the transition to the Mecklenburg Bight.  相似文献   

9.
Permafrost degradation influences the morphology, biogeochemical cycling and hydrology of Arctic landscapes over a range of time scales. To reconstruct temporal patterns of early to late Holocene permafrost and thermokarst dynamics, site‐specific palaeo‐records are needed. Here we present a multi‐proxy study of a 350‐cm‐long permafrost core from a drained lake basin on the northern Seward Peninsula, Alaska, revealing Lateglacial to Holocene thermokarst lake dynamics in a central location of Beringia. Use of radiocarbon dating, micropalaeontology (ostracods and testaceans), sedimentology (grain‐size analyses, magnetic susceptibility, tephra analyses), geochemistry (total nitrogen and carbon, total organic carbon, δ13Corg) and stable water isotopes (δ18O, δD, d excess) of ground ice allowed the reconstruction of several distinct thermokarst lake phases. These include a pre‐lacustrine environment at the base of the core characterized by the Devil Mountain Maar tephra (22 800±280 cal. a BP, Unit A), which has vertically subsided in places due to subsequent development of a deep thermokarst lake that initiated around 11 800 cal. a BP (Unit B). At about 9000 cal. a BP this lake transitioned from a stable depositional environment to a very dynamic lake system (Unit C) characterized by fluctuating lake levels, potentially intermediate wetland development, and expansion and erosion of shore deposits. Complete drainage of this lake occurred at 1060 cal. a BP, including post‐drainage sediment freezing from the top down to 154 cm and gradual accumulation of terrestrial peat (Unit D), as well as uniform upward talik refreezing. This core‐based reconstruction of multiple thermokarst lake generations since 11 800 cal. a BP improves our understanding of the temporal scales of thermokarst lake development from initiation to drainage, demonstrates complex landscape evolution in the ice‐rich permafrost regions of Central Beringia during the Lateglacial and Holocene, and enhances our understanding of biogeochemical cycles in thermokarst‐affected regions of the Arctic.  相似文献   

10.
Luoto, T. P. & Sarmaja‐Korjonen, K. 2011: Midge‐inferred Holocene effective moisture fluctuations in a subarctic lake, northern Lapland. Boreas, 10.1111/j.1502‐3885.2011.00217.x. ISSN 0300‐9483. We examined fossil midge (Diptera: Chironomidae) assemblages from Lake Várddoaijávri, northern Finland to track Holocene effective moisture variability. Application of a midge‐based water‐depth calibration model showed that the early Holocene was characterized by a high water level compared with the Holocene average, but the inferred values decreased at c. 8000 cal. a BP and increased again towards c. 6000 cal. a BP. The inferred water level decreased at c. 5500 cal. a BP, but increased again towards c. 4000 cal. a BP. Between 4000 and 3000 cal. a BP the lake experienced two rapid events of lower water level. A relatively high water level detected at c. 3000 cal. a BP was followed by a lowering towards c. 2000 cal. a BP. The time period from c. 2000 cal. a BP onwards was characterized by a general rise in lake level towards the present. Overall, the present reconstruction shows a close correspondence in its trends to previous lake‐level records in the region. Two common core taxa, Paratanytarsus and Corynocera ambigua, did not correlate significantly with water depth in the calibration data, creating a potential error source for the present lake‐level reconstruction. However, statistical analysis showed a clear community response to long‐term lake‐level changes, and therefore the major trends in Holocene effective moisture patterns were revealed. The present palaeoclimatic information can also serve as valuable background data when assessing the effects of the present climate change.  相似文献   

11.
Understanding Arctic glacier sensitivity is key to predicting future response to air temperature rise. Previous studies have used proglacial lake sediment records to reconstruct Holocene glacier advance–retreat patterns in South and West Greenland, but high‐resolution glacier records from High Arctic Greenland are scarce, despite the sensitivity of this region to future climate change. Detailed geochemical analysis of proglacial lake sediments close to Zackenberg, northeast Greenland, provides the first high‐resolution record of Late Holocene High Arctic glacier behaviour. Three phases of glacier advance have occurred in the last 2000 years. The first two phases (c. 1320–800 cal. a BP) occurred prior to the Little Ice Age (LIA), and correspond to the Dark Ages Cold Period and the Medieval Climate Anomaly. The third phase (c. 700 cal. a BP), representing a smaller scale glacier oscillation, is associated with the onset of the LIA. Our results are consistent with recent evidence of pre‐LIA glacier advance in other parts of the Arctic, including South and West Greenland, Svalbard, and Canada. The sub‐millennial glacier fluctuations identified in the Madsen Lake succession are not preserved in the moraine record. Importantly, coupled XRF and XRD analysis has effectively identified a phase of ice advance that is not visible by sedimentology alone. This highlights the value of high‐resolution geochemical analysis of lake sediments to establish rapid glacier advance–retreat patterns in regions where chronological and morphostratigraphical control is limited.  相似文献   

12.
The Qarun Lake in the Faiyum Oasis (Egypt) provides a unique record of Holocene environmental and climate change in an arid area largely devoid of fossil proxy records. Multiple lithological, palaeontological and geochemical proxies and 32 radiocarbon dates from the 26‐m‐long core FA‐1 provide a time series of the lake's transformation. Our results confirm that a permanent lake appeared in the Holocene at c. 10 cal. ka BP. The finely laminated lake sediments consist of diatomite, in which diatoms and ostracods together with lower concentrations of ions indicate a freshwater environment at the end of the early and middle Holocene. This freshwater supply was closely associated with regular inflows of the Nile water during flood seasons, when the Intertropical Convergence Zone (ITCZ) migrated northwards in Africa, although it has probably never reached the Faiyum Oasis. Local rainfall, possibly connected with a northern atmospheric circulation, may have been important during winter. Several phases in the lake's evolution are recognized, represented by oscillations between deep open freshwater conditions during more humid climate and shallow fresh to brackish water during drier episodes. After a long freshwater phase, the lake setting has become more brackish since c. 6.2 cal. ka BP as indicated by diatoms and increasing contents of evaporite ions in the sediment. This clearly shows that since that time the lake has occasionally become partly desiccated. This is a result of reduced discharge of the Nile. In the late Holocene the lake was mostly brackish and then gradually turned into a saline lake. This natural process was interrupted about 2.3 cal. ka BP when a man‐made canal facilitated water inflow from the Nile. The examined FA‐1 core can be used as a reference age model of climate change in the Holocene and its impact on the development and decline of ancient civilizations in northeastern Africa.  相似文献   

13.
The interplay of eustatic and isostatic factors causes complex relative sea‐level (RSL) histories, particularly in paraglacial settings. In this context the past record of RSL is important in understanding ice‐sheet history, earth rheology and resulting glacio‐isostatic adjustment. Field data to develop sea‐level reconstructions are often limited to shallow depths and uncertainty exists as to the veracity of modelled sea‐level curves. We use seismic stratigraphy, 39 vibrocores and 26 radiocarbon dates to investigate the deglacial history of Belfast Lough, Northern Ireland, and reconstruct past RSL. A typical sequence of till, glacimarine and Holocene sediments is preserved. Two sea‐level lowstands (both max. ?40 m) are recorded at c. 13.5 and 11.5k cal a bp . Each is followed by a rapid transgression and subsequent periods of RSL stability. The first transgression coincides temporally with a late stage of Meltwater Pulse 1a and the RSL stability occurred between c. 13.0 and c. 12.2k cal a bp (Younger Dryas). The second still/slowstand occurred between c. 10.3 and c. 11.5k cal a bp . Our data provide constraints on the direction and timing of RSL change during deglaciation. Application of the Depth of Closure concept adds an error term to sea‐level reconstructions based on seismic stratigraphic reconstructions.  相似文献   

14.
Both monsoons and westerlies have exerted influence on climate dynamics over the Tibetan Plateau (TP) since the last deglaciation, producing complex patterns of paleohydroclimatic conditions. Diverse proxy records are essential to forge a robust understanding of the climate system on the TP. Currently, there is a general lack of understanding of the response of inland lakes over the TP to climate change, especially glacier‐fed lakes. Paleohydrological reconstructions of such lakes could deepen our understanding of the history of lake systems and their relationship to regional climate variability. Here we use records of n‐alkanes and grain size from the sediments of Bangong Co in the western TP to reconstruct paleohydrological changes over the past 16,000 years. The Paq record (the ratio of non‐emergent aquatic macrophytes versus emergent aquatic macrophytes and terrestrial plants) is generally consistent with the variations in summer temperature and precipitation isotopes. The changes in grain‐size distributions show a similar trend to Paq but with less pronounced fluctuations in the early‐middle Holocene. The new data combined with previous results from the site demonstrate that: 1) Bangong Co experienced relatively large water‐level fluctuations during the last deglaciation, with a steadily high lake‐level during the early‐middle Holocene and a decreasing lake‐level in the late Holocene; 2) The lake level fluctuations were driven by both high summer temperatures via the melting water and monsoon precipitation. However, the dominant factor controlling lake level changed over time. The lake‐level history at Bangong Co deduced from the n‐alkanes and grain‐size records reveals the past hydrological changes in the catchment area, and stimulates more discussion about the future of glacier‐fed lakes under the conditions of unprecedented warming in the region.  相似文献   

15.
Pollen‐based quantitative estimates of seasonal precipitation from Lake Pergusa and lake‐level data from Lake Preola in Sicily (southern Italy) allow three successive periods to be distinguished within the Holocene: an early Holocene period before ca. 9800 cal a BP with rather dry climate conditions in winter and summer, a mid‐Holocene period between ca. 9800 and 4500 cal a BP with maximum winter and summer wetness, and a late Holocene period after 4500 cal a BP with declining winter and summer wetness. This evolution observed in the south‐central Mediterranean shows strong similarities to that recognized in the eastern Mediterranean. But, it contrasts with that reconstructed in north‐central Italy, where the mid‐Holocene appears to be characterized by a winter (summer) precipitation maximum (minimum), while the late Holocene coincided with a decrease (increase) in winter (summer) precipitation. Maximum precipitation at ca. 10 000–4500 cal a BP may have resulted from (i) increased local convection in response to a Holocene insolation maximum at 10 000 cal a BP and then (ii) the gradual weakening of the Hadley cell activity, which allowed the winter rainy westerlies to reach the Mediterranean area more frequently. After 4500 cal a BP, changes in precipitation seasonality may reflect non‐linear responses to orbitally driven insolation decrease in addition to seasonal and inter‐hemispheric changes of insolation. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

16.
The history of sea‐level change and sediment accumulation since the last deglaciation along the German North Sea coast is still controversial because of a limitation in the quantity and quality of chronological data. In the current study, the chronology of a 16‐ka coastal sedimentary record from the Garding‐2 core, retrieved from the Eiderstedt Peninsula in Schleswig‐Holstein, northern Germany, was established using OSL and AMS 14C dating techniques. The robust chronology using 14 radiocarbon and 25 OSL dates from the Garding‐2 core is the first long‐term record that covers the Holocene as well as the last deglaciation period in one succession in the German North Sea area. It provides a new insight into understanding the Holocene transgression and coastal accumulation histories. The combined evidence from the sedimentology and chronology investigations indicates that an estuarine environment dominated in Eiderstedt Peninsula from 16 to 13 ka, followed by a depositional hiatus between 13 and 8.3 ka, attributed to erosion caused by the Holocene transgression; the onset of the Holocene transgression at the core site occurred at around 8.3 ka. The sea level continued to rise with a decelerated rate until around 3 ka. Since 3 ka, the shoreline has begun to prograde. Foreshore (tidal flat) sediments have been deposited at the drilling site with a very high sedimentation rate of about 10 m ka?1. At around 2 ka, a sandy beach deposit accumulated in the sedimentary succession, indicating that the coastline shifted landward, which may represent a small‐scale transgression in the late Holocene. At around 1.5 ka, terrestrial clastic sediment started to accumulate, indicating a retreat of the relative sea level in this area, which may be related to local diking activities undertaken since the 11th century.  相似文献   

17.
《Sedimentology》2018,65(6):1891-1917
Coastal back‐barrier perched lakes are freshwater bodies that are elevated over sea‐level and are not directly subjected to the inflow of seawater. This study provides a detailed reconstruction of the Doniños back‐barrier perched lake that developed at the end of a small river valley in the rocky coast of the north‐west Iberian Peninsula during the Holocene transgression. Its sequence stratigraphy was reconstructed based on a core transect across the system, the analyses of its lithofacies and microfossil assemblages, and a high‐resolution radiocarbon‐based chronology. The Doniños perched lake was formed ca 4·5 ka bp . The setting of the perched lake was favoured by Late Holocene sea‐level stabilization and the formation of a barrier and back‐barrier basin, which was contemporaneous with the high systems tract period. This basin developed over marine and lagoonal sediments deposited between 10·2 ka bp and 8·0 ka bp , during rapidly rising sea‐level characteristic of the transgressive systems track period. At 1·1 ka bp , the barrier was breached and the perched lake was partially emptied, causing the erosion of the back‐barrier basin sediments and a significant sedimentary hiatus. Both enhanced storminess and human intervention were likely to be responsible for this event. After 1 ka bp , the barrier reclosed and the present‐day lake was reformed, with the water level reaching as high as 5 m above mean sea‐level. The depositional evolution of the Doniños system serves as a model of coastal back‐barrier perched lakes in coastal clastic systems that have developed over gently seaward‐dipping rugged substrates at small distances from the shoreline and under conditions of rising sea‐level and high sediment supply. A review of estuaries, back‐barrier lagoons, pocket beaches and back‐barrier perched lakes in the rocky coast of north‐west Spain shows that the elevation of the bedrock is the main factor controlling the origin and evolution of these systems.  相似文献   

18.
Palynological and sedimentological analyses of a sedimentary sequence sampled at Hauterive/Rouges‐Terres, Lake Neuchâtel (Switzerland) provide documentation of changes in vegetation and lake‐level during the Bølling, Younger Dryas and Preboreal pollen zones, and have allowed a comparison with sequences covering the same period from other sites located in the western part of the Swiss Plateau. The Juniperus–Hippophaë zone (regional pollen assemblage zone (RPAZ) CHb‐2, first part of the Bølling, ca. 14 650–14 450 cal. yr BP) was characterised by a generally low lake‐level. A weak rise occurred during this zone. The Juniperus–Hippophaë to Betula zone transition coincided with a lake‐level lowering, interrupted by a short‐lived but marked phase of higher lake‐level recorded at the neighbouring site of Hauterive‐Champréveyres, but not present at Hauterive/Rouges‐Terres owing to an erosion surface. Shortly after the beginning of the Betula zone (RPAZ CHb‐3, second part of the Bølling, ca 14 450–14 000 cal. yr BP), a marked rise in lake‐level occurred. It was composed of two successive periods of higher level, coinciding with high values of Betula, separated by a short episode of relatively lower lake‐level associated with raised values in Artemisia and other non‐arboreal pollen. The last part of RPAZ CHb‐3 saw a fall in lake‐level. The lower lake‐levels during RPAZ CHb‐2 to early RPAZ CHb‐3 can be correlated with the abrupt warming at the beginning of the Greenland Interstadial (GI) 1e thermal maximum. The successive episodes of higher lake‐level punctuating the GI 1e might be linked to the so‐called Intra‐Bølling Cold Oscillations identified from several palaeoclimatic records in the North Atlantic area, and also documented in oxygen‐isotope data sets from Swiss Plateau lakes. The Hauterive/Rouges‐Terres lake‐level record provides evidence for marked climatic drying through the second part of the Younger Dryas event (GS1), during the GS1–Preboreal (RPAZ CHb‐4b–4c) transition (except for a rise at ca. 11 450–11 400 cal. yr BP), and at the RPAZ CHb‐4c–5 (Preboreal–Boreal) transition, following the Preboreal Oscillation (after 11 150 cal. yr BP). The Preboreal Oscillation coincided with higher lake‐levels, its end being followed by a rapid expansion of Corylus, Quercus, Ulmus and Tilia. The Hauterive/Rouges‐Terres lake‐level record suggests that radiocarbon plateau at 12 600, 10 000 and 9500 14C yr BP corresponded to periods of generally lower lake‐level. This suggests that an increase in solar activity may have contributed to both climatic dryness and a decrease in atmospheric radiocarbon content. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

19.
New accelerator mass spectrometer radiocarbon ages from gastropods in shore deposits within the pluvial Lake Chewaucan basin, combined with stratigraphical and geomorphological evidence, identify an abrupt rise and fall of lake level at ca. 12 14C ka. The lake‐level high is coeval with lake‐level lows in the well‐dated records of palaeolakes Bonneville and Lahontan, and with a period of relatively wet conditions in the more southerly Owens Lake basin. This spatial pattern of pluvial lake levels in the western USA at 12 14C ka indicates a variable synoptic response to climate forcing at this time. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

20.
This study presents a synthesis of the geomorphology, facies variability and depositional architecture of ice‐marginal deltas affected by rapid lake‐level change. The integration of digital elevation models, outcrop, borehole, ground‐penetrating radar and high‐resolution shear‐wave seismic data allows for a comprehensive analysis of these delta systems and provides information about the distinct types of deltaic facies and geometries generated under different lake‐level trends. The exposed delta sediments record mainly the phase of maximum lake level and subsequent lake drainage. The stair‐stepped profiles of the delta systems reflect the progressive basinward lobe deposition during forced regression when the lakes successively drained. Depending on the rate and magnitude of lake‐level fall, fan‐shaped, lobate or more digitate tongue‐like delta morphologies developed. Deposits of the stair‐stepped transgressive delta bodies are buried, downlapped and onlapped by the younger forced regressive deposits. The delta styles comprise both Gilbert‐type deltas and shoal‐water deltas. The sedimentary facies of the steep Gilbert‐type delta foresets include a wide range of gravity‐flow deposits. Delta deposits of the forced‐regressive phase are commonly dominated by coarse‐grained debrisflow deposits, indicating strong upslope erosion and cannibalization of older delta deposits. Deposits of supercritical turbidity currents are particularly common in sand‐rich Gilbert‐type deltas that formed during slow rises in lake level and during highstands. Foreset beds consist typically of laterally and vertically stacked deposits of antidunes and cyclic steps. The trigger mechanisms for these supercritical turbidity currents were both hyperpycnal meltwater flows and slope‐failure events. Shoal‐water deltas formed at low water depths during both low rates of lake‐level rise and forced regression. Deposition occurred from tractional flows. Transgressive mouthbars form laterally extensive sand‐rich delta bodies with a digitate, multi‐tongue morphology. In contrast, forced regressive gravelly shoal‐water deltas show a high dispersion of flow directions and form laterally overlapping delta lobes. Deformation structures in the forced‐regressive ice‐marginal deltas are mainly extensional features, including normal faults, small graben or half‐graben structures and shear‐deformation bands, which are related to gravitational delta tectonics, postglacial faulting during glacial‐isostatic adjustment, and crestal collapse above salt domes. A neotectonic component cannot be ruled out in some cases.  相似文献   

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