Models of near-bed dynamics and sediment movement at the Iberian margin

A three dimensional time-dependent baroclinic hydrodynamic model, including sediment transport and incorporating a turbulence energy sub-model, is used in cross sectional form to examine sediment movement at the shelf edge off North West Iberia at 42°40.5’N where measurements were made as part of the OMEX-II programme. These calculations are complemented by a simpler, in essence time-independent model, which is used to examine the sensitivity of the sediment distribution over the slope (from a shelf-break source) to changes in the specified values of horizontal and vertical diffusion coefficients. The philosophy of the paper is to use idealized tidal, wind and wind wave forcing to examine changes in sediment distribution resulting from these processes. Calculations with the time-dependent and steady state models give insight into both the role of events and long-term effects. The steady state model focuses on the off-shelf region, whilst the time-dependent model considers on-shelf events.Tidal calculations showed that for the stratification used here the internal tide in the OMEX region was primarily confined to the shelf edge and ocean. A mean on-shelf sediment transport in the surface layer and off-shelf transport at the bed was found. Across-shelf circulations produced by up-welling/down-welling favourable winds gave rise to on-shelf/off-shelf currents in the bottom boundary layer with an opposite flow in the surface layer. In the case of an up-welling favourable wind, sediment suspension was at a maximum in the near coastal region, with sediment being advected off shore in the surface layer. With a down-welling favourable wind, surface sediment was advected towards the shore, but there was offshore transport at the bed. Near the shelf edge any upwelling flow had the tendency to return this sediment to the surface layer from whence it was transported on-shore. So in essence the sediment was trapped within an on-shelf circulation cell. Wind waves effects increased the total bed stress and hence the sediment concentration and its transport, although its pattern was determined by tidal and wind forcing.The time independent model with increased/decreased lateral diffusivity gave an enhanced/reduced horizontal sediment distribution for a given settling velocity. As the settling velocity increases, the down-slope movement of sediment is increased, with a reduction in the thickness of the near-bed sediment layer, but with little change in its horizontal extent.     

A derivation of the Isted formula for average mineral grade of a triangular prism

The Isted formula, which provides an exact calculation of the spatial average of mineral grade in a triangular prism under the assumption of linear change, is derived, and its limitations outlined. This formula may be used to estimate the mean of any trivariate function whenever variation in the vertical direction can be considered negligible.     

An atlas of photospheric magnetic field observations and computed coronal magnetic fields: 1976–1985

Daily magnetogram observations of the large-scale photospheric magnetic field have been made at the John M. Wilcox Solar Observatory at Stanford since May of 1976. These measurements provide a homogeneous record of the changing solar field through most of solar cycle 21.Using the photospheric data, the configuration of the coronal and heliospheric fields can be calculated using a Potential Field - Source Surface model. This provides a three - dimensional picture of the heliospheric field evolution during the solar cycle.In this note we announce the publication of UAG Report No. 94, an Atlas containing the complete set of synoptic charts of the measured photospheric magnetic field, the computed field at the source surface, and the coefficients of the multipole expansion of the coronal field. The general underlying structures of the solar and heliospheric fields, which determine the environment for solar-terrestrial relations and provide the context within which solar activity related events occur, can be approximated from these data.     

Biomonitoring of heavy metal availability in the marine environment

Biomonitors can be used to establish geographical and/or temporal variations in the bioavailabilities of heavy metals in the marine environment, offering time-integrated measures of those portions of the total ambient metal load that are of direct ecotoxicological relevance. Heavy metal biomonitors need to conform to certain required characteristics, not least being metal accumulators. Use of a suite of biomonitors allows recognition of the presence and relative magnitude of different metal sources. For example, a macrophytic alga responds essentially to dissolved metal sources only, a suspension feeder like a mussel responds to metal sources in dissolved and suspended phases, and a deposit feeder responds to metal available in the sediment. Examples are given of suitable heavy metal biomonitors in the coastal waters of Europe, New Zealand, Hong Kong and China. It is not valid to compare absolute accumulated metal concentrations in biomonitors interspecifically, although interspecific comparisions of rank orders do allow cross correlations of relative bioavailabilities of heavy metals to different biomonitors at the same sites. There is a need to identify widespread cosmopolitan biomonitors to allow intra-specific comparisons of bioavailabilities between geographical areas. Such cosmopolitan biomonitors may include the alga Ulva lactuca, mussels of the genera Mytilus and Perna, the oysters Ostrea and Crassostrea, barnacles like Balanus amphitrite and Tetraclita squamosa, and the talitrid amphipod Platorchestia platensis. A major caveat in the use of such cosmopolitan biomonitors remains the need for reliable, specific taxonomic identification.     

Alkali exchange equilibria between a silicate melt and coexisting magmatic volatile phase: an experimental study at 800°C and 100 MPa

Many experimental studies have been performed to evaluate the composition of coexisting silicate melts and magmatic volatile phases (MVP). However, few studies have attempted to define the relationship between melt chemistry and the acidity of a chloride-bearing fluid. Here we report data on melt composition as a function of the HCl concentration of coexisting brines. We performed 35 experimental runs with a NaCl-KCl-HCl-H₂O brine (70 wt% NaCl [equivalent])-silicate melt (starting composition of Qtz_0.38Ab_0.33Or_0.29, anhydrous) assemblage at 800°C and 100 MPa. We determined an apparent equilibrium constant

     

Chlorinated pesticides and PCBs in the sea-surface microlayer and seawater samples of Singapore

Sea-surface microlayer (SML) and seawater samples collected from Singapore's coastal marine environment were analyzed for selected chlorinated pesticides and polychlorinated biphenyls (PCBs). The SML is a potential site of enrichment of persistent organic pollutants (POPs) compared to the underlying water column. The concentration ranges of SigmaHCH, SigmaDDT and SigmaPCB in subsurface (1 m depth) seawater were 0.4-27.2 ng/l (mean 4.0 ng/l), 0.01-0.6 ng/l (mean 0.1 ng/l) and 0.05-1.8 ng/l (mean 0.5 ng/l) respectively. In the SML, the concentration ranges of SigmaHCH, SigmaDDT and SigmaPCB were 0.6-64.6 ng/l (mean 9.9 ng/l), 0.01-0.7 ng/l (mean 0.2 ng/l) and 0.07-12.4 ng/l (mean 1.3 ng/l) respectively. High spatial and temporal distribution was observed for all POPs measured. However, overall levels measured in the SML were lower than levels reported in the literature for SML samples from temperate coastal regions-possibly due to loss of semi-volatile compounds in the tropical climate of Singapore. Atmospheric wet deposition during the monsoon season may be an important source of POPs to the SML. This study provides the first scientific data on POP concentrations and enrichment factors in the SML for Southeast Asia.     

Tectonic impact on the Lake Sevan environment (Armenia)

 The geology of Lake Sevan is characterized by the structure of a pull-apart basin. Methane emission from the lake surface correlates with evidence of physical and chemical water properties, remote sensing, etc., and suggests its possible emission from active fault zones at the lake bottom, as well as emission of biogenic methane from the fermentation of bottom organic remains. Correlation with anomalies in the lake bottom geochemistry suggests a high permeability of active fault zones and suggests that geodynamic, seismic and geochemical activity across the fault zones during the buildup to the M=7.1 Spitak earthquake led to plankton depression, which resulted in a large number of fish dying in 1984, and ended with a seismic disaster in 1988. Received: 2 November 1999 · Accepted: 27 January 2000     

Radar characteristics of small craters: Implications for Venus

Shuttle radar images (SIR-A) of volcanic and impact craters were examined to assess their appearance on radar images. Radar characteristics were determined for: (a) 9 maarlike craters in the Pinacate volcanic field, Sonora, Mexico; (b) the caldera of Cerro Volcan Quemado, in the Bolivian Andes; (c) Talemzane impact crater, Algeria; and (d) Al Umchaimin, a possible impact structure in Iraq. SIR-A images were compared with conventional photographs and with results from field studies. Consideration was then given to radar images available for Venus, or anticipated from the Magellan mission. Of the criteria ordinarily used to identify impact craters, some can be assessed with radar images and others cannot be used; planimetric form — expressed as circularity — and ejecta-block distribution can be assessed on radar images, but rim and floor elevations relative to the surrounding plain and disposition of rim strata are difficult or impossible to determine. We conclude that it will be difficult to separate small impact craters from small volcanic craters on Venus using radar images, and suggest that it will be necessary to understand the geological setting of the areas containing in order to determine their origin.Department of Geology.Center for Meteorite Studies, Arizona State University, Tempe, Ariz., U.S.A.