Mysterious circles in the Namib Desert: review of hypotheses on their origin

Circular, slightly concave depressions, devoid of vegetation and often surrounded by a fringe of tall grasses occur in a broken belt in the pro-Namib zone of the west coast of southern Africa. Several hypotheses have been proposed to explain the origin of these so-called fairy circles. The most important of these relate to areas of localized radioactivity; termite activity; or allelopathic compounds released by dead Euphorbia damarana plants. No evidence of increased radioactivity could be detected in soil samples collected from these sites. Although termites occur at these localities no evidence was found to link termite activity directly to the formation of barren patches. Bioassays conducted on soil collected from the centre of the barren patch clearly demonstrated an inhibition of plant growth, while soil collected from the edge of the barren patch had a stimulatory effect on plant growth. No indication of growth inhibition was found in soil collected beneath E. damarana plants. At this stage none of the proposed hypotheses can satisfactorily explain the origin of the fairy circles.     

西北干旱地区大气降水δ18O的特征及水汽来源

In order to reveal the characteristics and climatic controls on the stable isotopic composition of precipitation over Arid Northwestern China, eight stations have been selected from Chinese Network of Isotopes in Precipitation (CHNIP). During the year 2005 and 2006, monthly precipitation samples have been collected and analyzed for the composition of δD and δ18O. The established local meteoric water line δD=7.42 δ18O+1.38, based on the 95 ob-tained monthly composite samples, could be treated as isotopic input function across the region. The deviations of slope and intercept from the Global Meteoric Water Line indicated the specific regional meteorological conditions. The monthly δ18O values were characterized by a positive correlation with surface air temperature (δ18O (‰) =0.33 T (℃)-13.12). The amount effect visualized during summer period (δ18O (‰) = -0.04P (mm)-3.44) though not appeared at a whole yearly-scale. Spatial distributions of δ18O have properly portrayed the atmospheric circulation background in each month over Arid Northwestern China. The quan-titative simulation of δ18O, which involved a Rayleigh fractionation and a kinetic fractionation, demonstrated that the latter one was the dominating function of condensation of raindrops. Furthermore, the raindrop suffered a re-evaporation during falling processes, and the pre-cipitation vapor might have been mixed with a quantity of local recycled water vapor. Multiple linear regression equations and a δ18O-T relation have been gained by using meteorological parameters and δ18O data to evaluate physical controls on the long-term data. The estab-lished δ18O-T relation, which has been based on the present-day precipitation, could be considered as a first step of quantitatively reconstructing the historical environmental climate.     

The need to correct for the Suess effect in the application of δ<Superscript>13</Superscript>C in sediment of autotrophic Lake Tanganyika,as a productivity proxy in the Anthropocene

The change in dissolved inorganic δ¹³C in the ocean resulting from the change in δ¹³C in atmospheric CO₂ owing to anthropogenic activities (the Suess effect) is well known. The need to correct for the Suess effect when applying δ¹³C in organic matter in lacustrine sediment deposited during the anthropocene as a productivity proxy, is widely although not universally acknowledged. This paper reviews conceptions about the Suess effect in lacustrine δ¹³C_org and methods to adjust for the Suess effect when δ¹³C_org is used to infer recent changes in aquatic productivity. Lake Tanganyika is used as an example to illustrate the necessity of the correction. When the Suess effect is not considered, interpretations of sediment core data can result that are opposite to those achieved with the correction applied, as is here shown in Lake Tanganyika and in other lakes. A new method to correct for the Suess effect is provided which has the advantage of being applicable to data for a larger period (1700–2000) than methods currently available. In addition, Lake Tanganyika is shown to be a net sink for CO₂.     

1200 years of fire impact on biogeochemistry as inferred from high resolution diatom analysis in a kettle lake from the Picea mariana-moss domain (Quebec, Canada)

The consequences of fire on water chemistry are important considering that major changes in the frequency and intensity of forest fires are anticipated as a result of global warming. Due to the important differences in succesionnal vegetative trends after fire between mixed-wood and coniferous-dominated forests in Quebec (Canada), we undertook a long-term paleoecological study of the impact of fires on the biogeochemistry of Lac à la Pessière, a small lake located in a conifer-dominated boreal forest ecoregion (Picea mariana-moss domain). The paleolimnological study was carried out using diatom assemblages (class:Bacillariophyceae) to reconstruct changes in environmental variables of limnological interest [pH, total phosphorus (TP), total nitrogen (TN), dissolved organic carbon (DOC) and, epilimnetic carbon dioxide (CO₂)] potentially associated with fire over the last 1200 calendar years. Diatom composition and related reconstructed limnological variables were compared before and after fire events. No significant changes were systematically observed in lake chemistry associated with fire events. However, diatom-inferred epilimnetic CO₂showed a clear decreasing trend over the last 400 cal. yrs BP. The results suggest that fire-induced changes in lake chemistry are limited in catchments dominated by black spruce (Picea mariana). We hypothesize that this fact result of excess moisture associated to the thick humus layer, which likely limits the mobilization of nutrients and major ions even during a fire event.