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1.
Peat and organic rich sediments at coastal sites in extreme northwest Ireland have accumulated in a wide variety of environments, often strongly influenced by late Holocene changes in relative sea level and by geomorphic processes. A deep peat sequence on the coast of Aranmore Island accumulated initially in a lake and subsequently in a freshwater marsh environment. The long pollen record serves as a template for regional events. It extends over much of the Holocene and shows relatively high levels of Pinus pollen up to just before the disappearance of this taxon at c . 3600 BP. Coastal peat occurrences elsewhere are much thinner and have accumulated over shorter periods; they contain further evidence to show that coastal areas were well-wooded compared with today, and that Pinus was an important woodland component prior to c . 4000 BP. At sites in Gweebarra Bay intertidal peats record the closure of small estuaries by geomorphological events during the past 5000 years. Coastal sites at Ballyness, Clonmass, and Trawenagh display regressive stratigraphies ˜ minerogenic marine sediments are overlain by silty peats capped by highly organic freshwater peats. Basal radiocarbon dates range from 4500 to 3300 BP. The silty peats are interpreted as having formed in salt-marsh environments and contain distinctive pollen spectra, marked by high levels of Pinus and Compositae Liguliflorae pollen. The data suggest that relative sea level attained levels close to that of today by the mid-Holocene in this region. The pattern of relative sea-level change agrees well with that predicted by geophysical modelling.  相似文献   

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3.
Small mounds of peat rise several metres above the level of the water‐table at Melaleuca Inlet and Louisa Plains on the buttongrass plains in southwest Tasmania. Possible origins of the peat mounds have been explored by pollen analysis and radiocarbon dating of a set of samples taken from a vertical section of one peat mound at Melaleuca. The peat accumulation is entirely of Holocene age although the mound is underlain by sapric peats preserving a cold climate palynoflora of probable Late Pleistocene age. Peats at and near the base of the mound accumulated under a heath sedgeland during the earliest Holocene while after about 7630 a BP the peat‐forming vegetation was shrub‐dominated. The radiocarbon data indicate two main phases of overall peat accumulation, between 7630 and 5340 a BP (Middle Holocene) and between 4450 and 450 a BP (Late Holocene), that were interrupted by a wildfire which burnt into the surface peats. The maintenance of high surface and internal levels of moisture almost certainly was the critical factor behind the low incidence of in situ fires burning into the surface peats on the mound. The perennial influx of groundwater below the mound is a possible origin that fits well with our observations, although the expansion and contraction of soils cannot be discounted as an initiating factor. Enhanced nutrient input from birds may have helped promote growth in the peat‐forming communities. The data do not support the mounds being eroded remnants of a former blanket peat cover or being due to periglacial activity. The peat mounds of southwest Tasmania deserve maximum protection because of their rarity in the Australian landscape and, it seems, elsewhere.  相似文献   

4.
Stratigraphic analyses of peat composition, LOI, pollen, spores, macrofossils, charcoal and AMS ages are used to reconstruct the peatland. vegetation and climatic dynamics in the Pur-Taz region of western Siberia over 5000 years (9300-4500 BP). Section stratigraphy shows many changes from shallow lake sediment to different combinations of forestcd or open sedge, moss, and Equisetum fen and peatland environments. Macrofossil and pollen data indicate that Larix sibirica and Beth pubescens trees were the first to arrive, followed by Picea obovata . The dominance of Picea macrofossils 6000-5000 BP in the Pur-Taz peatland along with regional Picea pollen maxima indicate warmer conditions and movement of the spruce treeline northward at this time. The decline of pollen and macrofossils from all of these tree species in uppermost peats suggests a change in the environment less favorable for their growth, perhaps cooler tempratures and/or less moisture. Of major significance is the evidence for old ages of the uppermost peats in this area of Siberia, suggesting a real lack of peat accumulation in recent millennia or recent oxidation of uppermost peat.  相似文献   

5.
Radiocarbon-dating and analyses of fossil testate amoebae (Protozoa) have established changes in soil moisture conditions on the developing surface of a Sphagnum -dominated peatland near Emo in northwestern Ontario.
The distribution and composition of modern testate amoebae communities were studied from peatlands in the region of Ontario and Minnesota as a guide to interpreting fossil assemblages. Although the core spans all of the Holocene, fossil testate amoebae were recovered only from the part post-dating 6500 BP. Earliest testate amoebae assemblages associated with bryophytic and cyperaceous-rich fen peat are dominated by species in the genera Cyclopyxis and Centropyxis . By 5000 BP, Amphitrema Jraaum, Assulina muscorum, Heleopera sphugni and Hyalosphenia subjaoa become important species as Sphagnum-rich peat accumulated at the site. Present-day microtopographic differentiation probably developed during historic time when the site became progressively drier, as indicated by a change of Nebela griseola, N. militaris and Trigonopyxis arcula . Although it is possible to derive quantitative estimates of changing soil moisture conditions from testate amoebae, care should be taken in interpreting results, particularly from non-Sphagnum-rich peats, until more is learned about the distribution and ecology of modern faunas.  相似文献   

6.
Petrographic studies indicate that lateral variations in the decomposition levels of peat are associated with the predominantly occurring peat macerals. Source Rock Analyzer (SRA) results indicate lateral variation in peat organic matter types from type II to III and back again to type II, occurring laterally within the top 0-m to 0.5-m layer at the basin margin to the midsection and further towards the near-center areas of the peat dome. This variation is most likely caused by a combination of factors: (a) Horizontal zonation and lateral variation of the dominant species of plant assemblages (b) Fibric (marginal) peats and hemic to sapric peats associated with type II organic matter (kerogen). Sample organic matter (coal-equivalent kerogen) typing indicates that the relative abundance of phytoclasts and palynomorphs generally supports the organic matter classification obtained by the SRA method. Lateral variations in the peat organic matter types may support the lateral vegetation variation concept. The classification of peat organic matter types (interpreted from visual analyses of palynological slides) occurring from the basin periphery to the mid-section and further towards the basin center yields organic matter of type II to type III and mixed types II to III (coal kerogen-equivalent), respectively.  相似文献   

7.
Palynological studies undertaken in the mountainous regions of the northwest of the Iberian Peninsula are few in number and have been concerned largely with the second half of the Holocene. New pollen data from two Galician sierras, the Courel and Queixa Sierras, provide a 10000‐yr record of vegetation and climate change. In the Courel Sierra before 9750 yr BP, Laguna Lucenza (1420 m a.s.l.) reflects a period of open landscape covered by Poaceae and heliophilous plants, which may be correlated with the Younger Dryas. The onset of the Holocene is characterised by the expansion of oak woodland, prior to 9300 yr BP, following a short phase of birch along with the gradual decline of pine. The oak values reach a peak at 8350 ± 80 yr BP. Towards 8800 yr BP Corylus begins to expand, followed by Alnus (7500 yr BP) and Ulmus. During this period, the Fraga pollen assemblage (Queixa Sierra, 1360 m a.s.l.) indicates Betula woodland surrounding the site, masking the regional predominance of oak. After 5000 yr BP there is a gradual decrease in arboreal pollen values in both Sierras. Castanea appears in Laguna Lucenza (Courel Sierra) at 4075 ± 75 yr BP. There is widespread deforestation during the last 4000 yr. During this period the presence of large quantities of microcharcoal particles points to the occurrence of fire. The reduction in forest is associated with the arrival of cultivation at 4000 yr BP at low altitudes in the Queixa Sierra. At higher altitudes the first agricultural activity is dated at later than 2000 yr BP. This coincides with the first record of cereal cultivation at high altitude in the Courel Sierra. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

8.
Detailed palynological analyses, including pollen preservation and charcoal counts, and sedimentological analyses (loss-on-ignition, percentage dry weight, dry bulk density and humification), supported by 15 14C dates, were applied to a small upland (350 m OD) peat basin near Pitlochry, east-central Grampians, Scotland. Stratigraphical analyses and radiocarbon dating were also undertaken on the valley peat surrounding the basin. Organic sediment began to accumulate in the basin and on the valley floor at ca. 9800 yr BP. ‘Recurrence surfaces’ within the valley floor peat correlate with the end of a trend to increased aridity in the deep basin, from 8900 to 8500 yr BP. Pine may have colonised the hills, with birch, before the migration of hazel at 9150 yr BP. From around 8000 yr BP elm grew on the base-rich soils. Alder may have been present by 7000 yr BP but it did not expand until immediately after the first major anthropogenic disturbance, at 4800 yr BP. Above 8000 yr BP silt was commonly deposited in the basin by streams, but several phases of exceptionally intense inwashing are identified, and at least two of these correlate with periods of increased precipitation. Grazing pressure during the elm decline marks the beginning of a long history of generally low intensity pastoralism, interspersed with periods of heightened anthropogenic activity.  相似文献   

9.
The Holocene development of a treed palsa bog and a peat plateau bog, located near the railroad to Churchill in the Hudson Bay Lowlands of northeastern Manitoba, was traced using peat macrofossil and radiocarbon analyses. Both sites first developed as wet rich fens through paludification of forested uplands around 6800 cal. yr BP. Results show a 20th-century age for the palsa formation and repeated periods of permafrost aggradation and collapse at the peat plateau site during the late Holocene. This timing of permafrost dynamics corroborates well with that inferred from previous studies on other permafrost peatlands in the same region. The developmental history of the palsa and peat plateau bogs is similar to that of adjacent permafrost-free fens, except for the specific frost heave and collapse features associated with permafrost dynamics. Permafrost aggradation and degradation is ascribed to regional climatic, local autogenic and other factors. Particularly the very recent palsa development can be assessed in terms of climatic changes as inferred from meteorological data and surface hydrological changes related to construction of the railroad. The results indicate that cold years with limited snowfall as well as altered drainage patterns associated with infrastructure development may have contributed to the recent palsa formation.  相似文献   

10.
This is the first of a series of articles presenting the results of a multi-proxy investigation aimed at reconstructing changes in the ecosystem and climate of Whitrig Bog, SE Scotland, during the last glacial-inter-glacial transition (Devensian Lateglacial, c . 14–10 ka BP). We present here the results from sediment lithology, chemistry, pollen, and plant macrofossil analyses. These data are used to infer the nature of the local catchment soils and both local and regional terrestrial vegetation. The interstadial period ( c . 13–11 ka BP) is characterized by a successional sequence developing from a landscape with bare, poorly developed minerogenic soils supporting a sparse herbaceous flora into open birch woodland with juniper scrub and stable organic soils. At c . 11 ka BP the Younger Dryas climatic cooling event caused an abrupt reversion to an open herbaceous arctic/alpine flora (e.g. macrofossil evidence of Silene furcata and Oxyria digyna ) and high levels of minerogenic erosion into the basin, indicating environmental response to a cold Arctic climate. In addition to this Younger Dryas climatic reversal, two lesser reversion episodes occurred earlier during the interstadial. The more pronounced of the two, late in the intersladial, is characterized by high levels of erosion and a change from birch/juniper woodland to an open herbaceous flora. The older oscillation occurs approximately mid-way through the interstadial sequence and is marked by similar pollen changes, albeit shorter lived and more subtle.  相似文献   

11.
Cores from five high alpine basins in the northern San Juan Mountains show several fluctuations in lithology. Typically, peats are interbedded with coarser clastic sediments or else woody peats alternate with fibrous peat. Twenty 14C dates provide radiometric control. Sediment rates averaged about 2.5 cm/100 yr but varied at the different sites between 1.19 and 50 cm/100 yr. Rates were lower during the middle of the Holocene. Basal radiocarbon dates indicate that these high (ca. 3600 m a.s.l.) northeasterly facing cirques were icefree by 9000 BP. There is some evidence in the cores for a short climatic reversal sometime between 8000 and 7000 BP. A major change occurred in the high basins very close to 5000 BP and thereafter there are several intervals of increased clastic sedimentation which may be related to Neoglacial climatic fluctuations. Analysis of a 2.15 m core near Hurricane Basin indicates significant fluctuation of pollen and macrofossils occurred during the 9000 ± year record. The Picea/Pinus ratios are used to delimit changes in the apparent elevation of the site: the ratios indicate that a short drop of “treeline” occurred about 8000 BP and then remained near present level until about ≥1800 BP when the apparent elevation of the site rose. Macrofossils indicate that spruce was present in the Hurricane Basin (and others) at specific periods and confirms the general results of the Picea/Pinus ratios. The San Juan Mountains do not possess a glacial Neoglacial record but the stratigraphy of these high cirque basins can be used to define glacial stades (cf. Jardine, 1972). The interpreted climatic response record on vegetation and sediment flux has both similarities and differences from other records in the western mountains of North America.  相似文献   

12.
In general, mires develop by autogenic succession from more groundwater‐fed to more rainwater‐fed. This study from a calcareous mire in the West Carpathians (Slovakia) describes a similar development in the Early Holocene, followed by a reverse development in the Middle and Late Holocene. Pollen, macrofossil and testate amoeba analyses show that the site started as a minerotrophic open fen woodland. After 10 700 cal a BP autogenic succession led to the accumulation of at least 1 m of Sphagnum fuscum peat. Around 9000 cal a BP, as climate could no longer sustain a stable water regime, the bog desiccated and a fire broke out. The fire removed part of the peat layer and as a consequence relative water levels rose, leading to the establishment of a wet minerotrophic swamp carr with Thelypteris palustris, Equisetum sp. and Alnus sp. with extremely slow peat accumulation. After 600 cal a BP, rapid peat accumulation with calcareous tufa formation resumed as a result of anthropogenic deforestation and hydrological changes in the catchment and resulting increased groundwater discharge. At present the mire still hosts a wealth of relict and endangered plant and animal species typical of calcareous fens and fen meadows. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

13.
A combined pollen, charcoal and climatic record is presented from Cranes Moor, southern England, covering the period c. 10 500–5850 cal a BP. It is shown that the occurrence of burning is closely related to natural processes, including prevailing climatic conditions and vegetation composition. These burning events are often linked to an increase in the summer moisture deficit, implying that the timing of burning events is linked to periods of warmer/drier climate during the Holocene Thermal Maximum (c. 11 000–5000 cal a BP). These events play an important role in the vegetation composition and succession around the site. The nature of the burning recorded at the site shows strong similarities with other records from northern Europe. This study throws caution on suggestions that fire in the Holocene record from areas such as the British Isles is linked only to human activity, and enhances the possibility that natural fire incidence played an important role in natural woodland structure dynamics.  相似文献   

14.
At Pedregal, more than 40 m of sediments are exposed within a ‘fan complex’ formed between lateral moraines of the adjacent Mucuchache and El Caballo valleys. Early and late Mérida (Wisconsinan) glaciations are represented by till and till plus proglacial sediments, respectively. A middle Wisconsinan interstadial event, here termed the Pedregal interstade, began at the end of the Early Mérida glaciation at approximately 60 ka BP. Following the retreat of ice from the small Pedregal Basin, a lake formed when the local drainage was blocked due to movement of the Mesa de Caballo along the Boconó Fault. Shallow lake or no-lake phases lasted approximately a few hundred to, at most, 2000 years, and each lake phase was marked by peat accumulation. Four of seven peats identified formed during sufficiently long intervals for soil profiles (incipient to mature Spodosols) also to develop. The Spodosol with the strongest development (Eb/Bsb/Coxb/Cub horizons) is found adjacent to the lowest peat and reflects ongoing early Mérida stadial (MIS 3) conditions; the youngest peats, associated with weak podzolic soils (Eb/Bsb horizons), formed under slightly warmer interstadial conditions, presumably with less soil water. Cyclic lacustrine deposition is related to lake level and relative depth fluctuations, due in part to variable shoreline/delta progradation and shallowing as the lake deepened in general. Whereas final drainage of the lake is related to movement of the Boconó Fault and breach of the moraines that form the Mesa de Caballo, earlier lake level fluctuations appear related to climate change. Radiocarbon dating of the peats suggests they are related to warmer periods and may tentatively correlate with small ‘interstadials’ or ‘D-O events’ detected in the oxygen-isotope record of Greenland ice cores and North Atlantic marine sediments.  相似文献   

15.
The Holocene environmental history of the eastern slope of the Polar Ural Mountains has been reconstructed using pollen, spores, algae and other microfossils from the Chernaya Gorka palsa section (67°05'N, 65°21'E, 170 m a.s.l.). An initial oligotrophic lake was formed at the study site c. 9800-9500 14C yr BP. Although tundra communities dominated the vegetation in the area, birch and larch trees might have grown at lower elevations. Dry and disturbed soil habitats also occurred around the lake. Algae (mostly Pediastrum and Botryococcus) started to expand in the lake as climate gradually improved after c. 9500 14C yr BP. However, the role of mosses (mostly Calliergon and Drepanocladus) was most important for the infilling of the lake basin. Increased temperatures and subsequent improvement of hydrological conditions resulted in vegetation changes: stands of willows developed rapidly and the role of tree birch in the local vegetation increased. The lake was completely filled at c. 8600 14C yr BP. Peat accumulation started with Bryales mosses and, later, Sphagnum became dominant. Stands of Larix, Picea and Betula became well developed during the Boreal climate optimum. Tree birch began to spread into the tundra. Different Bryales mosses formed peat c. 8000-6500 14C yr BP. Cyperaceae later became the main peat-forming element. Dense spruce canopies with Larix sibirica and Betula pubescens surrounded the study site during the Atlantic period, pointing to the warmest climate during the Holocene. Summer temperatures might have been up to 3-4°C higher than today. However, a decline of spruce and an increase of birch around 6700-6300 14C yr BP may reflect some climate deterioration. There are no dated deposits younger than 6000 14C yr BP. It is assumed that Subboreal climate deterioration resulted in the development of permafrost and formation of the palsa at the site. The deposits, now protruding above the surrounding terrain, were eroded by wind, water and cryogenic processes.  相似文献   

16.
The South Sumatra basin is among the most important coal producing basins in Indonesia. Results of an organic petrography study on coals from Tanjung Enim, South Sumatra Basin are reported. The studied low rank coals have a mean random huminite reflectance between 0.35% and 0.46% and are dominated by huminite (34.6–94.6 vol.%). Less abundant are liptinite (4.0–61.4 vol.%) and inertinite (0.2–43.9 vol.%). Minerals are found only in small amounts (0–2 vol.%); mostly as iron sulfide.Based on maceral assemblages, the coals can be grouped into five classes: (1) humotelinite-rich group, (2) humodetrinite-rich group, (3) humocollinite-rich group, (4) inertinite-rich group and (5) humodetrinite–liptinite-rich group. Comparing the distribution of maceral assemblages to the maceral or pre-maceral assemblages in modern tropical domed peat in Indonesia reveals many similarities. The basal section of the studied coal seams is represented typically by the humodetrinite–liptinite-rich group. This section might be derived from sapric or fine hemic peat often occurring at the base of modern peats. The middle section of the seams is characterized by humotelinite-rich and humocollinite-rich groups. The precursors of these groups were hemic and fine hemic peats. The top section of the coal seams is typically represented by the humodetrinite-rich or inertinite-rich group. These groups are the counterparts of fibric peat at the top of the modern peats. The sequence of maceral assemblages thus represents the change of topogenous to ombrogenous peat and the development of a raised peat bog.A comparison between the result of detailed maceral assemblage analysis and the paleodepositional environment as established from coal maceral ratio calculation indicates that the use of coal maceral ratio diagrams developed for other coal deposits fails to deduce paleo-peat development for these young tropical coals. In particular, mineral distribution and composition should not be neglected in coal facies interpretations.  相似文献   

17.
Forested peatlands are widespread in boreal regions of Canada, and these ecosystems, which are major terrestrial carbon sinks, are undergoing significant transformations linked to climate change, fires and human activities. This study targets millennial‐scale vegetation dynamics and related hydrological variability in forested peatlands of the Clay Belt south of James Bay, eastern Canada, using palaeoecological data. Changes in peatland vegetation communities were reconstructed using plant macrofossil analyses, and variations in water‐table depths were inferred using testate amoeba analyses. High‐resolution analyses of macroscopic charcoal >0.5 mm were used to reconstruct local fire history. Our data showed two successional pathways towards the development of present‐day forested peatlands influenced by autogenic processes such as vertical peat growth and related drying, and allogenic factors such as the occurrence of local fires. The oldest documented peatland initiated in a wet rich fen around 8000 cal. a BP shortly after land emergence and transformed into a drier forested bog rapidly after peat inception that persisted over millennia. In the second site, peat started to accumulate from ~5200 cal. a BP over a mesic coniferous forest that shifted into a wet forested peatland following a fire that partially consumed the organic layer ~4600 cal. a BP. The charcoal records show that fires rarely occurred in these peatlands, but they have favoured the process of forest paludification and influenced successional trajectories over millennia. The macrofossil data suggest that Picea mariana (black spruce) persisted on the peatlands throughout their development, although there were periods of more open canopy due to local fires in some cases. This study brings new understanding on the natural variability of boreal forested peatlands which may help predict their response to future changes in climate, fire regimes and anthropogenic disturbances.  相似文献   

18.
In 2008, the very extensive tropical peats were estimated to be about 182 million ha spanning South America, Asia and Africa. About 20.3%(36.9 million ha) of this area exist in Asia. Peats are classified based on their degree of decomposition, namely Fibrists, Hemists, Saprists and Folists. This makes them different in characteristics. The activities of microorganisms vary in different types of peat due to, for example, the sapric layer of well humified peat can provide water and food to microorganisms during heat stress. In another scenario, deeper peat is older and typically has lower levels of labile carbon to provide substrate for microbes compared to surface peat. A complete understanding of the microbial communities in different layers of peat is essential as microorganisms play major roles in peat decomposition and are important to ecosystem processes. These peats are a very important global carbon(C)store or reserve and could severely impact climate change if not managed well. Peatlands can store as much as 40 to 90 Gt C. Mis-management of peats could severely impact the environment particularly the emission of carbon into the atmosphere. For instance, clearing of peatlands using fire has been reported to release an estimated 88 t C ha~(-1) to the atmosphere. There are several factors which influence the environmental consequences of tropical peat especially in relation to climate change. The main influences are:(i) changes in temperature,(ii) changes in precipitation or rainfall,(iii) changes in atmospheric composition, and(iv) fire and haze. This paper is a brief review on these four influences in relation to climate change. It is apparent from the brief review that there is a need for continued short and long-term research to better understand tropical peats and how they affect our climate. This will hopefully provide the basis for predicting better what could happen under various scenarios.  相似文献   

19.
Pollen and plant macrofossil analyses from Svanåvatnet in northern Norway provide records of past vegetation and climate in this region from c . 8700 cal. yr BP until the present. Pollen accumulation rates and the presence of plant macrofossils indicate that Betula pubescens (birch) was present from c . 8600 cal. yr BP and Pinus sylvestris (pine) from c . 8200 cal. yr BP. Quantitative climate is reconstructed using modern pollen-climate transfer functions based on weighted-averaging partial least squares regression. A rapid increase in mean July temperature (Tjul) and mean annual precipitation (Pann) is inferred for the early Holocene. At times when tree abundance is at its highest and most diverse, inferred Tjul indicates maximum temperatures during the mid-Holocene of about 2°C warmer than at present. During the same time period, inferred Pann is 200–300 mm above present-day conditions until c . 3000 cal. yr BP. Mean January temperatures (Tjan) are reconstructed to be about 2°C warmer than today from 8000 to 3500 cal. yr BP. After 3500 cal. yr BP until today, a gradual decrease is seen in all the reconstructed climate parameters, together with a reduction in tree abundance and the development of a mosaic of open vegetation with grasses, dwarf shrubs and wet areas, and of woodland containing B. pubescens , P. sylvestris and Picea abies (spruce).  相似文献   

20.
Pollen analysis, 14C datings and peat stratigraphy from blanket mires overlying two of the six drumlins in the Momyr area NW of the Trondheimsfjord in Sør-Trøndelag county are presented in order to trace the peat development and vegetational history. 14C datings of the mineral soil/peat transition in 9 of the II profiles indicate that peat formation started about 7,800 years ago on the drumlin plateaux which at that time had a vegetation of scattered birch trees. From the plateaux the peat formation spread slowly down the slopes. Eventually, the mire surface bore a cover of pines which disappeared about 4,900 years ago, at the same time as peat formation commenced at the foot of the drumlins. This was probably a result of a change in the water table and onset of erosion in the already existing peat on the drumlin plateaux. During a period of about 1,000 years this new peat buried a dense birch vegetation existing at the lower part of the drumlins. Peat growth then spread upslope, and the drumlins' overall blanket of peat was completed well before the Subatlantic chronozone.  相似文献   

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