Silicon isotopes in meteorites and planetary core formation

The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ³⁰Si differences are resolved between chondrites (δ³⁰Si = −0.49 ± 0.15‰, 2σ_SD) and achondrites (δ³⁰Si = −0.47 ± 0.11‰, 2σ_SD), although enstatite chondrites are consistently lighter (δ³⁰Si = −0.63 ± 0.07‰, 2σ_SD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ³⁰Si_{BSE−meteorite∗} = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal-silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (ε ∼0.89), the Δ³⁰Si_{BSE−meteorite∗} value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.     

Catchments as reactors: a comprehensive approach for water fluxes and solute turnover

Sustainable water quality management requires a profound understanding of water fluxes (precipitation, run-off, recharge, etc.) and solute turnover such as retention, reaction, transformation, etc. at the catchment or landscape scale. The Water and Earth System Science competence cluster (WESS, http://www.wess.info/) aims at a holistic analysis of the water cycle coupled to reactive solute transport, including soil–plant–atmosphere and groundwater–surface water interactions. To facilitate exploring the impact of land-use and climate changes on water cycling and water quality, special emphasis is placed on feedbacks between the atmosphere, the land surface, and the subsurface. A major challenge lies in bridging the scales in monitoring and modeling of surface/subsurface versus atmospheric processes. The field work follows the approach of contrasting catchments, i.e. neighboring watersheds with different land use or similar watersheds with different climate. This paper introduces the featured catchments and explains methodologies of WESS by selected examples.     

Holocene and Pleistocene sea-level indicators at the coast of Jericoacoara,Ceará, NE Brazil

Beach-rock exposures provide a record of Holocene sea-level rise along the 560-km-long northeast-facing coast of Ceará, Brazil, that differs from the record available along the other 4300 km of Brazilian coastline further south. Whereas documentation is available from southern Brazil to show Holocene sea levels as much as 5 m above today's level, our observations along the northeastern coast indicate that sea level here was not above the present-day level during the Holocene. Near Jericoacoara, about 240 km northwest of Fortaleza, characterized by strong surf, Precambrian rocks crop out from under a temporary cover of sand in small protected locations with less surf. Here in this upper tidal zone beach rock is being formed, while it is being dismembered synchronously by erosion at lower tide levels. This shows a rising sea level. Along the entire coast of Ceará west of Ponta Grossa the absence of beach rock higher than spring tide level indicates that sea-level was not above its present-day level during the Holocene.Notches in bedrock situated between 2 m and 6 m above spring-tide high-water level that we formerly described as Holocene, are now believed to be Sangamonian.     

Carbonate diagenesis and feldspar alteration in fracture-related bleaching zones (Buntsandstein,central Germany): possible link to CO<Subscript>2</Subscript>-influenced fluid–mineral reactions

Fracture-related bleaching of Lower Triassic Buntsandstein red beds of central Germany was related to significant carbonate diagenesis and feldspar alteration caused by CO₂-rich fluids. Using cathodoluminescence microscopy and spectroscopy combined with electron microprobe analysis and stable carbon isotope study, two major fluid–mineral interactions were detected: (1) zoned, joint-filling calcites and zoned pore-filling calcite cements, the latter replacing an earlier dolomite, were formed during bleaching. During the calcite formation and dolomite–calcite transformation, iron was incorporated into the calcite cement crystal cores due to Fe availability from the coeval bleaching. The dedolomitisation was ultimately associated with a volume increase. The related permeability decrease implies a certain degree of sealing and increasing retention of CO₂, and the volume increase offers a minor CO₂ sink. Carbonate-rich sandstone, therefore, can provide advantages for underground CO₂ storage especially when situated in the fringes of the reservoir. (2) Alkali-feldspar alteration due to the bleaching fluids is reflected in cathodoluminescence spectra predominantly by the modulation of a brown luminescence emission peak (~620 nm). This peak represents a newly discovered effect related to alkali-feldspar alteration not solely associated with bleaching. Its modulation by the bleaching is interpreted to be due to Na depletion or a lattice defect in the Si–O bonds of the SiO₄-tetrahedron. Alteration reflected by this luminescence feature has a destructive effect on the feldspars implying the possibility of diminished rock integrity due to bleaching and, hence, CO₂-rich fluids. Two further CL spectral changes related to bleaching occur, (a) decreased intensity between around 570 nm assigned to Mn-depletion, and (b) increased amplitude and wavelength shift of the red (~680 nm) band. Converging evidence from carbonate and feldspar diagenesis, stable carbon isotope data and analysis of fracture directions suggests that CO₂ fluids contributed to a significant extent to the bleaching phenomena and alteration in the studied Buntsandstein strata.     

Impacts of rising water demands in the Juba and Shabelle river basins on water availability in south Somalia

ABSTRACT

Somalia has frequently been affected by droughts, famines and water-related humanitarian crises. Water is scarce and the only perennial streams, the Juba and Shabelle rivers, are trans-boundary with river flows mainly originating from the Ethiopian highlands. In both riparian countries water demands are projected to increase. This paper reveals the impact of rising regional water abstractions on stream flows by illustrating sectoral demands and joining them into scenarios of medium and high population and economic growth. These scenarios are associated to the time horizons of 2035 and 2055, respectively. The scenarios disclose alarming trends especially for the Shabelle River: in the medium and high growth scenarios, water demands surpass the available river flows by 200 and 3500 hm³, respectively. The calculated deficits partly derive from conflicting assumptions about river flows by the two main riparian countries, an obstacle to any integrated planning efforts and sustained regional development.

EDITOR Z.W. Kundzewicz; ASSOCIATE EDITOR F. Hattermann     

Development of storage capacity and morphology of the Aswan High Dam Reservoir

ABSTRACT

A digital elevation model (DEM), as well as satellite images, was used to detect the changes in the morphology and storage capacity of the Aswan High Dam Reservoir (AHDR) over the past five decades. Study findings indicate that the total storage capacity of the AHDR estimated by the DEM decreased by 12%. This decrease is mainly in the live and the dead storage capacities. The morphology of the reservoir changed in the southern part and at the entrance of the lacustrine part. A gradual conversion of the entrance to the riverine part was observed. The surface area of the AHDR decreased at low water levels because of sedimentation. The average reduction of the surface area between water levels of 140 and 168 m a.s.l. is about 15%, which is equivalent to almost 10 km³.     

Structural uncertainty assessment in a discharge simulation model

Abstract

A major goal in hydrological modelling is to identify and quantify different sources of uncertainty in the modelling process. This paper analyses the structural uncertainty in a streamflow modelling system by investigating a set of models with increasing model structure complexity. The models are applied to two basins: Kielstau in Germany and XitaoXi in China. The results show that the model structure is an important factor affecting model performance. For the Kielstau basin, influences from drainage and wetland are critical for the local runoff generation, while for the XitaoXi basin accurate distributions of precipitation and evapotranspiration are two of the determining factors for the success of the river flow simulations. The derived model uncertainty bounds exhibit appropriate coverage of observations. Both case studies indicate that simulation uncertainty for the low-flow period contributes more to the overall uncertainty than that for the peak-flow period, although the main hydrological features in these two basins differ greatly.

Citation Zhang, X. Y., Hörmann, G., Gao, J. F. & Fohrer, N. (2011) Structural uncertainty assessment in a discharge simulation model. Hydrol. Sci. J. 56(5), 854–869.