Acigöl rhyolite field,Central Anatolia (part 1): high-resolution dating of eruption episodes and zircon growth rates

Protracted pre-eruptive zircon residence is frequently detected in continental rhyolites and can conflict with thermal models, indicating briefer magma cooling durations if scaled to erupted volumes. Here, we present combined U-Th and (U-Th)/He zircon ages from the Acigöl rhyolite field (Central Anatolia, Turkey), which is part of a Quaternary bimodal volcanic complex. Unlike other geochronometers, this approach dates crystallization and eruption on the same crystals, allowing for internal consistency testing. Despite the overall longevity of Acigöl rhyolite volcanism and systematic trends of progressive depletion in compatible trace elements and decreasing zircon saturation temperatures, we find that zircon crystallized in two brief pulses corresponding to eruptions in the eastern and western part of the field during Middle and Late Pleistocene times, respectively. For Late Pleistocene zircon, resolvable differences exist between interior (average: 30.7 ± 0.9 ka; 1σ error) and rim (21.9 ± 1.3 ka) crystallization ages. These translate into radial crystal growth rates of ~10^?13 to 10^?14 cm/s, broadly consistent with those constrained by diffusion experiments. Rim crystallization and (U-Th)/He eruption ages (24.2 ± 0.4 ka) overlap within uncertainty. Evidence for brief zircon residence at Acigöl contrasts with many other rhyolite fields, suggesting that protracted zircon crystallization in, or recycling from, long-lived crystal mushes is not ubiquitous in continental silicic magma systems. Instead, the span of pre-eruptive zircon ages is consistent with autochthonous crystallization in individual small-volume magma batches that originated from basaltic precursors.     

Structural and geochronological studies on the Liba goldfield of the West Qinling Orogen, Central China

The Liba goldfield, located to the northeast of the Zhongchuan Granite in the West Qinling Orogen (WQO) of mainland China, contains the largest known gold resource of 2.8 Moz in the Zhongchuan area. Devonian metasedimentary rocks host the structurally controlled gold mineralization, which is associated with silica–sericite–chlorite–carbonate alteration. Two major styles of mineralization occurred at the goldfield, which are disseminated sediment-hosted and quartz vein hosted types. Pyrite, arsenopyrite, and arsenian pyrite are major gold carriers and gold also occurs as native gold grains and electrum spatially associated with the sulfides. Numerous felsic/intermediate dykes have a similar structural control as the mineralization, and their contacts with host rocks are recognized as favorable zones for mineralization. Detailed fieldwork in conjunction with geochronological studies has helped to define the deformation history and gold metallogenesis of the goldfield. Three major phases of deformation have been recognized in the Zhongchuan area. The first deformation (D₁) event was compressional in broadly a N–S orientation, the second (D₂) event was also compressional and orientated in a NE–SW direction, and the third (D₃) event was post-mineralization and was associated with the emplacement of barren calcite and anhydrite veins. Compression related to D₂ is the key process that controlled the distribution of igneous dykes and gold mineralization in the Liba goldfield. Both igneous and hydrothermal fluids preferentially focused along dilational jogs under local trans-extension, which took place during the late stage of D₂. Precise dating with high-resolution ion microprobe (SHRIMP) U–Pb on zircon and ⁴⁰Ar/³⁹Ar on muscovite, biotite, hornblende, and plagioclase of crosscutting pre-mineralization granitic porphyry and diorite dykes have constrained the mineralization age to after ca. 227 Ma. ⁴⁰Ar/³⁹Ar analysis of minerals formed in hydrothermal alteration zones associated with gold mineralization indicates that there was a widespread ca. 216 Ma hydrothermal event that affected almost all lithologies in the area. This detailed investigation is the first study to tightly constrain the timing of gold mineralization in the WQO. The broadly overlapping timing and similar structural control of the mineralization and igneous dykes show a promising correlation, which could be potentially used to map this Late Triassic gold mineralization event in the WQO.     

An operational MODIS aerosol retrieval algorithm at high spatial resolution,and its application over a complex urban region

Aerosol retrieval algorithms for the MODerate Resolution Imaging Spectroradiometer (MODIS) have been developed to estimate aerosol and microphysical properties of the atmosphere, which help to address aerosol climatic issues at global scale. However, higher spatial resolution aerosol products for urban areas have not been well-researched mainly due to the difficulty of differentiating aerosols from bright surfaces in urban areas. Here, an aerosol retrieval algorithm using the MODIS 500-m resolution bands is described, to retrieve aerosol properties over Hong Kong and the Pearl River Delta region. The rationale of our technique is to first estimate the aerosol reflectances by decomposing the top-of-atmosphere reflectances from surface reflectances and Rayleigh path reflectances. For the determination of surface reflectances, a Minimum Reflectance Technique (MRT) is used, and MRT images are computed for different seasons. For conversion of aerosol reflectance to aerosol optical thickness (AOT), comprehensive Look Up Tables specific to the local region are constructed, which consider aerosol properties and sun-viewing geometry in the radiative transfer calculations. Four local aerosol types, namely coastal urban, polluted urban, dust, and heavy pollution, were derived using cluster analysis on 3 years of AERONET measurements in Hong Kong. The resulting 500 m AOT images were found to be highly correlated with ground measurements from the AERONET (r² = 0.767) and Microtops II sunphotometers (r² = 0.760) in Hong Kong. This study further demonstrates the application of the fine resolution AOT images for monitoring inter-urban and intra-urban aerosol distributions and the influence of trans-boundary flows. These applications include characterization of spatial patterns of AOT within the city, and detection of regional biomass burning sources.     

The Environment Act 1995 and quiet enjoyment: implications for countryside recreation in the national parks of England and Wales, UK

Summary National parks in England and Wales have traditionally been seen as places of quiet, less hurried recreation. The phrase 'quiet enjoyment' was adopted to represent this notion, and this paper traces the policy-making process involved in attempting to conserve this view. The debate during the enactment of the Environment Act 1995 brought the use and meaning of the phrase to prominence; amongst other issues, the use of the phrase in tenancy law precluded its use in legislation. The loss of the phrase 'quiet enjoyment' could have prolonged effects on the way in which national parks are used for recreational purposes.     

Management application of an empirical model of sardine–climate regime shifts

In previous work, we show that accumulated anomalies of physical indices are proportional to California sardine landings and that the accumulated anomaly curves change the sign of their slope, showing maxima (minima) when climate is favorable (unfavorable) to successful completion of the sardine life cycle change. Here, we find unique time series characteristics of the periods when the climate changed for sardines in the 1930–2004 period. Only one 50–70 year cycle is examined but the consistency of the dominant signals in measurements taken independently within the sardine's environment at locations separated by thousands of kilometers, supports the argument that the events affecting the ocean-climate of the California Current region and consequently sardine life-cycle are large-scale and persist over multi-decadal periods. Year-to-year monitoring of the climate regime-state in the physical environment and its accumulating effects on sardine populations is also described. The ability to analyze climate shifts and monitor their effects on the sardine populations can reduce uncertainty in making resource management, social and business decisions. Possible effects of management decisions affecting transboundary fisheries issues within United States (US) and between the US and its Pacific neighbors are clarified. The methods presented will add an analysis of low-frequency events to the current management oriented analyses of interannual events, which are part of the existing Pacific Fishery Management Council sardine management plan.     

A history of violence: magma incubation,timing and tephra distribution of the Los Chocoyos supereruption (Atitlán Caldera,Guatemala)

The climactic Los Chocoyos (LCY) eruption from Atitlán caldera (Guatemala) is a key chronostratigraphic marker for the Quaternary period given the extensive distribution of its deposits that reached both the Pacific and Atlantic Oceans. Despite LCY tephra being an important marker horizon, a radioisotopic age for this eruption has remained elusive. Using zircon (U–Th)/He geochronology, we present the first radioisotopically determined eruption age for the LCY of 75 ± 2 ka. Additionally, the youngest zircon crystallization ²³⁸U–²³⁰Th rim ages in their respective samples constrain eruption age maxima for two other tephra units that erupted from Atitlán caldera, W-Fall (130 ⁺¹⁶/₋₁₄ ka) and I-Fall eruptions (56 ^+8.2/_−7.7 ka), which under- and overlie LCY tephra, respectively. Moreover, rim and interior zircon dating and glass chemistry suggest that before eruption silicic magma was stored for >80 kyr, with magma accumulation peaking within ca. 35 kyr before the LCY eruption during which the system may have developed into a vertically zoned magma chamber. Based on an updated distribution of LCY pyroclastic deposits, a new conservatively estimated volume of ~1220 ± 150 km³ is obtained (volcanic explosivity index VEI > 8), which confirms the LCY eruption as the first-ever recognized supereruption in Central America.     

Compositing and subsampling to reduce costs and improve power in benthic infaunal monitoring programs

Monitoring of soft-sediment infauna can be a useful tool in impact assessment, but sorting of specimens from sediments may be very time consuming and expensive. This can force a reduction in the intended sampling program, leading to low statistical power and increased uncertainty in decision making. We pooled field samples of marine infauna into composites, then exhaustively subsampled the composites to examine the statistical distribution of organisms in the subsamples. Abundances of organisms among subsamples were generally consistent with a random distribution, with deviations from randomness occurring more frequently for standard replicates than for subsamples. For the most abundant taxa, subsamples had much smaller variances than replicates, and in 76% of cases, the type of sample (i.e., subsample or replicate) had no effect on the mean abundance of organisms recorded. As our subsamples appeared generally unbiased, we then considered the cost and power for hypothetical monitoring programs using an MBACI analysis, and found substantial cost savings and increases in power to detect impact by using compositing and subsampling.     

Geochemical and hydrologic controls on the mobilization of arsenic derived from herbicide application

The fate and transport of As was examined at an industrial site where soil- and groundwater contamination are derived from the application of As₂O₃ as a herbicide. Application of arsenical herbicides was discontinued in the 1970s and soils in the source area were partially excavated in 2003. Arsenic contamination (up to 280 mg/kg) remains in the source area soils and a plume of As-contaminated groundwater persists in the surficial aquifer downgradient of the source area with maximum observed As concentrations of 1200 μg/L near the source area. The spatial extent of As contamination as defined by the 10 μg/L contour appears to have remained relatively stable over the period 1996–2006; the boundary of the 1000 μg/L contour has retreated over the same time period indicating a decrease in total As mass in the surficial groundwater.In column experiments conducted with source area soil, the As concentrations in the column effluent were comparable to those observed in groundwater near the source area. A substantial fraction of the As could be leached from the source area soil with ammonium sulfate and ammonium phosphate. Exhaustive extraction with background groundwater removed most of the total As. These results indicate that As in the source area soils is geochemically labile. Source area soils are low in extractable Fe, Mn and Al, and characterization by X-ray absorption spectroscopy and electron microscopy indicated that As is present primarily as arsenate sorbed to (alumino)silicate minerals. Batch sorption experiments showed much less sorption on surficial aquifer sediments than on sediments from the Jackson Bluff Formation (JBF), a presumed confining layer. This limited capacity of the surficial aquifer sediments for As sorption is consistent with the similar As contents observed for these sediments within and upgradient of the As plume. The apparent stability of the As plume cannot be explained by sequestration of As within the surficial aquifer. Sorption to JBF sediments may contribute to As sequestration, but As enrichment in JBF sediments within the plume (i.e., as compared with JBF sediments upgradient) was not observed. These results indicate that neither the persistence of As in the source area soils or the apparent stability of the plume of As-contaminated groundwater at this site can be explained by geochemical controls on As mobility. The absence of demonstrable geochemical bases for such observations suggests that possible hydrologic controls should be further investigated at this site.