Origin of a complex and spatially diverse dune-field pattern, Algodones, southeastern California

The Algodones Dune Field of southeastern California shows a complex and spatially diverse dune-field pattern that is superimposed upon a series of topographic lineations. Analysis of dune-field pattern parameters (dune crest length, crest orientation, dune spacing and defect density) derived from aerial images indicates that the dune-field pattern represents two constructional generations. Prominent compound crescentic dunes formed during the first constructional generation. A younger generation consists of a variety of simple crescentic dunes, linear dunes and zibars. Statistical differences in the pattern parameters between the dune groups within the second generation are resolved through consideration of the boundary conditions under which the dune pattern evolved, and provide explanations for: (1) diversity of dune types, (2) range in implied constructional times, (3) range in crest orientations, and (4) the anomalous nature of the population of linear dunes. The boundary conditions that have modified pattern development include orographic effects, grain size, vegetation, areal extent and antecedent conditions. Topographic lineations in the Algodones range from the Western Ramp, which defines the field margin, to subtle features masked by the pattern of dunes. Imaging of the Western Ramp using Ground Penetrating Radar shows high-angle cross-strata migrating perpendicular to the lineation trend. The most plausible hypothesis for the origin of the lineations is as dune ridges sequentially shed from adjacent Lake Cahuilla, which is the source of Algodones sands. The overall geomorphic complexity of the Algodones originates from the emplacement of the dune ridges during stages of Lake Cahuilla, the two generations of dune-field construction, and the controls exerted by boundary conditions.     

64层锥束CT扫描的优化系统

近年来X射线断层成像(CT)技术获得了突飞猛进的发展,自1998年推出4层螺旋CT后,CT扫描设备在容积覆盖,空间分辨率,扫描速度,切片数方面取得了长足进展.这不仅给医学应用带来了深远的影响,同时也给CT系统设计提出了巨大的挑战.容积CT(VCT)的设计过程引入了各种策略来战胜其复杂性.这些方法学包括:理论分析,系统性能预测的图像分析工具,各种基于专家背对背评价的参数优化.本文论述了64层CT系统设计中的一些考虑因素及优化过程.这些设计过程保证了锥束CT的优化性能.首批客户的应用反馈显示了我们设计实践的有效率性.     

Managing Ontong Java: Social institutions for production and governance of atoll resources in Solomon Islands

This paper explores the changing nature of social institutions and organizations for resource management on Ontong Java and their central role in maintaining livelihoods. Using detailed field data for three time periods, 1970–72, 1986 and 2006–07, and drawing on earlier secondary data, a longitudinal analysis of changes in governance and livelihoods is undertaken. Increased exploitation of marine resources has widened the resource base and increased people's access to goods and services. Following the ban in 2005 on the bêche-de-mer trade, however, livelihoods have reverted to being more subsistence oriented. For almost two decades after 1978, a communal organization, the Area Council, succeeded in regulating exploitation of the atoll's marine resources to ensure sustainable use, but in the 1990s it collapsed. In addition there was a paralysis of institutions that had once settled land disputes. This failure of atoll governance is a greater threat to future 'sustainability' than the usual processes that are invoked for atolls and small islands generally. As the Solomon Islands state is also failing, we argue that local institutions and organizations have a vital role to play in managing access to resources and the future of livelihoods on Ontong Java atoll.     

Simultaneous determination of global topography, tidal Love number and libration amplitude of Mercury by laser altimetry

Solar tidal forces generate elevation changes of Mercury's surface of the order 1 m within one Hermean year, and solar torques on the non-symmetric permanent mass distribution of the planet cause an uneven rotation of Mercury's surface with a libration amplitude of the order of 40 arcsec. Knowledge of the precise reaction of the planet to tidal forcing, expressed by the Love numbers h₂ and k₂, as well as accurate knowledge of the amplitude of forced libration Φ_lib, puts constraints on the internal structure, for example the state and the size of the core. The MESSENGER and BepiColombo missions to Mercury carry laser altimeters, whose primary goal is to accurately map the topography. Here we investigate if the Love number h₂ and the amplitude of forced libration can be determined together with the static topography of the planet from a global altimetry record. We do this by creating synthetic altimeter data for the nominal orbit of BepiColombo over the nominal mission duration of approximately four Mercury years and inverting them for the static and time-dependent parts of the topography. We assume purely Gaussian noise. We find that it is possible to extract both parameters h₂ and Φ_lib with an accuracy of approximately 10%, while the static topography coefficients of a spherical harmonic expansion can be determined simultaneously with an accuracy at the centimetre level. Extraction of the static topography to higher harmonic degrees improves the precision of the measurement of h₂ and Φ_lib. The simulation results demonstrate that it seems feasible to test current models on Mercury's interior with sufficient precision using BepiColombo Laser Altimeter data.     

Influence of vascular plant photosynthetic rate on CH4 emission from peat monoliths from southern boreal Sweden

Peat monoliths taken from a boreal peatland system were incubated at two different light intensities to investigate the effect of the photosynthetic rate of vascular plants ( Eriophorum angustifolium ) on net CH₄ emission. The experimental set-up consisted of six replicate monoliths as controls and six where the photosynthetic active radiation (PAR) was reduced by 60%. NEP and total system respiration decreased significantly in response to reduced PAR. No significant changes in CH₄ emission were found, but two different trends were noted. Methane emissions from the shaded monoliths initially seemed to be higher than emissions from the controls. After approximately four weeks the trend was reversed. The pattern may have been caused by "leakage" of organic compounds from inactivated roots that fueled CH₄ production. It is suggested that a new balanced exchange of potential substrate carbon between the plants and the surrounding peat was established. Comparably less easily degradable carbon compounds would then become available for CH₄ production. The fact that there appeared to be an effect of decreased carbon flow on CH₄ emission is further supported by a tendency for lower concentrations of organic acids in porewater in the shaded monoliths at the end of the experiment. These results indicate a possible lagtime on the order of weeks before changes in photosynthesis rates and NEP have an effect onCH₄ emission rates. Nevertheless it confirms the linkage between CO₂ and CH₄ cycling in wetland ecosystems.     

On the role of domain size and resolution in the simulations with the HIRHAM region climate model

We investigate the simulated temperature and precipitation of the HIRHAM regional climate model using systematic variations in domain size, resolution and detailed location in a total of eight simulations. HIRHAM was forced by ERA-Interim boundary data and the simulations focused on higher resolutions in the range of 5.5–12 km. HIRHAM outputs of seasonal precipitation and temperature were assessed by calculating distributed model errors against a higher resolution data set covering Denmark and a 0.25° resolution data set covering Europe. Furthermore the simulations were statistically tested against the Danish data set using bootstrap statistics. The results from the distributed validation of precipitation showed lower errors for the winter (DJF) season compared to the spring (MAM), fall (SON) and, in particular, summer (JJA) seasons for both validation data sets. For temperature, the pattern was in the opposite direction, with the lowest errors occurring for the JJA season. These seasonal patterns between precipitation and temperature are seen in the bootstrap analysis. It also showed that using a 4,000 × 2,800 km simulation with an 11 km resolution produced the highest significance levels. Also, the temperature errors were more highly significant than precipitation. In similarly sized domains, 12 of 16 combinations of variables, observation validation data and seasons showed better results for the highest resolution domain, but generally the most significant improvements were seen when varying the domain size.     

Behaviour of rubble-mound breakwater in directional and uni-directional waves

The application of directional waves in design processes has clearly received much interest during recent years. Thus, in model testing with moored ships and offshore structures, significant deviations have been revealed between results obtained in traditional uni-directional wave trains and those obtained in directional wave fields. Whether the same tendency is valid in connection with the design of rubble mound breakwaters is studied in the present project. A breakwater with a front slope of 1:2 is tested at the scale 1:40. The weight and diameter of the stones in the core, filter and armour layers were carefully selected, and the sources of scatter in repeated tests were minimized. Furthermore, the incident wave energy was accurately adjusted to keep the same level in both uni-directional and directional waves. For the actual rubble-mound breakwater unambiguous results were obtained. When uni-directional waves were applied, the damage increased by 30–50% relative to the directional wave situation.     

Velocities of a natural mid-ocean ridge basalt glass

We report, for the first time, ultrasonic velocity values for a pure (>95%), natural, submarine basalt glass of mid-ocean ridge basalt composition, from 10 to 1000 MPa at room temperature. These new data show that basalt glass, abundant in the upper oceanic crust, has the lowest velocity of any primary solid component of the oceanic crust. In addition, natural basalt glass has a steeper pressure-dependence of velocity than previously measured in more crystalline samples, indicating that cracks in natural basalt glass are weaker than in more crystalline rocks. To obtain values for the pure glass phase, we correct the natural glass data for the low-pressure closure of cracks, and the presence of minor mineralogic components and vesicles. These new data provide a baseline for evaluating the effect of abundant basalt glass and glassy mesostasis in oceanic upper crust on in situ seismic velocities. In addition, data on the elastic and seismic properties of natural glasses are useful for a better understanding of glass structure, and glass relaxation, with potential applications to submarine volcanology.