The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex(SFVC) comprise three distinct petrological groups based ontheir whole-rock geochemistry: (1) oceanic tholeiites; (2) transitionalalkali basalts and glasses; (3) high-Al, low-Ti tholeiites.Major and trace element, and Sr–Nd–Pb isotopic dataindicate that the oceanic tholeiites formed as low-degree partialmelts of normal mid-ocean ridge basalt (N-MORB)-source asthenospheresimilar in isotope composition to the East Pacific Rise today;the alkalic lavas were derived from an enriched source similarto that of E-MORB. The high-Al, low-Ti lavas resemble second-stagemelts of a depleted MORB-source asthenosphere that formed bymelting spinel lherzolite at low pressures. Trace element systematicsof the high-Al, low-Ti basalts show the influence of an enrichedcomponent, which overprints generally depleted trace elementcharacteristics. Tectonic discrimination diagrams show thatthe oceanic tholeiite and alkali suites are similar to present-daybasalts generated at mid-oceanic ridges. The high-Al, low-Tisuite resembles primitive arc basalts with an enriched, alkalibasalt-like overprint. Isotopic data show the influence of recycledcomponents in all three suites. The SFVC was constructed ona substrate of normal Coast Range ophiolite in an extensionalforearc setting. The close juxtaposition of the MORB-like olivinetholeiites with alkali and high-Al, low-Ti basalts suggestsderivation from a hybrid mantle source region that includedMORB-source asthenosphere, enriched oceanic asthenosphere, andthe depleted supra-subduction zone mantle wedge. We proposethat the SFVC formed in response to collision of a mid-oceanridge spreading center with the Coast Range ophiolite subductionzone. Formation of a slab window beneath the forearc duringcollision allowed the influx of ridge-derived magmas or themantle source of these magmas. Continued melting of the previouslydepleted mantle wedge above the now defunct subduction zoneproduced strongly depleted high-Al, low-Ti basalts that werepartially fertilized with enriched, alkali basalt-type meltsand slab-derived fluids. KEY WORDS: CRO; oceanic basalts; California     

Pan-African Tectonism in the Western Maud Belt: P-T-t Path for High-grade Gneisses in the H.U. Sverdrupfjella, East Antarctica

Extensive high-grade polydeformed metamorphic provinces surroundingArchaean cratonic nuclei in the East Antarctic Shield recordtwo tectono-thermal episodes in late Mesoproterozoic and lateNeoproterozoic–Cambrian times. In Western Dronning MaudLand, the high-grade Mesoproterozoic Maud Belt is juxtaposedagainst the Archaean Grunehogna Province and has traditionallybeen interpreted as a Grenvillian mobile belt that was thermallyoverprinted during the Early Palaeozoic. Integration of newU–Pb sensitive high-resolution ion microprobe and conventionalsingle zircon and monazite age data, and Ar–Ar data onhornblende and biotite, with thermobarometric calculations onrocks from the H.U. Sverdrupfjella, northern Maud Belt, resultedin a more complex P–T–t evolution than previouslyassumed. A c. 540 Ma monazite, hosted by an upper ampibolite-faciesmineral assemblage defining a regionally dominant top-to-NWshear fabric, provides strong evidence for the penetrative deformationin the area being of Pan-African age and not of Grenvillianage as previously reported. Relics of an eclogite-facies garnet–omphaciteassemblage within strain-protected mafic boudins indicate thatthe peak metamorphic conditions recorded by most rocks in thearea (T = 687–758°C, P = 9·4–11·3kbar) were attained subsequent to decompression from P >12·9 kbar. By analogy with limited U–Pb singlezircon age data and on circumstantial textural grounds, thisearlier eclogite-facies metamorphism is ascribed to subductionand accretion around 565 Ma. Post-peak metamorphic K-metasomatismunder amphibolite-facies conditions is ascribed to the intrusionof post-orogenic granite at c. 480 Ma. The recognition of extensivePan-African tectonism in the Maud Belt casts doubts on previousRodinia reconstructions, in which this belt takes a pivotalposition between East Antarctica, the Kalahari Craton and Laurentia.Evidence of late Mesoproterozoic high-grade metamorphism duringthe formation of the Maud Belt exists in the form of c. 1035Ma zircon overgrowths that are probably related to relics ofgranulite-facies metamorphism recorded from other parts of theMaud Belt. The polymetamorphic rocks are largely derived froma c. 1140 Ma volcanic arc and 1072 ± 10 Ma granite. KEY WORDS: Maud Belt; Pan-African orogeny; geochronology; P–T–t path, East Antarctica     

High-resolution historical records from Pettaquamscutt River basin sediments: 1. Pb and varve chronologies validate record of Cs released by the Chernobyl accident

Cesium-137 derived from the explosion of the Chernobyl reactor in 1986 was preserved in anoxic sediments from a coastal environment in southern Rhode Island. Although the radioactive plume was detected in surface air samples at several locations in the United States, this is the first known record of a Chernobyl ¹³⁷Cs peak in sediments from North America. The inventory of Chernobyl ¹³⁷Cs that was preserved in the Pettaquamscutt River is small compared to European counterparts and should only be detectable for the next 15-20 yr. However, the presence of two ¹³⁷Cs peaks (1963 and 1987) identifies a well-dated segment of the sediment column that could be exploited in understanding the decomposition and preservation of terrestrial and aquatic organic matter. Different methods for calculating the ²¹⁰Pb chronology were also evaluated in this study and checked against independent varve counting. The end result is a detailed chronology of a site well suited for reconstruction of historical records of environmental change.     

A model for oxygen and sulfur isotope fractionation in sulfate during bacterial sulfate reduction processes

We present a model of bacterial sulfate reduction that includes equations describing the fractionation relationship between the sulfur and the oxygen isotope composition of residual sulfate (δ³⁴S_{SO4_residual}, δ¹⁸O_{SO4_residual}) and the amount of residual sulfate. The model is based exclusively on oxygen isotope exchange between cell-internal sulfur compounds and ambient water as the dominating mechanism controlling oxygen isotope fractionation processes. We show that our model explains δ³⁴S_{SO4_residual} vs. δ¹⁸O_{SO4_residual} patterns observed from natural environments and from laboratory experiments, whereas other models, favoring kinetic isotope fractionation processes as dominant process, fail to explain many (but not all) observed δ³⁴S_{SO4_residual} vs. δ¹⁸O_{SO4_residual} patterns. Moreover, we show that a “typical” δ³⁴S_{SO4_residual} vs. δ¹⁸O_{SO4_residual} slope does not exist. We postulate that measurements of δ³⁴S_{SO4_residual} and δ¹⁸O_{SO4_residual} can be used as a tool to determine cell-specific sulfate reduction rates, oxygen isotope exchange rates, and equilibrium oxygen isotope exchange factors. Data from culture experiments are used to determine the range of sulfur isotope fractionation factors in which a simplified set of equations can be used. Numerical examples demonstrate the application of the equations. We postulate that, during denitrification, the oxygen isotope effects in residual nitrate are also the result of oxygen isotope exchange with ambient water. Consequently, the equations for the relationship between δ³⁴S_{SO4_residual}, δ¹⁸O_{SO4_residual}, and the amount of residual sulfate could be modified and used to calculate the fractionation-relationship between δ¹⁵N_{NO3_residual}, δ¹⁸O_{NO3_residual}, and the amount of residual nitrate during denitrification.     

Anaerobic diagenesis of silica and carbon in continental margin sediments: Discrete zones of TCO2 production

Pore water profiles of dissolved Si, Ca²⁺, SO₄^2-, CH₄, and TCO₂ (Dissolved Inorganic Carbon; DIC) were determined from multicores and gravity cores collected at nine sites off Southern California, the west coast of Mexico, and within the Gulf of California. These sites were located within the eastern North Pacific oxygen minimum zone at depths of 400 to 900 m and in settings where bottom water oxygen concentrations were <3 μM and sediments were laminated. Pore water profiles were defined at a resolution of millimeters (whole core squeezing), centimeters (sectioning and squeezing) and meters (gravity core sectioning and squeezing), and diffusive fluxes were calculated for different zones within the sediment column. The flux of dissolved silica across the sediment-water interface (SWI) ranged from 0.3 to 3.4 mmol Si m^-2d^-1, and TCO₂ fluxes ranged from 0.8 to 4.6 mmol C m^-2d^-1. A positive correlation (r = 0.74) existed between these fluxes, yet these two constituents exhibited significantly different diagenetic behavior downcore; dissolved Si generally reached a constant concentration (between 450 and 900 μM) in the upper few cm, whereas TCO₂ concentrations increased monotonically with depth.Methane was detected at micromolar levels in sediment intervals between 0 and 60 cm and at five sites, increased to millimolar levels at depths of 80 to 170 cm. At the horizon marking the appearance of millimolar levels of methane, there was a distinct change in slope of the sulfate and TCO₂ gradients. A flux budget for this horizon was determined by using linear fits to pore water profiles; these budgets indicate that the upward TCO₂ flux away from this horizon is 40 to 50% greater than the downward sulfate flux to this horizon. Given that the TCO₂ flux to this horizon from below was quite small, this imbalance suggests that anaerobic oxidation of methane by sulfate is not the only process producing TCO₂ within this horizon. A budget for TCO₂ at this horizon is balanced when 40 to 80% of the sulfate flux is attributed to organic carbon remineralization. Of the DIC that diffuses across the SWI, 20 to 40% is generated by reactions occurring within or below this deep reaction horizon.     

Concentration, isotopic composition, and sources of lead in Southern Ocean air during 1999/2000, measured at the Cape Grim Baseline Air Pollution Station, Tasmania

Southern Ocean aerosols were collected at the Cape Grim Baseline Air Pollution Station from onshore air under baseline conditions between February 1999 and April 2000. Thermal ionization techniques (TIMS) and isotope dilution mass spectrometry (IDMS) were used to measure the isotopic composition and concentration of lead in the air giving concentrations as low as 0.6 ± 0.1 pg · m⁻³. Air collected under baseline conditions for 12 months (May 1999-April 2000) yielded an overall lead concentration of 11.0 ± 0.2 pg · m⁻³ and isotopic composition of ²⁰⁶Pb/²⁰⁷Pb = 1.154, ²⁰⁸Pb/²⁰⁷Pb = 2.387 and ²⁰⁶Pb/²⁰⁴Pb = 17.93. The range in isotopic ratios was consistent with the mixing of lead from major population centers in the Southern Hemisphere in the mid to high latitudes, except for the presence of highly radiogenic lead in some samples. Contributions from radiogenic lead of up to ∼0.8% were observed. Three periods with the highest percentage contribution of radiogenic lead (>0.5%) were investigated in more detail, and 4-d back-trajectories and radon concentrations were used to help identify the sources. The sources are probably associated with the mining and processing of uranium rich ores in southern Africa and possibly South Australia.     

WATER QUALITY STUDIES IN MALAYSIAN DISUSED MAN MADE LAKFS FOR SUITABILITY OF AN AQUACULTURE PROJECT

A study has been conducted on the status of water quality in two disused man made lakes, which have potential for cage aquaculture project. This study highlights the quality of water analyzed with reference to various physical - chemical parameters in two selected lakes and their suitability for fish farming. A number of parameters were measured including the amount of NH3 - N, NO3 - N, NO2 - N and total phosphorus while in - situ measurement including DO, pH, temperature, conductivity, TDS and Secchi disk visibility. Beside the physical -chemical of water, the location, morphometry and climate conditions were also investigated. The chemical data analyzed for six months indicated that Lake A and Lake B are characterized by relatively high DO, slightly neutral pH and low TSS. The concentration of NH3 -N and NO2 -N was very low （0. 058 mg/L and 0. 04 mg/L, respectively） and total phosphorus was usually in low concentration and sometimes would seem negligible. Besides, NO3 - N occurred in slightly higher concentration （1.75 mg/L）. Results obtained indicated that the study site has high potentiality for development of inland fisheries practices.     

Refined modeling and movement characteristics analyses of irregularly shaped particles

Irregularly shaped (IRS) particles widely exist in many engineering and industrial fields. The macro physical and mechanical properties of the particle system are governed by the interaction between the particles in the system. The interaction between IRS particles is more complicated because of their complex geometric shape with extremely irregular and co‐existed concave and convex surfaces. These particles may interlock each other, making the sliding and friction of IRS particles more complex than that of particles with regular shape. In order to study the interaction of IRS particles more efficiently, a refined method of constructing discrete element model based on computed tomography scanning of IRS particles is proposed. Three parameters were introduced to control the accuracy and the number of packing spheres. Subsequently, the inertia tensor of the IRS particle model was optimized. Finally, laboratory and numerical open bottom cylinder tests were carried out to verify the refined modeling method. The influence of particle shape, particle position, and mesoscopic friction coefficient on the interaction of particles was also simulated. It is noteworthy that with the increase of mesoscopic friction coefficient, the fluidity of IRS particle assembly decreases, and intermittent limit equilibrium state may appear. Copyright © 2014 John Wiley & Sons, Ltd.