A conservative, unconditionally stable, second-order three-point differencing scheme for the diffusion–convection equation

A three-point differencing scheme for the diffusion–convection equation is presented that offers all the advantages of both the central and the one-sided ('upwind') differencing scheme without suffering from their drawbacks. Specifically, the scheme is conservative, unconditionally stable, and second-order-accurate in space. It is free of oscillations and over- or undershoots, simple to code, and requires essentially no more computing time than the one-sided scheme. Although known for a relatively long time in numerical mathematics, the scheme apparently has not received sufficient attention from modellers of hydrothermal systems or contaminant transport in the geosciences. In order to fill this gap a comparison is made between this scheme and the widely used one-sided scheme for the transient diffusion–convection equation in different time discretizations. The results are discussed taking into account other approaches towards minimizing numerical diffusion.     

Wadi Bakht revisited: Holocene climate change and prehistoric occupation in the Gilf Kebir region of the Eastern Sahara,SW Egypt

Geoarchaeological and chronological evidence from the remote Gilf Kebir Plateau in southwest Egypt suggests a new model for the influence of early and mid‐Holocene precipitation regimes on land‐use strategies of prehistoric settlers in what is now the center of the largest hyperarid area on earth. We hypothesize that the quantitatively higher, daytime, monsoon summer rainfall characteristic of the early Holocene (9300–5400 ¹⁴C yr B.P./8400–4300 yr B.C.) resulted in less grass growth on the plateau compared to the winter rains that presumably fell in the cool nights during the terminal phase of the Holocene pluvial (5400–4500 yr B.P./4300–3300 yr B.C.). The unparalleled climatic transition at 5400 yr B.P. (4300 yr B.C.) caused a fundamental environmental change that resulted in different patterns of human behavior, economy, and land use in the canyon‐like valleys and on the plains surrounding the plateau. The model emphasizes the crucial impact of seasonal rainfall distribution on cultural landscapes in arid regions and the lower significance of annual precipitation rates, with implications for future numeric climate models. It also serves as an example of how past climate changes have affected human societies. © 2004 Wiley Periodicals, Inc.     

Mineralogical and geochemical patterns of urban surface soils, the example of Pforzheim, Germany

This study presents a combined geochemical and mineralogical survey of urban surface soils. Many studies on urban soils are restricted to purely chemical surveys in order to investigate soil pollution caused by anthropogenic activities such as traffic, heating, industrial processing, waste disposal and many more. In environmental studies, chemical elements are often distinguished as lithogenic and anthropogenic elements. As a novel contribution to those studies, the authors combined the analysis of a broad set of chemical elements with the analysis of the main mineralogical phases. The semi-quantification of mineralogical phases supported the assignment of groups of chemical elements to lithogenic or anthropogenic origin. Minerals are important sinks for toxic elements. Thus, knowledge about their distribution in soils is crucial for the assessment of the environmental hazards due to pollution of urban soils. In Pforzheim, surface soils (0–5 cm depth) from various land use types (forest, agriculture, urban green space, settlement areas of various site densities) overlying different geological units (clastic and chemical sediments) were investigated. Urban surface soils of Pforzheim reflect to a considerable degree the mineral and chemical composition of parent rocks. Irrespective of the parent rocks, elevated concentrations of heavy metals (Zn, Cu, Pb, Sn, Ag) were found in soils throughout the whole inner urban settlement area of Pforzheim indicating pollution. These pollutants will tend to accumulate in inner urban surface soils according to the available adsorption capacity, which is normally higher in soils overlying limestone than in soils overlying sandstone. However, inner urban surface soils overlying sandstone show elevated concentrations of carbonates, phyllo-silicates and Fe and elevated pH values compared with forest soils overlying sandstone. Thus, in comparison to forest soils overlying sandstones, inner urban soils overlying sandstone affected by pollution concurrently possess elevated concentrations of mineral phases typically providing relatively high adsorption capacities for heavy metals.     

Towards a typology of urban centers and subcenters – evidence from German city regions

This study aims to characterize urban spatial structure with respect to its multidimensionality. Using an encompassing data set of socioeconomic variables, as well as variables pertaining to the built environment, accessibility and land use, we suggest a quantitatively based urban (sub-) center typology. The fine-grained spatial scale of 1 km² grid cells permits a higher spatial resolution than that used in most previous studies. Our suggested typology is two-layered: a “macro layer” based on cluster analyses sheds light on urban spatial configurations. A corresponding threshold-based “micro layer” identifies distinctive types of centers and subcenters at the local level. The application of this multi-scale and multivariate typology to four German city regions indicates both the core cities’ morphological dominance and the formation of subcenters with distinct profiles of regional economic importance, land use patterns and urban form. However, a substantial degree of spatial dispersion is observed because much activity is located in non-central locations.     

Lithium isotope composition of ordinary and carbonaceous chondrites, and differentiated planetary bodies: Bulk solar system and solar reservoirs

In order to better constrain the Li isotope composition of the bulk solar system and Li isotope fractionation during accretion and parent body processes, Li isotope compositions and concentrations were determined on a number of meteorite falls and finds. This is the first comprehensive study that systematically investigates a representative set of samples from carbonaceous chondrites (CI, CM2, CO3, CV3, CK4 and one ungrouped member), enstatite chondrites (EH, EL), ordinary chondrites (H, L, LL), and achondrites (one eucrite, diogenites, one pallasite, and a silicate inclusion from a IAB iron).

Carbonaceous chondrites have an average isotope composition of δ⁷Li = + 3.2‰ ± 1.9 (2σ) which agrees with the average composition of relatively pristine olivines (representative for the bulk composition) from the Earth primitive upper mantle (PUM). This is lighter than the average δ⁷Li of the basaltic differentiates of the Earth, Moon and Mars and the achondrites. It is an important observation, however, that the lighter end of the isotopic range of the differentiates always coincides with the averages of the mantle olivines and the carbonaceous chondrites. From this we conclude that the bulk of the inner solar system consists mostly of material from carbonaceous chondrites and that the variation seen in the differentiates is due to planetary body processes. Ordinary chondrites are significantly lighter than carbonaceous chondrites. No significant differences in δ⁷Li exist between enstatite chondrites (n = 3) and carbonaceous or ordinary chondrites. The difference between carbonaceous and ordinary chondrites and the variability within the chondrites could indicate the existence of distinct Li isotope reservoirs in the early solar nebula.     

Origin and karst geomorphological significance of the enigmatic Australian Nullarbor Plain ‘blowholes’

The Australian Nullarbor Plain, one of the world's largest limestone platforms (~200 000 km²), has few distinctive surface karst features for its size, but is known for its enigmatic ‘blowholes’, which can display strong barometric draughts. Thousands of these vertical tubes with decimetre–metre (dm–m) scale diameter puncture the largely featureless terrain. The cause and distribution of these has remained unclear, but they have been thought to originate from downward dissolution and/or salt weathering. To elucidate blowhole distribution and mode of formation we (i) correlated existing location data with Shuttle Radar Topography Mission (SRTM) data, which distinguishes the subtle undulations (< 10 m per km) of the landscape, (ii) surveyed blowhole morphology and (iii) determined their rock surface hardness. Over a sampled area of 4200 km², the distribution of 615 known blowholes is not correlated with present topography. Blowholes are often connected to small or, in some cases extensive, but typically shallow cavities, which exhibit numerous ‘cupolas’ (dome‐shaped pockets) in their ceilings. Statistical arguments suggest that cavities with cupolas are common, but in only a few cases do these puncture the surface. Hardness measurements indicate that salt weathering is not their main cause. Our observations suggest that blowholes do not develop downwards, but occur where a cupola breaks through the surface. Lowering of the land surface is suggested to be the main cause for this breakthrough. Although cupolas may undergo some modification under the current climate, they, as well as the shallow caves they are formed in, are likely to be palaeokarst features formed under a shallower water table and wetter conditions in the past. The findings presented have implications for theories of dissolutional forms development in caves worldwide. The environmental history of the Nullarbor platform allows testing of such theories, because many other factors, which complicate karst evolution elsewhere, have not interfered with landform evolution here. Copyright © 2011 John Wiley & Sons, Ltd.     

Earthquakes in Switzerland and surrounding regions during 2004

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2004. During this period, 677 earthquakes and 96 quarry blasts were detected and located in the region under consideration. With 22 events with M_L≥2.5, the seismic activity in the year 2004 was close to the average over the last 30 years. As in previous years, most of the activity was concentrated in the Valais and in Graubünden. In addition, several moderate earthquakes occurred in the lower crust below the northern Alpine foreland. Unusual was that five earthquakes were sufficiently strong to cause ground shaking of intensity IV over large portions of the territory. Two were located in Switzerland (Liestal, M_L 3.8, and Brugg, M_L 4.0). The epicenters of the other three strong events were located outside Switzerland (Besan?on in the French Jura, M_L 4.8, Waldkirch in southern Germany, M_L 5.1, and Lago di Garda in northern Italy, M_L 5.3).     

Prevalence and composition of fishing gear debris in the nests of northern gannets (Morus bassanus) are related to fishing effort

Bycatch and indirect mortality associated with global fishing operations affect non-target species. Northern gannets (Morus bassanus) and other seabirds incorporate marine debris, much of it originating in fisheries, into their nests, at times resulting in entanglement. We compared the prevalence and composition of marine debris in nests at two gannet colonies in Newfoundland before and after a basin-wide ground fish closure in 1992, and at the species' largest colony in the Gulf of St. Lawrence, where fishing effort is low. The proportion of nests with marine debris decreased following the fishery closure, and the proportion of nests with fishing gear was related exponentially to the number of gillnets set around breeding colonies. Assessing the composition of gannet nests could provide a useful index of the prevalence of fishing debris and could be used to assess entanglement risk of other animals in the marine environment over decadal scales.