The SCUBA Half Degree Extragalactic Survey (SHADES) – IX. The environment, mass and redshift dependence of star formation

We present a comparison between the SCUBA (Submillimetre Common User Bolometer Array) Half Degree Extragalactic Survey (SHADES) at 450 and  850 μm  in the Lockman Hole East with a deep Spitzer Space Telescope survey at  3.6–24 μm  conducted in guaranteed time. Using stacking analyses we demonstrate a striking correspondence between the galaxies contributing the submm extragalactic background light, with those likely to dominate the backgrounds at Spitzer wavelengths. Using a combination BRIzK plus Spitzer photometric redshifts, we show that at least a third of the Spitzer -identified submm galaxies at  1 < z < 1.5  appear to reside in overdensities when the density field is smoothed at 0.5–2 Mpc comoving diameters, supporting the high-redshift reversal of the local star formation–galaxy density relation. We derive the dust-shrouded cosmic star formation history of galaxies as a function of assembled stellar masses. For model stellar masses  <10¹¹ M_⊙  , this peaks at lower redshifts than the ostensible   z ∼ 2.2  maximum for submm point sources, adding to the growing consensus for 'downsizing' in star formation. Our surveys are also consistent with 'downsizing' in mass assembly. Both the mean star formation rates  〈d M _*/d t 〉  and specific star formation rates  〈(1/ M _*) d M _*/d t 〉  are in striking disagreement with some semi-analytic predictions from the Millenium Simulation. The discrepancy could either be resolved with a top-heavy initial mass function, or a significant component of the submm flux heated by the interstellar radiation field.     

Luminous red galaxy clustering at z≃ 0.7 – first results using AAOmega

We report on the AAT-AAOmega LRG Pilot observing run to establish the feasibility of a large spectroscopic survey using the new AAOmega instrument. We have selected luminous red galaxies (LRGs) using single epoch SDSS riz -photometry to   i < 20.5  and   z < 20.2  . We have observed in three fields including the COSMOS field and the COMBO-17 S11 field, obtaining a sample of ∼600 redshift   z ≳ 0.5  LRGs. Exposure times varied from 1–4 h to determine the minimum exposure for AAOmega to make an essentially complete LRG redshift survey in average conditions. We show that LRG redshifts to   i < 20.5  can be measured in ≈1.5 h exposures and present comparisons with 2SLAQ and COMBO-17 (photo)redshifts. Crucially, the riz selection coupled with the three to four times improved AAOmega throughput is shown to extend the LRG mean redshift from   z = 0.55  for 2SLAQ to   z = 0.681 ± 0.005  for riz -selected LRGs. This extended range is vital for maximizing the signal-to-noise ratio for the detection of the baryon acoustic oscillations (BAOs). Furthermore, we show that the amplitude of LRG clustering is   s ₀= 9.9 ± 0.7 h ⁻¹ Mpc  , as high as that seen in the 2SLAQ LRG Survey. Consistent results for this clustering amplitude are found from the projected and semi-projected correlation functions. This high amplitude is consistent with a long-lived population whose bias evolves as predicted by a simple 'high-peak' model. We conclude that a redshift survey of 360 000 LRGs over 3000 deg², with an effective volume some four times bigger than previously used to detect BAO with LRGs, is possible with AAOmega in 170 nights.     

Influence of Carbon and Microbial Community Priming on the Attenuation of Uranium in a Contaminated Floodplain Aquifer

The capacity for subsurface sediments to sequester radionuclide contaminants, such as uranium (U), and retain them after bioremediation efforts are completed is critical to the long‐term stewardship of re‐mediated sites. In U bioremediation strategies, carbon amendment stimulates bioreduction of U(VI) to U(IV), immobilizing it within the sediments. Sediments enriched in natural organic matter are naturally capable of sequestering significant U, but may serve as sources to the aquifer, contributing to plume persistence. Two types of organic‐rich sediments were compared to better understand U release mechanisms. Sediments that were artificially primed for U removal were retrieved from an area previously biostimulated while detrital‐rich sediments were collected from a location never subject to amendment. Batch incubations demonstrated that primed sediments rapidly removed uranium from the groundwater, whereas naturally reduced sediments released a sizeable portion of U before U(VI)‐reduction commenced. Column experiments confirmed that U release persisted for 65 pore volumes in naturally reduced sediments, demonstrating their sink‐source behavior. Acetate addition to primed sediments shifted the microbial community from sulfate‐reducing bacteria within Desulfobacteraceae to the iron‐reducing Geobacteraceae and Firmicutes, associated with efficient U(VI) removal and retention, respectively. In contrast, Geobacteraceae communities in naturally reduced sediments were replaced by sequences with similarity to Pseudomonas spp. during U release, while U(VI) removal only occurred with enrichment of Firmicutes. These investigations stress the importance of characterizing zones with heterogeneous carbon pools at U‐contaminated sites prior to the determination of a remedial strategy to identify areas, which may contribute to long‐term sourcing of the contaminants.     

Highway congestion during evacuation: examining the household’s choice of number of vehicles to evacuate

Hurricane evacuations in coastal counties have been reviewed and analyzed for the role of household preparedness and decisions before and during a disaster. However, one of the several emerging problems in the hurricane evacuation is transportation. Transportation issues have become more important in coastal evacuations as traffic problems impinge on people’s ability to get out of harm’s way and ultimately influence their decisions to evacuate. To add to the complexity, when families evacuate in multiple vehicles, it leads to additional vehicular traffic on roads and increased pressure on the transportation systems. However, little has been investigated on the characteristics that influence a household’s decision to evacuate in one or multiple vehicles. The outcome from such an analysis can help both the emergency managers and the transportation planners to targets groups that report taking more vehicles to develop policies that result in efficient evacuation. This study investigates the responses of evacuees surveyed after Hurricane Rita in the counties of Galveston, Brazoria and Harris County. The ordinary least square regression analysis revealed that access to transportation characteristics of a household such as number of registered vehicles in a household and number of eligible drivers was positively and significantly related to evacuating in more vehicles. Meanwhile, the risk of reaching destination safely was negatively related to taking more vehicles for evacuation even though both the risk index and deterrence index were positively significant. The time of decision and evacuation did not report any statistical significance.     

Physiochemical controls on the crystal-chemistry of Ni in birnessite: Genetic implications for ferromanganese precipitates

Sorption of Ni to birnessite is a fundamental control on the concentration of Ni in natural waters and associated sediments. Recent XAS work suggests the crystal-chemistry of Ni in birnessite is dependent on both structural and physiochemical factors. This work investigates the physiochemical controls on Ni crystal-chemistry in hexagonal birnessite with EXAFS of time series and pH series Ni-birnessite sorption experiments. At circumneutral pH Ni surface adsorbed above/below Mn octahedral vacancy sites in the phyllomanganate layers is progressively structurally incorporated into the vacancy sites with time (30% Ni incorporation after 408 h contact time). Ni structural incorporation into hexagonal birnessite occurs via rearrangement of surface adsorbed Ni with time rather than direct Ni incorporation from solution. At low pH (pH 4) Ni surface adsorbed at the vacancies is structurally incorporated into the vacancies upon increasing solution pH to circumneutral (∼20% Ni incorporation after 24 h contact time at circumneutral pH); newly incorporated Ni is stable with increasing contact time at pH circumneutral. However, upon decreasing solution pH from circumneutral back to pH 4, EXAFS shows a significant decrease in the proportion of Ni structurally incorporated; preliminary results indicate structural incorporation of Ni in hexagonal birnessite is reversible with decreasing pH. Time series results at circumneutral pH help explain the significant enrichment of Ni in marine ferromanganese precipitates; work here is combined with previous studies to provide a model correct at the molecular-level for Ni uptake by marine vernadite-rich ferromanganese precipitates. With Ni migrating from surface adsorbed to structurally incorporated, results suggest formation of a solid solution between an end-member vernadite and an end-member Ni-vernadite phase, with surface adsorption acting as a mechanism of transition from one composition to the other. pH series results call into question the viability of Ni crystal-chemistry in natural vernadite as a paleo-proxy for paleo-pH conditions in freshwater systems and challenge the traditional view that structurally incorporated trace-metals are stable within the Eh-pH field of the host mineral phase. Natural birnessite may not be as permanent a sink for Ni as first suggested by Ni crystal-chemistry.     

Ultrahigh-pressure metamorphic rocks and the thermal evolution of continent collision belts

Abstract Coesite-bearing eclogites exposed in the Alpine, Qinling-Dabie (China), Caledonian, and Ural orogenic belts provide insight into the time-dependent thermal structure of continent collision belts. Coesite-bearing eclogites record peak metamorphic temperatures of 550-900°C at pressures ≥ 2.5 GPa reflecting anomalously cool conditions at depths of 90 km or more. The low temperatures recorded by coesite-bearing eclogites strongly suggest formation in a convergent plate margin where the downward advection of cool lithosphere depresses isotherms on a regional scale. Subduction zone pressure-temperature (P-T) paths calculated using a two-dimensional finite-difference model predict steady-state temperatures of 450-650°C at 100 km depth at the slab-mantle interface for convergence rates of 10 to 100 mm/yr. Coesite-bearing eclogites record peak temperatures ~100-250°C higher, possibly reflecting (i) formation during the early stages of convergence prior to the achievement of thermal steady state; (ii) attainment of peak metamorphic temperatures during decompression (exhumation); (iii) formation during slow, <10 mm/yr, convergence; or (iv) uncertainties in the modeling parameters. Retrograde P-T paths determined for coesite-bearing eclogites from the western Alps and China indicate cooling during decompression from depths of ~100 km. Cooling of eclogite terrains during exhumation requires loss of heat downward into lithosphere that continues to subduct beneath the eclogites, loss of heat upward into the cooler hanging wall of a large-scale normal fault/shear zone, or a combination of the two scenarios.     

Seismic velocities in fractured rocks: an experimental verification of Hudson's theory1

Flow of fluids in many hydrocarbon reservoirs and aquifers is enhanced by the presence of cracks and fractures. These cracks could be detected by their effects on propagation of compressional and shear waves through the reservoir: several theories, including Hudson's, claim to predict the seismic effects of cracks. Although Hudson's theory has already been used to calculate crack densities from seismic surveys, the predictions of the theory have not yet been tested experimentally on rocks containing a known crack distribution. This paper describes an experimental verification of the theory. The rock used, Carrara marble, was chosen for its uniformity and low porosity, so that the effect of cracks would not be obscured by other influences. Cracks were induced by loading of laboratory specimens. Velocities of compressional and shear waves were measured by ultrasound at 0.85 MHz in dry and water-saturated specimens at high and low effective pressures. The cracks were then counted in polished sections of the specimens. In ‘dry’ specimens with both dry and saturated cracks, Hudson's theory overpredicted observed crack densities by a constant amount that is attributed to the observed value being systematically underestimated. The theory made poor predictions for fully saturated specimens. Shear-wave splitting, caused by anisotropy due to both crystal and crack alignment, was observed. Cracks were seen to follow grain boundaries rather than the direction of maximum compression due to loading. The results demonstrate that Hudson's theory may be used in some cases to determine crack and fracture densities from compressional- and shear-wave velocity data.