A catalogue of Spanish archaeomagnetic data

A total of 58 new archaeomagnetic directions has been determined from archaeological structures in Spain. Together with five previous results they allow the compilation of the first archaeomagnetic catalogue for Spain, which includes 63 directions with ages ranging between the 2nd century BC and the 20th century AD. Characteristic remanence directions have been obtained from stepwise thermal and alternating field demagnetization. The hierarchical structure has been respected in the calculation of the mean site directions. Rock magnetic experiments reveal that the main magnetic carrier is magnetite or titanomagnetite with different titanium contents. The age estimate of the studied structures is generally well justified by archaeological constraints. For six structures the proposed date is also supported by physical methods. The data are in close agreement with the French secular variation (SV) curve. This catalogue represents the first step in the construction of a SV curve for the Iberian Peninsula, which will be of much use in archaeomagnetic dating and in modelling of the Earth's magnetic field in Western Europe.     

Association of X-ray arches with chromospheric neutral lines

Daily maps of magnetic neutral lines derived from H observations have been superimposed on solar X-ray images for the period 15–30 June 1973. Nearly all X-ray-emitting structures consist of systems of arches covering chromospheric neutral lines. Areas of low emissivity, coronal holes, appear as the areas between arcades of arches. The presence of a coronal hole, therefore, is determined by the spacing between neutral lines and the scale of the arches over those neutral lines. X-ray emissivity on the solar disk extends from neutral lines in proportion to the vertical and horizontal scale of the arches over those neutral lines. Increasing scale of arches corresponds with increasing age of magnetic fields associated with the neutral line. All X-ray filament cavities coincided with neutral lines, but filaments appeared under cavities for only part of their length and for only a fraction of the disk passage.A substantial portion of this work was done while a visitor at American Science and Engineering, Cambridge, Mass. 02139, U.S.A.     

STAGES: the Space Telescope A901/2 Galaxy Evolution Survey

We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multicluster system at   z ∼ 0.165  has been the subject of an 80-orbit F606W Hubble Space Telescope (HST) /Advanced Camera for Surveys (ACS) mosaic covering the full     span of the supercluster. Extensive multiwavelength observations with XMM–Newton , GALEX, Spitzer , 2dF, Giant Metrewave Radio Telescope and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star formation rate, nuclear activity and stellar mass. In addition, with the multiwavelength data set and new high-resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of an environment we will be able to evaluate the relative importance of the dark matter haloes, the local galaxy density and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction and creation of a master catalogue. We perform the Sérsic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star formation rates for this field. We define galaxy and cluster sample selection criteria, which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues.     

Quantifying the value of historical climate knowledge and climate forecasts using agricultural systems modelling

A well tested agricultural systems model was used together with 114 years of historical climate data to study the performance of a dryland wheat–fallow system as impacted by climate variations and nitrogen input levels in southeast Australia, and to investigate the value of: (1) historical climate knowledge, (2) a perfect climate forecast, and (3) various forecasts of targeted variables. The potential value of historical climate records increases exponentially with the number of years of data. In order to confidently quantify the long term optimal nitrogen application rate at the study site at least 30 years of climate data are required. For nitrogen management only, the potential value of a perfect climate forecast is about $54/ha/year with a reduction of excess nitrogen application of 20 kg N/ha/year. The value of an ENSO based forecast system is $2/ha/year. Perfect forecasting of three or six categories of growing season rainfall would have a value of $10–12/ha/year. Perfect forecasts of three or six categories of simulated crop yield would bring about $33–34/ha/year. Choosing integrated variables as a forecasting target, for example crop yield derived from agricultural modelling, has the potential to significantly increase the value of forecasts.     

An improved age framework for late Quaternary silicic eruptions in northern Central America

 Five new stepwise-heating ⁴⁰Ar/³⁹Ar ages and one new high-sensitivity ¹⁴C date of ash-fall and ash-flow deposits from late Quaternary silicic volcanoes in northern Central America document the eruption rates and frequencies of five major rhyodacite and rhyolite calderas (Atitlán, Amatitlán, Ayarza, Coatepeque, and Ilopango) located north of the basalt, andesite, and dacite stratovolcanoes of the Central American volcanic front. These deposits form extensive time-stratigraphic horizons that intercalate regionally, and knowledge of dates and stratigraphy provides a valuable framework for age determinations of more localized volcanic and nonvolcanic events. The new data, especially when integrated with previous stratigraphic and dating work, show that all five calderas erupted several times in the past 200 ka and, despite a lack of historic activity, should be considered as active centers that could produce highly explosive eruptions again. Because of their locations near the highly vulnerable economic hearts of Guatemala and El Salvador, the risks of eruptions from these calderas should be carefully considered along with risks of major earthquakes and volcanic front volcanoes, which are much more frequent but inflict less severe and extensive damage. This investigation also includes some examples of dating efforts that failed to produce reasonable results. Received: 15 May 1998 / Accepted: 18 January 1999     

A framework to enhance semantic flexibility for analysis of distributed phenomena

While some geographic phenomena hold uniform properties, such as land‐use zones, many geographic phenomena are distributed such that their properties vary across an extended area. While such distributed phenomena are best represented as continuous surfaces, individual objects (or features) often emerge among clusters of high or low values in a field. For example, areas of relatively high elevation may be viewed as hills, while flat low‐lying areas are perceived as plains in a terrain. A comprehensive spatial analysis of distributed phenomena should examine both the spatial variance of its attribute surfaces and the characteristics of individual objects embedded in the field. An immediate research challenge to meet such spatial analysis needs is that these emerging features often have vague boundaries that vary according to the use and the user. The nature, and even existence, of these objects depend upon the range of values, or thresholds, used to define them. We propose a representation framework that takes a dual raster‐vector approach to capture both field‐ and object‐like characteristics of distributed phenomena and maintain multiple representations of embedded features delineated by boundaries that are likely to be relevant for the expected uses of the data. We demonstrate how boundaries influence the analysis and understanding of spatiotemporal characteristics of distributed phenomena. Using precipitation as a proof of concept, we show how the proposed framework enhances semantic flexibility in spatiotemporal query and analysis of distributed phenomena in geographic information systems.     

Chemical and isotopic tracers of the contribution of microbial gas in Devonian organic-rich shales and reservoir sandstones,northern Appalachian Basin

In this study, the geochemistry and origin of natural gas and formation waters in Devonian age organic-rich shales and reservoir sandstones across the northern Appalachian Basin margin (western New York, eastern Ohio, northwestern Pennsylvania, and eastern Kentucky) were investigated. Additional samples were collected from Mississippian Berea Sandstone, Silurian Medina Sandstone and Ordovician Trenton/Black River Group oil and gas wells for comparison. Dissolved gases in shallow groundwaters in Devonian organic-rich shales along Lake Erie contain detectable CH₄ (0.01–50.55 mol%) with low δ¹³C–CH₄ values (−74.68 to −57.86‰) and no higher chain hydrocarbons, characteristics typical of microbial gas. Nevertheless, these groundwaters have only moderate alkalinity (1.14–8.72 meq/kg) and relatively low δ¹³C values of dissolved inorganic C (DIC) (−24.8 to −0.6‰), suggesting that microbial methanogenesis is limited. The majority of natural gases in Devonian organic-rich shales and sandstones at depth (>168 m) in the northern Appalachian Basin have a low CH₄ to ethane and propane ratios (3–35 mol%; C₁/C₂ + C₃) and high δ¹³C and δD values of CH₄ (−53.35 to −40.24‰, and −315.0 to −174.6‰, respectively), which increase in depth, reservoir age and thermal maturity; the molecular and isotopic signature of these gases show that CH₄ was generated via thermogenic processes. Despite this, the geochemistry of co-produced brines shows evidence for microbial activity. High δ¹³C values of DIC (>+10‰), slightly elevated alkalinity (up to 12.01 meq/kg) and low SO₄ values (<1 mmole/L) in select Devonian organic-rich shale and sandstone formation water samples suggest the presence of methanogenesis, while low δ¹³C–DIC values (<−22‰) and relatively high SO₄ concentrations (up to 12.31 mmole/L) in many brine samples point to SO₄ reduction, which likely limits microbial CH₄ generation in the Appalachian Basin. Together the formation water and gas results suggest that the vast majority of CH₄ in the Devonian organic-rich shales and sandstones across the northern Appalachian Basin margin is thermogenic in origin. Small accumulations of microbial CH₄ are present at shallow depths along Lake Erie and in western NY.