Contrasting flow pathways within tropical forest slopes of Ultisol soils

There are very few experimental studies identifying hydrological pathways within rain forest slopes. Such knowledge is, however, necessary to understand why forest disturbance affects rainfall–riverflow response and nutrient migration. This study examines flow pathways within lowland rain forest slopes comprising Udults of the Ultisol soil order. Experimentation was conducted on four SE Asian hillslope units (each 5 × 5 m in plan) in the Bukit Timah catchment (Singapore Island), and in the W8S5 catchment (Sabah, Borneo Island). The flow pathways were identified by artificial tracer experiments. We evaluated how well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns. The tracer work indicated much faster subsurface flows at Bukit Timah than W8S5 for the storms studied. Some explanation of the greater subsurface waterflows at Bukit Timah in comparison to W8S5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries. Vertical flow of the tracer through the upper 1 m of soil predominated (>90 per cent of percolation) in the Bukit Timah slopes. In some contrast, a major component (approximately 60 per cent) of the tracer percolation was directed laterally within the W8S5 slopes. The flow vectors calculated using the hydrometric methods did, however, grossly under‐estimate the degree of lateral deflection of waterflow generated at W8S5 and to a lesser extent over‐estimated it at Bukit Timah. In part, these errors may relate to the inability of traditional hydrometric techniques to fully characterize the effect of the large and small ‘natural soil pipes’ present within both catchments. In conclusion, the study indicates that marked variations in flow vectors exist within the Udult great group of SE Asian soils and hydrometric calculations may be poor predictors of these dominant pathways. Copyright © 2005 John Wiley & Sons, Ltd.     

On the Behavior of the Stable Boundary Layer and the Role of Initial Conditions

Previous studies of the stable atmospheric boundary layer using techniques of nonlinear dynamical systems (MCNIDER et al., 1995) have shown that the equations support multiple solutions in certain parameter spaces. When geostrophic speed is used as a bifurcation parameter, two stable equilibria are found—a warm solution corresponding to the high-wind regime where the surface layer of the atmosphere stays coupled to the outer layer, and a cold solution corresponding to the low-wind, decoupled case. Between the stable equilibria is an unstable region where multiple solutions exist. The bifurcation diagram is a classic S shape with the foldback region showing the multiple solutions. These studies were carried out using a simple two-layer model of the atmosphere with a fairly complete surface energy budget. This allowed the dynamical analysis to be carried out on a coupled set of four ordinary differential equations. The present paper extends this work by examining additional bifurcation parameters and, more importantly, analyzing a set of partial differential equations with full vertical dependence. Simple mathematical representations of classical problems in dynamical analysis often exhibit interesting behavior, such as multiple solutions, that is not retained in the behavior of more complete representations. In the present case the S-shaped bifurcation diagram remains with only slight variations from the two-layer model. For the parameter space in the foldback region, the evolution of the boundary layer may be dramatically affected by the initial conditions at sunset. An eigenvalue analysis carried out to determine whether the system might support pure limit-cycle behavior showed that purely complex eigenvalues are not found. Thus, any cyclic behavior is likely to be transient.     

Groundwater recharge and evapotranspiration for two natural ecosystems covered with oak and heather

The evapotranspiration and groundwater recharge from two natural areas with high (oak) and low (heath) vegetation were estimated by calibrating a semi-physical numerical soil water and heat model to fit 8 and 7 years of TDR-measurements of water content, respectively. The measurements were made between the surface and 7 m depth. For the oak stand, the estimated annual recharge for the years 1992–1999 is 390 mm, the evaporation from soil and interception is 205 mm, and the transpiration is 285 mm. For the heath area estimation was carried out for the years 1993–1999. However, the heath was struck by a heavy beetle attack in 1994, which strongly affected the vegetation and thus the water balance for the following 3 years. For years not affected, the estimated recharge is 733 mm (about 50% larger than for the oak stand for the same years), the evaporation is 316 mm, and the transpiration is 128 mm. The estimated recharge values compare fairly well to estimates obtained from bromide tracer experiments. However, the recharge estimates obtained from the tracer experiments are very uncertain. The uncertainty is mainly due to spatial heterogeneity making the three replicate samples taken here for each time and depth insufficient.

The analyses of TDR-measurements and tracer data showed that water front movement depends on the antecedent soil water content. Some layers are bypassed, especially at low water contents, and at high soil water contents preferential flow was observed at the heath site.     

Nb/Zr fractionation on the Moon and the search for extinct Nb

A comprehensive Zr isotopic study was conducted on eleven lunar basalts and highland rocks to search for evidence of the extinct nuclide ⁹²Nb, which decays to ⁹²Zr with a half-life of 36 Ma. Internal isochrons were determined for two early highland rocks, 77215 and 60025. No resolvable Zr isotopic variations were detected in this wide range of lunar samples and thus there is no evidence for the former existence of live ⁹²Nb on the Moon. The Nb/Zr ratios of lunar ilmenites and bulk rock samples vary by only a factor of two to three relative to the chondritic Nb/Zr ratio. No evidence for larger Nb/Zr fractionation was found. This limited fractionation and late isotopic closure of the source region prevents the formation of measurable ⁹²Zr anomalies in high-Ti mare basalts. As a consequence, it is not possible to draw conclusions from the ⁹²Nb-⁹²Zr chronometer about the timing of early lunar differentiation and to constrain the role of ilmenite in the source region of high-Ti mare basalts. However, the fractionation is still sufficient to deduce an upper limit for the initial ⁹²Nb/⁹³Nb ratio of the solar system of <5 × 10⁻⁴.     

Thallium isotope composition of the upper continental crust and rivers—An investigation of the continental sources of dissolved marine thallium

The thallium (Tl) concentrations and isotope compositions of various river and estuarine waters, suspended riverine particulates and loess have been determined. These data are used to evaluate whether weathering reactions are associated with significant Tl isotope fractionation and to estimate the average Tl isotope composition of the upper continental crust as well as the mean Tl concentration and isotope composition of river water. Such parameters provide key constraints on the dissolved Tl fluxes to the oceans from rivers and mineral aerosols.The Tl isotope data for loess and suspended riverine detritus are relatively uniform with a mean of ε²⁰⁵Tl = −2.0 ± 0.3 (ε²⁰⁵Tl represents the deviation of the ²⁰⁵Tl/²⁰³Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 10⁴). For waters from four major and eight smaller rivers, the majority were found to have Tl concentrations between 1 and 7 ng/kg. Most have Tl isotope compositions very similar (within ±1.5 ε²⁰⁵Tl) to that deduced for the upper continental crust, which indicates that no significant Tl isotope fractionation occurs during weathering. Based on these results, it is estimated that rivers have a mean natural Tl concentration and isotope composition of 6 ± 4 ng/kg and ε²⁰⁵Tl = −2.5 ± 1.0, respectively.In the Amazon estuary, both additions and losses of Tl were observed, and these correlate with variations in Fe and Mn contents. The changes in Tl concentrations have much lower amplitudes, however, and are not associated with significant Tl isotope effects. In the Kalix estuary, the Tl concentrations and isotope compositions can be explained by two-component mixing between river water and a high-salinity end member that is enriched in Tl relative to seawater. These results indicate that Tl can display variable behavior in estuarine systems but large additions and losses of Tl were not observed in the present study.     

Oxygen isotope fractionation during synthesis of CaMg-carbonate and implications for sedimentary dolomite formation

Hydrous CaMg-carbonate was synthesized at temperatures of 40°, 60° and 80°C in the laboratory. This material has very similar mineralogical characteristics to natural disordered dolomite from the Coorong region in South Australia. Besides the dolomite variable amounts of amorphous carbonate are present in all samples. The oxygen isotope compositions of synthesized bulk carbonate samples (e.g., amorphous carbonate plus dolomite) plot significantly lower than the Northrop and Clayton (1966) dolomite-water equilibrium. Fractionated degassing of the samples, however, revealed relatively low oxygen isotope values for fast-reacting (using 100% H₃PO₄) amorphous carbonate. In contrast, slow-reacting dolomite has more positive oxygen isotope values, and calculated carbonate-water oxygen isotope fractionation values are close to strongest known dolomite-water oxygen isotope fractionation published earlier on. Variations of reaction/stabilization temperatures during synthesis gave evidence for dolomite formation from hypersaline solutions by a dissolution/reprecipitation process. It is likely that amorphous carbonate has been a problem in defining the dolomite-water fractionation in the past. Moreover, dolomite-associated amorphous carbonate contents probably led to incorrect speculations about lower oxygen isotope fractionation in a so-called protodolomite-water system.     

Tellurium isotopic composition of the early solar system—A search for effects resulting from stellar nucleosynthesis, Sn decay, and mass-independent fractionation

New precise Te isotope data acquired by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) are presented for selected extraterrestrial and terrestrial materials. Bulk samples of carbonaceous, ordinary and enstatite chondrites as well as the metal and sulfide phases of iron meteorites were analyzed to search for nucleosynthetic isotope anomalies and to find evidence of formerly live ¹²⁶Sn, which decays to ¹²⁶Te with a half-life of 234,500 yr. None of the meteorites show evidence of mass dependent Te isotope fractionations larger than 2‰ for δ^126/128Te. Following internal normalization of the data to ¹²⁵Te/¹²⁸Te, the Te isotope ratios of all analyzed meteorites were found to be identical to a terrestrial standard, within uncertainties. This provides evidence that the regions of the solar disk that were sampled during accretion of the meteorite parent bodies were well mixed and homogeneous on a large scale, with respect to Te isotopes. The data acquired for bulk carbonaceous chondrites indicate that the initial ¹²⁶Sn/¹¹⁸Sn ratio of the solar system was <4 × 10⁻⁵, but this is dependent on the assumption that no redistribution of Sn and Te occurred since the start of the solar system. Five Archean sedimentary sulfides that display both mass dependent and mass-independent isotope effects for S yield internally normalized Te isotope data, which indicate that mass-independent Te isotope effects are absent. The mass dependent fractionations in these samples are constrained to be less than ∼1‰ for δ^126/128Te.     

There’s nothing inherent about scale: political ecology, the local trap, and the politics of development in the Brazilian Amazon

This paper argues that research in political ecology would benefit from more explicit and careful attention to the question of scale and scalar politics. Although political ecologists have extensively considered scale as a methodological question, they have yet to develop an explicit theoretical approach to scale as an object of inquiry. We highlight one principal drawback to this underdeveloped approach to scale: what we call “the local trap” in which political ecologists assume that organization, policies, and action at the local scale are inherently more likely to have desired social and ecological effects than activities organized at other scales. Over the past 10 years or so, an increasingly sophisticated literature on scale has been developing among scholars in geography working in the political economy tradition. This literature has argued that scale is socially produced rather than ontologically given. Therefore, there is nothing inherent about any scale, and so the local scale cannot be intrinsically more desirable than other scales. We suggest that a greater engagement with this scale literature offers political ecology a theoretical way out of the local trap. As a first approximation of the kind of scalar analysis we advocate, we present a case study that examines the scalar politics that have shaped environmental change in the Brazilian Amazon.