A study of the long-term variability in radio emission of active galaxies

A long-term study of 13 active galaxies has been carried out at 10.7 GHz over the period September 1980 to April 1982. The sources were observed at approximately monthly intervals and most of them were selected because they are also X-ray sources. The data obtained are used to measure their respective variabilities and to search for any correlations with other source parameters. No obvious relationship to radio or X-ray luminosity was found, though there does appear to be two distinct levels of radio variability for these sources and clear evidence of higher X-ray than radio variability.     

A paleomagnetic study of the Mull lava succession

Summary. A paleomagnetic study has been made of a succession of 139 non-overlapping basaltic lavas, representing 91 per cent of the longest remaining succession of flows associated with the Paleogene Mull volcano. All the lavas have experienced considerable hydrothermal alteration, probably at up to several million years after initial magnetization and frequently with alteration to the opaque minerals and the production of new potentially magnetic phases. The question of whether directional remagnetization has taken place while preserving within-unit directional consistency and discreteness of unit mean direction is discussed. Extensive directional remagnetization is excluded as an explanation for the data. If stable directions obtained by alternating field remanence cleaning coincide with original TRM directions then a mean pole position for all temporally independent lava directions from the British Tertiary igneous province is at 71.9° N, 167.2° E, with k:22 and α₉₅:3.0°. This pole is significantly different from the geographic pole. If the difference in palaeomagnetic and geographic poles is interpreted in terms of absolute plate motion, then 2010 km of northwards motion of the western part of the Eurasian Plate, at 3.7 cm/yr, has taken place over the last 55 Myr. This motion has implications for the geological history of the Arctic and for the complexity of mantle motions.     

MAVIS–an upper crustal seismic refraction experiment in the Midland Valley of Scotland

The results of the Midland Valley Investigation by Seismology (MAVIS) are described. The experiment consists of three refraction profiles of between 80 and 100 km length across the Upper Palaeozoic basins of the Scottish Midland Valley.
Three refractors are recognized, thus defining four upper crustal layers. The top two layers are interpreted as Carboniferous plus Upper Old Red Sandstone, and Lower Old Red Sandstone plus Lower Palaeozoic. The structure of the refractor, an unconformity, separating the two layers mirrors surface structure. Deeper refractors, associated with Midland Valley crystalline basement, show no correlation with surface structure. The MAVIS survey allows the refinement of earlier gross interpretation of upper crustal structure in the Midland Valley, but confirms the existence of anomalously shallow (6.4 km s^–1) basement, and shows, in conjunction with results of previous work, that a large area of the central and southern Midland Valley is characterized by one velocity signature, probably indicative of a single terrane.     

The British Tertiary igneous province: palaeomagnetism of the dyke swarm along the Sleat coast of Skye

Summary. A palaeomagnetic investigation has been made of a swarm of more than 400 dykes along the south coast of Skye, Scotland, by the Sound of Sleat.
Seven red lamprophyre dykes have palaeomagnetic directions inconsistent with Tertiary age, and not inconsistent with their previously held Caledonian age. The remaining 409 dykes have palaeomagnetic directions that are consistent with a Lower Tertiary age. We present evidence suggesting that the Tertiary dykes might have been emplaced during a short time, over which the geomagnetic polarity occupied as few as three polarity intervals (NRN or RNR).
Certain 'intermediate' directions of magnetization have also been found, and are presented here.     

The yield and isotopic composition of radiolytic H2, a potential energy source for the deep subsurface biosphere

The production rate and isotopic composition of H₂ derived from radiolytic reactions in H₂O were measured to assess the importance of radiolytic H₂ in subsurface environments and to determine whether its isotopic signature can be used as a diagnostic tool. Saline and pure, aerobic and anaerobic water samples with pH values of 4, 7, and 10 were irradiated in sealed vials at room temperature with an artificial γ source, and the H₂ abundance in the headspace and its isotopic composition were measured. The H₂ concentrations were observed to increase linearly with dosage at a rate of 0.40 ± 0.04 molecules (100 eV)⁻¹ within the dosage range of 900 to 3500 Gray (Gy; Gy = 1 J Kg⁻¹) with no indication of a maximum limit on H₂ concentration. At ∼2000 Gy, the H₂ concentration varied only by 16% across the experimental range of pH, salinity, and O₂. Based upon this measured yield and H₂ yields for α and β particles, a radiolytic H₂ production rate of 10⁻⁹ to 10⁻⁴ nM s⁻¹ was estimated for the range of radioactive element concentrations and porosities typical of crustal rocks. The δD of H₂ was independent of the dosage, pH (except for pH 4), salinity, and O₂ and yielded an αD_H2O-H2 of 2.05 ± 0.07 (αD_H2O-H2 = (D/H)_H2O to (D/H)_H2), slightly less than predicted radiolytic models. Although this radiolytic fractionation value is significantly heavier than that of equilibrium isotopic exchange between H₂ and H₂O, the isotopic exchange rate between H₂ and H₂O will erase the heavy δD of radiolytic H₂ if the age of the groundwater is greater than ∼10³ to 10⁴ yr. The millimolar concentrations of H₂ observed in the groundwater of several Precambrian Shields are consistent with radiolysis of water that has resided in the subsurface for a few million years. These concentrations are well above those required to support H₂-utilizing microorganisms and to inhibit H₂-producing, fermentative microorganisms.     

Monitoring of Irgarol 1051 concentrations with concurrent phytoplankton evaluations in East Coast areas of the United States

The objectives of this study were to measure: (1) Irgarol and GS26575 (major metabolite) during the peak 2004 boating season at selected marinas and reference areas in the Carolinian Zoogeographic Province of the Eastern United States; (2) Irgarol and GS26575 at selected stations during the summer months in the Back Creek/Severn River area in Maryland in 2003 and 2004; and (3) structural and functional characteristics of resident phytoplankton communities concurrently with Irgarol and GS26575 monitoring in Back Creek/Severn River area. Irgarol concentrations from 14 marinas in the Carolinian Province ranged from non-detectable (<1 ng/L) to 85 ng/L; concentrations were less than 16 ng/L at all reference sites. The probability of exceeding the plant 10th centile for Irgarol (251 ng/L) was less than 0.6% for all marinas and 0.01% for all reference areas. These data suggest low ecological risk from Irgarol exposure for both marina and reference areas in the Carolinian Province. Irgarol concentrations ranged from 5 ng/L at the Severn River reference site to 1,816 ng/L in Port Annapolis marina during the two year study. Ecological risk from Irgarol exposure was high for the Port Annapolis marina sites based on a probability of exceeding the plant 10th centile. However, risk was low for Severn River and Severn River reference sites. Functional and structural measures of resident phytoplankton communities in the Back Creek and Severn River did not suggest that these target species are impaired in the Port Annapolis marina area where probabilistic analysis predicted adverse effects from Irgarol exposure.     

The eruptive history of the Tequila volcanic field, western Mexico: ages, volumes, and relative proportions of lava types

The eruptive history of the Tequila volcanic field (1600 km²) in the western Trans-Mexican Volcanic Belt is based on ⁴⁰Ar/³⁹Ar chronology and volume estimates for eruptive units younger than 1 Ma. Ages are reported for 49 volcanic units, including Volcán Tequila (an andesitic stratovolcano) and peripheral domes, flows, and scoria cones. Volumes of volcanic units 1 Ma were obtained with the aid of field mapping, ortho aerial photographs, digital elevation models (DEMs), and ArcGIS software. Between 1120 and 200 kyrs ago, a bimodal distribution of rhyolite (~35 km³) and high-Ti basalt (~39 km³) dominated the volcanic field. Between 685 and 225 kyrs ago, less than 3 km³ of andesite and dacite erupted from more than 15 isolated vents; these lavas are crystal-poor and show little evidence of storage in an upper crustal chamber. Approximately 200 kyr ago, ~31 km³ of andesite erupted to form the stratocone of Volcán Tequila. The phenocryst assemblage of these lavas suggests storage within a chamber at ~2–3 km depth. After a hiatus of ~110 kyrs, ~15 km³ of andesite erupted along the W and SE flanks of Volcán Tequila at ~90 ka, most likely from a second, discrete magma chamber located at ~5–6 km depth. The youngest volcanic feature (~60 ka) is the small andesitic volcano Cerro Tomasillo (~2 km³). Over the last 1 Myr, a total of 128±22 km³ of lava erupted in the Tequila volcanic field, leading to an average eruption rate of ~0.13 km³/kyr. This volume erupted over ~1600 km², leading to an average lava accumulation rate of ~8 cm/kyr. The relative proportions of lava types are ~22–43% basalt, ~0.4–1% basaltic andesite, ~29–54% andesite, ~2–3% dacite, and ~18–40% rhyolite. On the basis of eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and chemical composition, the lavas do not reflect the differentiation of a single (or only a few) parental liquids in a long-lived magma chamber. The rhyolites are geochemically diverse and were likely formed by episodic partial melting of upper crustal rocks in response to emplacement of basalts. There are no examples of mingled rhyolitic and basaltic magmas. Whatever mechanism is invoked to explain the generation of andesite at the Tequila volcanic field, it must be consistent with a dominantly bimodal distribution of high-Ti basalt and rhyolite for an 800 kyr interval beginning ~1 Ma, which abruptly switched to punctuated bursts of predominantly andesitic volcanism over the last 200 kyrs.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial responsility: J. Donnelly-NolanThis revised version was published online in January 2005 with corrections to Tables 1 and 3.An erratum to this article can be found at     

Numerical experiments on the onset of convective instability in the Earth's mantle

Summary. A simplified model of convection in the mantle is used to investigate the transient effect of cooling a fluid layer from above, The model, representing the mantle overlain by the lithosphere, consists of a two-dimensional fluid layer overlain by a solid conducting lid. The initial temperature of both layers is the same, with the top surface of the lid kept at 0°C throughout. We observe the onset of small-scale flow in the model. In the absence of internal heating the behaviour of the system is controlled by the Rayleigh number, R , and the ratio of the thicknesses of the two layers, a . The onset time of convection as defined by reference to conduction temperature profiles is related simply to a boundary layer critical Rayleigh number. The mean temperature profiles for the convection model are also compared with the observed depth—age relation for oceanic lithosphere and the results are used to estimate the viscosity of the mantle.