The wetland dynamics in eastern Amazon region during the last thousand years were studied by pollen analysis and AMS radiocarbon date. Three sediment cores were sampled from the Marajó Island, which is located at the mouth of the Amazon River. Marajó Island is covered mainly by Amazon coastal forest, herbaceous and varzea vegetation. One core was obtained from Lake Arari, which is surrounded by herbaceous vegetation flooded by freshwater. Two cores were sampled from an herbaceous plain that lies 45 km southeast from the Lake Arari. The pollen records indicate that the herbaceous vegetation began its development approximately 800 cal yr B.P. The pollen percentages between 750 and 500 cal yr B.P. indicate the expansion of mangroves. The predominance of herbaceous and varzea pollen occurs between 500 and 200 cal yr B.P. and the varzea pollen percentage increased during the last 200 years. The vegetation changes proposed by the pollen records from the Lake Arari, probably, represent the regional vegetation dynamics of the Marajó Island, while the pollen profiles from herbaceous plain present a smaller spatial representativeness of the vegetation. The alternation between fresh and brackish water vegetation in the wetlands may indicate changes in the physicochemical characteristics of the sediment during the time interval studied.
On the high volcanoes of the eastern Codillera Neovolcánica of Mexico, a glacier advance occurred around 12,000 B.P. which is characterized by a short duration (less than 200 years). According to the investigations of the moraines and other glacial and fluvial sediments, this 12,000 B.P. glaciation was caused by intense precipitation rather than by a temperature decrease. A comparison of the late Quaternary climatic history of Mexico (ca. 14,000 to 10,000 B.P.) with recent results from North America, Europe, and other parts of the world yield new insights concerning the interpretation of the ‘12-ka event’ of the Gulf of Mexico and of the Older Dryas climatic fluctuation of Europe.ZusammenfassungAn den hohen Vulkanen der Cordillera Neovolcánica Mexikos kam es um 12000 B.P. zu einem Gletschervorstoß, der sich durch seine kurze Dauer (weniger als 200 Jahre) auszeichnet. Aufgrund der Untersuchungen von Moränen und anderer glazialer und fluvialer Sedimente wird die Ursache des 12000 B.P.-Gletschervorstoßes in heftigen Niederschlägen und nicht in einer Temperatursenkung gesehen. Die spätquartäre Klimaentwicklung Mexikos (ca. 14000 bis 10000 B.P.) zeigt im Vergleich mit jüngsten Befunden aus Nordamerika, Europa und anderen Teilen der Erde, daß hinsichtlich des sog. ‘12-ka event’ des Golfs von Mexiko und der Klimaschwankung der Alteren Dryaszeit Europas neue Interpretationen möglich sind.
Because of the limitations associated with traditional methods of measuring rates of soil erosion, such as erosion plots, the fallout radionuclide caesium-137 (137Cs) has been increasingly used in recent years as an alternative approach to estimating rates of soil redistribution on both cultivated and noncultivated areas. The successful application of the 137Cs approach depends heavily on the availability of reliable conversion models for converting measurements of 137Cs redistribution, relative to the local reference inventory, to estimates of soil redistribution rates. In the absence of empirical conversion models, most studies have made use of theoretical conversion models. The assumptions made by such theoretical models are frequently untested and they thus remain largely unvalidated. This contribution describes the results of a measurement programme involving nine experimental plots located in southern Italy, aimed at validating several of the basic assumptions commonly associated with the use of mass balance models for estimating rates of soil redistribution on cultivated land from 137Cs measurements. Overall, the results confirm the general validity of these assumptions. However, several other assumptions and process representations incorporated into such models still require testing and elucidation.
Buyukcekmece Reservoir, located in the western outskirts of Istanbul, is one of the major water resources of Istanbul, and supplies drinking water to about 4 million people. Erosion in the catchment of the reservoir is an important problem in terms of its longer-term sustainability for water supply. There is an urgent need to obtain reliable quantitative data regarding erosion and deposition rates within the catchment to assess the magnitude of the problem and to plan catchment management strategies. In the absence of existing data, attention has focussed on the potential for using 137Cs measurements to provide retrospective estimates of medium-term soil erosion rates within the catchment over the past ca. 40 years. To date, the 137Cs approach has not been used to document soil redistribution rates in Turkey and this contribution reports an attempt to confirm the viability of the approach and the results of a preliminary investigation of rates of soil loss from uncultivated areas within the catchment. The soil redistribution rates estimated using the profile distribution conversion model varied from − 16.11 (erosion) to 4.59 (deposition) t/ha/year.
The paper describes the application of caesium-137 measurements for estimating soil erosion rates in a small catchment in the Three Gorges region of the Yangtze River, China. The construction of the Three Gorges Dam has drawn attention to the impact of erosion and sedimentation, but there are relatively few quantitative estimates of sediment transfer for this area. The suitability of the fallout radionuclide, caesium-137, for the rapid appraisal of soil redistribution in the steep and dissected terraced landscape of the Three Gorges is investigated here. Previous applications of the technique in Chinese agricultural environments have indicated the difficulty of obtaining reliable baseline fallout estimation. The integration of monthly rainfall data with a model of global strontium-90 fallout is developed to provide an independent estimate of baseline fallout, which is consistent with field measurements. The method also enables a mass balance model of caesium-137 mobility to be calibrated. Mean annual soil loss during the last four decades is estimated at 4500 t km−2 yr−1. Erosion rates are strongly related to field slope angles but highly variable spatially.
Sediment that is transported fluvially is subject to the processes of sorting, abrasion and breakage. Each of these processes may affect the properties used to determine the origin of the sediment; so they should be evaluated in sediment tracer studies. As part of a study to determine the dominant sources of sediment in Tarago Reservoir, southeastern Victoria, Australia, the effects of disaggregation and grain abrasion on the distribution of with respect to particle size fractions were studied in soils derived from the three different rock types of the Tarago catchment. The main action of abrasion over short distances is aggregate breakage. In each soil, the material produced by abrasion had approximately the same concentration as the corresponding size fractions of unabraded soil. The <40 μm size fraction, which is the dominant sediment size in the Tarago Reservoir, includes material generated by partial disaggregation and grain abrasion as well as unabraded soil. Consequently, determining the sources of sediment in the Tarago Reservoir using is unlikely to be significantly affected by grain abrasion or disaggregation. These results improve the confidence with which can be used to trace the origin of sediment at other sites.
The uplifting of the Himalayas has resulted in the development of a three-tiered landform in Southern China, the Yunnan–Guizhou Plateau is the middle tier of this landform on the eastern slope of the Himalayas. Lake Hongfeng and Lake Baihua in central Guizhou are about 1000 km away from Lake Erhai in western Yunnan. Sediment cores were collected from Lakes Erhai and Lugu in west Yunnan and Lakes Hongfeng and Baihua in central Guizhou, along with 28 surface soil cores from the watersheds of these lakes. The accumulation characteristics of 7xxxxBe and 137yyyyCs show that: (1) 137Cs activities in the soil cores of central Guizhou were higher than that in the western Yunnan. The activities and maximum penetrative depth of 7Be were similar for the two regions. The activity ratio of 7Be/137Cs in surface soil reached 100–1000 in western Yunnan, but only 10–100 for central Guizhou region. (2) 7Be inventories in soil cores at most sampling sites of central Guizhou were low, reflecting severe soil erosion. 7Be inventories in the soil cores of Lake Lugu watershed in summer–autumn were higher than that in Lake Hongfeng/Baihua watershed and also higher than that in Lake Erhai watershed. This could be related to high 7Be precipitation in summer and altitude. (3) 7Be inventories in sediment cores of Lake Erhai, Lake Lugu and Lake Baihua were 237±73, 322±19 and 783±44 Bq m−2, respectively. Based on activity values that were corrected to deposition, prior to 1986, 137Cs inventories in these lakes were 519±26, 937±13 and 3704±56 Bq m−2, respectively. (4) Modeling indicated that 7Be and 137Cs accumulations in Lake Hongfeng and Lake Baihua were dominated by watershed erosion, and controlled by the retention factor in Lake Erhai and Lake Lugu, but the 137Cs inventory ratios obtained in the sediment core and from direct atmospheric fallout were similar, and the ratios were smaller for 7Be. (5) 7Be inventories from direct atmospheric fallout in Lake Erhai, Lake Lugu and Lake Baihua were 0.07±0.02, 0.29±0.02 and 0.08±0.01 Bq m−2, respectively. Prior to 1986, 137Cs inventories were 0.11±0.01, 0.22±0.01 and 0.37±0.01 Bq m−2, respectively, showing that 137Cs deposition has a regional difference. This phenomenon may reflect that uplift of the Himalayas has a screening effect on the fallout of globally distributed atmospheric pollutants in western Yunnan, China.
Quantitative assessment of soil redistribution in landscapes remains a challenging task. In this study we used radioactive soil redistribution tracer 137Cs together with soil morphological characteristics and empirically-based modeling for quantitative assessment of long-term soil conservation effectiveness. Three pairs of arable slopes were selected, all located within the territory of the Novosil experimental station (the Orel Region, central European Russia). One slope in each pair undergone creation of artificial terraces with forest shelter belts located parallel to topography contour lines and spaced at approximately 100 m from each other.Preliminary results have shown that slopes with soil-protective measures are characterized by a 11–80% reduction of average soil redistribution rates, as shown by soil profile morphology and 137Cs methods. Discrepancy in values obtained can be attributed to differences in temporal resolution of methods as well as possible influence of individual extreme events on results yielded by the 137Cs method. On the other hand, more significant decrease in average soil degradation rates on slopes with soil conservation (62–75% for each pair of slopes) was predicted by the model.The 137Cs method overestimates gross and net soil redistribution rates, as a result of the influence of extreme erosion prior to tillage mixing of a fresh fallout isotope, not accounted for by calibration models used. Another shortcoming of the estimations obtained is that sediment redeposition directly within forest belts was not taken into account. Therefore, net erosion rates obtained for slopes with forest belts should be regarded as overestimation. Nevertheless, it can be generally concluded that the multi-technical approach has allowed acquiring much more detailed information on temporal and spatial variability of soil redistribution rates than single method-based studies.
Total soil erosion is the integrated result of all forms of soil erosion — wind, water and tillage. It has been recognized that in topographically complex landscapes, individual soil erosion processes and their interactions all contribute towards total soil erosion. In this study, two field sites, representing different landscapes in the northern region of the North American Great Plains, were examined. Water and tillage erosions were estimated using the established water and tillage erosion models and total soil erosion was estimated using the 137Cs technique.We determined that the patterns of water and tillage erosion across the landscapes are mainly dependent on topographic features and they are fundamentally different within topographically complex landscapes. On the slope of undulating landscapes, tillage and water erosion both contribute considerably to total soil erosion. On the knoll of hummocky landscapes, tillage erosion dominates the pattern of total soil erosion. Tested against the Cs measurements, the patterns of total soil erosion cannot be well estimated by water or tillage erosion model alone unless one of the two erosion processes predominate over the other erosion processes. Combining water and tillage erosion models generally provides better estimations of total soil erosion than the component models on their own. Most soil properties and crop yield were found to be closely correlated with total soil erosion. For a given erosion process, the soil erosion patterns estimated using different models with reasonable parameter settings were similar to each other. However, it is necessary to choose an optimal model and to obtain accurate parameters for the purpose of accurate assessments of the erosion rates.
The area effected by wind erosion in England is estimated to be small, but the magnitude of the problem within this area is unknown. Direct measurement of the process is difficult because of very high spatial and temporal variability, selectivity and its slow, insidious nature. The artificial radionuclide caesium-137 (137Cs), offers an alternative method. It was used here to estimate the net (ca. 35 year) soil flux (erosion and deposition) in a 5×5 km area (ca. 19 km2 area sampled) of East Anglia. It is the first study in the UK to investigate the continuous spatial variation of 137Cs over an area of such a size and one of only a few in the world to focus specifically on the redistribution of soil by wind. A two-stage, nested sampling frame captured approximately 50% of the variation that occurred between and within fields. A total of 148 samples were taken and their analysis was used to produce a variogram of the spatial variation. A spherical model was fitted to the experimental variogram using a weighted least-squares procedure. Simulations of sampling configurations on a regular grid did not provide a practical improvement to the nested sampling frame within the specified tolerance. Instead, the parameters were used in ordinary kriging to map 137Cs every 50 m at unsampled locations across the area. A tentative value for the newly established 137Cs reference inventory for the region was 2068±130 Bq m−2. Owens' mass–balance model of the relationship between 137Cs movement and soil redistribution was modified to the spatially distributed situation and also so that it could account better for the factors that control wind erosion and deposition (Towards improved interpretation of caesium-137 measurements in soil erosion studies. Unpublished PhD thesis, Exter University). A calibration relationship for each field was used to calculate the net soil flux at every 50 m. The sample region was found to approximately balance with a net soil loss of 0.6 t ha−1 year−1; the range was −32.6 to +37.5 t ha−1 year−1. Soil from the high-loss fields was accumulating in field boundaries. Despite little of the material leaving the region, the effect of soil nutrient loss on the fields may be considerable. It appears that wind erosion may have as great or greater impact in the areas where it is active than does water erosion. Net soil flux was inferred to be the result of wind erosion, but soil loss on harvested crops (primarily sugar beet), and perhaps losses during tillage (pulverizing erosion) may be other contributors, and this requires future study. Moreover, the quantity of soil lost may not be as important as its quality. Future work is therefore also required to investigate the mass–balance of soil nutrient enrichment and depletion as a consequence of soil erosion and fertiliser application.
Concentrations in the soil of anthropogenic and natural radionuclides have been investigated in order to assess the applicability of the 137Cs technique in an area of typical Mediterranean steep slopes. This technique can be used to estimate net soil redistribution rates but its potential in areas with shallow and stony soils on hard rock lithology have not been evaluated so far. In this research, the validity of using this technique in stony shallow soils at very steep slopes is discussed together with the relations between radionuclide concentrations and other soil properties, lithology, slope morphology and land use in a Mediterranean environment. Both natural Potassium-40 (40K), Uranium-238 (238U), Thorium-232 (232Th) and anthropogenic Caesium-137 (137Cs) radionuclides have been determined in samples taken along slope transects on uncultivated serpentinite soils and cultivated gneiss soils. In addition to the radionuclide concentrations, parameters such as slope position, slope angle, aspect, soil depth, surface stone cover, moss, litter, vegetation cover, soil crust, stone content and bulk density have been quantified.
Over the past years the Caesium-137 (137Cs) technique has been successfully applied in numerous environments all over the world. This technique is using the worldwide distribution of the anthropogenic 137Cs radionuclide and its redistribution associated with soil particles as an effective estimation of net soil-loss rates. In contrast to numerous studies on deep, often cultivated, clay soils with gentle to intermediate slopes, typical Mediterranean shallow stony soils on steep slopes have received less attention. In this paper, the landscape evolution over the past 37 years has been evaluated using the 137Cs technique for two lithological different areas under contrasting land use in the Álora case study area (Málaga, Spain). In soils on gneiss and serpentinite bedrock several transects have been selected on steep slopes up to 35° with mean soil depths from 37 cm for gneiss to 24 cm for serpentinite. Estimating net soil redistribution rates from radionuclide distributions depends on the calculation of the local area reference inventory and the used calibration technique. Several methods have been tested and final results were found to differ considerably. After careful parameter selection, the resulting net soil redistribution estimates for the different transects have been compared with simulations of a simple landscape evolution model, resulting in different possible scenarios of erosional response. Total net soil-loss for the study area range from 2.3±0.25 to 69.1±7.8 t ha−1 year−1 for serpentinite and gneiss slopes, respectively. Differences in total slope sediment budgets as well as differences along the transects reveal influences of landscape representation and land use. In this case the impact of soil translocation by tillage and resulting erosion rates are far more important than possible parent material induced differences. However, comparing the two sampled areas not only net rates but spatial patterns as well reveal important differences in distribution over the landscape of net erosion and net sedimentation zones.
Differences between measured 137Cs activity–depth profiles and idealised undisturbed profiles generated from an exponential model suggest that faunal turbation has redistributed 137Cs in mineral and organic upland soils in southern Scotland. Bioturbation is also demonstrated by the vertical displacement of other inputs to the soils of known age (non-native tree pollen and spheroidal carbonaceous particles, SCPs). The causes and mechanisms of bioturbation were further investigated by soil micromorphology. Well-drained mineral soils with active populations of earthworms are the most bioturbated, showing near-complete homogenisation to depths of about 20 cm. Enchytraeids also seem to remobilise 137Cs by the digestion of organic matter and may be the main cause of 137Cs redistribution in organic-rich upland soils. Relative rates of mixing are evaluated by comparing 137Cs depth profiles.
Artists have depicted soils in paintings since medieval times, and most landscape and soil paintings were made in the eighteenth century. The systematic study of soils started later and soil science was dominated by agricultural chemistry for most of the nineteenth century. Since the late 1800s soil profile drawings, paintings and photographs have been depicted in hundreds of textbooks. This paper traces the depiction of soil profiles in the scientific literature from the late 1700. The first soil profile depictions were simple diagrams illustrating different layers and soil processes. Photographs started to appear in textbooks at the end of the nineteenth century. In the 1950s, several books contained water paintings and from the 1970s onwards textbooks had colour photographs. Soil profile depictions were merely used to illustrate different orders in a classification system. Since the 1990s, efforts have been made to depict the soil profile in 3D. The overall trends and pattern found in this analysis not only reflect the technical ability to depict soils (e.g. camera, printing techniques) but also reflect the state of knowledge on the key properties and processes that have formed a soil.
In recent years, morphology and hydrogeology of the Castel di Sangro plain have undergone significant man-made changes. The plain is located in a very complex geological area where the Latium-Abruzzi carbonate platform, the Morrone-Rotella-Pizzone carbonate platform and the Molise nappe meet. Hydrology, hydrogeology, morphology and neotectonics of the area are conditioned by interlocking and overlapping of such palaeogeographic units. Some of them can be clearly recognised in the upper and middle Sangro area.After outlining the geological, structural and geomorphological features of the area, the authors discuss the morphological changes which occurred in the Sangro River pattern and its alluvial plain from 1875 to 1992 as a result of construction of major hydraulic structures (dams, river bed cementation, urbanisation of flood plain and spring areas, wells). Elements inferred from comparison of maps and aerial photographs were also considered. The study provides a concise description of objective observations on both natural and man-induced fluvial processes and groundwater flows.
This study used GIS technology, remote sensing images of 1949, 1966, 1981, 1984, 1988, 1992, 1996 and 2000, to analyze the spatial process of urbanization and its impact on the soil resources in Yixing city, southeast China. The results showed that the urban area grew more than three-fold during 51 years and the loss of paddy soil accounted for 82.9% of all soil loss. According to the expansion dynamics and the results of spatial metrics, the urban expansion process in Yixing could be divided into four steps, the initial step (1949–1966), the almost standstill step (1966–1981), the preparation step (1981–1984) and the rapid growing step (after 1984). The study also indicated that the urban expansion was encouraged by the transportation system, however, restricted by the rivers, lakes and hills. The local government's decision also affected the spatial process of urban expansion. The spatial process of Yixing urban expansion might reflect the spatial process of urbanization of southeast China, the region with a rapidly growing economy.
Rapid economic development has spurred land use change in China since the Chinese government initiated its economic reform in 1978. Although many papers have analyzed the characteristics of land use change, especially cropland conversion to non-agricultural use affected by economic development in the developed regions of China, relatively less attention has been paid to studying the characteristics of land use change affected by both economic development and environmental changes in its undeveloped regions. This paper analyzes the land use change and its driving forces in Dulan County, Qinghai Province on the Tibetan Plateau during 1990–2000. The land use change was studied based on the landscape metrics change and transition matrix of land use types, while its driving forces were analyzed according to climatic changes and socioeconomic development. The study indicates that the increase of land use benefits was given great attention; however, the protection of arid environment did not attracted much attention. The study suggests that the land use should be based on the sustainable protection of arid environment on the Tibetan Plateau.
Overbank floodplain deposits associated with a major flood which occurred in 1995 were collected shortly after the event from transects at 14 locations within the catchment of the River Ouse, Yorkshire, UK. At most sites, the sediment deposited nearest to the channel consisted primarily of sand-sized material, and the absolute particle size composition of the mineral fraction of the deposits evidenced fining with increasing distance from the channel. Most of the sand-sized material was deposited within 20 to 40 m of the channel, although at some sites, sand was present at distances > 100 m from the channels. There was no obvious downstream trend in the texture of the sediment deposited on the floodplains. In most places, the overbank sediment was considerably coarser than the suspended sediment transported in the study rivers, although at several sites, thed50 of the latter was similar to that of overbank sediment deposited furthest from the channel. Although the C:N ratio of the organic fraction of the overbank sediment was approximately constant across the width of the floodplain, the organic carbon and137Cs content of the deposited sediment increased with distance from the channel, and both properties were positively related to the magnitude of the < 63 μm fraction. The texture of sediment in cores collected from two of the sites in 1994 was generally finer than that of the overbank deposits from the 1995 flood at the same locations, confirming the extreme nature of the 1995 flood event.
After 2 days of intense precipitation, devastating flooding and debris flows affected the central coast of Venezuela on December 15–16, 1999. These events killed nearly 30,000 people and destroyed or damaged 126,000 homes. I visited the disaster area in March 2000; coastal valleys affected by debris flows were extensively covered by an interstitial matrix that included large amounts of often impressive boulders, up to 35–40 tons in weight. Granulometric analyses of four matrix samples from the Naiguatá valley showed they were all similarly coarse; their main constituent was sand, and grains <0.063 mm (silt and clay) made up only 1.3–14.2% by weight of the matrix. Median (D50) matrix grain-size was used to calculate—by interpolation on published diagrams (Water Resources Research, 21 (1985) 1511)—the minimum sediment concentration needed for debris flows to reach the yield-strength threshold required to mobilize large blocks. The data imply that debris flows needed at least 53–62% sediment by volume to transport matrix-supported boulders. Given the possibility that some of the fines may have been lost in suspension during flow or by subsequent flooding, I recalculated median matrix grain-sizes assuming that either 50% or 75% of the silt and clay had not been preserved in the deposits sampled. The new D50 values did not alter results substantially, as a 50–57% sediment concentration would have been still needed for such rough matrix to support boulders. Alternatively, the textures discussed here may approximately describe the actual particle-size distribution of the matrix involved in transporting boulders during the debris flows. This is suggested by the presence, 6 months after the flooding events, of a remnant sandy matrix still preserved beneath and between large boulders (Wieczorek, personal communication, 2000 and 2001. United States Department of the Interior, U.S. Geological Survey, Reston, VA 20192, USA). The above implies that the Venezuelan debris flows were of the granular, noncohesive type (cf. Scott, K.M., Vallance, J.W., Pringle, P.T., 1995. Sedimentology, behavior, and hazards of debris flows at Mount Rainier, Washington. U.S. Geological Survey Professional Paper 1547, 56 pp.).Data suggest the following events must have taken place in coastal Venezuela. Torrential rains quickly caused flooding along creeks and rivers, which must have rapidly attained hyperconcentrated streamflow. Bulking of stream alluvium in flood surges may also have taken place. However, the storm triggered widespread landsliding along the steep, unstable flanks of the Coastal Cordillera; many slides started in upper-basin areas, underlain by gneiss bedrock (Peña de Mora). Such slides released vast amounts of coarse, sandy debris, which mixed with turbulent floodwaters to produce numerous granular, noncohesive debris flows. These flows, despite their coarse matrix, exhibited intense particle collisions and interactions, thus were able to effectively mobilize vast masses of large boulders.
Our study demonstrates the importance of detailed lithostratigraphic and biostratigraphic analyses of deposits of lake-side terraces and alluvial fans in attempts to identify fluctuations of climate characteristics such as precipitation and evaporation rates, and of local conditions for water balance changes in the central part of Turkey during the last 20,000 years.Lake Tuz basin, central part of the Anatolian Plateau, Turkey, had two high lake-level stages dated as about 20,000–17,000 years ago and about 13,000 years ago. At the high lake stages, water level rose more than 15 m higher than the present lake level, and the lake area spread widely to near the southern edge of the basin, as inferred by lithologic and chronological investigation of drilling cores. In the Holocene, lake level fell and the southeast part of the basin became extensive lake terraces.Alluvial fans have formed along the edge of the upland area on the terraces in the Holocene. Stratigraphy and dating of the fan deposits show the fluctuations in the depositional rates of the fans. Major formative stages of the fans are dated about 7000–5500 and 2500–2000 years BP, and several times of minor stages (probably 4300 and 3000 years BP) occurred between the two major stages. Our archaeological research shows that there are strong relationships between the distributions of archaeological sites of this area and the above geo-environmental histories.
The sediment record of the Estanya lakes (Pre-Pyrenees, Spain) indicates that this region was intensely affected by land use during the Middle and Modern ages, which contrasts with the present-day poor socioeconomic development of the region. The correlation between sediment and biotic data and well-known historical events suggests a reliable history of environmental changes in this area during the last 2000 years. Changes in lake level were mainly controlled by climatic factors, whereas properties such as lithology, organic carbon, ostracods and pollen content are mainly associated with lake management and land use. Major sedimentological changes resulted from the use of the lake for hemp retting at the start of the XIVth century. Subsequent depopulation of the area in the XXth century led to the progressive recovery of the lacustrine ecosystem.
Environmental magnetism, elemental chemistry, pollen, macrofossil, and radioisotopes were studied on top-metre cores from Ballydoo Lough, Connemara, western Ireland, to reconstruct the impact of changing farming practices on soil erosion in a lake catchment. Documentary evidence, including detailed agricultural statistics, gave an independent land-use history over the period represented in the sediment record, i.e. most of the last two centuries. Maximum soil erosion, which involved unweathered coarse material as well as topsoil, was associated with a rapidly rising rural population and increased arable activity during the early 19th century. The Great Famine (1845–1847) halved the population, but arable and especially pastoral farming quickly recovered and were maintained at a high level from the late 1850s until the late 1890s. During this time, the rate of soil erosion slowed down considerably and the contribution of coarse mineral material relative to topsoil declined. Erosion of peaty soils was less important as potato cultivation declined and the middle slopes of the catchment were no longer cultivated. From the end of the 18th century to the 1970s, sediment accumulation and erosion rates were less and progressive podzolisation occurred as pastoral and particularly arable farming declined. The uppermost sediments record a sharp increase in soil erosion, involving mainly organic-rich soils from the upper slopes. This increase is attributed to a sharp rise in livestock numbers, especially sheep. A feature of the pollen record is the high arboreal pollen representation (43%–66% of total terrestrial pollen), which consists mainly of secondary pollen. It indicates a substantial input of pollen-bearing organic detritus derived from peaty soils and peat cuttings in the catchment.
Accelerated soil erosion is thought to couple with population growth, land exploitation and environmental degradation, leading to a major pressure on sustainable development of agro-ecosystem. However, implementation of wise policy and application of sound new techniques combined with traditional knowledge, may lead to less soil erosion or limited erosion under the tolerable value while intensive land use or exploitation is adopted for the population and economic growth. Such process of breaking the link between “environmental bads” and “economic goods” was termed as “decoupling”. In the present paper, decoupling process of soil erosion from human activities has been analyzed with the conception of decoupling indicators on the Chinese Loess Plateau. Before 1949, intensive soil erosion, as indicated by the high suspended sediment yield in the Yellow River, was induced by the wars within the nation and against the invasion of foreign countries. A distinct decoupling occurred after 1949, especially since the 1980s because of adoption of watershed-based comprehensive soil-water conservation measures. It implies that soil erosion could be decoupled from intensive land use exploration and the increasing population by a wise policy. And the decoupling indicator could be a sensitive indicator for agri-environment assessment.
A sedimentary sequence from the Espejo de los Lirios lake in the Metropolitan Zone of Mexico City (MZMC) was analyzed in detail for 210Pb and 137Cs activities, allowing to assess the evolution of the accretion rates in the lake during the last ∼84±4 years. The core revealed the deposition of contemporaneously indigenous lake sediments to the site interspersed with eroded bedrock from the catchment, evidencing episodic increases in sediment accretion which varied from 0.14 to 0.93 cm year−1. Increasing trends in sedimentation rates were found to be closely related to demographic expansion of the Metropolitan Zone of Mexico City and explained on the basis of the erosive processes produced as a result of deforestation created as a consequence of the urban and industrial development of the MZMC.
Pleistocene to Holocene as well as recent trends in climate have an influence on the composition of savanna–forest vegetation fringes in Africa, dominated mainly by savanna (C4) and mainly forest (C3) groups of plants. The modified vegetation cover plays an important role on the runoff processes and on the discharge of the draining river systems. Because the majority of forest–savanna borders in Central Africa is situated on geologically old planation surfaces, the main sources of palaeoenvironmental information are alluvial sediments of rivers. Therefore, this study focuses on the examination of alluvial soils and the determination of stable carbon isotopes (δ13C) of organic sediments on the Mbomou plateau and in the Mbari valley in the southeast of the Central African Republic (CAR). It has been shown that there is some evidence for an ongoing increase in C3-dominated forest plants, reducing the recent extension of savannas in the study area. The most important reasons for this trend are sufficient amount of annual rainfall (>1500 mm), decrease in bushfire frequency, and negative migration processes of the rural population due to the economic crisis in Central Africa. δ13C values in fossil soil horizons show that a greater extent of forest on the Mbomou plateau occurred around 7–7.5 ka and between 2.5 and 3 ka. Drier, savanna-dominated vegetation patterns were found at 5 ka and from 1 ka to the present. The more humid and arid climate periods during the Holocene partly correspond with high and low lake levels of Lake Chad. The findings also seem to be confirmed by other studies neighbouring Central African regions as Cameroon, Gabon and Congo–Brazzaville (Batéké Plateau), which indicate a more general validity of the findings from the Mbomou plateau, especially for the period since 3 ka.
