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"loess"
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Approaches and challenges to the study of loess
In September 2016, the annual meeting of the International Union for Quaternary Research's Loess and Pedostratigraphy Focus Group, traditionally referred to as a LoessFest, met in Eau Claire, Wisconsin, USA. The 2016 LoessFest focused on thin loess deposits and loess transportation surfaces. This LoessFest included 75 registered participants from 10 countries. Almost half of the participants were from outside the United States, and 18 of the participants were students. This review is the introduction to the special issue for Quaternary Research that originated from presentations and discussions at the 2016 LoessFest. This introduction highlights current understanding and ongoing work on loess in various regions of the world and provides brief summaries of some of the current approaches/strategies used to study loess deposits.
Journal Article
Human health risk assessment of groundwater nitrogen pollution in Jinghui canal irrigation area of the loess region, northwest China
Nitrogen pollution of groundwater is becoming more and more serious due to intense and extensive industrial and agricultural activities. This may exert great influence on human health. In this paper, human health risk due to groundwater nitrogen pollution in Jinghui canal irrigation area in Shaanxi Province of China where agricultural activities are intense was assessed. Forty-seven groundwater samples were collected from shallow wells and analyzed for physicochemical indices in the study area. Water samples were analyzed for pH, total dissolved solids (TDS), total hardness (TH), major ions (Na+, K+, Ca2+, Mg2+, HCO3−, CO32−, Cl− and SO42−), nitrate (NO3–N), nitrite (NO2–N) and ammonia nitrogen (NH4–N). General groundwater chemistry was described by statistical analysis and the Piper diagram. Water quality was quantified via comprehensive water quality index (CWQI), and human health risk was assessed considering the age and exposure pathways of the consumers. The results show that the shallow groundwater is slightly alkaline and groundwater types are HCO3·SO4·Cl–Mg and HCO3·SO4·Cl–Na. Rock weathering and evaporation are main natural processes regulating the groundwater chemistry. The CWQI indicates that groundwater in the study area is seriously polluted by TH, TDS, SO42−, Cl− and NO3−. Human health risk is high because of high concentrations of nitrate in drinking water. The results also show that children are at higher risk than adults. The health risk through dermal contact is much lower than that through drinking water intake and can be ignored.
Journal Article
Characterization of inherent spatial variability of loess deposit properties in Shaanxi Province, China
PurposeInherent spatial variability of properties of loess deposit along depth direction plays important roles in soil reclamation, agricultural irrigation, and risk assessment and management of geo-hazards on the Loess Plateau. This study aims to develop a database for loess deposit properties in Shaanxi Province first, followed by evaluating and reporting comprehensively the abovementioned spatial variability along depth.Materials and methodsA comprehensive literature review and numerous laboratory tests were conducted for 37, 81, and 177 loess profiles respectively for sandy, silty, and clayey loess to examine their properties in Shaanxi Province, China. For each property collected in the database, the means, coefficients of variation (COV), most suitable probability distribution functions, and spatial correlation lengths along the depth direction were evaluated and reported.Results and discussionThe physical and index properties and strength properties of the three loess soil generally exhibit relatively low variabilities with a mean COV of less than 20.0% in most cases. In contrast, the deformation properties exhibit medium to high variabilities with a mean COV varying from roughly 27.0 to 85.0%. For all of these properties mentioned above, the spatial correlation length along depth was estimated to be approximately 3 ~ 8 m.Conclusions(1) The COV of loess properties at one site can be significantly different from that at other sites. Thus, the site-specific variabilities of loess deposit properties shall be considered in geological design and analysis. (2) When site-specific data of properties for loess deposits are not available for on-going geological/geotechnical projects, the typical ranges of loess properties obtained in this study, including their statistics and correlation lengths, can be used as guidelines for approximation.
Journal Article
Characterizing magnetic mineral assemblages of surface sediments from major Asian dust sources and implications for the Chinese loess magnetism
Eolian dust plays an important role in the Earth’s climate system. Environmental magnetism has been widely used to trace dust variations at different spatial and temporal scales. However, the magnetic properties of sediments from key dust sources have not been well determined. In this study, surface samples from potential dust sources in inner Eastern Asia were systematically investigated. Our results indicate that ferrimagnetic and antiferromagnetic minerals are both present in surface sediments and that they have broad grain size distributions. Ferrimagnetic components are dominated by partially oxidized coarse-grained (pseudo-single domain and multi-domain) lithogenic magnetite particles with minor contributions from pedogenic fine-grained (single domain and superparamagnetic) particles. Antiferromagnetic hematite can be classified into three groups in terms of diffuse reflectance spectroscopy (DRS) band positions (
P
560 nm
,
P
545 nm
, and
P
535 nm
, where numbers indicate the DRS band wavelength for hematite). The first group (
P
560 nm
) is the coarse-grained hematite of lithogenic origin and is mostly confined to western China. The
P
535 nm
group is of pedogenic origin. The
P
545 nm
group is an intermediate phase that is present both in surface samples from the source regions and in loess. Therefore, the
P
545 nm
and
P
535
nm
groups are related to eolian inputs to the Chinese Loess Plateau and pedogenic processes, respectively. In addition, significant differences exist between the magnetic properties of eolian material from sources and depositional regions due to gravitational sorting. These insights provide strong constraints on interpretation of dust signals recorded by the Chinese loess and marine sediments from the North Pacific Ocean.
Journal Article
The 3.6-Ma aridity and westerlies history over midlatitude Asia linked with global climatic cooling
Midlatitude Asia (MLA), strongly influenced by westerlies-controlled climate, is a key source of global atmospheric dust, and plays a significant role in Earth’s climate system . However, it remains unclear how the westerlies, MLA aridity, and dust flux from this region evolved over time. Here, we report a unique high-resolution eolian dust record covering the past 3.6 Ma, retrieved from the thickest loess borehole sequence (671 m) recovered to date, at the southern margin of the Taklimakan desert in the MLA interior. The results show that eolian dust accumulation, which is closely related to aridity and the westerlies, indicates existence of a dry climate, desert area, and stable land surface, promoting continuous loess deposition since at least ∼3.6 Ma. This region experienced long-term stepwise drying at ∼2.7, 1.1, and 0.5 Ma, coeval with a dominant periodicity shift from 41-ka cyclicity to 100-ka cyclicity between 1.1 Ma and 0.5 Ma. These features match well with global ice volume variability both in the time and frequency domains (including the Mid-Pleistocene Transition), highlighting global cooling-forced aridity and westerlies climate changes on these timescales. Numerical modeling demonstrates that global cooling can dry MLA and intensify the westerlies, which facilitates dust emission and transport, providing an interpretive framework. Increased dust may have promoted positive feedbacks (e.g., decreasing atmospheric CO₂ concentrations and modulating radiation budgets), contributing to further cooling. Unraveling the long-term evolution of MLA aridity and westerlies climate is an indispensable component of the unfolding mystery of global climate change.
Journal Article
Implications of the geochemistry of L1LL1 (MIS2) loess in Poland for paleoenvironment and new normalizing values for loess-focused multi-elemental analyses
Loess paleoenvironmental reconstructions on regional to supra-regional scales have recently gained much attention. Geochemistry comparisons in relation to reference datasets, such as the Upper Continental Crust (UCC) data, have furthered our understanding of the climatic and geomorphological conditions under which terrestrial sites have developed. However, UCC data differs from loess, thereby obscuring important features, and the existing “average loess” datasets also are not sufficient for modern investigations. In this study, we examine the youngest Polish loess (L1LL1 = MIS 2, ca. 26–15 ka) for its suitability as a new, loess-focused reference dataset. Eighty-nine samples from seven sites were analyzed, using inductively coupled plasma spectrometry. The loess had assumedly been homogenized during transportation and/or sedimentary recycling (LaN/SmN = 3.34–4.06, median 3.78; Eu/Eu* = 0.46–0.66, median 0.55; GdN/YbN = 1.08–1.49, median 1.26), and weakly affected by pre- or post-depositional weathering (CIA = 53.64–69.12, median 57.69). The statistically significant differences between sites in elemental medians were mostly conditioned by variations in grain size and in the “fresh” to “re-deposited” sediment ratio. Nonetheless, the overall geochemical composition homogeneity provided a basis for the estimation of Polish Median Loess (PML) data, as determined for 41 chemical elements. When used, PML data highlight differences between loess regions in Europe, thereby providing a tool for cross-continental comparisons.
Journal Article
Quantification of the pore structures of Malan loess and the effects on loess permeability and environmental significance, Shaanxi Province, China: an experimental study
The pore microstructures of loess control the porosity and permeability of the loess, affecting the patterns of groundwater flow and the transport of contaminants. In the present study, the pore microstructure of Malan loess was investigated quantitatively in samples from five different loess layers. Specimens were examined via SEM, and pore microstructure parameters were determined using the digital image analysis method. Pore structures (including pore area ratio (PAR), pore size distribution, pore shape and pore morphology), the effects of the pore structure on loess permeability and the environmental significance of these factors are discussed in this paper. The results indicate that the samples of five loess layers in this study are considered to be porous soil, according to both the PAR and porosity values. The number of micropores, small pores, mesopores and macropores decreased significantly. The differences in number were mainly due to the significant reduction in the number of round pores, indicating that round pores tend to be small. In terms of area, micropores, small pores, mesopores and macropores were dominated by elongated and irregular pores. The macro- and mesopore structural characteristics of loess determined the transfer characteristics of groundwater and pollutants in the loess. Additionally, the 62% area reduction in the area of elongated or irregular macro- and mesopores and the decreasing connectivity from top to bottom layers caused by the deformation and destruction of the pores due to the overlying soil reduced the permeability and the water/pollutant migration rate with depth.
Journal Article