Dynamics and driving factors of late Holocene gullying in the Main Ethiopian Rift (MER)

The issue of determining the driving factors in gullying, apart from land use, is somewhat lagging in comparison with the study of their physical modelling and control technology. Available information focuses on the basic ideas of climatic control, anthropic determinism and internal “authigenic” dynamics. High resolution chronology of cyclic systems, common in extensively gullied areas, can provide a clue to the weight of each factor. This paper reports a study of this kind, focusing on two gully catchments in the Main Ethiopian Rift (MER), but backed by an extensive regional survey. By integrating tracing and correlation of unconformity-bounded stratigraphic units and soils with radiocarbon dating, a detailed chronology was obtained for the last 5000 years. This could be compared with proxy climate reconstructions of similar detail. Clear evidence of climatic control emerged; gully filling is triggered by decreased stream transport capacity and increased sediment supply during transitions towards drier climate phases, while gully entrenchment appears to take place at the start of moist intervals, for the reverse reasons. A broader consideration of geological setting, however, puts forward a more general interpretation. These gullies actually formed, in the beginning, as part of the land surface response to sudden, very recent tectonic events, which created accommodation space for temporary sediment stores. They should then be seen in the frame of the Discontinuous Ephemeral Stream (DES) concept; as such, they are intrinsically non-linear and complex phenomena, whose response is linearized by a strong climatic–vegetational forcing, acting on both channel flow and sediment supply.     

Interannual variations of the Antarctic Ocean CO2 uptake from 1986 to 1994

The interannual variations of CO₂ sources and sinks in the surface waters of the Antarctic Ocean (south of 50°S) were studied between 1986 and 1994. An existing, slightly modified one-dimensional model describing the mixed-layer carbon cycle was used for this study and forced by available satellite-derived and climatological data. Between 1986 and 1994, the mean Antarctic Ocean CO₂ uptake was 0.53 Pg C year⁻¹ with an interannual variability of 0.15 Pg C year⁻¹.Interannual variation of the Antarctic Ocean CO₂ uptake is related to the Antarctic Circumpolar Wave (ACW), which affects sea surface temperature (SST), wind-speed and sea-ice extent. The CO₂ uptake in the Antarctic Ocean has increased from 1986 to 1994 by 0.32 Pg C. It was found that over the 9 years, the surface ocean carbon dioxide fugacity (fCO₂) increase was half that of the atmospheric CO₂ increase inducing an increase of the air–sea fCO₂ gradient. This effect is responsible for 60% of the Antarctic Ocean CO₂ uptake increase between 1986 and 1994, as the ACW effect cancels out over the 9 years investigated.     

Wood storage in three mountain streams of the Southern Andes and its hydro‐morphological effects

This study analyses large wood (LW) storage and the associated effects on channel morphology and flow hydraulics in three third‐order mountain basins (drainage area 9–12 km²) covered in old‐growth Nothofagus forests, ranging from the temperate warm Chilean Andean Cordillera to the sub‐Antarctic Tierra del Fuego (Argentina). Amount, characteristics and dimensions of large wood (>10 cm diameter, >1 m long) were recorded, as well as their effects on stream morphology, hydraulics and sediment storage. Results show that major differences in LW abundance exist even between adjacent basins, as a result of different disturbance histories and basin dissection. Massive LW volumes (i.e. >1000 m³ ha^?1) can be reached in basins disturbed by fires followed by mass movements and debris flows. Potential energy dissipation resulting from wood dams is about a quarter of the total elevation drop in two streams, with a gross sediment volume stored behind wood dams of around 1000 m³ km^?1, which appears to be of the same order as the annual sediment yield. Finally, the presence of wood dams may increase flow resistance by up to one order of magnitude. Copyright © 2007 John Wiley & Sons, Ltd.     

Indentation of a continent with a built-in thickness change: experiment and nature

Orogens oblique to the direction of plate convergence are currently attributed to obliquity between the margins of one or both of the sutured continents to their direction convergence. We use a single analogue experiment and natural examples to illustrate a potential additional factor: variations in strength of the indented continent at a high angle to the convergence direction. The wavelengths of structures in laterally shortened lithosphere depend on the strength of the most competent layers. Lateral variations in crustal thickness must therefore lead to structures oblique to any applied lateral compression.

An analogue experiment was performed to explore this phenomenon. A two-layer ‘indented continent’ was modelled by a brittle upper crust of sand above a lower crust of high-viscosity polymer floating on a single layer of low-viscosity syrup representing the mantle. The well-known strike-slip structures allowing lateral escape to distant weak boundaries were hindered by lateral boundaries in front of the indenter. This allowed us to focus on the effects of a thickness change built into the ‘indented continent’ along a zone parallel to the direction in which a vertical rigid wall advancing at a steady rate represented the indenter. Vertical escape led to an ‘orogenic belt’ oblique to the advancing wall; this obliquity influences subsequent lateral escape. Model scaling and interpretations are based on Extended Thin Sheet Approximation (ETSA) and standard theories of faulting.

Four sectors of the Alpine–Himalayan orogen (Iran, Tunisia, the Eastern Alps and the Himalaya) are oblique to the continental convergence direction, and we point to thickness changes at high angles to the suture that may account for this geometry. As crustal thicknesses north of oblique sectors of the Himalayas are not yet known, we speculate on them.

We infer from the main difference between our experiment and all our examples chosen from nature that vertical orogenic escape was oblique to our model suture but can be parallel to natural sutures.     

Gas Dynamics in Planetary Nebulae: From Macro-structures to FLIERs

Purpose of this paper is to clarify how Planetary Nebulae (PNe) are very interesting laboratories to study cosmic gas dynamics. I first recall the history of PNe which are generated from low and intermediate mass stars through successive mass loss processes starting in the Reg Giant phase of evolution and continuing also after the termination of the pulsed AGB phase, where most of the nebular mass is believed to be ejected. The correpondings tellar winds are the ingredients of the nebula. Their initial properties and subsequent mutual interactions, under the action of the evolving stellar radiation field, are responsible for the properties of the nebula. The observed structures of PNe are considered in detail. Larger scale macroscopic structures (MACS) are examined separately from quite smaller scale microscopic structures (MICS). The formation of MACS, at least in cases of round to moderately elliptical PNe, is shown to be reasonably well understood in terms of existing hydrodynamical models. Considering the kinematical behaviour, MICS can be separated into FLIERs (Fast Low Ionization Emitting Regions) and SLOWERs (slowly moving). Attention is focussed on FLIERs and on the proposed mechanisms to interpret them. Recent observations with the Hubble Space Telescope have provided us with a wealth of detailed (subarcsec) information on the nebular structures. The inner structure of FLIERs is here illustrated to consist of substructures of various shapes with an high degree of individually from object to object, also within the same PN. These new data call for deeper thoretical efforts to solve the problems of cosmic gas dynamics, posed by their observed properties. An ample account is given of the most relevant original works, in an effort to allow the non specialist reader to quickly become acquainted with the status of art in the various aspects of the subject. This revised version was published online in July 2006 with corrections to the Cover Date.     

Impact of Monthly Radioxenon Source Time-Resolution on Atmospheric Concentration Predictions

The general characterisation of the global radioxenon background is of interest for the verification of the Comprehensive Nuclear-Test-Ban Treaty. Since the major background sources are only a few isotope production facilities, their source term has an emphasized influence on the worldwide monitoring process of radioxenon. In this work, two different datasets of source terms are applied through atmospheric transport modelling, to estimate the concentration at two radioxenon detection stations in Germany and Sweden. One dataset relies on estimated average annual emissions; the other includes monthly resolved measurements from an isotope production facility in Fleurus, Belgium. The quality of the estimations is then validated by comparing them to the radioxenon concentrations that have been sampled at two monitoring stations over the course of 1 year.     

砂土的流体动力学方程与本构模型的比较

目前砂土材料连续力学方程建模有两种途径,一种是源自于论证Navier-Stokes方程时所用的方法,称为流体动力学方法,主要针对普通固体、普通流体、超流、液晶、高聚物、颗粒物质等材料,它的基本做法是先给出守恒变量（如能量、动量、熵等）和对称破缺变量（如弹性应变、量子相位等）的一般运动方程,再用与具体材料有关的热力学函数和迁移系数来封闭它们;另一种方法是本构模型法,针对有塑性固体、非牛顿流体、砂土材料等材料,该做法特点是直接构建应力、应变、应力率、应变率、速度、密度、温度等变量之间的函数关系,并以此来封闭连续和牛顿方程。近年来采用这两种方法建立的砂土方程在文献上都有报道,因此,有必要对两者的特色、科学基础、适用范围,包括概念上的和具体方程等,进行比较,作为颗粒物理与土力学之间跨学科交流的一种尝试和沟通。通过比较得出,两种建模途径对表征砂土状态的完备变量集、屈服面、与塑性有关的耗散等使用了很不一样的思路和方程。     

Force reduction factors for steel buildings with welded and post-tensioned connections

The seismic responses of steel buildings with perimeter moment resisting frames (MRF) with welded connections (WC) are estimated and compared to those of similar buildings with semi-rigid post-tensioned connections (PC). The responses are estimated in terms of ductility reduction factors (R _µ,), ductility demands (µ _G ) and force reduction factors (R). Two steel model buildings, which were modeled as complex-3D-MDOF systems, were used in the study. Results indicate that the reduction magnitude of global response parameters is larger than that of local response parameters, contradicting the same reduction implicitly assumed in the static equivalent lateral force procedure, implying that non-conservative design may result. The value of 8 for R, suggested in many codes for ductile steel MRF, and the value of 1 suggested in the well known Newmark and Hall procedure for the ratio of R to µ _G , cannot be justified. The reason for this is that SDOF systems were used to model actual structures, where higher mode effects, energy dissipation and structural overstrength weren’t explicitly considered. The codes should be more transparent regarding the magnitude and the components involved in the force reduction factors. The seismic performance of steel buildings with PC may be superior to that of the buildings with WC, since their force reduction factors are larger and their ductility demands smaller, implying that PC buildings could be designed for smaller lateral seismic forces. The conclusions of this paper are for the particular structural systems and models considered. Much more research is needed to reach more general conclusions.