Petrogenesis of oceanic plagiogranites by partial melting of gabbros: an experimental study

We performed hydrous partial melting experiments at shallow pressures (0.2 GPa) under slightly oxidizing conditions (NNO oxygen buffer) on oceanic cumulate gabbros drilled by ODP (Ocean Drilling Program) cruises to evaluate whether the partial melting of oceanic gabbro can generate SiO₂-rich melts with compositions typical of oceanic plagiogranites. The experimental melts of the low-temperature runs broadly overlap those of natural plagiogranites. At 940 °C, the normalized SiO₂ contents of the experimental melts of all systems range between 60 and 61 wt%, and at 900 °C between 63 and 68 wt%. These liquids are characterized by low TiO₂ and FeO^tot contents, similar to those of natural plagiogranites from the plutonic section of the oceanic crust, but in contrast to Fe and Ti-rich low-temperature experimental melts obtained in MORB systems at ~950 °C. The ~1,500-m-long drilled gabbroic section of ODP Hole 735B (Legs 118 and 176) at the Southwest Indian Ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high-temperature shearing. The compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins, while the compositions of the crystals of low-temperature runs correspond to those of minerals from high-temperature microscopic veins occurring in the gabbroic section of the Hole 735B. This suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water-rich fluid phase. If the temperature estimations for high-temperature shear zones are correct (up to 1,000 °C), and a water-rich fluid phase is present, the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust.Editorial responsibility: J. Hoefs     

A one-dimensional modeling study of the diurnal cycle in the equatorial Atlantic at the PIRATA buoys during the EGEE-3 campaign

A one-dimensional model is used to analyze, at the local scale, the response of the equatorial Atlantic Ocean under different meteorological conditions. The study was performed at the location of three moored buoys of the Pilot Research Moored Array in the Tropical Atlantic located at 10° W, 0° N; 10° W, 6° S; and 10° W, 10° S. During the EGEE-3 (Etude de la circulation océanique et de sa variabilité dans le Golfe de Guinee) campaign of May–June 2006, each buoy was visited for maintenance during 2 days. On board the ship, high-resolution atmospheric parameters were collected, as were profiles of temperature, salinity, and current. These data are used here to initialize, force, and validate a one-dimensional model in order to study the diurnal oceanic mixed-layer variability. It is shown that the diurnal variability of the sea surface temperatures is mainly driven by the solar heat flux. The diurnal response of the near-surface temperatures to daytime heating and nighttime cooling has an amplitude of a few tenths of degree. The computed diurnal heat budget experiences a net warming tendency of 31 and 27 W m⁻² at 0° N and 10° S, respectively, and a cooling tendency of 122 W m⁻² at 6° S. Both observed and simulated mixed-layer depths experience a jump between the nighttime convection phase and the well-stabilized diurnal water column. Its amplitude changes dramatically depending on the meteorological conditions occurring at the stations and reaches its maximum amplitude (~50 m) at 10° S. At 6° and 10° S, the presence of barrier layers is observed, a feature that is clearer at 10° S. Simulated turbulent kinetic energy (TKE) dissipation rates, compared to independent microstructure measurements, show that the model tracks their diurnal evolution reasonably well. It is also shown that the shear and buoyancy productions and the vertical diffusion of TKE all contribute to the supply of TKE, but the buoyancy production is the main source of TKE during the period of the simulation.     

Comparative Dietary Diversity and Trophic Ecology of Two Estuarine Mesopredators

Quantifying the trophic dynamics of mesopredators in coastal habitats is an essential precursor to understanding their role in linking multiple trophic levels. Traditional dietary analyses may miss key aspects of a species’ feeding ecology and may thus fail to identify trophic linkages between predators and economically important prey populations. We applied stomach content and stable isotope analyses to estimate diet and trophic dynamics and investigated intraspecific dietary diversity of bonnetheads (Sphyrna tiburo) and red drum (Sciaenops ocellatus). Both juvenile and adult bonnetheads and juvenile red drum had diets comprised mainly of crustaceans, notably portunid crabs and penaeid shrimp, with varying degrees of dietary and isotopic niche overlap. Juvenile and adult bonnetheads had high dietary and isotopic niche overlap, whereas the degree of overlap between bonnetheads and red drum varied. Our findings indicate that bonnetheads are dietary specialists whereas red drum are dietary generalists. Further analysis identified intraspecific variability in the diet of each species; adult bonnetheads had the highest prey diversity whereas red drum had the lowest. We show that dietary and isotopic niche overlap and intraspecific variation in diet exist among juvenile and adult bonnetheads and juvenile red drum, though to varying degrees. Our findings demonstrate the importance of fully examining the trophic ecology of species that share habitats and resources, both at individual and population levels.     

Development and erosion in the Brazilian Amazon: A geochronological case study

A ²¹⁰Pb chronology of sedimentation in a small floodplain lake in the Jamari river basin (Rondonia, Brazil) provided insight into the relationship between development and erosion in the Brazilian Amazon. Positive correlations between increased sedimentation in the lake and levels of deforestation and mining in the Jamari basin suggested a major increase in erosion associated with these activities. This increase in erosion could diminish agricultural yields in the basin and significantly reduce the useful life of a hydro-electric plant recently built on the river.     

Use of principal component analysis for identification of Rockland and Trego Hot Springs tephras in the Hat Creek Graben,northeastern California,USA

Discontinuous tephra layers were discovered at Burney Spring Mountain, northern California. Stratigraphic relationships suggest that they are two distinct tephras. Binary plots and standard similarity coefficients of electron probe microanalysis data have been supplemented with principal component analysis to correlate the two tephra layers to known regional tephras. Using principal component analysis, we are furthermore able to bound our uncertainty in the correlation of the two tephra layers. After removal of outliers, within the 95% prediction interval, we can say that one tephra layer is likely the Rockland tephra, aged 565–610 ka, and the second layer is likely from Mt. Mazama, the Trego Hot Springs tephra, aged ~ 29 ka. In the case of the Rockland tephra, the new findings suggest that dispersal to the north was highly restricted. For Trego Hot Springs ash, the new findings extend the distribution to the southwest, with a rapid thinning in that direction. Coupled with considerations of regular tephra dispersal patterns, the results suggest that the primary dispersal direction for both tephras was to the south, and that occurrences in other directions are unlikely or otherwise anomalous.     

Vertical structure of Jupiter’s Oval BA before and after it reddened: What changed?

To constrain the properties of Oval BA before and after it reddened, we use Hubble methane band images from 1994 to 2009 to find that the distribution of upper tropospheric haze atop the oval and its progenitors remained unchanged, with reflectivity variations of less than 10% over this time span. We quantify measurement uncertainties and short-term fluctuations in velocity fields extracted from Cassini and Hubble data, and show that there were no significant changes in the horizontal velocity field of Oval BA in 2000, 2006, and 2009. Based on models of the oval’s dynamics, the static stability of the oval’s surroundings was also unchanged.The vertical extent of the oval did not change, based on the unchanged haze reflectivity and unchanged stratification. Published vortex models require Brunt-Väisälä frequencies of about 0.08 s⁻¹ at the base of the vortex, and we combine this value with a review of prior constraints on the vertically variable static stability in Jupiter’s troposphere to show that the vortex must extend down to the condensation level of water in supersolar abundance.The only observable change was an increase in short-wavelength optical absorption that appeared not at the core of the oval, but in a red annulus. The secondary circulation in the vortex keeps this red annulus warmer than the vortex core. Although the underlying cause of the color change cannot be proven, we explore the idea that the new chromophores in the red annulus may be related to a global or hemispheric temperature change.     

Simulating Australian Urban Climate in a Mesoscale Atmospheric Numerical Model

We develop an urban canopy scheme coupled to a mesoscale atmospheric numerical model and evaluate the simulated climate of an Australian city. The urban canopy scheme is based on the Town Energy Budget approach, but is modified to efficiently represent the predominately suburban component of Australian cities in regional climate simulations. Energy conservation is improved by adding a simple model of air-conditioning to prevent the urban parametrization acting as an energy sink during the Australian summer. In-canyon vegetation for suburban areas is represented by a big-leaf model, but with a largely reduced set of prognostic variables compared to previous approaches. Although we have used a recirculation/venting based parametrization of in-canyon turbulent heat fluxes that employs two canyon wall energy budgets, we avoid using a fixed canyon orientation by averaging the canyon fluxes after integrating over 180° of possible canyon orientations. The urban canopy scheme is evaluated by simulating the climate for Melbourne, Australia after coupling it to The Air Pollution Model. The combined system was found to predict a realistic climatology of air temperatures and winds when compared with observations from Environmental Protection Authority monitoring stations. The model also produced a plausible partitioning of the urban energy budget when compared to urban flux-tower studies. Overall, the urban canyon parametrization appears to have reasonable potential for studying present and predicting changes in future Australian urban climates in regional climate simulations.