Upper limit on dimming of cosmological sources by intergalactic grey dust from the soft X-ray background

Active galactic nuclei (AGN) produce a dominant fraction  ( F _AGN∼ 80 per cent)  of the soft X-ray background (SXB) at photon energies  0.5 < E < 2 keV  . If dust pervaded throughout the intergalactic medium, its scattering opacity would have produced diffuse X-ray haloes around AGN. Taking account of known galaxies and galaxy clusters, only a fraction   F _halo≲ 10 per cent  of the SXB can be in the form of diffuse X-ray haloes around AGN. We therefore limit the intergalactic opacity to optical/infrared photons from large dust grains, with radii in the range   a = 0.2–2.0 μm  , to a level  τ_GD≲ 0.15( F _halo/10 per cent)(F_AGN/80 per cent)⁻¹  to a redshift   z ∼ 1  . Our results are only weakly dependent on the grain size distribution in this size range or the redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can be used to improve our constraints and diminish the importance of dust as a source of systematic uncertainty for future supernova surveys which aim to improve the precision on measuring the redshift evolution of the dark energy equation-of-state.     

Groundwater flow dynamics in the complex aquifer system of Gidabo River Basin (Ethiopian Rift): a multi-proxy approach

Hydrochemical and isotope data in conjunction with hydraulic head and spring discharge observations were used to characterize the regional groundwater flow dynamics and the role of the tectonic setting in the Gidabo River Basin, Ethiopian Rift. Both groundwater levels and hydrochemical and isotopic data indicate groundwater flow from the major recharge area in the highland and escarpment into deep rift floor aquifers, suggesting a deep regional flow system can be distinguished from the shallow local aquifers. The δ¹⁸O and δ²H values of deep thermal (≥30 °C) groundwater are depleted relative to the shallow (<60 m below ground level) groundwater in the rift floor. Based on the δ¹⁸O values, the thermal groundwater is found to be recharged in the highland around 2,600 m a.s.l. and on average mixed with a proportion of 30 % shallow groundwater. While most groundwater samples display diluted solutions, δ¹³C data of dissolved inorganic carbon reveal that locally the thermal groundwater near fault zones is loaded with mantle CO₂, which enhances silicate weathering and leads to anomalously high total dissolved solids (2,000–2,320 mg/l) and fluoride concentrations (6–15 mg/l) exceeding the recommended guideline value. The faults are generally found to act as complex conduit leaky barrier systems favoring vertical mixing processes. Normal faults dipping to the west appear to facilitate movement of groundwater into deeper aquifers and towards the rift floor, whereas those dipping to the east tend to act as leaky barriers perpendicular to the fault but enable preferential flow parallel to the fault plane.     

Minerals of the viridine hornfels from Darmstadt,Germany

Viridine containing the highest amounts of Mn₂O₃ detected thus far (up to 20.5 mol % “Mn₂SiO₅”) coexists in a metasedimentary hornfels with spessartine, Mn-phlogopite (mangan-ophyllite), Mn-phengite (alurgite), hematite, quartz and probably some primary braunite. In layers poorer in viridine spessartine is absent but piemontite appears as an additional phase. Microprobe analyses of all these phases are presented which indicate very strong fractionation of Mg and Mn in coexisting phlogopite and garnet, and of Fe and Mn in coexisting hematite and braunite. Sericitic aggregates consisting of phengitic muscovite and braunite are interpreted as retrograde alteration products of viridine, but might partly be pinitic alterations of a former Mg-rich cordierite. Due to the occurrence of the assemblage spessartine-viridine-quartz Mn-cordierite cannot have been a stable phase prior to retrograde alterations. In general the stability field of viridine is extended towards higher temperatures as compared to that of pure andalusite, Al₂SiO₅. Due to the coexistence of phlogopite and muscovite (phengite) the temperature of contact metamorphism cannot have exceeded some 550°–650° C depending on the prevailing water pressure.     

Did dilution limit the phytoplankton response to iron addition in HNLCLSi sub-Antarctic waters during the SAGE experiment?

An in situ iron addition experiment (SAGE) was carried out in high-nitrate low-chlorophyll low-silicic acid (HNLCLSi) sub-Antarctic surface waters south-east of New Zealand. In contrast to other iron addition experiments, the phytoplankton response was minor, with a doubling of biomass relative to surrounding waters, with the temporal trends in dissolved iron and macronutrients instead dominated by physical factors such as mixing and dilution. The initial increase in patch surface area indicated a lateral dilution rate of 0.125 d⁻¹, with a second estimate from a model of the decline in peak SF₆ concentration yielding a higher lateral dilution rate of 0.16-0.25 d⁻¹. The model was tested on the SOIREE SF₆ dataset and provided a lateral dilution of 0.07 d⁻¹, consistent with previous published estimates. MODIS ocean colour images showed elevated chlorophyll coincident with the SF₆ patch on day 10 and 12, and an elevated chlorophyll filament at the SAGE experiment location 3-4 days after ship departure, which provided additional lateral dilution estimates of 0.19 and 0.128 d⁻¹. Dissolved iron at the patch centre declined by 85% within two days of the initial infusion, of which dilution accounted for 50-65%; it also decreased rapidly after the 2nd and 3rd infusions but remained elevated after the fourth infusion. Despite decreases in nitrate and silicic acid from day 7 and 10, respectively, the final nutrient concentrations in the patch exceeded the initial concentrations due to supply from lateral intrusion and mixed-layer deepening. The low Si:N loss ratio suggested that the observed limited response to iron was primarily by non-siliceous phytoplankton. Algal growth rate exceeded the minimum dilution rate during two periods (days 3-6 and 10-14), and coincided with net chlorophyll accumulation. However, as the ratio of algal growth to dilution was the lowest reported for an iron addition experiment, dilution was clearly a significant factor in the SAGE experiment recording the lowest phytoplankton response to mesoscale iron addition.     

High-magnesium staurolite in a sapphirine-garnet rock from the Limpopo Belt,Southern Africa

Unusually Mg-rich staurolite with F=Fe/(Fe+ Mg) = 0.51 occurs in a peraluminous silica-deficient sapphirine-garnet-gedrite-spinel-corundum-phlogopite rock as relic inclusions within pyrope-almandine garnet (F=0.43–0.48), from which it is generally separated by sapphirine (F=0.19). Gedrite has F=0.24–0.27, phlogopite 0.19, and spinel 0.43. The gradual disappearance of staurolite is due to a shift of the 3-phase AFM assemblage staurolite-garnetsapphirine to more Fe-rich compositions according to probably prograde reactions between staurolite and garnet to form sapphirine together with either quartz or gedrite. Thus still more Mg-rich staurolite seems to have existed in the rock at earlier stages of metamorphism characterized by lower temperatures and perhaps higher pressures. The local equilibria now frozen in indicate minimum pressures of 7–8 kbar and temperatures in excess of about 800° C.Staurolite is free from Zn, very low in Si, and richer in Ti than any other staurolite found thus far. Most notable is the unique Mg/Fe fraction between staurolite and garnet with staurolite being the more Fe-rich phase (K_D=1.32).     

Mutual phenomena of Jupiter's satellites

Observations of an occultation of Europa by Io are fitted by a model light curve. The model has five free parameters, namely the radius of Europa, the impact parameter, the brightness ratio of the satellites, the time of midevent and the mean relative velocity. The model assumes a fixed value for the radius of Io and for the solar phase angle α, and that Europa has a uniform surface brightness. The OC residuals of the best fitting light curve are very small (～0.002 mag) and of a purely random nature; there is no evidence of albedo features. Taking α = 0 does not affect significantly the quality of the fit. Six mutual eclipses were also observed, and their times of minima agree well with the predictions of Aksnes Icarus21 (1974). For two events these predictions differ by about 20 min from those of Brinkmann and Millis Sky & Telescope45 (1973).     

The correlation between star formation and 21-cm emission during the reionization epoch

Reionization is thought to be dominated by low-mass galaxies, while direct observations of resolved galaxies probe only the most massive, rarest objects. The cross-correlation between fluctuations in the surface brightness of the cumulative Lyα emission (which serves as a proxy for the star formation rate) and the redshifted 21-cm signal from neutral hydrogen in the intergalactic medium (IGM) will directly probe the causal link between the production of ionizing photons in galaxies and the reionization of the IGM. We discuss the prospects for detecting this cross-correlation for unresolved galaxies. We find that on angular scales ≲10 arcmin detection will be practical using wide-field near-infrared (near-IR) imaging from space in combination with the forthcoming Mileura Wide-field Array – Low Frequency Demonstrator. When redshifted 21-cm observations of the neutral IGM are combined with space-based near-IR imaging of Lyα emission, the detection on angular scales ≲3 arcmin will be limited by the sensitivity of the 21-cm signal, even when a small-aperture optical telescope (∼2 m) and a moderate field of view (∼10 deg²) are used. On scales ≳3 arcmin, the measurement of cross-correlation will be limited by the accuracy of the foreground sky subtraction.     

The SOLAS air-sea gas exchange experiment (SAGE) 2004

The SOLAS air-sea gas exchange experiment (SAGE) was a multiple-objective study investigating gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the expansive subpolar zone of the southern oceans. This paper provides a general introduction and summary of the main experimental findings. The release site was selected from a pre-voyage desktop study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the south-east of New Zealand and the experiment was conducted between mid-March and mid-April 2004. In common with other mesoscale iron addition experiments (FeAX’s), SAGE was designed as a Lagrangian study, quantifying key biological and physical drivers influencing the air-sea gas exchange processes of CO₂, DMS and other biogenic gases associated with an iron-induced phytoplankton bloom. A dual tracer SF₆/³He release enabled quantification of both the lateral evolution of a labelled volume (patch) of ocean and the air-sea tracer exchange at tenths of kilometer scale, in conjunction with the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the gas exchange coefficient on windspeed that is widely applicable and describe air-sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties and windspeed were quantified to further assist the development of gas exchange models for high-wind environments.There was a significant increase in the photosynthetic competence (F_v/F_m) of resident phytoplankton within the first day following iron addition, but in contrast to other FeAX’s, rates of net primary production and column-integrated chlorophyll a concentrations had only doubled relative to the unfertilised surrounding waters by the end of the experiment. After 15 days and four iron additions totalling 1.1 ton Fe²⁺, this was a very modest response compared to other mesoscale iron enrichment experiments. An investigation of the factors limiting bloom development considered co-limitation by light and other nutrients, the phytoplankton seed-stock and grazing regulation. Whilst incident light levels and the initial Si:N ratio were the lowest recorded in all FeAXs to date, there was only a small seed-stock of diatoms (less than 1% of biomass) and the main response to iron addition was by the picophytoplankton. A high rate of dilution of the fertilised patch relative to phytoplankton growth rate, the greater than expected depth of the surface mixed layer and microzooplankton grazing were all considered as factors that prevented significant biomass accumulation. In line with the limited response, the enhanced biological draw-down of pCO₂ was small and masked by a general increase in pCO₂ due to mixing with higher pCO₂ waters. The DMS precursor DMSP was kept in check through grazing activity and in contrast to most FeAX’s dissolved dimethylsulfide (DMS) concentration declined through the experiment. SAGE is an important low-end member in the range of responses to iron addition in FeAX’s. In the context of iron fertilisation as a geoengineering tool for atmospheric CO₂ removal, SAGE has clearly demonstrated that a significant proportion of the low iron ocean may not produce a phytoplankton bloom in response to iron addition.     

Human perturbations on the global biogeochemical cycles of coupled Si–C and responses of terrestrial processes and the coastal ocean

We present a model of the global biogeochemical cycle of silicon (Si) that emphasizes its linkages to the carbon cycle and temperature. The Si cycle is a crucial part of global nutrient biogeochemistry regulating long-term atmospheric CO₂ concentrations due to silicate mineral weathering reactions involving the uptake of atmospheric CO₂ and production of riverine dissolved silica, cations and bicarbonate. In addition and importantly, the Si cycle is strongly coupled to the other nutrient cycles of N, P, and Fe; hence siliceous organisms represent a significant fraction of global primary productivity and biomass. Human perturbations involving land-use changes, burning of fossil fuel, and inorganic N and P fertilization have greatly altered the terrestrial Si cycle, changing the river discharge of Si and consequently impacting marine primary productivity primarily in coastal ocean waters.