Release of Arsenic from Volcanic Rocks through Interactions with Inorganic Anions and Organic Ligands

The effects of a number of inorganic anions (F⁻, HCO₃ ⁻, B(OH)₄⁻, Cl⁻, I⁻) and of the siderophore DFO-B on the release of As from volcanic rocks were investigated in batch experiments. While previously reported field and laboratory data support a role of inorganic anions on As mobilization into aquifers, the role of siderophores on As-induced mobilization was less investigated. Fluoride, bicarbonate and DFO-B have shown a significant influence on the release of As from the rocks. Lava was mostly affected among the investigated rocks at pH 6 and 20°C by releasing 4% of its initial As content in the presence of 0.01 M F⁻and 10% in the presence of 500 μM DFO-B. The effect of fluoride was larger at pH 6 than at pH 8.5 for all the rocks. In the case of DFO-B, there was also a larger effect at pH 6 compared to pH 8 for the various rocks except tuff. Bicarbonate played a role under alkaline conditions while its effect was negligible at pH 6. Anion exchange processes in the presence of fluoride and bicarbonate and complexation processes in the presence of the siderophore DFO-B appear to be the major processes responsible for the release of arsenic from the rocks. The siderophore DFO-B plays mainly an indirect role on the As release by complexing Al, Fe and Mn, thus favoring the dissolution of the rocks and the consequent release of As bound to surface Al, Fe and Mn oxy-hydroxides. These findings suggest that ionic interactions with fluoride, bicarbonate and siderophore may be a further triggering factor in the mobilization of As from aquifer rocks.     

Conditions in the protoplanetary disk as seen by the type B CAIs

Abstract— Type B coarse‐grained calcium‐aluminum‐rich inclusions (CAIs) are the oldest known materials to have formed in the solar system and are a unique source of information regarding conditions and processes in the protoplanetary disk around the young sun. Recent experimental results on the crystallization and evaporation of type B‐like silicate melts allow us to place the following constraints on the conditions in the protoplanetary disk during the formation of type B CAIs. 1) Once type B CAIs precursors have been condensed from a solar composition gas, they were reheated at 1250–1450 °C, as is indicated by their igneous texture. 2) The melilite mantles characteristic of type B1 CAIs could be formed by crystallization of magnesium‐ and silicon‐depleted melt in the outer part of the partially molten droplets. Such depletion can arise when evaporation is fast compared to chemical diffusion in the melt. This requires the pressure of the surrounding solar composition gas to be at least 10⁻⁴ bars during the initial crystallization of melilite mantle. Type B2 CAIs with uniform distribution of melilite are expected to form at pressures less than 10⁻⁵ bars. 3) Evaporation calculations are used to place bounds on the thermal history of the type B CAIs. Observed compositional zoning in melilite suggests that the temperatures in the protoplanetary disk where the type B CAIs resided after crystallization could not have exceeded ˜1000 °C for more than a few tens of thousands of years. A recent calculation of the physical conditions associated with nebular shocks produced transient temperatures and gas pressures very much like what we find is required to melt reasonable CAI precursors and evaporate these sufficiently quickly to make a type B1 CAI.     

Isotopic constraints on the Si-biogeochemical cycle of the Antarctic Zone in the Kerguelen area (KEOPS)

Estimation of the silicon (Si) mass balance in the ocean from direct measurements (Si uptake-dissolution rates …) is plagued by the strong temporal and spatial variability of the surface ocean as well as methodological artifacts. Tracers with different sensitivities toward physical and biological processes would be of great complementary use. Silicon isotopic composition is a promising proxy to improve constraints on the Si-biogeochemical cycle, since it integrates over longer timescales in comparison with direct measurements and since the isotopic balance allows to resolve the processes involved, i.e. uptake, dissolution, mixing. Si-isotopic signatures of seawater Si(OH)₄ and biogenic silica (bSiO₂) were investigated in late summer 2005 during the KEOPS experiment, focusing on two contrasting biogeochemical areas in the Antarctic Zone: a natural iron-fertilized area above the Kerguelen Plateau (< 500 m water depth) and the High Nutrient Low Chlorophyll area (HNLC) east of the plateau (> 1000 m water depth). For the HNLC area the Si-isotopic constraint identified Upper Circumpolar Deep Water as being the ultimate Si-source. The latter supplies summer mixed layer with 4.0 ± 0.7 mol Si m^? 2 yr^? 1. This supply must be equivalent to the net annual bSiO₂ production and exceeds the seasonal depletion as estimated from a simple mixed layer mass balance (2.5 ± 0.2 mol Si m^? 2 yr^? 1). This discrepancy reveals that some 1.5 ± 0.7 mol Si m^? 2 yr^? 1 must be supplied to the mixed layer during the stratification period. For the fertilized plateau bloom area, a low apparent mixed layer isotopic fractionation value (?³⁰Si) probably reflects (1) a significant impact of bSiO₂ dissolution, enriching the bSiO₂ pool in heavy isotope; and/or (2) a high Si uptake over supply ratio in mixed layer at the beginning of the bloom, following an initial closed system operating mode, which, however, becomes supplied toward the end of the bloom (low Si uptake over supply ratio) with isotopically light Si(OH)₄ from below when the surface Si(OH)₄ pool is significantly depleted. We estimated a net integrated bSiO₂ production of 10.5 ± 1.4 mol Si m^? 2 yr^? 1 in the AASW above the plateau, which includes a significant contribution of bSiO₂ production below the euphotic layer. However, advection which could be significant for this area has not been taken into account in the latter estimation based on a 1D approach of the plateau system. Finally, combining the KEOPS Si-isotopic data with those from previous studies, we refined the average Si-isotopic fractionation factor to ? 1.2 ± 0.2‰ for the Antarctic Circumpolar Current.     

In situ survey of graphite in unequilibrated chondrites: Morphologies,C, N,O, and H isotopic ratios

Abstract— We performed in situ morphological and isotopic studies of graphite in the primitive chondrites Khohar (L3), Mezö‐Madaras (L3), Inman (L3), Grady (H3), Acfer 182 (CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (EH5). Various graphite morphologies were identified, including book, veins, fibrous, fine‐grained, spherulitic, and granular graphite, and cliftonite. SIMS measurements of H, C, N, and O isotopic compositions of the graphites revealed large variations in the isotopic ratios of these four elements. The δ¹⁵N and δ¹³C values show significant variations among the different graphite types without displaying any strict correlation between the isotopic composition and morphology. In the Khohar vein graphites, large ¹⁵N excesses are found, with δ¹⁵N_max ～+955‰, confirming previous results. Excesses in ¹⁵N are also detected in fine‐grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ¹⁵N ranging up to +440‰. The ¹⁵N excesses are attributed to ion‐molecule reactions at low temperatures in the interstellar molecular cloud (IMC) from which the solar system formed, though the largest excesses seem to be incompatible with the results of some recent calculation. Significant variations in the carbon isotopic ratios are detected between graphite from different chondrite groups, with a tendency for a systematic increase in δ¹³C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to small‐ and large‐scale carbon isotopic variations in the solar nebula.     

Precipitable water evaluations from infrared sun‐photometric measurements analyzed using the atmospheric hygrometry technique

Measurements of direct solar irradiance were taken employing 4 different sun‐photometers at near infrared wavelengths, suitable for use in atmospheric hygrometry. This technique utilising a set of spectral ratios, in and out of selected water vapour absorption bands, was applied to the measurements to obtain accurate evaluations of precipitable water. For all the hygrometric ratios given by the 4 sun‐photometers used at the 3 stations of Sagres, Monchique and Mt. Foia, during the CLEARCOLUMN experiment, we determined the calibration curves by correcting them for the Rayleigh scattering effects and, then, plotting the natural logarithms of such corrected ratios versus the square root of the water vapour mass present along the atmospheric slant path. The regression lines drawn for the various scatter diagrams were estimated to give evaluations of precipitable water with an uncertainty of less than 5%, 3% and 10% at the 3 stations, respectively. The calibration curves of the sun‐photometer located at the Sagres station were determined using the precipitable water evaluations obtained from the local radiosounding measurements taken on 5 clear‐sky days. Those of the sun‐photometers used at the Monchique and Mt. Foia stations were instead determined through intercomparison between subsets of measurements simultaneously taken with various instruments at Sagres and Mt. Foia. Using these calibration curves, we examined all the field measurements determining the time‐patterns of precipitable water at the 3 stations. During the period from 16 June to 25 July 1997, precipitable water was found to vary between 1.1 and 3.7 g cm⁻² at the Sagres station (with an accuracy within ±13%), between 1.0 and 2.8 g cm⁻² at Monchique (±11%) and between 0.8 and 3.0 g cm⁻² at the top of Mt. Foia (±26%).     

A HED Laboratory Astrophysics Testbed Comes of Age: JET Deflection via Cross Winds

We present new data from High-Energy Density (HED) laboratory experiments designed to explore the interaction of a heavy hypersonic radiative jet with a cross wind. The jets are generated with the MAGPIE pulsed power machine where converging conical plasma flows are produced from a cylindrically symmetric array of inclined wires. Radiative hypersonic jets emerge from the convergence point. The cross wind is generated by ablation of a plastic foil via soft-X-rays from the plasma convergence region. Our experiments show that the jets are deflected by the action of the cross wind with the angle of deflection dependent on the proximity of the foil. Shocks within the jet beam are apparent in the data. Analysis of the data shows that the interaction of the jet and cross wind is collisional and therefore in the hydrodynamic regime. We consider the astrophysical relevance of these experiments applying published models of jet deflection developed for AGN and YSOs. We also present results of 3-D numerical simulations of jet deflection using a new astrophysical Adaptive Mesh Refinement code. These simulations show highly structured shocks occurring within the beam similar to what was observed in the experiments.     

An Approach to Improving the Retrieval Accuracy of Oceanic Constituents in Case Ⅱ Waters

In the present paper, a method is proposed to improve the performance of Artificial Neural Network(ANN)based algorithms for the retrieval of oceanic constituents in Case Ⅱ waters. The ANN-based algorithms have been developed based on a constraint condition, which represents, to a certain degree, the correlation between suspended particulate matter(SPM)and pigment(CHL), coloured dissolved organic matter(CDOM)and CHL, as well as CDOM and SPM, found in Case Ⅱ waters. Compared with the ANN-based algorithm developed without a constraint condition, the performance of ANN-based algorithms developed with a constraint conditions is much better for the retrieval of CHL and CDOM, especially in the case of high noise levels; however, there is not significant improvement for the retrieval of SPM.