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1.
The sequence of Late-glacial and Holocene alluvial sedimentation in the middle Caquetá River Basin of Colombian Amazonia is described, based on the study of the sediments and palynology of several river bank sections and on 30 radiocarbon dates. An early Late-glacial sedimentation cycle is recognised, followed by a minor late Late-glacial erosion phase. The Holocene valley fill consists of grey clays (often present in the lower part of the sections) deposited in open water and silty clays often with faint yellow mottling, deposited under a regime of seasonal flooding. The base of the Holocene sections is formed by sands, where exposed. In two places the transition of sand to open-water grey clay was dated around 10 000 yr BP and there is a suggestion that open water may have been more common at the beginning of the Holocene than later, when sedimentation by seasonal flooding became important. In many places much of the earlier Holocene sediments may have been removed by erosion and replaced by younger sediments, by a process of lateral aggradation. A considerable part of the present valley fill is younger than ca. 3500 yr. However, in several places older Holocene sediments are found, apparently only little affected by later erosion, lying below younger varzea silty clays. During the Holocene more organic sediments were formed in periods with reduced river discharge, related to drier climates in the Andes and possibly in Amazonia. These dry periods, deduced from data in the Caquetá River area, correspond well with dry phases in other parts of northwestern South America (e.g. between approximately 2700-1900 yr BP and approximately 3200-3800 yr BP). Rates of average net sedimentation, calculated from dated sections that apparently lack major hiatuses caused by erosion, were high in the lower Holocene, low during the middle Holocene and increase again in the upper Holocene. Levee deposits became coarser and the high river level of the Caquetá increased during the late Holocene. These phenomena may be explained by the increasing influence of man on the vegetation cover in the Andean headwater areas and possibly also in the Amazonian catchment area of the Caquetá River.  相似文献   

2.
Late Pleistocene and Holocene vegetational and climatic change have been studied palynologically at a site at 1750 m elevation in the subandean vegetation belt near Popayán, in the southern Colombian Andes. Time control on the pollen record is based on six AMS 14C ages, ranging from possibly Middle Pleniglacial time (around 50000 yr BP) to 1092 ± 44 yr BP. Because of the presence of two hiatuses only the Middle Pleniglacial and Late Holocene periods (the last 2300 yr BP) are represented. Pollen data indicate the presence of closed subandean forest during glacial time. Changes in the contribution of pollen originating from the uppermost and lowermost subandean forest belts, changes in the contribution of a number of other subandean forest taxa, and changes in species composition between the three pollen zones, suggest that the climate during the Middle Pleniglacial was markedly colder, and perhaps also wetter, than during the Late Holocene. Pollen assemblages from the Late Holocene indicate that the landscape has been affected by deforestation and agriculture since at least 2300 yr BP, but that human impact decreased in the last 780 yr BP. © 1998 John Wiley & Sons, Ltd.  相似文献   

3.
Fine-grained fluvial residual channel infillings are likely to reflect systematic compositional changes in response to climate change, owing to changing weathering and geomorphological conditions in the upstream drainage basin. Our research focuses on the bulk sediment and clay geochemistry, laser granulometry and clay mineralogy of Late-glacial and Early Holocene River Meuse (Maas) unexposed residual channel infillings in northern Limburg (The Netherlands). We demonstrate that residual channel infillings register a systematic bulk and clay compositional change related to climate change on a 1–10 k-yr time-scale. Late-glacial and Holocene climatic amelioration stabilised the landscape and facilitated prolonged and intense chemical weathering of phyllosilicates and clay minerals due to soil formation. Clay translocation and subsequent erosion of topsoils on Palaeozoic bedrock and loess deposits increased the supply of smectite and vermiculite within River Meuse sediments. Smectite plus vermiculite contents rose from 30–40% in the Pleniglacial to 60% in the Late Allerød and to 70–80% in the Holocene. Younger Dryas cooling and landscape instability caused almost immediate return to low smectite and vermiculite contents. Following an Early Holocene rise, within about 5000 yr, a steady state supply is reached before 5 ka (Mid-Holocene). Holocene sediments therefore contain higher amounts of clay that are richer in high-Al, low-K and low-Mg vermiculites and smectites compared with Late (Pleni-)glacial sediments. The importance of clay mineral provenance and loess admixture in the River Meuse fluvial sediments is discussed. © 1998 John Wiley & Sons, Ltd.  相似文献   

4.
Cryoturbated organic beds and channel fills, intercalated with sandy and gravelly fluvial units, have been studied in an opencast brown‐coal mine near Nochten (Niederlausitz), eastern Germany. The fluvial–aeolian sequence covers parts of the Early, Pleni‐ and Late‐glacial. The detailed chronology is based on 11 radiocarbon and 12 OSL dates, covering the period between ca. 100 kyr and 11 kyr BP. Basal peat deposits are correlated with an Early Weichselian interstadial. During this period boreal forests were present and minimum mean summer temperatures were > 13°C. Early Pleniglacial deposits are absent. The Middle and Late Pleniglacial environments were treeless and different types of tundra vegetation can be recognised. Minimum mean summer temperatures varied between 10 and 15°C. Vegetation and climate is reconstructed in detail for the periods around 34–38 kyr BP and 24–25 kyr BP. Around 34–38 ka, a mixture between a low shrub tundra and a cottongrass tussock–subshrub tundra was present. The botanical and sedimentological data suggest that from the Middle to the Late Pleniglacial, the climate became more continental, aridity and wind strength increased, and the role of a protecting winter snow cover decreased. A sedge–grass–moss tundra dominated around 24 and 25 kyr BP. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

5.
Radiocarbon dates from two sites in the Andes (Ecuador and Peru) confirm that glaciers culminated a readvance after 11 000 yr BP. A moraine stage, equivalent in altitude and position relative to existing glaciers, is present in most glacierized ranges, but its age is equivocal. Broadly limiting dates from Colombia and Peru suggest that the stage may be Late-glacial, as it is younger than 12100 yr BP, but formed before the early Holocene; in southern Chile a comparable moraine stage is older than ca. 9100 yr BP. Andean glaciers appear to have advanced at least twice during the Late-glacial interval. Glacier reconstruction from these moraine limits suggests depression of the equilibrium line altitude by at least 300–400 m in the northern and north-central Andes, and possibly less than this farther south. Late-glacial climatic change occurred globally and possibly reflects North Atlantic temperature and circulation changes forced by deglaciation of the northern ice sheets, migrations north and south of the Atlantic Polar Front, and the switching off and on of a ‘dust pump’ in low midlatitudes.  相似文献   

6.
New pollen, micro-charcoal, sediment and mineral analyses of a radiocarbon-dated sediment core from the Serra Sul dos Carajás (southeast Amazonia) indicate changes between drier and wetter climatic conditions during the past 25,000 yr, reflected by fire events, expansion of savanna vegetation and no-analog Amazonian forest communities. A cool and dry last glacial maximum (LGM) and late glacial were followed by a wet phase in the early Holocene lasting for ca. 1200 yr, when tropical forest occurred under stable humid conditions. Subsequently, an increasingly warm, seasonal climate established. The onset of seasonality falls within the early Holocene warm period, with possibly longer dry seasons from 10,200 to 3400 cal yr BP, and an explicitly drier phase from 9000 to 3700 cal yr BP. Modern conditions with shorter dry seasons became established after 3400 cal yr BP. Taken together with paleoenvironmental evidence from elsewhere in the Amazon Basin, the observed changes in late Pleistocene and Holocene vegetation in the Serra Sul dos Carajás likely reflect large-scale shifts in precipitation patterns driven by the latitudinal displacement of the Inter-Tropical Convergence Zone and changes in sea-surface temperatures in the tropical Atlantic.  相似文献   

7.
Foraminifera, pollen, lithology and radiocarbon dates from a core in the southern Kattegat provide a rare opportunity to obtain data relating to environmental conditions during the Middle Weichselian in the offshore Kattegat. This core is also correlated with an adjacent second core. Redeposited Eemian foraminifera and pollen occur in the Middle Weichselian sediments. This is interpreted as a result of reworking by an active Middle Weichselian ice present in, or advancing from, a northeasterly to easterly direction. During a second phase the Middle Weichselian sediments were compacted, probably a result of overriding by an ice from the northeast during the Middle Weichselian and/or the Late Weichselian Maximum. The Middle Weichselian sequence is overlain by a Holocene sequence which, in turn, is overlain by an admixture of Middle Weichselian and Holocene sediments. This mixing may be a result of tectonic activity some time between 7300 and 1000 BP. The core ends in Holocene fine sediments representing the last c. 1000 years.  相似文献   

8.
Burial Lake in northwest Alaska records changes in water level and regional vegetation since ∼ 39,000 cal yr BP based on terrestrial macrofossil AMS radiocarbon dates. A sedimentary unconformity is dated between 34,800 and 23,200 cal yr BP. During all or some of this period there was a hiatus in deposition indicating a major drop in lake level and deflation of lacustrine sediments. MIS 3 vegetation was herb-shrub tundra; more xeric graminoid-herb tundra developed after 23,200 cal yr BP. The tundra gradually became more mesic after 17,000 cal yr BP. Expansions of Salix then Betula, at 15,000 and 14,000 cal yr BP, respectively, are coincident with a major rise in lake level marked by increasing fine-grained sediment and higher organic matter content. Several sites in the region display disrupted sedimentation and probable hiatuses during the last glacial maximum (LGM); together regional data indicate an arid interval prior to and during the LGM and continued low moisture levels until ∼ 15,000 cal yr BP. AMS 14C dates from Burial Lake are approximately synchronous with AMS 14C dates reported for the Betula expansion at nearby sites and sites across northern Alaska, but 1000-2000 yr younger than bulk-sediment dates.  相似文献   

9.
Previous radiocarbon ages of detrital moss fragments in basal organic sediments of Lake Emma indicated that extensive deglaciation of the San Juan Mountains occurred prior to 14,900 yr B.P. (Carrara et al., 1984). Paleoecological analyses of insect and plant macrofossils from these basal sediments cast doubt on the reliability of the radiocarbon ages. Subsequent accelerator radiocarbon dates of insect fossils and wood fragments indicate an early Holocene age, rather than a late Pleistocene age, for the basal sediments of Lake Emma. These new radiocarbon ages suggest that by at least 10,000 yr B.P. deglaciation of the San Juan Mountains was complete. The insect and plant macrofossils from the basal organic sediments indicate a higher-than-present treeline during the early Holocene. The insect assemblages consisted of about 30% bark beetles, which contrasts markedly with the composition of insects from modern lake sediments and modern specimens collected in the Lake Emma cirque, in which bark beetles comprise only about 3% of the assemblages. In addition, in the fossil assemblages there were a number of flightless insect species (not subject to upslope transport by wind) indicative of coniferous forest environments. These insects were likewise absent in the modern assemblage.  相似文献   

10.
Molluscs, ostracodes, diatoms, pollen, plant macrofossils, peat, and wood have been found in glacial Lake Algonquin sediments, and estuarine-alluvial sediments of the same age, in southern Ontario. Molluscs and ostracodes are particularly abundant and widespread. Pollen analysis of Lake Algonquin sediments, bogs on the Algonquin terrace, and upland bogs above the Algonquin terrace, indicate that Lake Algonquin was still in existence at the time of the spruce-pine pollen transition, previously dated at an average of 10,600 yr BP at a number of sites in Michigan, Ohio, and southern Ontario. Wood in estuarine-alluvial sediments graded to the Algonquin level is of similar radiocarbon age. Evidence from several sites in the eastern Great Lakes area suggests the presence of a preceding low-water stage (Kirkfield outlet stage); drowned and alluviated valleys and fining-upward sediment sequences have been identified in this study as further supporting evidence. Lake Algonquin drained from the southern sites by isostatic tilting and eventual opening of the “North Bay outlet” some time shortly after 10,400 yr BP.Our radiocarbon dates suggest the low-water stage has an age of about 11,000 yr BP, and that Lake Algonquin drained 10,000–15,000 y. a. Dates previously published for the Lake Michigan basin are generally too young in comparison with ours, and dates on the Champlain Sea are generally too old. More critical evaluation of all dating results is desirable.From fossil remains we suggest a rapidly expanding fauna in the waters of Lake Algonquin. The spruce pollen period was a time of rapid faunal and floral migration, when the ice front was retreating from Kirkfield to North Bay, Ontario. Diversity of some species and fossil numbers increased substantially at the transition from spruce to pine just before Lake Algonquin drained.  相似文献   

11.
Radiocarbon dates are described from a section through Lateglacial and early Flandrian sediments at Llanilid, Mid-Glamorgan, South Wales. Comparisons between age determinations on the alkali soluble (humic) and alkali insoluble (humin) organic fractions from 12 biostratigraphic horizons reveal the extent of contamination by both older and younger carbon residues. The Llanilid time-scale suggests that for the Lateglacial, the earliest organic sediments date from around 13 200 yr BP, the early Interstadial Juniperus maximum occurred at ca. 12 400-12 500 yr BP with a marked decline some 200 years later, the main Betula phase lasted only from ca. 11 700 to 11 400 yr BP and the end of the Interstadial occurred around 11 100 yr BP. The beginning of the Flandrian dates from ca. 10 000 yr BP, the Juniperus maximum occurred approximately 200 years later, the expansion of birch woodland began around 9600 yr BP, while the first hazel arrived in the area at ca. 9300 yr BP. These age determinations are discussed in the context of radiocarbon dates from comparable biostratigraphic horizons in western Britain and the dating of Lateglacial events in the ocean core records from the North Atlantic.  相似文献   

12.
Two sediment cores were studied from Comprido Lake, a black water floodplain lake located near Monte Alegre City, Eastern Amazonian Basin. The total organic carbon (TOC), nitrogen content (TN), δ13CTOC, sedimentary chlorophyll, diatom record and mineralogical composition revealed different hydrological and climatic regimes during the Holocene. Between 10,300 and 7800 cal yr BP, a dry climate was suggested by low values of TOC and chlorophyll derivatives concentrations that are related to the development of a C4 grasses on unflooded mud banks. A gap in sedimentation due to a complete dryness of the lake occurred between 7800 and 3000 cal years BP corresponding to the Middle Holocene dry phase. From 3000 cal years BP onwards a gradual increase of the TOC, chlorophyll derivatives and Aulacoseira sp. suggest an increase in the productivity and in water lake level due to the high water flow of the Amazon River and the catchment area as well. The Comprido Lake record indicates that the Late Holocene in this region was characterized by a wetter climate, as also observed in other records of the Amazonian Basin.  相似文献   

13.
The late Pleniglacial and Late-glacial Maas valley, south of Nijmegen, contains four terraces. Three river systems are described based on the morphology of channel scars on these terrace surfaces and by sediment characteristics. The River Maas reacted to climatic warming at the start of the Weichselian Late-glacial by changing its river system slowly, from a braided system to a transitional phase between braiding and meandering and finally to a highly sinuous meandering system. The Maas reacted rapidly to the Younger Dryas climate deterioration by again establishing a braiding system. At the onset of the Holocene, the river changed abruptly to a meandering river without a transitional phase. The triggering factor for change in the Maas river pattern is almost certainly the changing climate in the Late Glacial. Gradient lines on the terrace surfaces show that tectonic activity did not modify the morphology of the channels. A division of the terraces is shown, the morphological, sedimentological and petrographical characteristics are presented and the linking of changing fluvial patterns with climatic changes or tectonic movements is discussed. © 1997 John Wiley & Sons, Ltd.  相似文献   

14.
Neptune's Cave in the Velfjord–Tosenfjord area of Nordland, Norway is described, together with its various organic deposits. Samples of attached barnacles, loose marine molluscs, animal bones and organic sediments were dated, with radiocarbon ages of 9840 ± 90 and 9570 ± 80 yr BP being derived for the barnacles and molluscs, based on the superseded but locally used marine reservoir age of 440 years. A growth temperature of c . 7.5 °C in undiluted seawater is deduced from the δ13C and δ18O values of both types of marine shell, which is consistent with their early Holocene age. From the dates, and an assessment of local Holocene uplift and Weichselian deglaciation, a scenario is constructed that could explain the situation and condition of the various deposits. The analysis uses assumed local isobases and sea-level curve to give results: that are consistent with previous data, that equate the demise of the barnacles to the collapse of a tidewater glacier in Tosenfjord, and that constrain the minimum extent of local Holocene uplift. An elk fell into the cave in the mid-Holocene at 5100 ± 70 yr BP, after which a much later single 'bog-burst' event at 1780 ± 70 yr BP could explain the transport of the various loose deposits further into the cave.  相似文献   

15.
The Weichselian Late Pleniglacial, Lateglacial and Holocene fluvial history of the middle Tisza valley in Hungary has been compared with other river systems in West and Central Europe, enabling us to define local and regional forcing factors in fluvial system change. Four Weichselian to Holocene floodplain generations, differing in palaeochannel characteristics and elevation, were defined by geomorphological analysis. Coring transects enabled the construction of the channel geometry and fluvial architecture. Pollen analysis of the fine-grained deposits has determined the vegetation development over time and, for the first time, a bio(chrono)stratigraphic framework for the changes in the fluvial system. Radiocarbon dating has provided an absolute chronology; however, the results are problematic due to the partly reworked character of the organic material in the loamy sediments. During the Late Pleniglacial, aggradation by a braided precursor system of the Tisza and local deflation and dune formation took place in a steppe or open coniferous forest landscape. A channel pattern change from braided to large-scale meandering and gradual incision occurred during the Late Pleniglacial or start of the Lateglacial, due to climate warming and climate-related boreal forest development, leading to lower stream power and lower sediment supply, although bank-full discharges were still high. Alternatively, this fluvial change might reflect the tectonically induced avulsion of the River Tisza into the area. The climatic deterioration of the Younger Dryas Stadial, frequently registered by fluvial system changes along the North Atlantic margin, is not reflected in the middle Tisza valley and meandering persisted. The Lateglacial to Holocene climatic warming resulted in the growth of deciduous forest and channel incision and a prominent terrace scarp developed. The Holocene floodplain was formed by laterally migrating smaller meandering channels reflecting lower bank-full discharges. Intra-Holocene river changes have not been observed.  相似文献   

16.
A Holocene lake sediment record is presented from Lake N14 situated on Angissoq Island 15 km off the main coast of southern Greenland. The palaeoclimatic development has been interpreted on the basis of flux and percentage content of biogenic silica, clastic material, organic material and sulphur as well as sedimentation rate, moss content and magnetic susceptibility. A total of 43 radiocarbon dates has ensured a reliable chronology. It is argued that varying sediment composition mainly reflects changing precipitation. By analogy with the present meteorological conditions in southern Greenland, Holocene climate development is inferred. Between 11 550 and 9300 cal. yr BP temperature and precipitation increase markedly, but this period is climatically unstable. From 9300 yr BP conditions become more stable and a Holocene climatic optimum, characterised by warm and humid conditions, is observed from 8000 to 5000 cal. yr BP. From 4700 cal. yr BP the first signs of a climatic deterioration are observed, and from 3700 cal. yr BP the climate has become more dry and cold. Superimposed on the climatic long‐term trend is climate variability on a centennial time‐scale that increases in amplitude after 3700 cal. yr BP. A climatic scenario related to the strength and position of the Greenland high‐pressure cell and the Iceland low‐pressure cell is proposed to explain the Holocene centennial climate variability. A comparison of the Lake N14 record with a terrestrial as well as a marine record from the eastern North Atlantic Ocean suggests that the centennial climate variability was uniform over large areas at certain times. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

17.
The shoreline displacement history of the eastern James Bay lowlands in the last 7 ka has been investigated by means of AMS radiocarbon dating of sediments cored from wetlands. We present twelve radiocarbon dates on macrofossils from six sites spread along a gradient of increasing land age and elevation. Palynomorph analysis (pollen, spores, and dinoflagellate cysts) was used to define the isolation stratigraphy. During the last 7 ka the shoreline elevation has regressed at a decreasing rate. The rate of shoreline emergence was initially rapid (6. 5 m/ 100 yr) between 6850 and  6400 cal yr BP then slowed down to 1.4– 2 m/ 100 yr during the late Holocene. Examination of previous relative sea level data based upon mollusc shells reveals high levels of uncertainty that mask potential temporal variability.  相似文献   

18.
Palynological and paleontological investigations supported by the radiocarbon dates of the lacustrine sediments of two profiles from the temperate lake Saria Tal, in Naini Tal District, Kumaun Himalaya, have revealed the presence of a concealed fold at the region. The profile from bore cores represents the upper part of the Late Holocene and the profile from exposed sections from the Middle Holocene to the over middle part of the Late Holocene. The data generated from different investigations have uniformly indicated that the former profile represents normal superposition, while the latter represents the reverse order. The contemporary pollen as well as molluscan zones of both profiles are situated at different elevations but consist of similar bioremains – indicating continuation of the same strata in two profiles. The presence of reverse order of superposition, continuation of the same strata in two profiles at different elevations, and the orientation of biozones, have indicated that the revealed folding is of syncline type. The present study has also given an idea about the origin of this lake.  相似文献   

19.
Values of δ13C obtained from conventional bulk sediment radiocarbon dates encompassing the Pleistocene Holocene boundary have been compiled and plotted against 14C age. In all. 286 lake sediment dates from southern Sweden in the range 8.000 to 13.000 BP have been evaluated. A significant decrease in δ13C values, initiated shortly before 10.000 RP and amounting to 5%, is distinguished. This change is accompanied by increased limnic productivity. decreased erosive input and increased organic carbon content of the sediments. A probable explanation for the δ13C decline in organic material is decreased importance of dissolution of silicates at the transition to the Holocene. During the Late Weichselian. extensive weathering of exposed minerogenic material with subsequent input of bicarbonate to the lake water may have caused a relative enrichment of 13C in dissolved inorganic carbon. Furthermore, the early Holocene increase in terrestrial vegetation cover probably led to an increased supply of 13C depleted carbon dioxide to the lake water by root respiration. Altered limnic vegetation, presumably towards increased production of phytoplankton. could also have contributed to the observed decreasing δ13C trend. The importance of these processes compared to other possible influencing factors. mainly endogenic carbonate production and changes in the global carbon cycle. is discussed.  相似文献   

20.
The late‐glacial Bølling period was first identified by Johs. Iversen on the basis of pollen results from Lake Bølling Sø in Denmark. Because there were no radiocarbon dates from the sequence the Bølling Chronozone (12 000–13 000 14C yr BP) was later established on the basis of dates from other sites. A new project is reinvestigating the sediments from the Bølling Sø sequence with AMS radiocarbon dating and multiproxy analyses. Here we present results of AMS radiocarbon dating, macrofossil analyses, cladoceran analyses (Cladocera concentrations and chydorid ephippia) and Pediastrum analyses (concentrations). The AMS dates on land plant remains show that the lower part of the sequence is around 12 500 14C yr BP, and thus clearly pre‐dates the Allerød chronozone. However, construction of a chronology for the sequence was problematic, partly because of reworking of macroscopic plant remains. The climate ameliorated after glacial conditions to such an extent that growth of plants could begin at ca. 12 500 14C yr BP, but the results of multiproxy analyses show little evidence for a further warming period during the pre‐Allerød part of the sequence. Lake productivity was low, and tree birch rare or maybe absent. This may reflect widespread occurrence of dead ice, unstable soils, heavy in‐wash of minerogenic matter to the lake, resulting in turbid water and rapid sedimentation. The early pioneer vegetation was characterised by Salix polaris and Dryas octopetala, and by herbs. The Allerød Chronozone, and especially its initial part, appears to have been relatively warm but reduced cladoceran concentrations and increased proportion of chydorid ephippia suggest that climate cooled in the middle Allerød and that the late Allerød was colder than the early part. The early Younger Dryas was probably colder than the late Younger Dryas. Clear warming is apparent at the beginning of the Holocene, where the first macrofossil evidence of trees (Betula pubescens, Populus tremula) is found. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

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