Magnetostratigraphy and Anisotropy of Magnetic Susceptibility of the Lulehe Formation in the Northeastern Qaidam Basin

The timing of onset of deposition of the Lulehe Formation is a significant factor in understanding the genesis of the Qaidam basin and the evolution of the Tibetan Plateau. Here, we describe a detailed magnetostratigraphic and magnetic fabric study of the middle and lower parts of the Lulehe Formation. A total of 234 samples were collected from 117 sites throughout a thickness of almost 460 m of fluvial and lacustrine deposits at the Xitieshan section in the northeastern Qaidam basin. Out of these sites, 94 sites yielded well-defined characteristic remanent magnetization components by stepwise thermal demagnetization and were used to establish the magnetostratigraphy of the studied section. Based on correlation with the geomagnetic polarity timescale, the studied section spans the period from 53.8 Ma to 50.7 Ma. Our results show a three-fold decrease in sedimentation rates as well as marked change in facies from braided river to delta and shore-shallow lake around 52.6 Ma, which suggests tectonic uplift of the northeastern Qaidam basin margin ridge was rapid at the onset of formation of the Qaidam basin and subsequently weakened after 52.6 Ma. The anisotropy of magnetic susceptibility results indicate that tectonic compression stress had reached the northeastern Tibetan Plateau by the early stages of Indo-Eurasian plate collision and that the direction of stress in the study area was NE-SW. Furthermore, a weakening of tectonic compression stress around 52.6 Ma is consistent with sedimentary records. The age of initial deposition of the Qaidam basin （around 53.8 Ma） was almost synchronous with that of the Qiangtang, Hoh Xil, Xining, and Lanzhou basins, which implies that stress was transferred rapidly through the Tibetan Plateau during or immediately after the onset of Indo-Eurasian collision.

Co-occurrence, sources and co-enrichment mechanism of arsenic, fluoride in groundwater from Huaihe River Basin, China

Arsenic(As),fluoride(F^(−))are both ubiquitous in groundwater,and co-exposure to these elements through contaminated drinking water may cause detrimental effects on human health more in comparison with individual exposure.As,F^(−)co-occurrence in groundwater of the inland plain in Huaihe River Basin,China is a major concern,where inhabitants are rely on groundwater as the leading water source for drinking to date.This work employs an approach of hydrochemical analysis and modelling to identify the possible origin of As and F^(−),to analyze co-enrichment mechanism,and to estimate the associated exposure risk.The results shows presence of elevated As and F^(−)concentrations is an important factor affecting groundwater quality from 62 groundwater samples.The recorded As concentrations vary from 0.23 to 20.40μg/L,with a mean of 5.95μg/L,F^(−)concentrations vary from 0.54 to 2.60μg/L,with a mean of 1.29 mg/L,and 8%of samples are simultaneously above their permissible limits in drinking water by the WHO.Groundwater with As,F^(−)co-contamination is occurred within reducing and alkaline aquifers,and its chemical type is HCO_(3)–Na.The hydrochemical processes involved in the co-contamination are reductive desorption,evaporation,and ion exchange,which are controlled by local geology,geomorphology,and hydrochemistry.Groundwater As is derived and released by reductive desorption and F^(−)is mainly originated by fluorite dissolution.Groundwater As,F^(−)are geogenic sources,and the mechanisms for co-contamination are associated with high elemental abundance,flat terrain,alkaline and reductive groundwater conditions.The research provides a case study about groundwater As,F^(−)co-contamination,which may be enhance understanding the co-enrichment mechanism in semi-humid areas.

Defect engineering in N-doped OMC for lightweight and high-efficiency electromagnetic wave absorption

Developing low density and efficient dielectric loss materials has become a research hotspot,which can greatly meet the demands of modern radars and settle the problem of electromagnetic wave pollution.Herein,a series of N-doped ordered mesoporous carbon(OMC)materials with different nitrogen content were prepared via a modified self-assembly method and defect engineering in subsequent calcination treatment.It was discovered that the content and type of nitrogen doping can be effectively modulated by the amount of precursor dicyandiamide,resulting in the changes in porous structure,carbon defects,electromagnetic properties,microwave absorption(MA)performance and radar cross section(RCS)reduction values.Remarkably,as-fabricated OMC/N2.5 displays ideal MA performance,whose minimum reflection loss(RL(min))value reaches−35.3 dB at 7.76 GHz(3.0 mm)and its effective absorption bandwidth reaches 3.52 GHz(10.64-14.16 GHz,2.0 mm).Furthermore,the optimal RCS reduction values can be obtained as 12.01 dB·m^(2) when the detection theta is 30°,which validly reduces the chances of being detected by radar.Thus,this work opens up a novel way for the development of lightweight and high-efficiency MA materials.