Determining accurate temperature-time paths from U-Pb thermochronology: An example from the Kaapvaal craton, southern Africa

Thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure. However, many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited. This study uses apatite (T_c = 450-550 °C) and titanite (T_c = 550-650 °C) U-Pb ID-TIMS thermochronology at the single- to sub-grain scale to separate the variable effects of volume diffusion of Pb from metamorphic (over)growth above and below the T_c of a mineral. Data are presented from two ca. 3227 Ma tonalite samples from north and south of the Barberton Greenstone Belt (BGB), southern Africa. Two distinct populations of apatite from a sample north of the BGB record fast cooling followed by metamorphic growth ∼10 Myr later. Both apatite and titanite dates from south of the BGB show a strong correlation with the grain size and record 100 Myr of post-emplacement cooling. Complex core-rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the T_c. The age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite-difference numerical model to show that slow, non-linear, cooling and not thermal resetting is responsible for the observed distribution. The thermal histories from either side of the BGB are very different and provide unique insight into the BGB’s tectonic evolution: a ∼70 Myr period of apparent stability after ca. 3.2 Ga terrane assembly was followed by fast exhumation south of the BGB that led to lower-crustal melting and intrusion of granitic batholiths ca. 3.14-3.10 Ga.     

Hypoxic events in Narragansett Bay, Rhode Island, during the summer of 2001

Bottom water hypoxic events were observed in Narragansett Bay, Rhode Island during the summer of 2001 using a towed sensor, vertical casts at fixed stations, and continuous monitoring buoys. This combination of approaches allowed for both extensive spatial and temporal sampling. Oxygen concentrations below the U.S. Environmental Protection Agency (EPA) acute hypoxia criterion of 2.3 mg l^?1 were observed in the northern parts of Narragansett Bay, including the Providence River. We estimate 39% of the area of the Providence River was affected by acute hypoxia between July and September 2001. All other regions experienced only small areas of acute hypoxia (<5%), and no acute hypoxia was observed from Quonset Point south. The area encompassing oxygen concentrations below the EPA chronic hypoxia criterion of 4.8 mg l^?1 was much more extensive in the upper half of Narragansett Bay, sometimes covering the majority of the region, though it is unclear whether exposure to concentrations below this criterion persisted long enough to significantly affect marine species in these areas. Vertical profiles of dissolved oxygen typically exhibited a mid water oxygen minimum near the pycnocline, followed by a slight increase in oxygen with depth. The surface waters above the pycnocline were typically supersaturated with oxygen. The northern portions of the Bay where the most extensive hypoxia was observed corresponded to the regions with both the greatest thermohaline stratification, the highest nutrient inputs, and the highest primary productivity.     

The P-y Response of Laterally Loaded Flexible Piles in Residual Soil

This paper describes the main features related to lateral displacements with depth after successive lateral loading–unloading cycles applied to the top of reinforced-concrete flexible bored piles embedded in naturally bonded residual soil. The bored piles under study have a cylindrical shape, with 0.40-m in diameter and 8.0-m in length. Both bored piles types (P1 and P2) include an embedded steel pipe section in their center as longitudinal steel reinforcements: pile type P1 has another 16 steel rods as steel reinforcement to concrete while pile type P2 has no further steel reinforcement. Pile type P1 has three times as much stiffness (EI) and four and a half times the plastic moment (M_y) than pile type P2. A similar load–displacement performance was observed at initial loads as for small displacements of both piles. At this initial loading stage, the response of the reinforced concrete piles is a function of the soil characteristics and of a linear elastic pile deformation. During this stage, piles can even be understood as probes for evaluating soil reactions. For larger horizontal displacements, after the concrete section starts undergoing large deformations, approaching the ultimate bending moment, pile behavior and consequently the load–displacement relation starts to diverge for both piles. For pile P1 the values of relevant lateral displacements are extended to about 2.5-m in depth, while for pile P2 lateral displacements are mostly constrained to about 2.0-m in depth. Measurements of horizontal displacements of pile P1 against depth recorded with a slope indicator show that, after unloading, lateral loads at distinct stages (small and near failure loads), exhibits a much higher elastic phase of the system response. An analytical fitting model of soil reaction is proposed based on the measured displacements from slope indicator. The integration of a continuous model proposed for the soil reaction agrees fairly well with the measured displacements up to moments close to plastic limit. Results of load–displacement show that the stiffer pile (P1) was able to mobilize twice as much lateral load compared to pile P2 for a service limit displacement of about 20 mm. The paper shows results that enable the isolation of the structural variable through real scale pile load tests, thus granting understanding of its importance and enabling its quantitative visualization in examples of piles embedded in residual soil sites.

     

Do Spur-Throated Grasshoppers, <Emphasis Type="Italic">Melanoplus</Emphasis> spp. (Orthoptera: Acrididae), Exert Top-Down Control on Smooth Cordgrass <Emphasis Type="Italic">Spartina alterniflora</Emphasis> in Northern New England?

Recently, strong top-down (consumer) control of cordgrass (Spartina alterniflora) has been demonstrated. Here, we manipulated the densities of cordgrass consumers, acridid grasshoppers (Melanoplus bivittatus and Melanoplus femurrubrum), to examine their impact on cordgrass in the Plum Island Estuary (PIE), MA, USA. After 1 month, there was no detectable effect of grasshopper density on S. alterniflora biomass and grasshoppers at the highest densities (34 individuals per square meter) consumed only ~14% of the standing stock biomass. However, significant impacts of grasshopper density on grazing damage were seen. For example, plant damage and scarring length increased by 160% and 6,156%, respectively, at the highest grasshopper densities relative to exclusion (zero grasshoppers) densities. Plant height was significantly reduced with increasing grasshopper densities, although this may be a function of leaf tip removal instead of reduced plant growth. No other strong consumers of cordgrass (e.g., Littoraria irrorata, Prokelisia marginata) were observed in PIE and we suggest that consumer regulation of cordgrass is weak in this system.     

Analysis of soil drying incorporating a constitutive model for curling

This paper focuses on the shrinkage behavior of soil specimens involving sand, kaolinite, and kaolinite/sand mixtures subjected to desiccation under controlled conditions. Both, free and restrained shrinkage conditions are studied. The experiments show that pure soils do not curl upon unrestrained shrinkage; however, (under the same conditions) kaolinite/sand mixtures exhibited a marked curling. Furthermore, the mixture with the higher sand content broke through the middle of the sample after displaying a significant curling. Soils subjected to restricted shrinkage developed cracks with slight curling. To simulate the observed behavior, a mechanical model able to reproduce the detachment of the soil sample from the mold is proposed in this work and implemented in a fully coupled hydro-mechanical finite-element code. It is concluded that suction and differential shrinkage are key factors influencing the curling behavior of soils. The proposed framework was able to satisfactorily explain and reproduce the different stages and features of soil behavior observed in the experiments.

     

Enzymatic microbial Mn(II) oxidation and Mn biooxide production in the Guaymas Basin deep-sea hydrothermal plume

Microorganisms play important roles in mediating biogeochemical reactions in deep-sea hydrothermal plumes, but little is known regarding the mechanisms that underpin these transformations. At Guaymas Basin (GB) in the Gulf of California, hydrothermal vents inject fluids laden with dissolved Mn(II) (dMn) into the deep waters of the basin where it is oxidized and precipitated as particulate Mn(III/IV) oxides, forming turbid hydrothermal “clouds”. Previous studies have predicted extremely short residence times for dMn at GB and suggested they are the result of microbially-mediated Mn(II) oxidation and precipitation. Here we present biogeochemical results that support a central role for microorganisms in driving Mn(II) oxidation in the GB hydrothermal plume, with enzymes being the primary catalytic agent. dMn removal rates at GB are remarkably fast for a deep-sea hydrothermal plume (up to 2 nM/h). These rapid rates were only observed within the plume, not in background deep-sea water above the GB plume or at GB plume depths (∼1750-2000 m) in the neighboring Carmen Basin, where there is no known venting. dMn removal is dramatically inhibited under anoxic conditions and by the presence of the biological poison, sodium azide. A conspicuous temperature optimum of dMn removal rates (∼40 °C) and a saturation-like (i.e. Michaelis-Menten) response to O₂ concentration were observed, indicating an enzymatic mechanism. dMn removal was resistant to heat treatment used to select for spore-forming organisms, but very sensitive to low concentrations of added Cu, a cofactor required by the putative Mn(II)-oxidizing enzyme. Extended X-ray absorption fine structure spectroscopy (EXAFS) and synchrotron radiation-based X-ray diffraction (SR-XRD) revealed the Mn oxides to have a hexagonal birnessite or δ-MnO₂-like mineral structure, indicating that these freshly formed deep-sea Mn oxides are strikingly similar to primary biogenic Mn oxides produced by laboratory cultures of bacteria. Overall, these results reveal a vigorous Mn biogeochemical cycle in the GB hydrothermal plume, where a distinct microbial community enzymatically catalyzes rapid Mn(II) oxidation and the production of Mn biooxides.     

An exotic Cretaceous kimberlite linked to metasomatized lithospheric mantle beneath the southwestern margin of the Francisco Craton,Brazil

We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts,ilmenite,phlogopite,spinel,zircon,and uncommon minerals (ti...     

Efficacy of Coagulants and Ballast Compounds in Removal of Cyanobacteria (<Emphasis Type="Italic">Microcystis</Emphasis>) from Water of the Tropical Lagoon Jacarepaguá (Rio de Janeiro,Brazil)

Eutrophication is considered the most important water quality problem in freshwaters and coastal waters worldwide promoting frequent occurrence of blooms of potentially toxic cyanobacteria. Removal of cyanobacteria from the water column using a combination of coagulant and ballast is a promising technique for mitigation and an alternative to the use of algaecides. In laboratory, we tested experimentally the efficiency of two coagulants, polyaluminium chloride (PAC) and chitosan (made of shrimp shells), alone and combined with two ballasts: red soil (RS) and the own lagoon sediment, to remove natural populations of cyanobacteria, from an urban brackish coastal lagoon. PAC was a very effective coagulant when applied at low doses (≤8 mg Al L^?1) and settled the cyanobacteria, while at high doses (≥16 mg Al L^?1) large flocks aggregated in the top of test tubes. In contrast, chitosan was not able to form flocks, even in high doses (>16 mg L^?1) and did not efficiently settle down cyanobacteria when combined with ballast. The RS itself removed 33–47 % of the cyanobacteria. This removal was strongly enhanced when combined with PAC in a dose-dependent matter; 8 mg Al L^?1 was considered the best dose to be applied. The lagoon sediment alone did not promote any settling of cyanobacteria but removal was high when combined with PAC. Combined coagulant and ballast seems a very efficient, cheap, fast and safe curative measure to lessen the harmful cyanobacteria bloom nuisance in periods when particularly needed, such as around the 2016 Olympics in Jacarepaguá Lagoon.     

Anticipating ubiquitous computing: Logics to forecast technological futures

Visions of the future predict spaces apparently teaming with ever more novel and pervasive technologies. Significant amongst such forecasts is the notion of ‘ubiquitous computing’ (ubicomp), understood as an affordance or capacity tied (in)to people, places and things. This article stages an encounter between the futurity of ubicomp and recent debates in geography around anticipation. So, first, the future orientation in ubicomp research and development (R&D) is investigated as a mode of anticipation. ‘Knowledges’, and ‘logics’ of anticipation are subsequently, and second, discussed as the conceptual apparatus that constructs and perpetuates the ‘proximate future’ of ubicomp. This analysis connects recent discussion about ‘anticipation’ in social sciences research with the methods of ubicomp research, which fits with an emergent agenda around futurity in human geography. Third, the conceptual articulation of ‘anticipatory logic’ is applied to the analysis of empirical investigations of ubicomp R&D to identify the specific logics of anticipation at play. This article accordingly examines the logics of anticipation that both support and destabilise the certainty with which the future is imagined within ubicomp. In conclusion, the multiple ways of anticipating a future world and the ways in which they discipline understandings of futurity are framed as a politics of anticipation.     

Hydrogen isotope fractionation between OH-bearing minerals and water

Hydrogen isotope fractionation factors between hydroxyl-bearing minerals and water were determined at temperatures ranging between 400 and 850°C. The hydrogen isotope exchange rates for the mineral-water pairs examined were very slow. In most cases it was necessary to use an interpolation method for the determination of the hydrogen isotope equilibrium fractionation factor, α_e.For the temperature range of 450–850°C the hydrogen isotope fractionation factors for the mica-water and amphibole-water systems are simply expressed as a function of temperature and the molar fractions of the six-fold coordinated cations in the crystal, regardless of mineral species, as follows: 10³ In α_{e(mineral-water)} = ? 22.4 (10⁶T^?2) + 28.2 + (2X_Al ? 4X_Mg ? 68X_Fe), where X is the molar fraction of the cations. As the equation indicates, for any specific composition of the OH-bearing minerals, the change of α_e with temperature, over the temperature range investigated, is the same for all minerals studied. Thus for any specified values of X_Al, X_Mg, and X_Fe for these minerals, the relationship between α_e and T is 10³ In α_e = αT^?2 + k. Consequently, hydrogen isotope fractionation among coexisting minerals is temperature independent and cannot be used as a hydrogen isotope geothermometer.Some exceptions to the above general observations exist for minerals such as boehmite and kaolinite. In these minerals hydrogen bonding modifies the equilibrium hydrogen isotopic fractionation between mineral and water.