MIS-3晚期以来乌伦古湖古湖相沉积记录的初步研究

通过对现代乌伦古湖附近出露的古湖相沉积剖面的AMS~(14)C测年,粒度、总有机碳、总有机氮以及碳酸盐等环境代用指标的分析及其与全新世钻孔沉积记录的对比研究,结果发现:乌伦古湖在MIS-3晚期的33600-22500 cal a BP以及冰后期至早中全新世的16500-6500 cal a BP期间,维持着湖相沉积环境,湖面约比现在湖面高40 m.33600-22500 cal a BP的MIS-3晚期,气候相对温暖,乌伦古湖呈现高湖面特征,湖泊沉积物来源以流水搬运为主;22500-16500 cal a BP的末次冰期冰盛期,气候寒冷干燥,湖泊沉积物来源以风力搬运为主;16500-6500 cal a BP的冰后期以及早、中全新世期间,气候回暖,湖泊沉积物主要来源于河流径流作用.6500-5500 cal a BP,受高温干旱事件的影响,湖面收缩、水位剧降,除沉积中心外的其它钻孔位置出现沉积中断.5500 cal a BP后气候转冷变湿,湖泊重新恢复到现在的状态.乌伦古湖MIS-3晚期以来的古湖相沉积环境变化及其反映的古气候万年尺度上的干湿变化与周边区域气候环境变化记录有很好的一致性,响应了区域环境变化和全球气候突变事件.季风和西风的强度消长变化及其引起的环流条件改变以及温度变化引起的蒸发效应可能是区域气候环境变化的主要原因.这一古湖相沉积记录的研究可为MIS-3晚期以来北疆地区的古湖泊演化以及长时间尺度上西风和季风环流相互关系及其影响区的气候环境演化提供地质证据.     

Sediment accumulation rates in subarctic lakes: Insights into age-depth modeling from 22 dated lake records from the Northwest Territories,Canada

Age-depth modeling using Bayesian statistics requires well-informed prior information about the behavior of sediment accumulation. Here we present average sediment accumulation rates (represented as deposition times, DT, in yr/cm) for lakes in an Arctic setting, and we examine the variability across space (intra- and inter-lake) and time (late Holocene). The dataset includes over 100 radiocarbon dates, primarily on bulk sediment, from 22 sediment cores obtained from 18 lakes spanning the boreal to tundra ecotone gradients in subarctic Canada. There are four to twenty-five radiocarbon dates per core, depending on the length and character of the sediment records. Deposition times were calculated at 100-year intervals from age-depth models constructed using the ‘classical’ age-depth modeling software Clam. Lakes in boreal settings have the most rapid accumulation (mean DT 20 ± 10 yr/cm), whereas lakes in tundra settings accumulate at moderate (mean DT 70 ± 10 yr/cm) to very slow rates, (>100 yr/cm). Many of the age-depth models demonstrate fluctuations in accumulation that coincide with lake evolution and post-glacial climate change. Ten of our sediment cores yielded sediments as old as c. 9000 cal BP (BP = years before AD 1950). From between c. 9000 cal BP and c. 6000 cal BP, sediment accumulation was relatively rapid (DT of 20–60 yr/cm). Accumulation slowed between c. 5500 and c. 4000 cal BP as vegetation expanded northward in response to warming. A short period of rapid accumulation occurred near 1200 cal BP at three lakes. Our research will help inform priors in Bayesian age modeling.     

Aeolization on the Atlantic coast of Galicia (NW Spain) from the end of the last glacial period to the present day: Chronology,origin and evolution of coastal dunes linked to sea-level oscillations

The Atlantic coast of Galicia (NW Spain) is a high-energy environment where shingle beaches are currently developing. These coarser sediments alternate with sandy deposits which are also considered as beaches typical of a low-energy environment. The physical association of both types of sediment with contrasted sedimentary significance raises problems of interpretation. The study of four outcrops of fossil aeolianites on this coast has allowed us to reconstruct their evolution from the end of the Upper Pleistocene to the present day. Their chronology, estimated by optically stimulated luminescence between 35 and 14 ky at the end of the last glaciation (MIS2), coincides with a local sea level 120 m below the present one. This implies a coastline shifted several kilometres from its current location and the subaerial exposure of a wide strip of the continental shelf covered by sands. The wind blew sand to form dunes towards the continent, covering the coastal areas, which then emerged with no other limitation than the active river channels. Sea-level rise during the Holocene transgression has progressively swamped these aeolian deposits, leaving only flooded dunes, relict coastal dunes and climbing dunes on cliffs up to 180 m high. The aeolian process continued as long as there was a sandy source area to erode, although accretion finished when the sea reached its current level (Late Holocene). Since then, the wind turned from accretion to erosion of the dunes and sand beaches. This erosion exposes the older shingle beaches (probably of Eemian age) buried under the aeolian sands, as well as old, submerged forest remains and megalithic monuments. The destruction of sand beaches and dunes currently observed along the Galician coast is linked, according to most researchers, to anthropogenic global warming. However, their management should consider these evolutive issues.     

湖北江汉平原JZ-2010剖面沉积物粒度特征与环境演变

通过对江汉平原JZ-2010湖相沉积剖面AMS14C年代的精确测定与校正,以及代用指标粒度、磁化率的分析和研究,重建了研究区12.76 cal ka BP以来的环境演变过程:1)12.76-6.70 cal ka BP,区域环境处于偏干的晚冰期向湿润的早全新世发展阶段.2)6.70-4.47 cal ka BP,区域环境湿润,为湿度配置最佳的全新世适宜期,这是大溪文化向屈家岭文化转变时期自然背景.3)4.47-3.67 cal ka BP,区域气候环境偏干,其中经历了明显的4.2 cal ka BP干旱事件.4)3.67-1.22 cal ka BP时期,环境向湿润发展;在2.5 cal ka BP左右水动力条件明显变强,此时正对应于古云梦泽扩张时期.5)1.22 cal ka BP后,研究区处于湖泊面积减小的干旱时期;人类活动带来的影响达到高峰,围湖造田等农业活动也是江汉平原湖群趋于衰退的重要原因之一.     

The chronology of Late Quaternary fluvial activity in part of the Milfield Basin,northeast England

The sediment stratigraphy of a 4 m thick intercalated Holocene alluvial fill and valley floor peat at a site in the Milfield Basin, Northumberland, has been dated by a series of eight ¹⁴C assays, and related to a previously analysed pollen record. The sequence extends from the earliest Holocene until c. 2800 cal. BP . Prior to the onset of peat inception, substantial amounts of channel-trenching can be demonstrated to have occurred in the Milfield Basin during the Loch Lomond Stadial. There is no measurable early Holocene accelerated fluvial activity, but a major flooding event occurred at c. 7500 cal. BP , much earlier than recorded elsewhere in the region. The explanation for this is not clear. However, the cessation of mid-Holocene overbank sedimentation at c. 4000–3500 cal. BP is tentatively correlated with slope stability associated with woodland regeneration. © 1998 John Wiley & Sons, Ltd.     

Understanding Amazonian fluvial rias based on a Late Pleistocene–Holocene analog

Fluvial rias are elongated lakes at tributary mouths that can reach dozens of kilometers in length, constituting one of the most remarkable features in the Amazonian landscape. Thus far, definitive data which documents the genesis of fluvial rias have not been published. The main goal of this work was to integrate morphological, sedimentological and chronological information in order to characterize fluvial paleorias in the interfluve of the Purus and Madeira Rivers and discuss the most likely hypothesis for their genesis. These paleorias were first observed through remote sensing imagery as several elongated and interconnecting belts of open vegetation that are in sharp contact with the surrounding dense forest. The belts are branched and form a dendritic pattern similar to many modern drainage networks. The sedimentary record of these belts revealed the prevalence of sharp‐based sandstones and mudstones arranged into fining‐upward successions, which are compatible with deposition within channels. Active channel and abandoned channel deposits were recognized. These are topped by continuous mudstones related to rapid channel abandonment and formation of a low energy basin or ria environment. Radiocarbon dating of these deposits recorded only Late Pleistocene and Holocene ages ranging from 21 547–22 285 cal yr bp to 5928–6124 cal yr bp . This chronology for sediment deposition is not compatible with the hypothesis of Amazonian rias being formed by fluvial erosion during the Last Glaciation Maximum low sea level, with sediment accumulation during the subsequent Holocene transgression. Instead, the studied paleorias record previous tributaries of the Madeira River that became abandoned as the position of this river shifted southeastward and its interfluve tilted northward, inverting the drainage systems. Therefore, a neotectonic origin of some Amazonian paleorias seems most likely. This hypothesis should be considered in further investigations aiming at understanding the origin of numerous modern fluvial rias that typify the Amazonian landscape. Copyright © 2014 John Wiley & Sons, Ltd.     

Reconstruction of paleocoastlines for the northwestern South China Sea since the Last Glacial Maximum

The range of relative sea level rise in the northwestern South China Sea since the Last Glacial Maximum was over 100 m. As a result, lowland regions including the Northeast Vietnam coast, Beibu Gulf, and South China coast experienced an evolution from land to sea. Based on the principle of reconstructing paleogeography and using recent digital elevation model, relative sea level curves, and sediment accumulation data, this paper presents a series of paleogeographic scenarios back to 20 cal. ka BP for the northwestern South China Sea. The scenarios demonstrate the entire process of coastline changes for the area of interest. During the late glacial period from 20 to 15 cal. ka BP, coastline slowly retreated, causing a land loss of only 1×10⁴ km², and thus the land-sea distribution remained nearly unchanged. Later in 15–10 cal. ka BP coastline rapidly retreated and area of land loss was up to 24×10⁴km², causing lowlands around Northeast Vietnam and South China soon to be underwater. Coastline retreat continued quite rapidly during the early Holocene. From 10 to 6 cal. ka BP land area had decreased by 9×10⁴km², and during that process the Qiongzhou Strait completely opened up. Since the mid Holocene, main controls on coastline change are from vertical crustal movements and sedimentation. Transgression was surpassed by regression, resulting in a land accretion of about 10×10⁴km². Supported by Key Laboratory of Marginal Sea Geology, Chinese Academy of Sciences (Grant No. MSGL0711), the Guangdong Natural Science Foundation (Grant No. 04001309) and Open Fund of the Key Laboratory of Marine Geology and Environment, Chinese Academy of Sciences (Grant No. MGE2007KG04)     

Holocene vegetational and climatic variation in westerly-dominated areas of Central Asia inferred from the Sayram Lake in northern Xinjiang,China

Changes in the vegetation and climate of the westerly-dominated areas in Central Asia during the Holocene were interpreted using pollen-assemblages and charcoal data from a 300-cm-long sediment core of the Sayram Lake,northern Xinjiang.Accele-rator Mass Spectrometry(AMS) radiocarbon dating methods were applied to bulk organic matter of the samples.Artemisia spp./Chenopodiaceae ratios and results from principal component analysis were used to infer that the lake basin was dominated by desert vegetation before ca.9.6 cal.ka BP,which suggests a warm and dry climate in the early Holocene.Desert steppe/steppe expanded during 9.6-5.5 cal.ka BP,indicating a remarkable increase both in the precipitation and temperature during the mid-Holocene.Desert vegetation dominated between 6.5 and 5.5 cal.ka BP,marking an extreme warmer and drier interval.The steppe/meadow steppe recovered,and temperatures decreased from 5.5 cal.ka BP in the late Holocene,as indicated by the increased abundance of Artemisia and the development of meadows.Holocene temperatures and moisture variations in the Sayram Lake areas were similar to those of adjacent areas.This consistency implies that solar radiation was the main driving factor for regional temperature changes,and that the effect of temperature variations was significant on regional changes in humidity.The evolution of climate and environment in the Sayram Lake areas,which were characterized as dry in the early Holocene and relatively humid in the middle-late Holocene,are clearly different from those in monsoonal areas.Dry conditions in the early Holocene in the Sayram Lake areas were closely related to decreased water vapor advection.These conditions were a result of reduced westerly wind speeds and less evaporation upstream,which in turn were caused by seasonal changes in solar radiation superimposed by strong evaporation following warming and drying local climate.     

Geological setting as background for methane distribution in Holocene mud deposits, Århus Bay,Denmark

Detailed acoustic mapping have been carried out in the Århus Bay in order to establish the general Late Weichselian and Holocene stratigraphy, and to map the gas related acoustic blanking.The mapping results show that the oldest seismic unit is glacial till, probably related to the latest glacial advance in the region. The glacial till is covered by late-glacial ice-lake clay and silt reaching a thickness of up to 10 m. In the deeper part of the bay, early Holocene organic material and peat has been recorded in a few cores. A thin seismic unit is observed, which probably represents an early Holocene lowstand period, when most of the Århus Bay was dry land. The three upper seismic units are related to the Holocene transgression of the region representing different hydrographical conditions. The lowermost unit (Marine unit 1) partly drapes the basin area with clay sediments and partly shows prograding sandy coastal deposits around glacial ridges. Marine shells from this unit date back to 8700 cal. years BP which are the oldest marine shells found south of the threshold in the northern Great Belt. The next unit (Marine unit 2) consists in general of mud to sandy mud, which cover most of the western central part of the Århus Bay and in some places reach the present seabed in areas of erosion or non-deposition. The distribution of the youngest seismic unit (mud, Marine unit 3) is confined to the sub-recent to recent sedimentation basins in the eastern central part of the area. Acoustical blanking shows that the methane production takes place in the Holocene marine sediments. A map of the distribution and depth to free methane in the muddy sediments has been produced. Combined information from the different seismic equipment used allowed a mapping of the distribution and depth to free gas in the intervals 0.5–2, 2–4 and >4 m. The map shows that acoustic blanking is found in the central part of Århus Bay about 4 m below the seabed. In areas with high sedimentation rate, the acoustic blanking is found closer to the sediment surface and in selected key stations, pore-water chemistry have documented that depth to acoustical blanking is comparable to the methane saturation depth.Barotropic induced inflow dominates the present current system in the semi-enclosed Århus Bay. The inflow events create turbulence in the outer eastern parts of the bay, followed by high sedimentation rates. The recent situation is reflected in the seabed sediments in the eastern part of the bay, which are characterised as soft sandy mud deposits, with gas bubbles close to the seabed.Seismic investigations have not previously been used to identify recent sedimentation areas and the most vulnerable areas in respect to possible escape of methane and toxic hydrogen-sulphide. The results of the present survey show that future monitoring must be focused in shallow gas areas in the eastern-most part of Århus Bay.     

Holocene alluvial fan and terrace formation in the Bowland Fells,Northwest England

In the Bowland Fells, Lancashire, northwest England, in the headwater valleys of the Hodder river system, is a suite of Holocene fluvial landforms. Debris cones and alluvial fans at tributary junctions, and river terraces along the main valleys post-date late Pleistocene forms and pre-date the modern valley floor alluvial forms. Eight ¹⁴C dates from wood samples incorporated within the terrace and fan deposits have allowed two main phases of Holocene erosion to be identified with debris cone/fan deposition taking place after c. 5400 BP but before c. 1900 BP and again at c. 900 BP. Some of the fans and cones are complex with deposits attributable to both phases; others are simple and attributable only to the later phase. In the headwaters an upper terrace at c. 5400 BP pre-dates the cones and a lower terrace is contemporaneous with the first debris cone phase. Lower downvalley the youngest of three terraces date from c. 5000 BP or earlier indicating that the sequence is less complex downstream.     

Holocene vegetation succession and responses to climate change in the northern sector of Northeast China

Sediment pollen samples from the Huola Basin in the northern sector of northeast China, and surface pollen samples from its environs, were analyzed to reconstruct accurately the historical response of vegetation to climate change since 9100 cal yr BP. Pollen analysis of the Huola Section indicates that vegetation experienced a transformation from early-mid Holocene warm-cold mixed vegetation to late Holocene cold-temperate vegetation. From 9100 to 6000 cal yr BP, the study area was warmer and moister than at present, developing Corylus, Carpinus, Pinus, Picea, Betula and Larix-dominated forests. Two cooling events at 6000–5000 and 3500–2500 cal yr BP led to a decrease in Corylus, Carpinus and other warmth-loving vegetation, whereas cold temperate forests composed of Larix and Betula expanded. After 2500 cal yr BP, Larix and Betula dominated cold-temperate vegetated landscapes. The Holocene warm period in NE China(9100–6000 cal yr BP) suggests that such warming could have resulted in a strengthening of the influence from East Asian Summer Monsoon on northernmost NE China and would have benefited the development of warm-temperate forest vegetation and an improved plant load, which also provides the similarity model for the possible global warming in the future.     

湖泊沉积物磁学特征指示的埃及Faiyum盆地全新世以来的环境演变

利用埃及北部Faiyum盆地获得的高取芯率沉积物岩芯,进行沉积物多种磁性参数的测量,结合有机碳、介形虫、粒度等分析,在AMS14C加速器测年的基础上,建立全新世以来湖泊沉积物磁性特征变化的时间序列.结果表明,粒度效应以及沉积后的各种次生作用对沉积物的磁性特征没有明显的影响,磁性变化主要反映了沉积物不同来源组成的相对变化.全新世前沉积物磁性较弱,主要含不完全反铁磁性矿物,与周边沙漠的物质相似,结合其粒度特征,沉积物来源应以近源物质为主.全新世早中期(约10 5.4 ka BP)沉积物磁性变化相对稳定,有机质含量也较高,指示了来自尼罗河较为稳定的物质供应;而大约5.4 ka BP尤其最近约4.2 ka BP以来,磁性的明显变化反映了流域降水减少情况下,来自青尼河物质贡献的相对增加;最近约2.0 ka BP以来沉积物的磁性变化,则更多地与盆地人类活动的强化有关.总体而言,Faiyum盆地全新世以来的环境演变主要受控于全新世以来尼罗河与盆地的水力学联系.即:全新世前盆地未与尼罗河连通时,沉积物主要来源于周边沙漠的风成物质;而受全新世早-中期来流域季风降水增加的影响,泛滥的尼罗河为盆地提供了相对稳定的物质供应,湖泊也处于高湖面;全新世晚期以来,随着流域干旱化的加剧,尼罗河与盆地的连通性开始减弱,来自高磁性的青尼罗河物质贡献开始相对增加.最近约2.0 ka BP以来,虽然仍有人工运河连接尼罗河与盆地,但沉积物磁性的显著变化更多地反映了盆地人类活动的不断强化.     

Sedimentary archives of the French Atlantic coast (inner Bay of Vilaine,south Brittany): Depositional history and late Holocene climatic and environmental signals

The late Holocene is of particular interest to our understanding of the evolution of coastal sedimentary systems because this period encompasses warmer and cooler periods, and rising sea level in northern Europe. Based on an approach combining AMS ¹⁴C, sedimentological and rock magnetic analyses on sediment cores complemented with seismic data collected in the macrotidal Bay of Vilaine (south Brittany), we document the depositional history of the inner bay coeval to the mid- to late-Holocene transgression in south Brittany. Correlation between sedimentary archives revealed the main sedimentary infilling phases during the last 6000 years. Four units (U1–U4) are recognized in the coastal sediment wedge of the system, corresponding to the stepwise marine invasion of the bay. We show that (1) marine inundation, due to the steep morphology of the bedrock, is diachronous between distal and proximal records. A time lag of ～1000 years is inferred over a distance of less than 5 km; (2) in the outer areas, the sedimentation has been condensed since 3000 years; (3) proximal estuarine archives offer the best record of sedimentary processes covering the last 2000 years, including the Medieval Warm Period (MWP).Correlations in proximal records in the Bay of Vilaine assess the connection between coastal sedimentary dynamics, climatic conditions and anthropogenic activities during the MWP. We match the preservation of clay deposits to increased river-borne suspended matter transported to the estuary probably as a result of accelerated land-use development (higher soil erosion) in the catchment area between ca. 880 and 1050 AD. Because the preservation of estuarine sedimentary successions is favoured when coastal wave sediment reworking is minimal, it is proposed that the prevailing climatic regime in south Brittany during the MWP likely resembled to that of the preferred negative phase of the North Atlantic Oscillation (NAO). Our data are fairly consistent with other late Holocene records from northern Europe including the Atlantic seaboard. However, they outline the difficulty in interpreting climatic and anthropogenic signatures in coastal sedimentary records where high-resolution chronologies required to unravel their respective influences are still missing.     

Radiocarbon wiggle-match dating of proglacial lake sediments – Implications for the 8.2 ka event

The problem of insufficient age-control limits the utilisation of the 8.2 ka BP event for modelling freshwater forcing in climate change studies. High-resolution radiocarbon dates, magnetic susceptibility and lithostratigraphic evidence from a lake sediment core from Nedre Hervavatnet located at Sygnefjell in western Norway provide a record of the early Holocene. We use the method of radiocarbon wiggle-match dating of the lake sediments using the non-linear relationship between the ¹⁴C calibration curve and the consecutive accumulation order of the sample series in order to build a high-resolution age-model. The timing and duration of Holocene environmental changes is estimated using 38 AMS radiocarbon dates on terrestrial macrofossils, insects and chironomids covering the time period from 9750 to 1180 cal BP. Chironomids, Salix and Betula leaves produce the most consistent results. Sedimentological and physical properties of the core suggest that three meltwater events with high sedimentation rates are superimposed on a long-term trend with glacier retreat between 9750 and 8000 cal BP. The lake sediment sequence of Nedre Hervavatnet demonstrates the following: only a reliable high-resolution geochronology based on carefully selected terrestrial macrofossils allows the reconstruction of a more refined and complex environmental change history before and during the 8.2 ka event.     

Holocene salinity fluctuations of the East Korean lagoon related to sea level and precipitation changes

Detailed diatom records within core sediments from Maeho Lagoon along the Eastern coast of South Korea revealed that the ecological dynamics of the lagoon during the Holocene were associated with relative sea level and regional precipitation. Accelerator mass spectrometry (AMS) ¹⁴C dating indicates that sediment accumulation began prior to 8300 cal. year BP, and that the lagoon formed around 8000 cal. year BP. The salinity level of the lagoon increased until 5000 cal. year BP, and then decreased. Long‐term trends in salinity were dependent upon changes in sea level: periods of high salinity in the lagoon were generally coincident with periods of high sea levels along the east coasts of Korea and Japan. On the other hand, multicentennial‐scale fluctuations in diatom assemblages and magnetic susceptibility (MS) suggest that a 400 year fluctuation in salinity was regulated by changes in precipitation in the area of this lagoon. Changes in the westerly jet stream, controlled by variation in solar irradiance, had an important influence on precipitation volume in South Korea, suggesting that the patterns of the westerly jet stream fluctuate on a 400 year cycle.     

Biomarker evidence for changes in terrestrial organic matter input into the Yellow Sea mud area during the Holocene

We present lipid biomarker records of two cores(ZY1 and ZY3) from the central South Yellow Sea mud area to investigate the changes in sources and transport processes of the sedimentary organic matter(OM) throughout the Holocene. Based on the analysis of marine biomarker content(SPB(Phytoplankton Biomarker, total content of brassicasterol, dinosterol and C_(37)-alkenones) and crenarchaeol), and terrestrial biomarkers(Sn-alkanols and br GDGTs) as well as TMBR′ and BIT index values, the marine organic matter(MOM) and terrestrial organic matter(TOM) deposition history was reconstructed. Changes in TOM and MOM were related to variations in land vegetation density and marine productivity, as well as transport processes dominated by the oceanic circulation system. The marine biomarker contents from the South Yellow Sea have generally increased throughout the Holocene, indicating that the increased MOM contents were mainly controlled by the strengthening of the circulation system. The terrestrial biomarkers, on the other hand, were more variable, indicating more complex influence of TOM burial in the Yellow Sea. During the Early Holocene(7200–6000 cal yr BP), the moderate TOM input revealed by the terrestrial proxy records may result from abundant land source supply by strong river transport despite the lack of transport via circulation system. The Mid-Holocene(6000–3000 cal yr BP) was characterized by decreased terrestrial biomarker contents. The balance between the decrease in land source supply and increase of transportation by the current system of the TOM resulted in the lower but stable contents of TOM. During the Late Holocene(3000 cal yr BP to present), the TOM deposition in the South Yellow Sea increased as the current system was further enhanced and thus transported more TOM to the central South Yellow Sea, although the land supply of TOM was further reduced.     

Numerical study of the effects of check dams on erosion and sedimentation in the Pachang River

Scouring and deposition processes resulting from variable rainfall and typhoon occurrence in tropical climatic conditions induce significant changes in the riverbeds of Taiwan. Along the Pachang River of western Taiwan, severe damage occurs during typhoons due to large and sudden variations in discharge, erosion, sediment transport and deposition. In order to simulate this process, the NETSTARS numerical modeling tool was used in the present paper. The influence of existing and planned check dam structures on flow control was also analyzed to determine their capacity to resist river erosion or to not be buried beneath sediments. Not only does the modeling tool allow calculation of the erosion-deposition behavior at the scale of the whole river, but it also provides local determination of the optimum location and characteristics such as foundation depth and lateral encroachment of future check dams. The results of a 10-year, long-term modeling simulation in terms of riverbed stability and scouring potential thus provide insights about unsafe future behavior at 4 sites. This numerical model provides a better general understanding and useful information for the optimal prevention of both the scouring damage and the burial related to sediment deposition with large changes in discharge and sediment transport.     

The role of fault reactivation in the development of tropical montane lakes

This work details the role of fault reactivation in the development of tropical montane lakes by using basin morpho-structural analysis and seismostratigraphic studies. The upland lakes are severely faulted sinkholes, whose faults penetrate the Quaternary sedimentary units. Four main stages are related to the lake formation: (i) an Early Proterozoic tectonic deformation of the rocks along the southern border of the Carajás Structure, where the lake is placed; (ii) differential erosion by – and building of – the formation of the South Carajás Hill; (iii) Fe-rich crust formation by weathering and gravitational collapse faults following the E–W plateau border and the start of Violão Lake formation during the Pliocene–Pleistocene; and (iv) episodic fault-fracture reactivation by gravitational collapse causing pulses of subsidence in the lake and outlining its faulted borders. Dissolution of the lateritic crust and erosion by runoff drainage under wet climate conditions were coeval with fault activities, which allowed the deposition of relatively thick clastic deposits organized in three main seismostratigraphic units associated with major lake-level fluctuations. Initial fault reactivation under low-level water started lacustrine basin development with deposition of prograding fan deltas related to the main drainage. A second fault reactivation by gravitational collapse increased the lake accommodation space and resulted in the deposition of fine-grained sediments from dilute interflows or overflows until 36 000 cal year BP. At about 31 000 cal year BP, rapid decreases in the lake water level under redox conditions at the sediment/water interface allowed widespread siderite formation. A third gravitational collapse episode was responsible for the increase in the lake area and depth and the returning of clastic/organic deposition up to the present. This tropical montane lake can be seen as a representative example for understanding the formation of other upland lakes controlled by fault reactivation. © 2020 John Wiley & Sons, Ltd.