Baryon loading and the Weibel instability in gamma-ray bursts

The dynamics of two counter-streaming electron–positron–ion unmagnetized plasma shells with zero net charge is analysed in the context of magnetic field generation in gamma-ray burst internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios.     

中蒙干旱半干旱区冬、夏季地面气温时空变化特征分析(Ⅱ):7月

为整体地分析中蒙干旱半干旱区(中蒙干旱区)夏季气温的时空变化特征,利用中蒙干旱区内分布相对均匀的104个站点(包括5个“人造站”)1961—1997年7月的平均地面气温资料等,首先分析了该区夏季的平均气温场、气温年较差及日较差等背景;然后,对标准化的7月平均气温距平资料分别作了EOF和REOF分析。结果表明:(1)中蒙干旱区夏季的热量资源充足。夏季气温场分布的区域性特色更明显。(2)受纬度、地形高度及远离海洋等的影响,中蒙干旱区地面气温的年较差大,日较差也大,呈典型大陆性气候(大陆度高)的特色,但是区内的夏季气温年际变化小。(3)根据气温的EOF分析,中蒙干旱区夏季气温异常可粗分为全区一致型、准东西差异型、准南北差异型及准东北—西南差异型等4种常见分布模态;再根据REOF分析,可将该区夏季气温异常进一步细分为青海高原、高原东北侧、南疆、北疆、陕西甘南区、蒙古国西北部及东南部等7个分区。(4)近37年来,中蒙干旱区夏季气温增暖主要出现在高原东北侧和南疆区,强度比冬春季弱;在青海高原及蒙古国东南部气温还在变凉,区内各分区夏季气温变化也约有3~4年的周期。     

Anatomically preserved plants in siderite concretions in the shale split of the Foord Seam: mineralogy, geochemistry, genesis (Upper Carboniferous, Canada)

A shale split in the well-studied Foord Seam (Upper Carboniferous), Stellarton Basin, Nova Scotia, provides a rare opportunity to study little known siderite concretions with anatomically preserved plants from a limnic basin. Siderite concretions occur throughout the split, commonly as sheet-like, inter-connected lepidodendrid logs, or less frequently as spherical masses; they are typically vuggy. A sample of 14 stratigraphically oriented concretions in the split shows that primary siderite varies from 45% to 80% (wt.%) due to selective replacement by 5% to 40% ferroan dolomite or ankerite in plant tissues. These two minerals show large Mg/Fe and Mg/Ca variability and are Fezoned. Clay content ranges from trace to 20%, and quartz from 5% to 10%. Partial cell-wall destruction by erosion and changeable oxic/anoxic conditions, prior to siderite permineralization, resulted in preservation mainly of the more resistant plant tissues. On average, carbon from siderite is isotopically heavier (+4.8±1.9 0/00) than that from ‘dolomite' (−7.2±1.1 0/00). The distribution of and the positive correlation between δ¹³C and δ¹⁸O probably reflects the two fractionation processes associated with methanogenesis and ‘dolomitization', respectively, in the genetic history of the siderite concretions in the split. Siderite concretions formed diagenetically early from bicarbonate under reducing conditions in a slightly alkaline freshwater environment that precluded calcite deposition.     

Temporal patterns and controls on runoff magnitude and solution chemistry of urban catchments in the semiarid southwestern United States

Urban expansion and the scarcity of water supplies in arid and semiarid regions have increased the importance of urban runoff to localized water resources. However, urban catchment responses to precipitation are poorly understood in semiarid regions where intense rainfall often results in large runoff events during the short summer monsoon season. To evaluate how urban runoff quantity and quality respond to rainfall magnitude and timing, we collected stream stage data and runoff samples throughout the 2007 and 2008 summer monsoons from four ephemeral drainages in Tucson, Arizona. Antecedent rainfall explained 20% to 30% of discharge (mm) and runoff ratio in the least impervious (22%) catchment but was not statistically related to hydrologic responses at more impervious sites. Regression models indicated that rainfall depth, imperviousness and their combined effect control discharge and runoff ratios (p < 0.01, r² = 0.91 and 0.75, respectively). In contrast, runoff quality did not vary with imperviousness or catchment size. Rainfall depth and duration, time since antecedent rainfall and event and cumulative discharge controlled runoff hydrochemistry and resulted in five specific solute response patterns: (i) strong event and seasonal solute mobilization (solute flush), (ii) event chemostasis and strong seasonal flush, (iii) event chemostasis and weak seasonal flush, (iv) event and seasonal chemostasis and (v) late seasonal flush. Our results indicate that hydrologic responses of semiarid catchments are controlled by rainfall partitioning at the event scale, whereas wetting magnitude, frequency and timing alter solute stores readily available for transport and control temporal runoff quality. Copyright © 2012 John Wiley & Sons, Ltd.     

Modelling regional effects of artificial groundwater recharge in a multilayer aquifer characterized by perched water tables

This paper presents an approach to estimate the effects of a managed recharge experiment in a multilayer aquifer characterized by the presence of perched water tables in the Medina del Campo groundwater body, Douro basin, central Spain. A numerical model was developed to evaluate the effect of artificial recharge on the shallow sector of a regional-scale aquifer and on formerly active wetlands. The model was developed in the Visual MODFLOW Pro v.2011.1 environment in order to represent and analyse the regional impact of this artificial recharge event. Results suggest that the assumption of a single perched system may prove useful in regional contexts where data is limited. From a study site perspective, managed recharge is observed to increase shallow storage along the riverbanks, which is considered valuable for environmental purposes. However, downstream wetlands are unlikely to experience a significant recovery. Furthermore, only a small percentage of artificial recharge is expected to reach the deep regional aquifer. This method can be exported to settings characterized by the presence of perched aquifers and associated groundwater dependent ecosystems.     

The influence of salinity on the solubility of Zn and Cd sulphides in the Scheldt estuary

In the estuary of the river Scheldt, where an oxygen gradient exists in addition to the salinity gradient, redox processes will be of major importance for trace metal mobilisation. In this study, the influence of salinity and pH on the redox processes of dissolved Zn and Cd sulphides is investigated together with the effects on the ratio of the dissolved Zn and Cd concentrations. The speciation of these metals is calculated with the chemical equilibrium programme +. Zn sulphides are oxidised at lower oxygen concentrations than Cd sulphides, due to lower stability constants, causing a sudden increase or peak in the dissolved Zn/Cd ratio. The formation of dissolved Cd chloride complexes when oxidation occurs at high salinities (S=15) increases the mobility of Cd, causing a decrease in the Zn/Cd peak of the total dissolved concentrations. The peak is three to four times smaller at S=15 than when oxidation occurs at S=2. The simple model calculations compare very well with field data. The Scheldt estuary is suitable to illustrate these calculations. In the 1970s, the anoxic part of the estuary reached S=15–20, but since the early 1980s it has dropped to S=2–10. Historic data on metals in the estuary from 1978, 1987 and the 1990s were used to compare with the equilibrium calculations. The increase of the dissolved Zn/Cd peak at low salinity as a consequence, of the decreasing anoxic region is confirmed well by the data. The good agreement between model calculations and field data is a proof of the extreme importance of redox processes for the solubility of Zn and Cd sulphides in the estuary.     

Sample treatment with focused ultrasound and bath sonication as a powerful tool for the evaluation of cadmium pollution in estuarine waters

A new sample treatment was developed for the determination of dissolved cadmium in Tagus estuarine waters, based on focused ultrasound in conjunction with small volumes in the extraction steps for Cd pre-concentration. Cadmium was first pre-concentrated using a classical approach (APDC as the complexing agent and MIBK as the organic phase) and then back-extracted into HNO₃ with the aid of focused ultrasound, which reduced the acid concentration by more than one order of magnitude (from 4 to 0.1 mol L^− 3). This sample treatment was accomplished in less than 5 min, using low sample volume (20 mL), and low-volume, low-concentration reagents. The pre-concentration factor used in this work was 25, but different sample/organic volume ratios may be used in order to increase that value. The limit of detection and the limit of quantification in Tagus water samples were 0.03 nmol L^− 1 and 0.1 nmol L^− 3, respectively. Recoveries from spiked Tagus water were higher than 90%. The procedure was validated using the reference estuarine water NRC-SLEW-3. In the solubilization of Cd particulate, bath ultrasonication was used in conjunction with HNO₃ + HCl, followed by H₂O₂, which took about 2 instead of the usual 12 h (cooling included) when high-pressure microwave digestion is used.