The major-ion composition of silurian seawater

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg²⁺, Na⁺, and SO₄²⁻, and much higher concentrations of Ca²⁺ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca²⁺ in excess of SO₄²⁻. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO₄-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na⁺ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg²⁺/Ca²⁺ ratio of Silurian seawater was ∼1.4, and the K⁺/Ca²⁺ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg²⁺/Ca²⁺ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg²⁺/Ca²⁺ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg²⁺/Ca²⁺ ratio of seawater, not the pCO₂ of the Silurian atmosphere.     

A safer city centre for all? Senses of `community safety' in Newcastle upon Tyne

In recent years, many city authorities in Europe and North America have sought to attract people back to city centres for leisure, shopping, business and residence. The planning measures commonly adopted take explicit account of issues of crime and safety. The concept of `community safety' as applied to the city centre is problematic, however, as users have diverse lifestyles, interests and values which impact on their own and others' safety. This paper reports on a study of the experiences and perceptions of safety of different groups of city centre users in Newcastle upon Tyne, England. While social identities and roles shaped the extent of concern and subsequent impacts on people's use of the city centre space, the research suggested that many concerns are genuinely shared. The paper investigates the congruence of a range of planning strategies with city centre users' safety concerns. Situational measures were viewed with scepticism, as fear centres on the presence and behaviour of others.     

Placing geographies of public health

Following the move to a 'post–medical' geography, a large amount of research has come to focus on public health issues. This paper explores these current geographies of public health and argues for the development of a more critical perspective. In particular, it draws on commentary that has emerged out of debates that have taken place within a body of literature usually identified as the critical 'new' public health. The paper goes on to argue that such scholarship offers crucial insights for the production of a critical geography of public health.     

Atmospheric nitrogen inputs into the North Sea: effect on productivity

The ANICE (Atmospheric Nitrogen Inputs into the Coastal Ecosystem) project addressed the atmospheric deposition of nitrogen to the North Sea, with emphasis on coastal effects. ANICE focused on quantifying the deposition of inorganic nitrogen compounds to the North Sea and the governing processes. An overview of the results from modelling and experimental efforts is presented. They serve to identify the role of the atmosphere as a source of biologically essential chemical species to the marine biota. Data from the Weybourne Atmospheric Observatory (UK) are used to evaluate the effect of short episodes with very high atmospheric nitrogen concentrations. One such episode resulted in an average deposition of 0.8 mmol N m⁻² day⁻¹, which has the potential to promote primary productivity of 5.3 mmol C m⁻² day⁻¹. This value is compared to long-term effects determined from model results. The total calculated atmospheric deposition to the North Sea in 1999 is 948 kg N km⁻¹, i.e. 0.19 mmol N m⁻² day⁻¹ which has the potential to promote primary productivity of 1.2 mmol C m⁻² day⁻¹. Detailed results for August 1999 show strong gradients across the North Sea due to adjacent areas where emissions of NO_x and NH₃ are among the highest in Europe. The average atmospheric deposition to the southern part of the North Sea in August 1999 could potentially promote primary production of 2.0 mmol C m⁻² day⁻¹, i.e. 5.5% of the total production at this time of the year in this area of the North Sea. For the entire study area the atmospheric contribution to the primary production per m² is about two-third of this value. Most of the deposition occurs during short periods with high atmospheric concentrations. This atmospheric nitrogen is almost entirely anthropogenic in origin and thus represents a human-induced perturbation of the ecosystem.     

Role of the subducted slab,mantle wedge and continental crust in the generation of adakites from the Andean Austral Volcanic Zone

 All six Holocene volcanic centers of the Andean Austral Volcanic Zone (AVZ; 49–54°S) have erupted exclusively adakitic andesites and dacites characterized by low Yb and Y concentrations and high Sr/Y ratios, suggesting a source with residual garnet, amphibole and pyroxene, but little or no olivine and plagioclase. Melting of mafic lower crust may be the source for adakites in some arcs, but such a source is inconsistent with the high Mg# of AVZ adakites. Also, the AVZ occurs in a region of relatively thin crust (<35 km) within which plagioclase rather than garnet is stable. The source for AVZ adakites is more likely to be subducted oceanic basalt, recrystallized to garnet-amphibolite or eclogite. Geothermal models indicate that partial melting of the subducted oceanic crust is probable below the Austral Andes due to the slow subduction rate (2 cm/year) and the young age (<24 Ma) of the subducted oceanic lithosphere. Geochemical models for AVZ adakites are also consistent with a large material contribution from subducted oceanic crust (35–90% slab-derived mass), including sediment (up to 4% sediment-derived mass, representing approximately 15% of all sediment subducted). Variable isotopic and trace-element ratios observed for AVZ adakites, which span the range reported for adakites world-wide, require multistage models involving melting of different proportions of subducted basalt and sediment, as well as an important material contribution from both the overlying mantle wedge (10–50% mass contribution) and continental crust (0–30% mass contribution). Andesites from Cook Island volcano, located in the southernmost AVZ (54°S) where subduction is more oblique, have MORB-like Sr, Nd, Pb and O isotopic composition and trace-element ratios. These can be modeled by small degrees (2–4%) of partial melting of eclogitic MORB, yielding a tonalitic parent (intermediate SiO₂, CaO/Na₂O>1), followed by limited interaction of this melt with the overlying mantle (≥90% MORB melt, ≤10% mantle), but only very little (≤1%) or no participation of either subducted sediment or crust. In contrast, models for the magmatic evolution of Burney (52°S), Reclus (51°S) and northernmost AVZ (49–50°S) andesites and dacites require melting of a mixture of MORB and subducted sediment, followed by interaction of this melt not only with the overlying mantle, but the crust as well. Crustal assimilation and fractional crystallization (AFC) processes and the mass contribution from the crust become more significant northwards in the AVZ as the angle of convergence becomes more orthogonal. Received: 1 March 1995 / Accepted: 13 September 1995     

The Effects of Climate Change on Water Resources in the West: Introduction and Overview

The results of an experimental `end to end' assessment of the effects of climate change on water resources in the western United States are described. The assessment focuses on the potential effects of climate change over the first half of the 21st century on the Columbia, Sacramento/San Joaquin, and Colorado river basins. The paper describes the methodology used for the assessment, and it summarizes the principal findings of the study. The strengths and weaknesses of this study are discussed, and suggestions are made for improving future climate change assessments.     

Precise/ Small Sample Size Determinations of Lithium Isotopic Compositions of Geological Reference Materials and Modern Seawater by MC-ICP-MS

The Li isotope ratios of four international rock reference materials, USGS BHVO-2, GSJ JB-2, JG-2, JA-1 and modern seawater (Mediterranean, Pacific and North Atlantic) were determined using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). These reference materials of natural samples were chosen to span a considerable range in Li isotope ratios and cover several different matrices in order to provide a useful benchmark for future studies. Our new analytical technique achieves significantly higher precision and reproducibility (< ± O.3%o 2s) than previous methods, with the additional advantage of requiring very low sample masses of ca . 2 ng of Li.