Soluble salts in a Quaternary loess-soil sequence near Xining and the environmental implications

Dust composition is reflective to environmental conditions of the source areas, the transportation dynamics, and the post-depositional environments. In this study, soil chemical analysis and micromorphological approaches were conducted on a loess-soil sequence of the last 900ka near Xining at the western most part of the Loess Plateau. Thirty representative samples were selected to determine the pH, exchangeable sodium percentage (ESP), cation exchangeable capacity (CEC), and the exchangeable cations. Soil solutions of sixty representative samples were analyzed to determine the composition of soluble salts.All the analyzed samples show alkaline properties with the pH values varing from 8.5 to 10.0. pH values in loess layers (generally >9.0) are higher than in soil layers (generally <9.0). Except the Holocene soil S0 with weak alkaline property, ESP in loess varies from 15 ％ ~25 ％ while it ranges from 20 ％ ~40 ％ in soils. Both loess and soil layers contain a significant amount of soluble salts, including abundant sulfates that are thought to have significant climate impacts. Na+ represents a proportion of more than 70 ％ among cations. Among anions, CO2-3 and HCO-3 are more abundant in loess layers than in soil layers while SO2-4 and Cl- are more abundant in soils. These indicate that loess layers contain more Na2 CO3 and NaHCO3, while soil layers mainly contain NaCl and Na2SO4. Overall, soluble salts are much more abundant in soils than in loess layers.Micromorphological investigations show a weak weathering intensity of the studied soils, characterized by relatively fresh feldspars and biotite grains. All soil samples still contain detrital carbonates, suggesting their weakly developed steppe condition. These do not support a chemical weathering origin of the soluble salts. Because the studied section is located on high-terraces and no hydromorphic features were observed in either loess or soil layers, these soluble salts were unlikely to be originated from groundwater. Consequently, we interpret that these soluble salts were derived from eolian dust during the loess deposition. Salt-bearing dust should have been originated from the surrounding alkaline and saline lakes through eolian erosion.Our results therefore revealed a detrital origin for the abundant soluble salts. These indicate that alkaline/saline lakes have been constantly developed in the surrounding areas over the last 900ka. These lakes were probably seasonal and have experienced wind erosion during dry seasons. A most startling feature is the much higher content of soluble salts in loess layers than in soil layers, indicating that the interglacial dusts contain more soluble salts than glacial dusts. These might be attributable to the more abundant rainfalls associated with the higher temperatures and stronger evaporation during the interglacial times

Syn- and post-depositional modification of loess deposits and their paleoenvironmental significance

Loess-paleosol sequences are widely used for paleoenvironmental reconstructions. As a sub-aerial deposit, any level of a loess-soil section was exposed to land surface, and is subject to syn-depositional affections by surface processes. Also, because loess is sensible to erosion, post-depositional modifications are highly common in space. Here, some most commonly found syn-depositional and post-depositional features of loess deposits are studied mainly based on field and micromorphological investigations on the eolian deposits in northern China. Since most of these features are formed under specific environmental conditions, they may be used as indicators for paleoenvironmental reconstruction. Meanwhile, a number of modifications may affect the significance of some climate proxies that are frequently used in loess studies. Consequently, their influence should be taken into considerations in the climate interpretations. Firstly, we show that the bottom parts for most of the loess-soil sections usually contain some water-reworking features. This is because the substratum of loess deposits was never flat enough so that eolian dust deposited in the earliest stage was more or less affected by surface water. These processes may lead to a progressive flatting of the land surface, resulting in the formation of typical loess-soil sequences. We show also that syn-depositional affection of groundwater oscillations may result in ambiguous distinctions of loess and soil layers because of the disturbances on zonal pedogenic processes, while loess deposits without these affections have usually clear cyclical occurrences of loess and soil layers that are mostly regulated by Milankovitch cycles. Vegetation degradations under extremely dry conditions may cause eolian and sheet-water erosions of loess deposits. These features are highly common in the loess deposits in northern China. Wind and sheet-water erosion features are most frequently observed across the boundaries between loess and soil layers. These appear to suggest that climate transitions at orbital scales are usually marked by a severe dry event, leading to vegetation degradation. Freeze-thaw features are much less common in the loess deposits in China than in the loess of Europe and Americas, probably due to the much drier conditions. However, some weak frost features are observed in the northern and northwestern Loess Plateau while they appear to be absent in the southern Loess Plateau. These are consistent with the present-day climate pattern in the region, as is marked by warmer conditions in the southeastern part and cooler conditions in the northwestern part. A number of post-depositional modifications are also described in this report as they may influence the significance of some climate proxies. Also, because some features of the loess-soil sequences are easy to be confused with the modifications described above. Their basic morphological properties, formation processes and their significance are also discussed

Cenozoic evolution of geomorphic and sedimentary environments in the Tianshui-Qinan regions

In the western Loess Plateau in northern China,the Miocene eolian sequences reported in the recent years from different sites have extended to about 22Ma ago. The eolian origin of these sequences are evidenced by (1)their wide distribution mantling the broad highlands; (2)the spatially correlative magneto- and litho-stratigraphy,magnetic susceptibility and grain-size records; (3)the presence of several hundreds of paleosols with the interbedded loess layers; (4)the fine silty textures with the maximum grain-size mostly <120μm throughout the ca.16Ma sequence; (5)the similarity of quartz grain morphology and geochemical properties to Quaternary loess;(6)the well-preserved land snails and lack of aquatic species throughout the sequences; and (7)the cyclical changes of various climate proxies along the sequences,similar to those in Quaternary loess-soil sequences in China. These Miocene eolian sequences,combined with the well-known loess-soil sequences of the last 2.6Ma and the eolian Red-Clay (8.0～2.6Ma),provide a nearly continuous terrestrial climate record since the Early Neogene.In this study,we address the evolution of geomorphic and sedimentary environments in the Tianshui-Qin'an regions where Miocene loess deposits widely spread. Earlier geological studies showed that the Tianshui and Qin'an regions are within two distinct tectonic units,which have different tectonic histories. During the Paleogene,both regions are dominated by pluvial-plain landscapes as evidenced by the widely spread pluvial sandy-gravel deposits. A tectonic event occurred near the Oligocene-Miocene boundary,leading to the formation of plateau and basin settings in these regions. Since the early Miocene,typical loess-soil sequences started to form on the plateaus while secondary loess (loess-like deposits) were formed in the basins.During Late Miocene times,the Tianshui region experienced an intense erosion leading to the formations of some small shallow lakes. Their deposits are characterized by interbedded grey-green silty-clays with coarser fluvial sandy or gravel layers. Some of them are affected by pedogenesis. These indicate that these small lakes, were frequently dried up along with the fluctuations of paleoclimates. Loess deposits in the Qin'an region have also experienced erosions,resulting in the formations of some shallow depressions in which water-reworked loess or shallow-water deposits were formed. They sometimes contained large mammalian fossils. On the contrary, typical loess-soil sequences continued to form in flat areas, that contained only micromammalian and landsnail fossils. This Late Miocene erosion period,probably characterized by several erosion events,was started ca.11Ma ago and lasted until 7Ma. It may correspond to the Late Miocene growth of the Himalayan-Tibetan complex.The basic patterns of the hilly landscape in these regions were formed in Late Pliocene or Early Pleistocene times due to another intense erosion. The severely fragmented topography of the regions since then has not been favorable for latter loess deposition,thus the Pleistocene loess is purely preserved in these regions. This Late Pliocene or Early Pleistocene erosion may be linked to another tectonic event or to the climate changes relative to the northern hemispheric cooling

Higgs Boson Studies at the Tevatron

We combine searches by the CDF and D0 Collaborations for the standard model Higgs boson with mass in the range 90--200 GeV$/c^2$ produced in the gluon-gluon fusion, $WH$, $ZH$, $t{\bar{t}}H$, and vector boson fusion processes, and decaying in the $H\rightarrow b{\bar{b}}$, $H\rightarrow W^+W^-$, $H\rightarrow ZZ$, $H\rightarrow\tau^+\tau^-$, and $H\rightarrow \gamma\gamma$ modes. The data correspond to integrated luminosities of up to 10 fb$^{-1}$ and were collected at the Fermilab Tevatron in $p{\bar{p}}$ collisions at $\sqrt{s}=1.96$ TeV. The searches are also interpreted in the context of fermiophobic and fourth generation models. We observe a significant excess of events in the mass range between 115 and 140 GeV/$c^2$. The local significance corresponds to 3.0 standard deviations at $m_H=125$ GeV/$c^2$, consistent with the mass of the Higgs boson observed at the LHC, and we expect a local significance of 1.9 standard deviations. We separately combine searches for $H \to b\bar{b}$, $H \to W^+W^-$, $H\rightarrow\tau^+\tau^-$, and $H\rightarrow\gamma\gamma$. The observed signal strengths in all channels are consistent with the presence of a standard model Higgs boson with a mass of 125 GeV/$c^2$

Evidence for a Particle Produced in Association with Weak Bosons and Decaying to a Bottom-Antibottom Quark Pair in Higgs Boson Searches at the Tevatron

We combine searches by the CDF and D0 Collaborations for the associated production of a Higgs boson with a W or Z boson and subsequent decay of the Higgs boson to a bottom-antibottom quark pair. The data, originating from Fermilab Tevatron p[bar over p] collisions at √s=1.96  TeV, correspond to integrated luminosities of up to 9.7  fb[superscript -1]. The searches are conducted for a Higgs boson with mass in the range 100–150  GeV/c[superscript 2]. We observe an excess of events in the data compared with the background predictions, which is most significant in the mass range between 120 and 135  GeV/c[superscript 2]. The largest local significance is 3.3 standard deviations, corresponding to a global significance of 3.1 standard deviations. We interpret this as evidence for the presence of a new particle consistent with the standard model Higgs boson, which is produced in association with a weak vector boson and decays to a bottom-antibottom quark pair