Xingmin Meng

Palaeoclimate and palaeoenvironmental responses in the western Mediterranean over the last 140 ka: evidence from Mallorca, Spain

Climatic change in the western Mediterranean basin over the last 140 ka has been investigated from sediment and soil sequences along the coast of the northeastern part of Mallorca, Spain. Palaeoclimate and palaeoenvironments are reconstructed from sediment and soil structures, marine and terrestrial molluscan fauna, grain size distributions, mineral magnetics, sediment and soil chemistry, SEM and oxygen isotope values. Dating is by OSL and amino acid geochronometry along with marine molluscan faunal assemblages. All Oxygen Isotope stages and substages from OIS 6 to 1 are recorded. Mean annual temperatures (m.a.t.) of c. 19.2°C for the climatic optimum of the Last Interglacial compare with c. 17.3°C for the present, and are associated with high sea level, extensive woodland and stable soils, although wide temperature ranges are detected during this stage with values as low as c. 10.3°C. Through OIS 5d to 5a m.a.t. fluctuated from 8.2°C to 17.9°C. Low sea level, open vegetation, effective river activity and aeolian sand and dust deposition characterize 5d and 5b while relatively high sea-level and soil development on stable landscapes dominate OIS 5c and 5a. OIS 4 is represented by m.a.t. of c. 4.9°C with aeolian sand and loess deposition, limited river activity and weak soil development. Loess deposition persisted into OIS 3, but at this time the m.a.t. was c. 13°C and with higher humidity and vegetation growth. During the LGM (OIS 2) m.a.t. was c. 8.1°C and effective river, slope and aeolian processes, along with associated with limited vegetation cover, were responsible for the most extensive changes in the landscape. Major rates of change occur during periods of climatic deterioration when the high levels of geomorphic energy were imposed on a landscape undergoing a breakdown of vegetation cover.

The loess of north-central China: Geotechnical properties and their relation to slope stability

Abstract Loess is an aeolian deposit consisting of predominantly silt-sized quartz particles, and containing variable amounts of clay-sized minerals. Loess is generally classified as a water-softening material, because upon wetting the loess fabric rapidly weakens or collapses. The strain hardening of Malan loess and the brittle failure of Lishi and Wucheng loess are in strong contrast to the failure behaviour of these loess deposits in a remoulded state. From tests carried out on samples with varying moisture contents, using a modified Bromhead ring shear apparatus, it was found that the effective apparent cohesion gradually increases and the effective internal friction angle decreases with an increase in moisture content. When the moisture content reaches a material-specific threshold, the effective cohesion decreases rapidly and the effective internal friction angle stabilises at a residual value. The frequent failure of loess slopes in the western part of the Chinese Loess Plateau is closely related to progressive weathering along zones in these slopes, which causes a dramatic decrease in strength from the peak strength condition. Progressive weathering is common in the loess slopes in the western part of the Chinese Loess Plateau. During the process shear strength reduction along potential slip surfaces may be achieved by leaching of readily soluble salts, destruction of cementation bonds, and redistribution of particles. Localized collapse of the loess fabric causes internal deformation and consequently peak strength conditions are concentrated on a progressively smaller area of the failure plane. Therefore, the mode of failure of loess slopes is generally determined by brittle failure of the undisturbed, and unweathered, central parts of the slopes. It is important that both the weathered and unweathered strength of the loessmaterials in this area be established in order to analyze the stability of existing loessslopes, many of which are steepand lie directly above domestic and industrial urban areas.

Variations in loess and palaeosol properties as indicators of palaeoclimatic gradients across the Loess Plateau of North China

Abstract New data are presented on micromorphology, magnetic susceptibility and particle size characteristics of three soil/sediment units at four locations along a WNW-ESE transect across the Loess Plateau of North China. Differences and spatial trends are used to assess the sensitivity of such data as proxies of regional palaeoclimatic gradients. The steepest climatic gradient suggested by the magnetic susceptibility for the S1 palaeosol is at the eastern (humid) end of the traverse. Micromorphology and particle size analysis show that the loess units in the east are richer in clay than the best developed palaeosols in the semi-arid west, and that particle size in the west appears to provide a more sensitive record of climatically-driven changes than magnetic susceptibility. The use of surface and near-surface soils as a datum for inferring palaeoprecipitation values is reviewed in the light of recent evidence of substantial degradation of such soils. Micromorphological and particle size analysis of closely spaced samples from loess-palaeosol sequences is a sensitive addition to magnetic susceptibility stratigraphy and correlation.

Origin of the magnetic susceptibility signal in Chinese loess

Abstract The quality of the loess magnetic susceptibility (MS) record as a climate proxy has been compared to the ice core and deep-sea records, yet the source and causes of the MS signal remain poorly understood. Four explanations are currently in circulation. The dominant hypothesis, that MS variations largely result from post-depositional weathering and/or formation of magnetic phases by pedogenic processes, has led to the use of MS data as a palaeoprecipitation proxy. We present a different view based on analyses of samples of monthly dust falls over 4 years in North China. These have MS values comparable to those of Pleistocene loess and palaeosols but, unlike the latter, their variations are seasonal and so cannot be explained by weathering differences. Our experiments confirm that MS enhancement in the palaeosols is caused principally by fine-grain magnetic minerals, but, in contrast with previous proposals, the major source of the ultrafine magnetic minerals appears to be decomposed plant litter.

Climate change, loess and palaeosols: proxy measures and resolution in North China

The Loess Plateau of North China is a key region for the study of records of Quaternary climatic change. Various techniques to abstract climatically sensitive data were first applied to the classic loess—palaeosol site at Luochuan, and current views on climatic change in Asia have been strongly moulded by results from this central part of the Loess Plateau. Data including the magnetic susceptibility, granulometry, mineralogy, geochemistry, micromorphology, fossil content, and inorganic and organic carbon content of Asian loess—palaeosol series have been used, singly and in combination, as surrogate measures of palaeoclimates to infer climatic changes with particular reference to changes in the Asian monsoon. Some of this work is reviewed and the status of a number of parameters as climatic proxies is discussed. Some lithostratigraphic sequences from which proxy measures of climate have been derived are incomplete, as shown by erosion surfaces, water-laminated zones and truncated palaeosol profiles, while others provide records of relatively low resolution. Most descriptions of Quaternary climatic changes based on studies of the loess emphasize alternations in the relative dominance of warm and moist v. cool, dry and dust-laden monsoons. Some recent studies of the much higher resolution loess—palaeosol successions on the western margins of the Loess Plateau have suggested that such changes may sometimes be out of phase, with the inference that the current climate model is too simple. Means of testing this hypothesis are proposed.

SRTM DEM and its application advances

The Shuttle Radar Topography Mission (SRTM) provides for the first time a near-global high-resolution digital elevation model (DEM) with great advantages of homogeneous quality and free availability. The last 10 years or so have seen rapid advances in the data processing and applications of SRTM DEM. From the perspective of SRTM, we present in this article a brief overview of the principles, datasets, void filling and accuracy of SRTM DEM first. Special emphasis is on its application advances in various research fields including, but not limited to, geology, geomorphology, water resources and hydrology, glaciology, evaluation of natural hazards and vegetation surveys. Problems that arose from the applications and the future research interests are also addressed. We hope this study will greatly facilitate the ease of use of SRTM DEM in extensive fields.

Modelling the erosional susceptibility of landslide catchments in thick loess: Chinese variations on a theme by Jan de Ploey

Abstract In his 1990 paper (Catena, 17: 175–183), Jan de Ploey proposed a system whereby catchments are viewed as functional units in which geomorphic work results from the combined effects of water erosion and mass movements. His measure of erosional susceptibility of catchments, ES, introduced the gravitational potential in addition to the kinetic energy of the eroding agents. This modelling of the erosional susceptibility of catchments in terms of energy was being used to explore time-dependent variations of ES when Jan de Ploey died. Also at this time, he extended an invitation to the present authors to test and extend his model using our data base on the landslide-dominated catchments in the thick loess country of north-central China. This paper presents a series of new equations expressing variations in ES using a data base for over 200 landslides in the loess-covered mountainous terrain (relative relief up to 1500 m) of the Lanzhou region of eastern Gansu Province. These equations express the loss of potential energy of the landslide mass in relation to the relative relief and the pressure energy input in the form of rainfall. These indices of erosional susceptibility can be used to analyse the differences in response, from one catchment to another and through time, between the characteristics of vegetation cover including land use, hydrology, geomechanical behaviour, and mechanisms of movement on slopes. Using certain assumptions on percentage area of catchments affected by mass movement and long-term mean annual precipitation in the Lanzhou region, the Es equation suggests an estimate for non-clayey materials of about 10−4 m−2/s−2, which is a significantly higher susceptibility value than the mean worldwide value for non clayey materials obtained by De Ploey.

Introduction to Geohazards of Central China

This thematic set of papers is intended to raise awareness of the types, nature, effects and impacts of geohazards in Central China. The papers will be published in this and subsequent issues of QJEGH and demonstrate the significant impacts that are wrought on the lives and livelihoods of those who live and work in the mountainous areas of Central China. China is all too often in the news as a result of geohazards that have a severe impact on lives and livelihoods. The mountainous terrain of Central China is subject to frequent hazards of high intensity with recent examples such as the 2008 Wenchuan earthquake and the 2010 Zhouqu debris flow. Rapid economic development brings with it expansion of urban centres and infrastructure networks, which not only increases the exposure of the population to natural processes in a dynamic environment, but can also lead to further strains on a landscape that is only marginally stable. Frequent geohazard events have sparked much research in an effort to better understand processes and material properties that can be used to inform and implement effective strategies to mitigate against the negative consequences of these geohazards. Owing to their location along the eastern margins of the Tibetan Plateau the provinces of Gansu, Sichuan and Yunnan are particularly affected by geohazards (Fig. 1). This tectonically active region is strongly affected by continuing uplift and, towards the east and NE, crustal stress release has resulted in the formation of some very large systems of NNE–SSW- and WNW–ESE-trending strike-slip and thrust fault zones (e.g. Dijkstra et al . 1993). Differences in relative uplift and displacement along these faults have had a significant impact on the present physiography of Central China. Several important geohazard regions can be distinguished and in this introduction we briefly highlight issues in two of …

Collapsible Loess on the Loess Plateau of China

The aeolian silt accumulation making up the Loess Plateau of North China reaches more than 250m in thickness in the Lanzhou region. Regional studies have shown that there is a clear gradient in collapsibility from the dry north-west to the moist south-east of the Plateau. On the western margins, collapsibility is significantly greater in the young (Malan) loess: as the surface drape, it is also very vulnerable to human disturbance. High collapsibility is a factor in the extensive pipe-and-sinkhole morphology known as ‘loess karst’. Population pressure has led to abuse by mechanical disruption of thick loess. Also, unwise extension of irrigation waters enhances the natural effect of severe summer rainstorms, resulting in subsidence and landslides.

Comparison of proxy records of Late Pleistocene climate change from a high-resolution loess–palaeosol sequence in north-central China

A micromorphologically based pedosedimentary reconstruction of a 40-m loess–palaeosol sequence from the western part of the Chinese Loess Plateau provides a sensitive proxy record of changing processes and associated environmental conditions in this region between ca. 150 000 and 10 000 yr BP. Depth functions of more traditional climate proxies, such as magnetic susceptibility, calcium carbonate content and median grain size, support the broad pattern of environmental changes inferred from the micromorphology, although the bulk properties sometimes lag behind or are out of phase with each other and do not match all the fluctuations in the micromorphological record. The reasons for the disparities are probably complex, although they partly reflect differences in response rates and sensitivities of proxies to different climate parameters, as well as genuine out-of-phase changes in strength of monsoonal climate controls. This work illustrates the sensitivity of micromorphology in detecting past environmental changes within rapidly aggrading landscapes, and emphasises the need to understand more fully the local and regional significance of bulk proxies currently used in global correlations with marine- and ice-core records. Copyright