Non-equilibrium flocculation characteristics of fine-grained sediments in grid-generated turbulent flow

Results are presented from a series of settling column experiments investigating temporal variations in the flocculation characteristics of purely cohesive (kaolin clay) sediment suspensions and cohesive (kaolin) and non-cohesive (fine sand) sediment fraction mixtures. Experimental runs were conducted under controlled hydrodynamic conditions generated by a rigid array of in-phase oscillating grids. The results indicated that rapid initial floc aggregation occurred under low turbulent shear rates, with peak maximal and root-mean-square (r.m.s.) floc sizes (∼ 400 μm and ∼ 200 μm, respectively) attained after relatively short time periods, before reducing with time. By contrast, lower aggregation rates and smaller floc sizes were observed under higher shear conditions, with flocs retaining suspended in the settling column for longer time scales due to the increased turbulence. The mud input concentration displayed some correlation with maximal and r.m.s. floc sizes at higher shear rates but no correlation was apparent at low shear rates. This observed floc behaviour may be attributed to the differences in concentration gradients at high and low shear rates that affect both floc settling rate and time required for flocs to attain equilibrium size. The addition of the fine sand fraction to the kaolin clay suspension reduced both the initial floc formation (i.e. aggregation) rate and the maximal and r.m.s. floc sizes attained throughout the experiments. The reduction in maximal floc sizes appeared to be enhanced by an increase in the ratio of fine sand to kaolin clay content within the mixture.     

Mineralogy and petrogenesis of lunar magnesian granulitic meteorite Northwest Africa 5744

Lunar meteorite Northwest Africa (NWA) 5744 is a granulitic breccia with an anorthositic troctolite composition that may represent a distinct crustal lithology not previously described. This meteorite is the namesake and first‐discovered stone of its pairing group. Bulk rock major element abundances show the greatest affinity to Mg‐suite rocks, yet trace element abundances are more consistent with those of ferroan anorthosites. The relatively low abundances of incompatible trace elements (including K, P, Th, U, and rare earth elements) in NWA 5744 could indicate derivation from a highlands crustal lithology or mixture of lithologies that are distinct from the Procellarum KREEP terrane on the lunar nearside. Impact‐related thermal and shock metamorphism of NWA 5744 was intense enough to recrystallize mafic minerals in the matrix, but not intense enough to chemically equilibrate the constituent minerals. Thus, we infer that NWA 5744 was likely metamorphosed near the lunar surface, either as a lithic component within an impact melt sheet or from impact‐induced shock.     

Estimating Solar Flux Density at Low Radio Frequencies Using a Sky Brightness Model

Sky models have been used in the past to calibrate individual low radio frequency telescopes. In this article we generalize this approach from a single antenna to a two element interferometer, and formulate the problem in a way that allows us to estimate the flux density of the Sun using the normalized cross-correlations (visibilities) measured on a low resolution interferometric baseline. For wide field-of-view instruments, typically the case at low radio frequencies, this approach can provide robust absolute solar flux calibration for well characterized antennas and receiver systems. It can provide a reliable and computationally lean method for extracting parameters of physical interest using a small fraction of the voluminous interferometric data, which can be computationally prohibitively expensive to calibrate and image using conventional approaches. We demonstrate this technique by applying it to data from the Murchison Widefield Array and assess its reliability.     

Taxonomic Studies on a New Marine Ciliate, Apocoleps magnus gen. nov., spec. nov. （Ciliophora, Colepidae）, Isolated from Qingdao, China

The morphology and infraciliature of a new marine colepid ciliate, Apocoleps magnus gen. nov., spec. nov., are described based on living observations and silver impregnations. The new genus Apocoleps is characterized by having 8 (vs. 6 in most other related genera) armour tiers, spines at both ends of the cell, 3 adoral organelles and plates with 4 reniform uni-windows. Apocoleps magnus spec. nov. is defined by the following features: body elongated and slightly curved, about 100–120μm× 35–45 μm in vivo; an...     

运用nrDNA ITS 数据研究角叉菜属(Chondrus)在红藻门中的系统进化地位

测定了角叉菜属(Chondrus)5个代表种的核糖体DNA内转录间隔区(ITS)及5.8SrDNA基因序列。结果表明,角叉菜属ITS区(含ITS1、5.8SrDNA和ITS2)序列长度范围为704—714bp,G+C含量为44.6%—45.7%,变异位点69个,信息位点16个;其中,ITS1和ITS2的长度范围分别为147—149bp和398—404bp。5.8SrDNA长度为158bp,没有变异和信息位点。由MEGA3构建的系统进化树(ME和MP)显示:在进化尺度上,真红藻纲的松节藻科(Rhodomelaceae)与红毛菜纲(Bangiophyceae)亲缘关系较近。在真红藻纲内,杉藻目(Gigartinales)的进化地位相对较高,其次是海膜科(Halymeniaceae)、石花菜科(Gelidiaceae)、红叶藻科(Delesseriaceae)和粉枝藻科(Liagoraceae)等,而松节藻科进化地位相对较低。在杉藻目内,杉藻科(Gigartinaceae)和胶黏藻科(Dumontiaceae)进化关系密切,而形态学特征相似的角叉菜和马泽藻(Mazzaella)亲缘关系非常近。     

Honey, I cooled the cods: Modelling the effect of temperature on the structure of Boreal/Arctic fish ecosystems

Historically colder regions of the North Atlantic had fisheries dominated by only a few fish species; principally cod and capelin. Possible population dynamic mechanisms that lead to such dominance are investigated by considering how a charmingly simple published multispecies model of the North Sea would react if the system operated at a lower temperature. The existing model equations were modified to describe temperature effects on growth, fecundity and recruitment and the model was rerun based on typical temperatures for the North Sea and a colder system. The results suggest that total fish biomass in the colder system increases but the community is more vulnerable to a given rate of fishing mortality. In the colder system, within species density dependence is reduced but relative predation rates are higher. Consequently, intermediate-sized species are vulnerable to relatively high levels of predation throughout their life history and tend to be excluded, leading to a system dominated by small and large species. The model helps to explain how temperature may govern coexistence and competitive exclusion in fish communities and accounts for the observed dominance of small and large species in Boreal/Arctic ecosystems.     

Degradation of wetlands on the Qinghai-Tibet Plateau: A comparison of the effectiveness of three indicators

The Qinghai-Tibet Plateau encompasses a large quantity of wetlands, some of which have been degraded to varying severity levels. In the literature, a number of degradation indicators have been proposed to evaluate ecological health of wetlands, but their effectiveness in the plateau environment remains unknown. In this study, we assessed the effectiveness of three degradation indicators, soil moisture content at 10 cm deep, vegetative cover, and density of pika burrows. The degradation severity of wetlands in Maduo County on the Qinghai-Tibet Plateau is enumerated at four levels, intact, slight, moderate and severe. Analysis of 106 samples collected in the field demonstrates that the density of pika burrows is the least reliable indicator. By comparison, vegetative cover and underlying soil moisture content are more reliable, even though neither is a perfect indicator as the difference among adjacent levels of severity as revealed by t-test is not always statistically significant. The imperfection of vegetative cover as an indicator is due to its variation among different types of wetlands. The limitation of moisture content is attributed to its non-linear relationship with wetland degradation. Above the threshold of about 50% in moisture content wetlands are unlikely to be degraded. It is recommended that moisture be measured at the point near the surface and vegetative cover be further differentiated by species in order to improve their effectiveness.     

Warm frontal structure in association with a rapidly deepening extratropical cyclone

Abstract

The detailed characteristics of a CASPII warm frontal passage are presented in this article. This storm, Intensive Operating Period (IOP) 13 (February 26–27, 1992), was observed in detail with an array of diverse instruments. It has the advantage over earlier freezing precipitation studies of having simultaneous, in situ and remote sensing measurements by aircraft and ground‐based Doppler radar.

The associated precipitation was in the form of banded structures parallel to the front. Within these bands were embedded precipitation cores, some parallel to the band, some perpendicular. The warm front itself was characterized by major perturbations in its kinematic and thermodynamic features. The cores oriented parallel to the front were the result of embedded convection generated, at least in part, by the irregularities in the frontal surface.

The cores oriented perpendicular to the front were closely associated with the 0°C isotherm on the underside of the frontal inversion. Precipitation phase changes played a significant role in the occurrence of wide near 0°C regions, both vertically and horizontally. These regions had a profound influence on the observed precipitation types and led to complex precipitation‐thermodynamic‐dynamic interactions. Instabilities produced by these interactions are seen in wave‐like features observed by the Doppler radar in these regions, both parallel and perpendicular to the frontal zone.     

Trends,drivers and impacts of changes in swidden cultivation in tropical forest-agriculture frontiers: A global assessment

This meta-analysis of land-cover transformations of the past 10–15 years in tropical forest-agriculture frontiers world-wide shows that swidden agriculture decreases in landscapes with access to local, national and international markets that encourage cattle production and cash cropping, including biofuels. Conservation policies and practices also accelerate changes in swidden by restricting forest clearing and encouraging commercial agriculture. However, swidden remains important in many frontier areas where farmers have unequal or insecure access to investment and market opportunities, or where multi-functionality of land uses has been preserved as a strategy to adapt to current ecological, economic and political circumstances. In some areas swidden remains important simply because intensification is not a viable choice, for example when population densities and/or food market demands are low. The transformation of swidden landscapes into more intensive land uses has generally increased household incomes, but has also led to negative effects on the social and human capital of local communities to varying degrees. From an environmental perspective, the transition from swidden to other land uses often contributes to permanent deforestation, loss of biodiversity, increased weed pressure, declines in soil fertility, and accelerated soil erosion. Our prognosis is that, despite the global trend towards land use intensification, in many areas swidden will remain part of rural landscapes as the safety component of diversified systems, particularly in response to risks and uncertainties associated with more intensive land use systems.