U–Pb geochronology of two discrete Ordovician high-pressure metamorphic events in the Seve Nappe Complex, Scandinavian Caledonides

U–Pb geochronology for eclogites in two different areas of the Seve Nappe Complex (SNC) in Sweden confirms previous indications of discrete Ordovician high-pressure events affecting various parts of the complex. In Norrbotten, just north of the Arctic circle, eclogites from the Ts?kkok and Vaimok Lenses yield identical metamorphic zircon ages of 482 ± 1 Ma. Titanite in a metagabbro from the Vaimok Lens retains an older age of 607 ± 2 Ma, which may date a protolith coeval with mafic dikes in the overlying Sarek Lens; high-U zircon cores in one of the eclogites also indicate a similar age. Farther south, in J?mtland, the Tjeliken eclogite yields a significantly younger metamorphic age of 446 ± 1 Ma. Although they support the age discrepancy between the Norrbotten and J?mtland eclogites, the U–Pb ages of both eclogite suites are ca. 20 m.y. younger than previously reported Sm–Nd ages. The latter may either represent early prograde growth or be spuriously too old due to isotopic disequilibrium. The SNC has traditionally been taken to represent the outermost margin of Baltica, linking the Early Caledonian eclogite–forming events to subduction of Baltica below an offshore arc. Alternatively, the coincidence of these eclogite-forming events with orogenic phases recorded on the Laurentian margin may point to an origin from other regions of the Iapetus Ocean.     

Ecology of rocky reef fish of northeastern New Zealand: 50 years on

Abstract

This review examines the history and current understanding of the ecology of rocky reef fishes in northeastern New Zealand, marking the fiftieth anniversary of the Leigh Marine Laboratory. The cumulative number of publications increased steadily to c. 180 by 2012. Most attention has been on the snapper (Pagrus auratus, f. Sparidae), the largest in size and economically most important species, and on triplefins (Tripterygiidae), the smallest in size and most diverse family. A strong quantitative school emerged in the 1980s, a period when there was little research on temperate reef fishes elsewhere. A ‘bottom-up’ view of the effects of habitat structure on fish emerged, identifying the key roles of depth, topography and macroalgae. By 2000, attention shifted to using marine reserves as laboratories for ecological research. A ‘top-down’ view of fish as predators emerged, documenting cascading effects on prey communities and habitats. The two different viewpoints have not yet been integrated. Research gaps, including population connectivity and climate change, represent major challenges for the next 50 years.     

Fish recruitment into a South African temporarily open/closed temperate estuary during three different hydrological mouth phases

Quantitative recruitment of ichthyofauna into a South African temporarily open/closed estuary (TOCE) was investigated during three distinct hydrological phases: closed overwash, open outflow and tidal mouth conditions. Maximum recruitment densities occurred during outflow conditions and declined when the system became tidally inundated. Recruitment densities were lowest during overwash events. Selective diel variation was evident, with higher night-time densities for each phase. This study highlights the importance of seasonal timing and duration of open conditions and overwash events for structuring ichthyofaunal community composition of southern African TOCEs.     

华南西部地区晚中生代与W-Sn矿有关花岗岩的年代学格架及地质意义

华南西部地区是我国重要的大型-超大型锡多金属矿床分布区。本文根据近年来新获得的花岗岩LA-ICP-MS锆石U-Pb数据,结合近几年以来新报道的高精度成岩成矿年代学资料和地球动力学研究进展,总结发现华南西部地区中生代岩浆-成矿作用的时代主要集中在晚白垩世(76～98Ma)。通过分别总结华南东、西部地区晚白垩岩浆作用及相应的成矿作用的特征,发现华南东、西部的成岩成矿作用特征表现出明显的差异,但是这两地内部各自却表现出高度的一致性;且东部地区明显要比西部地区更为复杂,但二者之间的共同点在于该时期均处于岩石圈伸展的环境和壳幔间的相互作用明显。晚白垩世华南西部地区主要受控于华南陆块陆内背景下岩石圈伸展而导致的岩浆-成矿作用这一因素,而东部地区除此之外还受到太平洋古板块北北东向走滑带来的影响。     

Do medical marijuana centers behave like locally undesirable land uses? Implications for the geography of health and environmental justice

As of 2013, medical marijuana is legal in 20 US States and the District of Columbia, but few studies have investigated the impact of the retail centers that sell the drug. We draw upon the social construction literature to frame our research and help us determine whether medical marijuana centers in Denver, Colorado, are considered locally undesirable land uses (LULUs). The geography of health and environmental justice frameworks lead us to hypothesize that marijuana centers are more likely to be opened in Hispanic, Black, and poor neighborhoods than in non-Hispanic White and affluent neighborhoods. We also hypothesize that marijuana centers will tend to increase the minority composition and poverty of the neighborhoods in which they are located. Contrary to expectations, we find no empirical support for these two hypotheses. Instead, results suggest that marijuana centers are likely to be situated in neighborhoods with higher crime rates and more retail employment. Thus, despite the view by many planners and law enforcement officials that these centers are problematic, they do not take on LULU characteristics in siting and demographic changes. This finding, while limited to Denver, has important implications for policymakers who are considering similar marijuana policies.     

Potential effects of obliquity change on gas hydrate stability zones on Mars

Methane hydrate dissociation due to obliquity-driven temperature change has been suggested as a potential source of atmospheric methane plumes recently observed on Mars. This work uses both equilibrium and time-dependent models to determine how geothermal gradients change on Mars as a result of obliquity and predict how these changes affect gas hydrate stability zones (HSZs). The models predict that the depth to the HSZ decreases with increasing latitude for both CO₂ and CH₄ hydrate, with CO₂ hydrate occurring at shallower depths than CH₄ hydrate over all latitudes. The depth of the HSZ increases as surface temperatures warm and decreases as surface temperatures cool with changing obliquity, with the largest change in HSZ volume predicted near the equator and the poles. Therefore, changes in the depth to the HSZ may cause hydrate dissociation near the equator and poles as the geothermal gradient moves in and out of the hydrate stability field over hundreds of thousands of years. Sublimation of overlying ice containing diffused methane could account for recent observations of seasonal methane plumes on Mars. In addition, near-surface gas hydrate reservoirs may be preserved at mid-latitudes due to minimal changes in surface temperature with obliquity over geologic time scales. Comparisons of the predicted changes in the HSZ with hydrate dissociation and diffusion rates reveal that metastable hydrate may also remain in the near subsurface, especially at high latitudes, for millions to billions of years. The presence of methane hydrate in the near subsurface at midlatitudes could be an important analytical target for future Mars missions, as well as serving as a source of fuel for future spacecraft.     

Habitat characteristics,temporal variability,and macroinvertebrate communities associated with a mat-forming nuisance diatom (Didymosphenia geminata) in Catskill mountain streams,New York

Didymosphenia geminata has recently and rapidly greatly expanded its range and abundance, sometimes as an exotic invasive and other times as a nuisance (‘native invader’) within its hypothesized native range, including the northeastern United States. D. geminata mats are visually conspicuous and can grow >10 cm thick. Mats first appeared in the eastern Catskill mountains (New York) in 2009. Our objectives were to (1) document D. geminata growth in three impounded or regulated rivers in the eastern Catskill mountains from 2010 to 2012 and (2) measure the effects of D. geminata mats on macroinvertebrates. The highest D. geminata cell densities were downstream of reservoir outflows in two of three streams. D. geminata mat development peaked in the summer each year, but maximum coverage and cell density was variable among years. D. geminata cover was negatively correlated with 10 days maximum antecedent shear stress, and the year with lowest mean D. geminata cover had multiple tropical storms and floods, suggesting that low variation in flow allows for D. geminata mat proliferation. Across sites, D. geminata density was negatively correlated with nitrate concentrations. D. geminata density was negatively related to macroinvertebrate richness suggesting that D. geminata mats may negatively affect aquatic food webs. D. geminata appears to be a nuisance species with similar habitat characteristics and growth where it is both a native invader and an invasive species.     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

Disease environments and subnational patterns of under-five mortality in sub-Saharan Africa

This study examined the subregional spatial distribution of child mortality risk (CMR) in sub-Saharan Africa. Data for maps were obtained from Demographic and Health Surveys in the 1980s and 1990s for 95 provinces. Two distinct regions are identified: West Africa and a belt of countries spanning East to Southern Africa. Some countries are missing from the analysis. CMR was higher in West Africa. East/Southern Africa had the following patterns: low mortality in Central/South and North/West provinces of Namibia, Botswana, and all of Zimbabwe, except Manicaland and Mashonaland Central provinces (adjoining areas with moderately low mortality); low mortality in the Kenya/Tanzania highlands, with the Rift Valley, Central, Eastern, Nairobi, and northern highlands of Tanzania (adjacent provinces with moderately low or moderate mortality); high mortality in northern Zambia and Malawi (adjacent provinces with moderately high mortality); and moderate or moderately high mortality in the Lake Victoria basin and western provinces. West Africa had south/north or coastal/interior patterns. Coastal and southern provinces had low or moderately low mortality, while interior provinces had high mortality. Regression models indicate that the spatial patterns in West Africa were due to socioeconomic differences. In East/Southern Africa, mortality differences were due to differences in intensity of malaria transmission.     

Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway

The Solund–Hyllestad–Lavik area affords an excellent opportunity to understand the ultrahigh‐pressure Scandian orogeny because it contains a near‐complete record of ophiolite emplacement, high‐pressure metamorphism and large‐scale extension. In this area, the Upper Allochthon was intruded by thec. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle/Lower Allochthon, probably in the Wenlockian. The Middle/Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410–400 Ma. Within < 5–10 Myr, all these units were exhumed by the Nordfjord–Sogn detachment zone, producing shear strains > 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the near‐ultrahigh‐pressure rocks to within c. 3 km vertical distance from the low‐grade Solund Conglomerate.