A re-appraisal of the stratigraphy and volcanology of the Cerro Galán volcanic system,NW Argentina

From detailed fieldwork and biotite ⁴⁰Ar/³⁹Ar dating correlated with paleomagnetic analyses of lithic clasts, we present a revision of the stratigraphy, areal extent and volume estimates of ignimbrites in the Cerro Galán volcanic complex. We find evidence for nine distinct outflow ignimbrites, including two newly identified ignimbrites in the Toconquis Group (the Pitas and Vega Ignimbrites). Toconquis Group Ignimbrites (~5.60–4.51 Ma biotite ages) have been discovered to the southwest and north of the caldera, increasing their spatial extents from previous estimates. Previously thought to be contemporaneous, we distinguish the Real Grande Ignimbrite (4.68 ± 0.07 Ma biotite age) from the Cueva Negra Ignimbrite (3.77 ± 0.08 Ma biotite age). The form and collapse processes of the Cerro Galán caldera are also reassessed. Based on re-interpretation of the margins of the caldera, we find evidence for a fault-bounded trapdoor collapse hinged along a regional N-S fault on the eastern side of the caldera and accommodated on a N-S fault on the western caldera margin. The collapsed area defines a roughly isosceles trapezoid shape elongated E-W and with maximum dimensions 27 × 16 km. The Cerro Galán Ignimbrite (CGI; 2.08 ± 0.02 Ma sanidine age) outflow sheet extends to 40 km in all directions from the inferred structural margins, with a maximum runout distance of ~80 km to the north of the caldera. New deposit volume estimates confirm an increase in eruptive volume through time, wherein the Toconquis Group Ignimbrites increase in volume from the ~10 km³ Lower Merihuaca Ignimbrite to a maximum of ~390 km³ (Dense Rock Equivalent; DRE) with the Real Grande Ignimbrite. The climactic CGI has a revised volume of ~630 km³ (DRE), approximately two thirds of the commonly quoted value.     

Columnar jointing in vapor-phase-altered,non-welded Cerro Galán Ignimbrite,Paycuqui, Argentina

Columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits. However, the non-welded Cerro Galán Ignimbrite at Paycuqui, Argentina, contains well-developed columnar joints that are instead due to high-temperature vapor-phase alteration of the deposit, where devitrification and vapor-phase crystallization have increased the density and cohesion of the upper half of the section. Thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures, above 630°C, but the lack of welding textures indicates temperatures below the glass transition temperature. In order to remain below the glass transition at 630°C, the minimum cooling rate prior to deposition was 3.0 × 10⁻³–8.5 × 10⁻²°C/min (depending on the experimental data used for comparison). Alternatively, if the deposit was emplaced above the glass transition temperature, conductive cooling alone was insufficient to prevent welding. Crack patterns (average, 4.5 sides to each polygon) and column diameters (average, 75 cm) are consistent with relatively rapid cooling, where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer. The presence of regularly spaced, complex radiating joint patterns is consistent with fumarolic gas rise, where volatiles originated in the valley-confined drainage system below. Joint spacing is a proxy for cooling rates and is controlled by depositional thickness/valley width. We suggest that the formation of joints in high-temperature, non-welded deposits is aided by the presence of underlying external water, where vapor transfer causes crystallization in pore spaces, densifies the deposit, and helps prevent welding.     

Three years of photometry of IM pegasi = HR8703

Differential photometry of this K1 IV–III p RS VNn-type binary in 1978–79, 1979–80, and 1980–81 at eight different observatories shows that the variability, discovered orignally by Herbst, is periodic. The mean of three different determinations of the photometric period was 24^d.39±-0^d.03, about 1% shorter than the orbital period of 24^d.649 determined spectroscopically by Harper. The total range observed during these three years was 5 ^m .60<V<5 ^m .85. The light curve was nearly sinusoidal but always noticeably asymmetric, with the sense reversing sometime between 1979–80 and 1980–81.     

How effective are GIS in practice? A case study of British local government

Abstract

GIS have received increasing attention in recent years, particularly in local government. Despite the claims in the literature about the potential of the technology, to date there is still little information on the impact GIS applications are having on the organizations in which they are being implemented. With these considerations in mind this paper presents an evaluation of the findings of twelve case studies which were undertaken in a variety of British local government contexts during 1991 and 1992.

The evaluation of the findings of the case studies is divided into two parts. The first part indicates the limited impact GIS has had on the local authorities investigated even after at least two years experience. Given these findings the second part seeks to identify the issues which appear to be responsible for inhibiting the effective implementation of GIS in British local government. Overall it is suggested that some organizational cultures are inherently receptive and able to sustain the development of innovations such as GIS. However, whilst very few organizations are inherently innovative it was possible to identify four factors which appeared to enhance the chances of success. These are simple applications producing information which is fundamental to the work of potential users, user directed implementation involving the participation of all the stakeholders in the project, an awareness of the limitations of the organization with respect to the range of available resources, and a high degree of stability with respect to the general organizational environment, or alternatively an ability to cope with change. As a result the evaluation indicates that if GIS are to be effectively implemented much greater consideration must be given to the impact of organizational issues.     

Water quality along a river continuum subject to point and diffuse sources

The water quality along the River Kennet, in the Thames basin of southern England, was examined in terms of the influence of point- and diffuse-nutrient inputs. The river is supplied mainly from a Cretaceous Chalk aquifer and hence the waters are of a calcium bicarbonate type. The nitrate largely comes from agricultural sources, with concentrations decreasing downstream due to plant uptake and probable denitrification. In contrast, soluble reactive phosphorus (SRP) is largely associated with sewage inputs and concentrations increase downstream in line with effluents from major towns such as Newbury and Reading. Adjacent to the river in the lower half of the catchment is the Kennet and Avon Canal and the two are in places hydrologically connected. The canal inputs may influence calcium carbonate (calcite) precipitation and increase suspended sediment and particulate phosphorus concentrations in the river. Monitoring upstream and downstream of Marlborough sewage treatment works (STW) showed that SRP concentrations in the effluent were highly variable due to variable efficiency of P stripping and still sufficiently concentrated to dominate downstream river SRP with potential impacts on stream ecology. Biological recovery in this river following P stripping at STWs is complex and controlling those spikes in SRP that are above a threshold of 100 μg l⁻¹ may be a critical requirement. More stringent effluent targets than are currently recommended may be needed (less than 800 μg RP l⁻¹) to achieve good ecological status in this river depending on SRP concentrations upstream.     

Environmental control of short-term variation in the plankton community of inner Tokyo Bay, Japan

Water temperature, salinity, nutrient concentrations and the composition of the plankton community were recorded at three stations in inner Tokyo Bay over a period of 328 days (from June 8, 1995 to April 30, 1996) with a nominal sampling frequency of once per day. Inspection of the results revealed that the data could be divided into two blocs as an aid to analysis: the period from June to October was characterized by the development of stratification of temperature and salinity (stratification period), and November to March was characterized by uniform temperature and salinity in the water column due to vertical mixing (mixing period). Oxygen-depleted water forms in the bottom layer during the stratification period, but vertical mixing of the water column, due to changing wind and rainfall conditions caused by passing weather fronts, results in the breakdown of the oxygen-depleted water mass. Nutrient loads are high in the surface water due to the freshwater supply, but occasional pulses of primary production cause a depletion of phosphate in the surface water, suggesting that the phosphorus becomes a limiting nutrient for phytoplankton growth in this period. Several short-term peaks of plankton abundance (blooms) occurred as responses to temporal changes in water quality from June to November, with consequent species succession. Significant fluctuations in the densities of the diatom Skeletonema costatum and several species of ciliates corresponded to the daily changes in the physical and chemical characteristics of the coastal environment. During the mixing period, when water temperature and solar radiation decreased, there were no short-term variations in water quality and although nutrient concentrations gradually increased from November to February, primary production remained low. This study shows that the short-term dynamics of the phytoplankton community are closely coupled to fluctuations in environmental forcing, and that the degree of coupling is stronger during periods when solar radiation is greater. The results provide a novel typological understanding of seasonal plankton dynamics in a shallow, eutrophicated marine embayment, and suggest how such systems may be treated in simulation modeling.     

A gender analysis of quality of life in Saskatoon, Saskatchewan

While many community initiatives have attempted to assess the nature and determinants of their citizens’ quality of life (QOL), these initiatives have produced little about whether the key determinants of life quality differ by gender. Using both quantitative and qualitative data from a recent QOL research project in Saskatoon, Saskatchewan, Canada, this study explores whether factors that predict poor QOL are similar for women and men. The study also examines whether the nature of these gender differences varies across low, medium and high socio-economic status locales. Results showed that men and women were very similar in: (1) their QOL ratings, (2) reports of deterioration in neighborhood QOL, and (3) rating particular aspects of their neighborhood. Few gender differences were also found when analyzing data referring to place characteristics and social cohesion. Multivariate analysis revealed further similarities: (1) being divorced/widowed was a significant predictor of poor QOL, and (2) a negative view of community security issues was associated with deterioration in perceived neighborhood quality of life. For women, however, other variables were important predictors of QOL: being middle aged, being single, and having a poor opinion of the overall quality of their neighborhood. Qualitative analysis revealed that while men and women shared concern about general areas such as safety and neighborhood supports, their perceptions of the details around these issues differed. The multi-method results suggest that urban policy actions should include a gendered discussion of common QOL issues.     

Arsenic mobility in weathered gold mine tailings under a low-organic soil cover

Historical gold mining operations in Nova Scotia, Canada, resulted in numerous deposits of publicly accessible, arsenic (As)-rich mine waste that has weathered in situ for 75–150 years, resulting in a wide range of As-bearing secondary minerals. The geochemical heterogeneity of this mine waste creates a challenge for identifying a single remediation approach that will limit As mobility. A 30-cm-thick, low-organic content soil cover was evaluated in a laboratory leaching experiment where, to simulate natural conditions, the equivalent of 2 years of synthetic rainwater was leached through each column and two dry seasons were incorporated into the leaching protocol. Each column was a stratigraphic representation of the four major tailings types found at the historical Montague and Goldenville gold mine districts: hardpan tailings, oxic tailings, wetland tailings, and high Ca tailings. Hardpan tailings released acidic, As-rich waters (max 12 mg/L) under the soil cover but this acidity was buffered by surrounding oxic tailings. Leachate from the oxic tailings was circumneutral, with average As concentrations between 4.4 and 9.7 mg/L throughout the experiment. The presence of carbonates in the high Ca tailings resulted in near-neutral to weakly alkaline leachate pH values and average As concentrations between 2.1 and 6.1 mg/L. Oxidation of sulfides in the wetland tailings led to acidic leachate over time and a decrease in As concentrations to values that were generally less than 1 mg/L. This study shows that the use of a low-organic content soil cover does not create reducing conditions that would destabilize oxidized, As-bearing secondary phases in these tailings. However, oxygen penetration through the cover during dry seasons would continue to release As to tailings pore waters via sulfide oxidation reactions.     

The impact of resuspension on sediment mercury dynamics, and methylmercury production and fate: A mesocosm study

The objective of this study was to investigate the effects of resuspension on the fate and bioaccumulation of mercury (THg) and methylmercury (MeHg) in shallow estuarine environments, using mesocosms. Two 4-week experiments were conducted in July (Experiment 1) and October (Experiment 2) of 2001 with Baltimore Harbor sediments. Hard clams, Mercenaria mercenaria, were introduced into sediments for Experiment 2. Tidal resuspension (4 h on and 2 h off cycles) was simulated, with 3 replicate tanks for each treatment—resuspension (R) and non-resuspension (NR). Sediment cores were collected during the experiments for THg, MeHg, organic content and AVS analyses, and for the determination of methylation/demethylation using Hg stable isotopes (¹⁹⁹Hg(II) and CH₃¹⁹⁹Hg(II)). Zooplankton samples were collected once a week while clams were taken before and after Experiment 2 for THg and MeHg analyses. Our results suggest that the interplay between Hg methylation and MeHg degradation determines the overall MeHg pool in sediments. Sediment resuspension does not appear to directly impact the Hg transformations but can lead to changes in the association to Hg binding phases, influencing Hg methylation. The bioaccumulation results indicate that sediment resuspension can play an important role in transferring sediment MeHg into organisms.