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Basaltic volcanism which forms the oceanic crust at mid-ocean ridges is the result of pressure release melting associated with ascending mantle convection. We present a model that gives the distribution of melting beneath the ridge and the subsequent migration of magma through the asthenosphere. In order to produce the degree of partial melting associated with the basaltic rocks making up the ocean crust, melting must extend to a depth of at least 70 km. Small degrees of partial melting are expected to result in an interconnected permeability along grain intersections. Due to the differential buoyancy of the magma relative to the residual solid the magma will be rapidly driven upwards. Solid-state creep allows the solid matrix to collapse as the magma migrates upwards and the lithostatic pressure in the matrix is nearly equal to the fluid pressure in the magma. The percentage partial melt present is only slightly greater than that necessary for the development of interconnected permeability and is much less than the degree of partial melting. The first partial melt fraction produced at the greatest depths migrates upwards and mixes with the later partial melt fractions produced at shallower depths. The uniformity of this mixing will have a profound effect on the chemistry of the basalts of the oceanic crust. 相似文献
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Albert S O'Neil JM Udy JW Ahern KS O'Sullivan CM Dennison WC 《Marine pollution bulletin》2005,51(1-4):428-437
During the last decade there has been a significant rise in observations of blooms of the toxic cyanobacterium Lyngbya majuscula along the east coast of Queensland, Australia. Whether the increase in cyanobacterial abundance is a biological indicator of widespread water quality degradation or also a function of other environmental change is unknown. A bioassay approach was used to assesses the potential for runoff from various land uses to stimulate productivity of L. majuscula. In Moreton Bay, L. majuscula productivity was significantly (p<0.05) stimulated by soil extracts, which were high in phosphorus, iron and organic carbon. Productivity of L. majuscula from the Great Barrier Reef was also significantly (p<0.05) elevated by iron and phosphorus rich extracts, in this case seabird guano adjacent to the bloom site. Hence, it is possible that other L. majuscula blooms are a result of similar stimulating factors (iron, phosphorus and organic carbon), delivered through different mechanisms. 相似文献
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Scott G. Johnston Edward D. Burton Richard T. Bush Annabelle F. Keene Leigh A. Sullivan Douglas Smith Angus E. McElnea Col R. Ahern Bernard Powell 《Applied Geochemistry》2010
Tidal inundation was restored to a severely degraded tropical acid sulfate soil landscape and subsequent changes in the abundance and fractionation of Al, Fe and selected trace metals were investigated. After 5 a of regular tidal inundation there were large decreases in water-soluble and exchangeable Al fractions within former sulfuric horizons. This was strongly associated with decreased soil acidity and increases in pH, suggesting pH-dependent immobilisation of Al via precipitation as poorly soluble phases. The water-soluble fractions of Fe, Zn, Ni and Mn also decreased. However, there was substantial enrichment (2–5×) of the reactive Fe fraction (FeR; 1 M HCl extractable) near the soil surface, plus a closely corresponding enrichment of 1 M HCl extractable Cr, Zn, Ni and Mn. Surficial accumulations of Fe(III) minerals in the inter-tidal zone were poorly crystalline (up to 38% FeR) and comprised mainly of schwertmannite (Fe8O8(OH)6SO4) with minor quantities of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH). These Fe (III) mineral accumulations provide an effective substrate for the adsorption/co-precipitation and accumulation of trace metals. Arsenic displayed contrary behaviour to trace metals with peak concentrations (∼60 μg g−1) near the redox minima. Changes in the abundance and fractionation of the various metals can be primarily explained by the shift in the geochemical regime from oxic–acidic to reducing-circumneutral conditions, combined with the enrichment of reactive Fe near the soil surface. Whilst increasing sequestration of trace metals via sulfidisation is likely to occur over the long-term, the current abundance of reactive Fe near the sediment–water interface favours a dynamic environment with respect to metals in the tidally inundated areas. 相似文献
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Scott G. Johnston Annabelle F. Keene Richard T. Bush Edward D. Burton Leigh A. Sullivan Lloyd Isaacson Angus E. McElnea Col R. Ahern C. Douglas Smith Bernard Powell 《Chemical Geology》2011,280(3-4):257-270
Tidal inundation is a new technique for remediating coastal acid sulfate soils (CASS). Here, we examine the effects of this technique on the geochemical zonation and cycling of Fe across a tidally inundated CASS toposequence, by investigating toposequence hydrology, in situ porewater geochemistry, solid-phase Fe fractions and Fe mineralogy. Interactions between topography and tides exerted a fundamental hydrological control on the geochemical zonation, redistribution and subsequent mineralogical transformations of Fe within the landscape. Reductive dissolution of Fe(III) minerals, including jarosite (KFe3(SO4)2(OH)6), resulted in elevated concentrations of porewater Fe2+ (> 30 mmol L?1) in former sulfuric horizons in the upper-intertidal zone. Tidal forcing generated oscillating hydraulic gradients, driving upward advection of this Fe2+-enriched porewater along the intertidal slope. Subsequent oxidation of Fe2+ led to substantial accumulation of reactive Fe(III) fractions (up to 8000 μmol g?1) in redox-interfacial, tidal zone sediments. These Fe(III)-precipitates were poorly crystalline and displayed a distinct mineralisation sequence related to tidal zonation. Schwertmannite (Fe8O8(OH)6SO4) was the dominant Fe mineral phase in the upper-intertidal zone at mainly low pH (3–4). This was followed by increasing lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) at circumneutral pH within lower-intertidal and subtidal zones. Relationships were evident between Fe fractions and topography. There was increasing precipitation of Fe-sulfide minerals and non-sulfidic solid-phase Fe(II) in the lower intertidal and subtidal zones. Precipitation of Fe-sulfide minerals was spatially co-incident with decreases in porewater Fe2+. A conceptual model is presented to explain the observed landscape-scale patterns of Fe mineralisation and hydro-geochemical zonation. This study provides valuable insights into the hydro-geochemical processes caused by saline tidal inundation of low lying CASS landscapes, regardless of whether inundation is an intentional strategy or due to sea-level rise. 相似文献
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Full-disk images from the Cartesian Full-Disk Telescope no. 2 (CFDT2) were used to study the center-to-limb (CLV) variation of facular contrast in two colors. The CFDT2 images, which have 2.5 arc sec pixels, were obtained during the summer months of 1993, 1994 and 1995. In order to minimize the bias in finding faint facular features in continuum images, we have used coaligned images obtained in the Ca K-line to identify faculae. Faculae were sorted into 20 annular bins of equal width. To reduce the effects of seeing, faculae were not identified closer to the limb than =0.2. The facular pixel contrasts were fitted to various trial functions. The contrast in the blue filter (470.6 nm) rose from 0.122% at disk center to 12.2% at =0.2. The contrast in the red filter (672.3 nm) rose from 0.13% at disk center to 8.16% at =0.2. We have also analyzed the facular contrasts multiplied by their -value to obtain an estimate of facular flux tube contrasts. These flux tube contrasts increased roughly linearly from =0.95 to 0.25. The blue flux tube contrast reached a maximum of 2.48% near =0.25. The red flux tube contrast reached a maximum of 1.59% at =0.2. These contrast values are not corrected for the filling factor. The blue curve leveled off slightly betwen =0.25 and 0.2 while the red curve showed no deviation from its linear trend. These results may provide some support for the hot wall model of facular flux tubes. 相似文献
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Three sources of stress at active (Andean) continental margins are considered: body forces on the plates which drive their motion, thermal stresses generated within the cooling lithosphereand bending stresses due to the flexure of the lithosphere at an ocean trench. It is argued that the bending stresses dominate. The evolution of passive (Atlantictype) continental margins is also considered. Models for the free and locked flexure of the continental and oceanic lithosphere are given. Based on observed gravity anomalies, it is argued that the continental margin fault system must remain active throughout much of the evolution of the margin. These displacements accommodate both the subsidence of the oceanic lithosphere due to its cooling and thickeningand the sedimentary loading. This loading may be responsible for the seismicity on the eastern continental margin of the United States e.g., the Charleston, South Carolina earthquake of 1884. 相似文献
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Scott G. Johnston Richard T. Bush Leigh A. Sullivan Edward D. Burton Douglas Smith Michelle A. Martens Angus E. McElnea Col R. Ahern Bernard Powell Luisa P. Stephens Steve T. Wilbraham Simon van Heel 《Estuarine, Coastal and Shelf Science》2009
This study examines the remediation of surface water quality in a severely degraded coastal acid sulfate soil landscape. The remediation strategy consisted of partial restoration of marine tidal exchange within estuarine creeks and incremental tidal inundation of acidified soils, plus strategic liming of drainage waters. Time-series water quality and climatic data collected over 5 years were analysed to assess changes in water quality due to this remediation strategy. A time-weighted rainfall function (TWR) was generated from daily rainfall data to integrate the effects of antecedent rainfall on shallow groundwater levels in a way that was relevant to acid export dynamics. Significant increases in mean pH were evident over time at multiple monitoring sites. Regression analysis at multiple sites revealed a temporal progression of change in significant relationships between mean daily electrical conductivity (EC) vs. mean daily pH, and TWR vs. mean daily pH. These data demonstrate a substantial decrease over time in the magnitude of creek acidification per given quantity of antecedent rainfall. Data also show considerable increase in soil pH (2–3 units) in formerly acidified areas subject to tidal inundation. This coincides with a decrease in soil pe, indicating stronger reducing conditions. These observations suggest a fundamental shift has occurred in sediment geochemistry in favour of proton-consuming reductive processes. Combined, these data highlight the potential effectiveness of marine tidal inundation as a landscape-scale acid sulfate soil remediation strategy. 相似文献
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Kelley Lee Tony McMichael Colin Butler Mike Ahern David Bradley 《Global Change & Human Health》2002,3(1):16-19
Debates over the merits and demerits of globalisation for health are increasingly polarised. Conclusions range from globalisation being essentially positive for health, albeit with a need to smooth out some rough edges, to one of utter condemnation, with adverse effects on the majority of the world's population. Anyone wading into this debate is immediately confronted by seemingly irreconcilable differences in ideology, opinion and interests. Both camps agree that global changes are occurring, and with them many of the determinants of population health status. While some skepticism persists about whether “globalisation” has value beyond being a fashionable buzzword, most agree that we need better understanding of these changes. Two difficult questions arise: (i) What are the health impacts of these changes; and (ii) how can we respond more effectively to them? To move beyond the stand-offs that have already formed within the health community, this paper reviews the main empirical evidence that currently exists, summarises key points of debate that remain, and suggests some ways forward for the research and policy communities. In particular, there is need for an informed and inclusive debate about the positive and negative health consequences of globalisation. 相似文献
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The effective elastic thickness of the lithosphere in the Williston Basin region has been determined for several time intervals by fitting an elastic flexure equation to the shape of the basin over time. The elastic thickness increases from about 40 km just after the basin began subsiding (450 m.y. B.P.) to about 80 km at the present.These results agree well with the increasing elastic thickness with age predicted for a cooling, 250 km-thick plate (taking elastic thickness to be depth to the 450°C isotherm) if we assume a thermal age of the lithosphere of about 500 m.y. Radiometric dates of basement in the area, however, generally yield ages of 1.7 b.y. (Churchill Province) and 2.5 b.y. (Superior Province).Because basement ages are generally much greater than the inferred lithospheric thermal age in the Williston Basin region, we suggest that lithosphere in this area was thermally rejuvenated, about 500 m.y.B.P., by the same process(es) responsible for the thermal subsidence of the basin. Temperatures were not high enough in the upper crust to reset most radiometric clocks. Additional support for a thermal event 500 m.y.B.P. comes from fission-track ages and a single Rb-Sr age of basement rocks of about 500 m.y.Lithospheric rejuvenation has been modelled using a one-dimensional finite-difference thermal model. The base of a 250 km-thick plate is heated from 1333°C to 1850°C (approximately the solidus of basalt at that depth) for approximately 100 m.y. With conduction only, the depth to the 450°C isotherm slowly decreases, then slowly increases after heating ceases. Rapid thinning and slow thickening of the lithosphere can be achieved only if convective thinning is simulated in the model. The model with convection yields results consistent with the observed increase in lithospheric thickness as well as the observed subsidence of the basin over time. 相似文献
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