Role of a strong oxygen-deficient zone in the preservation and degradation of organic matter: a carbon budget for the continental margins of northwest Mexico and Washington State

Rates of organic carbon oxidation in marine sediments were determined for the continental margins of northwest Mexico and Washington State, with the goal of assessing the role of oxygen in the preservation of organic matter on a margin with a strong oxygen-deficient zone and on a typical western continental margin. Total carbon oxidation rates (including rates for individual electron acceptors: O₂, NO₃⁻, and SO₄⁼) were determined at depths ranging from 100 to 3000 m on both margins. Carbon oxidation rates were generally higher on the Washington margin than on the Mexican margin. The relative importance of the different electron acceptors varied across the two margins and was related primarily to the availability of O₂ and NO₃⁻ from the overlying water. The relative contribution of O₂ consumption increased in deeper sediments (>2000 m) as aerobic processes began to dominate the total carbon oxidation rate. Denitrification rates were highest in Washington sediments; however, denitrification represented a larger fraction of the total carbon oxidation rate in the Mexican sediments (∼40% for Mexico vs. ∼30% for Washington). Sulfate reduction accounted for as much as 79% of the total carbon oxidation rate in shallow sediments and less than 20% in deep sediments on both margins. The offshore trends in carbon oxidation rate appeared to be related to the organic carbon input rate. Pore-water O₂ and NO₃⁻ penetration depths were shallowest in nearshore stations and increased offshore. Regeneration ratios of C:N:P reveal “non-Redfield” behavior on both margins. Carbon budgets for the two margins demonstrate that off Mexico, a much greater percentage of the organic matter produced in the surface ocean reached the sediments (>15% vs. <8% for Mexico and Washington, respectively). On the Mexican margin, ∼8% of the primary production escaped oxidation in the surface sediments to be permanently buried, as compared with only ∼1.2% of the primary production on the Washington margin. This suggests that oxygen-deficient conditions on Mexican margin are linked to enhanced carbon preservation.     

Managing spatial uncertainty using attribute,geometric, and neighborhood measures in an empirical rule-based model approach

 A rule-based model for managing uncertainty in spatial databases is presented. The overall goal of the model is to allow a user to assign to a single map class each polygon whose class is not entirely certain using more information than only the map class attributes of such polygons (that are herein termed abjects). This situation might arise when multiple map realizations of an area are available and interpreters/cartographers are not in agreement as to what class is present at a given location or when a digital image is classified by algorithmic/probabilistic means. The scale-based model developed relies on attribute, geometric, and neighborhood measures of abjects arranged in a hierarchical rule-based structure. Structural knowledge of these measures leads to the procedural knowledge that determines what action – e.g., merge, reclassify, retain – is to be taken for a given abject. The wider applicability of the model and associated methodology is also discussed. Received: 5 July 2001 / Accepted: 11 April 2002     

Isovalent trace element partitioning between minerals and melts: A computer simulation study

We present a new approach for the rationalisation of trace element partitioning between silicate melts and minerals, which is not based on the empirical, parameterised continuum models in common use. We calculate the energetics of ion substitution using atomistic simulation techniques, which include an explicit evaluation of the relaxation energy (strain energy) contribution to this process. Solution energies are estimated for isovalent impurities in CaO, diopside, orthoenstatite, and forsterite. These show a parabolic dependence on ionic radius, similar to the variation of mineral-melt partition coefficients with ionic radius. The success of the empirical models, which often include only the strain energy, appear to have been due to the partial cancellation of energy terms, and to the empirical fitting of the parameters included in these models. Our approach can be readily extended to aliovalent substitution.     

“A Mirror and a Lamp”: The Role of Power in the Rural Landscape Trajectory of the Ovens Region of Australia

Power relationships structure discourse and its influence on policy and rural land use planning, but little research has examined how this might be observed in the dynamics of rural landscape transition. In a qualitative case study of the Ovens catchment, Victoria, Australia, discursive power is observed through sites of tension between informant interpretations of past, present, and future landscapes and contemporary local decision making. Three main tensions and an observation emerged that suggest discourse and power can be observed through the transfer and support of social memory narratives. We conclude that the identification of tensions between stakeholder perceptions of the past, present, and future of their landscape; awareness of the influence of those interpretations on current decision making; and attention to social memory narratives can provide invaluable insights for those seeking to understand local relationships of power.     

Mercury,Trace Elements and Organic Constituents in Atmospheric Fine Particulate Matter,Shenandoah National Park,Virginia, USA: A Combined Approach to Sampling and Analysis

Compliance with U.S. air quality regulatory standards for atmospheric fine particulate matter (PM_2.5) is based on meeting average 24 hour (35 μ m^?3) and yearly (15 μg m^?3) mass‐per‐unit‐volume limits, regardless of PM_2.5 composition. Whereas this presents a workable regulatory framework, information on particle composition is needed to assess the fate and transport of PM_2.5 and determine potential environmental/human health impacts. To address these important non‐regulatory issues an integrated approach is generally used that includes (1) field sampling of atmospheric particulate matter on filter media, using a size‐limiting cyclone, or with no particle‐size limitation; and (2) chemical extraction of exposed filters and analysis of separate particulate‐bound fractions for total mercury, trace elements and organic constituents, utilising different USGS laboratories optimised for quantitative analysis of these substances. This combination of sampling and analysis allowed for a more detailed interpretation of PM_2.5 sources and potential effects, compared to measurements of PM_2.5 abundance alone. Results obtained using this combined approach are presented for a 2006 air sampling campaign in Shenandoah National Park (Virginia, USA) to assess sources of atmospheric contaminants and their potential impact on air quality in the Park. PM_2.5 was collected at two sampling sites (Big Meadows and Pinnacles) separated by 13.6 km. At both sites, element concentrations in PM₂₅ were low, consistent with remote or rural locations. However, element/Zr crustal abundance enrichment factors greater than 10, indicating anthropogenic input, were found for Hg, Se, S, Sb, Cd, Pb, Mo, Zn and Cu, listed in decreasing order of enrichment. Principal component analysis showed that four element associations accounted for 84% of the PM_2.5 trace element variation; these associations are interpreted to represent: (1) crustal sources (Al, REE); (2) coal combustion (Se, Sb), (3) metal production and/or mobile sources (Mo, Cd, Pb, Cu, Zn) and (4) a transient marine source (Sr, Mg). Concentrations of Hg in PM_2.5 at background levels in the single pg m^?3 were shown by collection and analysis of PM_2.5 on filters and by an automated speciation analyser set up at the Big Meadows air quality site. The speciation unit revealed periodic elevation of reactive gaseous mercury (RGM) that co‐occurred with peaks in SO₂, indicating an anthropogenic source. GC/MS total ion current chromatograms for the two sites were quite similar indicating that organic signatures were regional in extent and/or that the same compounds were present locally at each site. Calculated carbon preference index values for n‐alkanes indicated that plant waxes rather than anthropogenic sources, were the dominant alkane source. Polycyclic aromatic hydrocarbons (PAHs) were detected, with a predominance of non‐alkylated, and higher molecular weight PAHs in this fraction, suggestive of a combustion source (fossil fuel or forest fires).     

Mapping Farmer Identity: why,how, and what does it tell us?

Rural areas in developed economies are becoming increasingly multi-functional in that their character is being shaped by a mix of production, consumption and conservation values. Agriculture may remain the dominant land use, but many landholders do not see themselves as farmers by occupation. Researchers have demonstrated that occupational identity influences land use and management. However, efforts to explore the influence of occupational identity often rely on surrogate measures and have largely ignored identity theory. We build on research demonstrating that collective identity theory can be used to develop a valid and reliable measure of farmer identity and to then classify rural landholders across a natural resource management (NRM) region. The contribution of this paper is to explain how that measure of farmer identity can be mapped and then demonstrate the relevance of doing so for regional NRM in Australia. As expected, farmer identity varied across the case study region with distance from urban centres, and across different environmental assets. Those findings should lead to a more targeted approach to landholder engagement in NRM. We suggest that farmer identity (i.e. farmer-collective occupational identity construct; F-COIC) might provide a next step for responding to the challenges of interpreting and mapping multi-functionality.     

Stochastic coalescence model for terrestrial planetary accretion

A nonequilibrium stochastic coalescence model for terrestrial planetary accretion is developed by using an approximation to the Safronov-Golovin solution for the scalar transport equation with linear kernel. According to this model, formation of comparatively massive objects occurs quite rapidly during the early stages of accretive evolution in a given terrestrial planetesimal population, while during late growth stages, an increasingly substantial fraction of total population mass becomes incorporated into progressively fewer, relatively very large bodies. The model also implies that the (conservative) growth rate of the population's largest member varies directly as its mass, and further suggests that this growth rate may not decline significantly until very nearly final planetary mass is attained.