A Pb-Ra-Th-U study of Klyuchevskoy and Bezymianny volcanoes, Kamchatka     

Simon Turner  Kenneth W.W. Sims  Craig Cook 《Geochimica et cosmochimica acta》2007,71(19):4771-4785

Lavas from Klyuchevskoy and Bezymianny volcanoes, Kamchatka, appear to show a link between the extent of partial melting in their mantle source region and the subsequent degree of fractionation suffered by the magmas during passage through the crust. This fractionation may have occurred on timescales significantly less than 1000 years if observed ²²⁶Ra excesses largely reflect variable residual porosity in the source melting region. Unlike most arc lavas, those with the highest MgO contents and Ba/Th ratios have the lowest ²²⁶Ra excess. Forward models suggest that those portions of the source which had undergone the greatest addition of U by fluids from the subducting plate also underwent the greatest extents of partial melting at the highest residual porosity. At Kluchevskoy, a change from eruption of high-MgO to high-Al₂O₃ basaltic andesites around 1945 is reflected in an increase in size of ²²⁶Ra excess which seems to require a simultaneous decrease in residual porosity and suggests a rapid changes in the melting regime. The eruption of andesites at Bezyminanny, simultaneous with the eruption of basaltic andesites at Klyuchevskoy, further suggests that different degree melts produced at differing residual porosity can be formed and extracted from the melt region at the same time. Thus, the melting processes beneath Klyuchevskoy and Bezyminanny are demonstrably complex. They have clearly been influenced by both fluid addition from the subducting plate and extension and decompression beneath the Central Kamchatka Depression. Finally, the ²¹⁰Pb data are, with one or two exceptions, in equilibrium with ²²⁶Ra, suggesting that there was restricted relative magma-gas movement in this highly productive magmatic system.  相似文献   

Inter-relationships of MnO₂ precipitation, siderophore-Mn complex formation, siderophore degradation, and iron limitation in Mn-oxidizing bacterial cultures     

Dorothy L. Parker  Takami Morita  Rebecca Verity  James K. McCarthy 《Geochimica et cosmochimica acta》2007,71(23):5672-5683

To examine the pathways that form Mn^(III) and Mn^(IV) in the Mn^(II)-oxidizing bacterial strains Pseudomonas putida GB-1 and MnB1, and to test whether the siderophore pyoverdine (PVD) inhibits Mn^(IV)O₂ formation, cultures were subjected to various protocols at known concentrations of iron and PVD. Depending on growth conditions, P. putida produced one of two oxidized Mn species - either soluble PVD-Mn^(III) complex or insoluble Mn^(IV)O₂ minerals - but not both simultaneously. PVD-Mn^(III) was present, and MnO₂ precipitation was inhibited, both in iron-limited cultures that had synthesized 26-50 μM PVD and in iron-replete (non-PVD-producing) cultures that were supplemented with 10-550 μM purified PVD. PVD-Mn^(III) arose by predominantly ligand-mediated air oxidation of Mn^(II) in the presence of PVD, based on the following evidence: (a) yields and rates of this reaction were similar in sterile media and in cultures, and (b) GB-1 mutants deficient in enzymatic Mn oxidation produced PVD-Mn^(III) as efficiently as wild type. Only wild type, however, could degrade PVD-Mn^(III), a process linked to the production of both MnO₂ and an altered PVD with absorbance and fluorescence spectra markedly different from those of either PVD or PVD-Mn^(III). Two conditions, the presence of bioavailable iron and the absence of PVD at concentrations exceeding those of Mn, both had to be satisfied for MnO₂ to appear. These results suggest that P. putida cultures produce soluble Mn^(III) or MnO₂ by different and mutually inhibitory pathways: enzymatic catalysis yielding MnO₂ under iron sufficiency or PVD-promoted oxidation yielding PVD-Mn^(III) under iron limitation. Since PVD-producing Pseudomonas species are environmentally prevalent Mn oxidizers, these data predict influences of iron (via PVD-Mn^(III) versus MnO₂) on the global oxidation/reduction cycling of various pollutants, recalcitrant organic matter, and elements such as C, S, N, Cr, U, and Mn.  相似文献   

Impact of storm runoff from tropical watersheds on coastal water quality and productivity     

E. Heinen De Carlo  Daniel J. HooverCharles W. Young  Rebecca S. HooverFred T. Mackenzie 《Applied Geochemistry》2007

Storm runoff in the steep watersheds in Hawaii leads to sediment and freshwater pulses to coastal waters that quickly affect nearshore water quality. This is particularly true in semi-enclosed embayments, such as Kaneohe Bay, Oahu, where water has a relatively long residence time compared to more open coastal areas of the islands. In this paper the authors discuss water quality and productivity in Kaneohe Bay after back-to-back rain events in late November and early December 2003, following a particularly dry summer. The short-term biogeochemical response of coastal waters and the ecosystem to runoff and physical forcing was evaluated through a combination of continuous in situ measurements and adaptive synoptic sampling carried out on a variety of temporal and spatial scales.  相似文献   

Hypoxia in the northern Gulf of Mexico: Does the science support the Plan to Reduce,Mitigate, and Control Hypoxia?     

N. N. Rabalais  R. E. Turner  B. K Sen Gupta  D. F. Boesch  P. Chapman  M. C. Murrell 《Estuaries and Coasts》2007,30(5):753-772

We update and reevaluate the scientific information on the distribution, history, and causes of continental shelf hypoxia that supports the 2001 Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001), incorporating data, publications, and research results produced since the 1999 integrated assessment. The metric of mid-summer hypoxic area on the LouisianaTexas shelf is an adequate and suitable measure for continued efforts to reduce nutrients loads from the Mississippi River and hypoxia in the northern Gulf of Mexico as outlined in the Action Plan. More frequent measurements of simple metrics (e.g., area and volume) from late spring through late summer would ensure that the metric is representative of the system in any given year and useful in a public discourse of conditions and causes. The long-term data on hypoxia, sources of nutrients, associated biological parameters, and paleoindicators continue to verify and strengthen the relationship between the nitratenitrogen load of the Mississippi River, the extent of hypoxia, and changes in the coastal ecosystem (eutrophication and worsening hypoxia). Multiple lines of evidence, some of them representing independent data sources, are consistent with the big picture pattern of increased eutrophication as a result of long-term nutrient increases that result in excess carbon production and accumulation and, ultimately, bottom water hypoxia. The additional findings arising since 1999 strengthen the science supporting the Action Plan that focuses on reducing nutrient loads, primarily nitrogen, through multiple actions to reduce the size of the hypoxic zone in the northern Gulf of Mexico.  相似文献   

Multiple phases of North African humidity recorded in lacustrine sediments from the Fazzan Basin, Libyan Sahara     

S.J. Armitage  N.A. Drake  S. Stokes  A. El-Hawat  M.J. Salem  K. White  P. Turner  S.J. McLaren 《Quaternary Geochronology》2007,2(1-4):181-186

The Fazzan Basin of south-west Libya is at present arid with less than 20 mm of rainfall per annum. However, regionally extensive limestones, lacustrine sands and coquina (fossiliferous carbonate rock) deposits show that the Fazzan Basin previously contained a large palaeolake, indicating that the climate in the past was more humid. Optically stimulated luminescence (OSL) dating techniques have been applied to key lacustrine deposits within the basin in an attempt to provide an internally consistent chronology for this humidity record. Results indicate that palaeolake sediments within the Fazzan Basin record a very long history of palaeohydrological change, ranging from present day arid conditions to humidity capable of sustaining a lake with an approximate area of 76,250 km². The existence of humid periods in mid oxygen isotope stage 5 and the early Holocene is confirmed. An older lacustrine event, tentatively correlated to oxygen isotope stage 11, is also recognized. In addition, evidence is presented for at least two humid phases beyond the age range over which the conventional OSL dating technique is applicable. This study demonstrates that OSL dating of palaeolake sediments within the Fazzan Basin offers the potential to provide a detailed record of North African humidity spanning several glacial–interglacial cycles.  相似文献   

Reconciling feminist theory and gendered resource management in Indonesia     

Rebecca Elmhirst 《Area》1998,30(3):225-235

Summary This paper considers the difficulties in reconciling a fluid and ambiguous conceptualization of gender difference with policy-based efforts to define women as 'target groups' in development interventions. The issue is explored with reference to gender issues in a transmigration resettlement area in Indonesia, where the instability of gender identities is particularly marked, and where gendered resource use and control are particularly blurred.  相似文献   

The equilibrium speciation of dissolved components in freshwater and sea water at 25°C and 1 atm pressure     

D.R Turner  M Whitfield  A.G Dickson 《Geochimica et cosmochimica acta》1981,45(6):855-881

A data base summarising the stability constants of more than 500 complexes is used to calculate speciation pictures for 58 trace elements in model seawater (pH 8.2) and freshwaters (pH 6 and 9). Consideration of the results provides a general summary of the chemical periodicity of the speciation of trace components in natural waters. The polarising power of an element ((cation charge)²/(radius), z²/r) provides a useful index to the degree of hydrolysis in aqueous solution. The fully hydrolysed elements with a high polarising power form distinct groupings in the periodic table. The relative magnitudes of the acid dissociation constants are summarised by Pauling's rules and the speciation of the fully hydrolysed elements in natural waters largely depends on pH and, to a lesser extent, on interactions with the major cations. The remaining cations of low and intermediate polarising power can be subdivided according to their tendency to form covalent bonds. An empirical parameter Δβ(= logβ⁰_MF ? log β⁰_MCl) is used to define (a)-type (Δβ > 2), borderline (a)-type (2 >Δβ > 0), (b)-type (Δβ < ?2) and borderline (b)-type (0 >Δβ > ?2) cations. Again these various categories form coherent groupings on the periodic table. By considering the interactions of cations from the various categories with the inorganic ligands commonly encountered in natural waters it is possible to assign the ligands themselves to ‘hard’ (e.g. F^?, SO₄^2?), ‘intermediate’ (e.g. OH^?, CO^2?₃) and ‘soft’ categories (e.g. Cl^?). These concepts can be summarised by constructing a Complexation Field Diagram in which the various cations are located on a plot of $\text{z}{}^2\text{r}$ vs δβ. The extension of the model to include redox equilibria and additional ligands is described.  相似文献   

Variability of the reverse-flow concentration technique of measuring plankton respiration     

R. Eugene Turner 《Estuaries and Coasts》1978,1(1):65-68

Results from the reverse-flow concentration method used to determine community plankton respiration vary significantly because of metabolic inactivation or losses of a portion of the concentrated sample during the filtering process. These losses seem to be nearly constant at concentrations greater than 150X. The within sample coefficient of variation is 20–40% and is primarily a result of clumping and inadequate mixing before subsampling. A rapid processing of the samples is critical for best results.  相似文献   

Biaxiality in relation to visible twinning in experimentally deformed calcite     

Francis J. Turner 《Contributions to Mineralogy and Petrology》1975,53(4):241-252

Moderate to strong biaxiality (2V = 10 °–45 °) in experimentally deformed calcite (in single crystals and in marble) is attributed to overlap between one or two thin {01¯12} twin lamellae and the enclosing host. A perfectly centered conoscopic figure (section normal to [0001]) is perceptibly asymmetric about the trace of the optic axial plane. This asymmetry is pronouned in thick sections (> 0.04 mm) and completely distrupts the biaxial configuration of the figure if the overlapping lamella exceeds about 0.0025 mm in thickness. In sections somewhat oblique to [0001] and cut at 20 ° or less to the plane of twinning the conoscopic figure may appear to be perfectly biaxial-expecially in thin sections ( 0.02 mm) enclosing thin ( 0.001 mm) but still visible twins.Similar values of 2V recorded for natural calcite likewise are attributed to twinning on a visible scale.  相似文献   

Magnetisation history of lower old red sandstones from the Gamrie Outlier,Scotland     

P. Turner  R. Archer 《Earth and Planetary Science Letters》1975,27(2):240-250

Red beds in the Lower Old Red Sandstone Gamrie Outlier on the southern margin of the Orcadian Basin originated through the diagenetic breakdown of iron silicates. The magnetic evidence indicates that haematite pigment was produced over a long time period because both normal and reversed components of magnetisation are present in individual specimens. A part of the magnetisation in these rocks is carried by coarse particle specularite but there is no evidence that this is DRM or PDRM. The composite nature of the magnetisation and its acquisition over a relatively long time period is also indicated by the fact that all sample/site mean directions are of the same polarity (reversed) but originated over a period when normal and reversed polarities were common. There is evidence that the production of early diagenetic pigment was necessary for the preservation of Devonian magnetisations in these sandstones.Another group of sandstones acquired their magnetisation in Permo-Carboniferous times and these occur in intimate association with those which retain the original composite Devonian direction. The stable NRM of these rocks is carried by specularite and they are thought to have been remagnetised because they originally contained a greater proportion of magnetite than those sandstones which retain the original magnetisation and also an unstable pigment magnetisation. In-situ oxidation of magnetite in Permo-Carboniferous times is thus thought to be responsible for the remagnetisation of these rocks.  相似文献