Natural variations of δSi ratios during progressive basalt weathering, Hawaiian Islands

Silicon stable isotopes can be used to trace the biogeochemical pathways of Si as it moves from its continental sources to its sink in ocean sediments. Along the way, Si is incorporated into clay minerals, taken up by plants where it forms plant opal, and leached into rivers, the major land-to-ocean conduit. Compared to igneous rocks, the waters that drain continents are enriched in heavy Si isotopes, but the mechanisms that control fractionation have not been elucidated. We studied Si isotope fractionation along a 4 million yr basaltic soil chronosequence on the Hawaiian Islands. Using the natural context of these samples in combination with laboratory experiments, we demonstrate that the isotopic composition of dissolved Si in weathering systems is determined by the combined effects of rock disintegration, clay mineral neosynthesis, and Si biocycling. Weathering preferentially releases ²⁸Si into solution, whereas secondary mineral formation preferentially removes ²⁸Si from solution. In humid environments, leached soils have lost large amounts of this soluble Si, thus creating a net loss of ³⁰Si from the entire soil system. As soils develop and greater fractions of Si reside in neoformed clay minerals, δ³⁰Si_{bulk soil} values change progressively toward more negative values; basalt δ³⁰Si values are about −0.5‰, but older soils have δ³⁰Si values up to −2.5‰. The difference between the solid and solution δ³⁰Si values remains more or less constant with progressive weathering, and therefore, soil water from older soils has a more negative δ³⁰Si composition. In the upper horizons of the Hawaiian soils, this weathering-driven δ³⁰Si shift is modified by the addition of unweathered primary minerals via dust, carrying δ³⁰Si values of about −0.5‰, and by biocycling of Si via plants, producing negative δ³⁰Si values in phytoliths and positive δ³⁰Si values in soil solutions derived from upper horizons. Due to the high concentrations of dissolved Si in these near-surface layers, rivers have more positive δ³⁰Si values than predicted based on the weathering status of the lower horizons. When combined with published δ³⁰Si values from large rivers worldwide, we find that the results from Hawaii point to weathering control of Si isotopes delivered to the oceans, and thus, to an important continent-ocean linkage that warrants further investigation.     

Systematics of xenocrystic contamination: preservation of discrete feldspar populations at McCullough Pass Caldera revealed by Ar/Ar dating

Single crystal ⁴⁰Ar/³⁹Ar dating of K-feldspars from silicic volcanic rocks containing xenocrysts often yields a spectrum of ages slightly older than those of juvenile sanidine phenocrysts. In contrast, feldspars from thin, low-volume units of the Tertiary (14 Ma) McCullough Pass Tuff define discrete age populations at 14 Ma, 15 Ma, and 1.3 Ga, reflecting the time of eruption, xenocrysts from an older ignimbrite exposed in the caldera wall, and Proterozoic basement K-feldspars, respectively. Conductive cooling and diffusion modelling suggests preservation of such discrete populations is likely only when xenocrystic material is incorporated into the magma very near or at the surface, or is engulfed in thin, rapidly cooled pyroclastic flows during emplacement. Incorporation of xenocrysts into the subvolcanic magma chamber, into thick rhyolite domes or lava flows, or into large, welded ignimbrite sheets will result in partial or total resetting of the K/Ar isotopic system. Similarly, petrographic evidence such as exsolution lamellae may be homogenized under these conditions but not in thin ignimbrites. Extremely low diffusion rates for disordering of the Al–Si tetrahedral siting of basement feldspars suggests that they will retain their ordered structural state given rhyolitic magma temperatures. Thus, even when petrographic and K/Ar isotopic evidence for xenocrystic contamination is obscured, it may be preserved in the form of Al–Si ordering.     

The cultural politics of local economic development: meaning-making, place-making, and the urban policy process

A major concern of work in urban and political geography in recent decades has been to analyze how and in whose interests local space economies are produced and reproduced. A common focus is on the role local elites play in gathering support for their development agendas. Drawing from these literatures, this paper focuses on how various visions of the future of localities are contested in the local policy process. It argues that this struggle can be usefully understood as a cultural politics in which meanings are defined and struggled over, where social values are naturalized, and by which `common sense' is constructed and contested. The use of the term `cultural politics of local economic development' is, then, intended to indicate that meaning-making and place-making occur simultaneously in struggles over the future of space economies. It is also an attempt to overcome the problematic distinction between `culture' and `economy' that continues to haunt a great deal of work on urban politics. Through a case study of urban politics in Lexington, Kentucky in which discursive strategies are highlighted, it is argued that this approach is useful in that it provides insight into non-elite perspectives on local economic development and that it underscores the role played by everyday life in constituting political action. The paper concludes by suggesting that any problematization of the conceptual distinction between `culture' and `economy' must be carried out in and through detailed analyses of how groups involved in social struggle frequently construct rhetorical strategies in reference to it.     

Ecosystem Evaluation, Climate Change and Water Resources Planning

This paper considers ecosystem evaluation under conditions of climate change in the context both of the U.S. Water Resources Council's Principles and Guidelines (P&G) and the more general Federal regulations governing environmental evaluation. Federal water agencies have responsibilities for protecting aquatic ecosystems through their regulatory programs and operations and planning missions. The primary concern of water resources and aquatic ecosystems planning in the United States is on the riparian or floodplain corridors of river systems. In the context of climate change, planning for these systems focusses on adaptation options both for current climate variability and for that engendered by potential climate change.Ecosystems appear to be highly vulnerable to climate change, as described in IPCC reports. Aquatic ecosystems are likely to be doubly affected, first by thermally induced changes of global warming and second by changes in the hydrologic regime. Perhaps as much as any of the issues dealt with in this issue, the evaluation of ecosystems is linked to fundamental questions of criteria as well as to the details of the Federal environmental planning system. That system is a densely woven, interlocking system of environmental protection legislation, criteria and regulations that includes a self-contained evaluation system driven by the National Environmental Policy Act (NEPA) procedural guidelines (United States Council on Environmental Quality, 1978) and Environmental Impact Statement (EIS) requirements. The Corps of Engineers must use both the P&G and the NEPA/EIS system in discharging its responsibilities.If U.S. Federal agencies are to take the lead in formulating and evaluating adaptation options, there needs to be a reexamination of existing evaluation approaches. Among the elements of the P&G that may require rethinking in view of the prospects of global climate change are those relating to risk and uncertainty, nonstationarity, interest rates, and multiple objectives. Within the government planning process, efforts must be made to resolve inconsistencies and constraints in order to permit the optimal evaluation of water-based ecosystems under global climate change. The interrelationships of the two systems are described in this paper, and alternative ways of viewing the planning process are discussed. Strategic planning and management at the watershed level provides an effective approach to many of the issues. Current NEPA/EIS impact analysis does not provide a suitable framework for environmental impact analysis under climate uncertainty, and site-specific water resources evaluation relating to climate change appears difficult at current levels of knowledge about climate change. The IPCC Technical Guidelines, however, provide a useful beginning for assessing the impacts of future climate states.     

Diatom succession in an urban reservoir system

A 43 cm by 5 cm diameter sediment core sample was obtained from Ford Lake reservoir in Washtenaw County, Michigan, and sectioned at 1 cm intervals. The purpose of this study was to determine whether diatom communities in this reservoir have undergone quantifiable changes in abundance and composition since its creation. Thirty-one cm of this core appeared to represent material deposited since the creation of the reservoir based on changes in diatom abundance, the physical composition of the sediment and the change in biogenic SiO₂ concentration. Fortyseven species of diatoms were identified total concentrations of diatom remains varied from 1×10⁴ g^-1 to 1×10⁷ g^-1. Prior to the establishment of the reservoir, the diatom flora was dominated by benthic taxa. Benthic diatoms were numerous throughout the entire core, but eutrophic taxa (e.g., Aulacoseira italica, Aulacoseira granulata, Stephanodiscus niagarae, Fragilaria crotonensis) dominated much of the core after the reservoir's creation. Total diatom density increased about tenfold in the about the first 10–15 years after the reservoir's creation before declining markedly.     

On the moments approximation method for constructing a Lagrangian Stochastic model

The exact Eulerian velocity probability density function (pdf) of a turbulent field is generally unknown, and one normally has available only partial information in the form of low order moments. We compare two alternative Lagrangian Stochastic (LS) approaches formed from this partial information, (i) the moments approximation approach (Kaplan and Dinar, 1993); and (ii) the well-mixed model (Thomson, 1987) that corresponds to the maximum missing information pdf formed from the available information. We show that the moments approximation model does not in general satisfy the well-mixed constraint, and can give an inferior prediction of dispersion.