Stability and equation of state of post-aragonite BaCO3

At ambient conditions, witherite is the stable form of BaCO₃ and has the aragonite structure with space group Pmcn. Above ~10 GPa, BaCO₃ adopts a post-aragonite structure with space group Pmmn. High-pressure and high-temperature synchrotron X-ray diffraction experiments were used to study the stability and equation of state of post-aragonite BaCO₃, which remained stable to the highest experimental P–T conditions of 150 GPa and 2,000 K. We obtained a bulk modulus K ₀ = 88(2) GPa with $K'$  = 4.8(3) and V ₀ = 128.1(5) ų using a third-order Birch-Murnaghan fit to the 300 K experimental data. We also carried out density functional theory (DFT) calculations of enthalpy (H) of two structures of BaCO₃ relative to the enthalpy of the post-aragonite phase. In agreement with previous studies and the current experiments, the calculations show aragonite to post-aragonite phase transitions at ~8 GPa. We also tested a potential high-pressure post–post-aragonite structure (space group C222 ₁ ) featuring four-fold coordination of oxygen around carbon. In agreement with previous DFT studies, ΔH between the C222 ₁ structure and post-aragonite (Pmmn) decreases with pressure, but the Pmmn structure remains energetically favorable to pressures greater than 200 GPa. We conclude that post–post-aragonite phase transformations of carbonates do not follow systematic trends observed for post-aragonite transitions governed solely by the ionic radii of their metal cations.     

Evaluation of mineral substrates for in situ iron removal from groundwater

Calcium carbonate-based materials (CCBM) have been found to remove Fe(II) and other divalent metal cations from aqueous solution and thus have the potential for incorporation into remediation systems to remove Fe(II) from groundwater at landfills. Research was conducted to examine the ability of a range of CCBM to remove Fe(II) and assess the mechanism of removal. Different CCBM (limestone, concrete, dolomite, marble), as well as gypsum, witherite, and quartz sand, were tested for their ability to remove Fe(II) from water using batch tests conducted under anaerobic conditions. Limestone (specific surface area of approximately 0.46 m²/g) was found to have the best removal effectiveness, and the final Fe(II) concentration was reduced from 50 to <0.01 mg/L. Kinetics experiments conducted over a 72 h period indicated that the removal process of Fe(II) by CCBM was a two-step process. The first step is rapid sorption of Fe(II) onto the CCBM surfaces within the first hour, and the second step is relatively slow co-precipitation of iron-containing solids formed through various chemical reactions. The two best performing CCBM (limestone and concrete) were evaluated for their removal ability based on media particle size (diameters of 3–5, 7–10, 15–25, and 40–50 mm) and revealed statistically significant (p < 0.01) increases in Fe(II) removal for each particle size class examined. SEM analysis of reacted materials revealed visible precipitates on the reactive material surface; XRD analysis was not able to detect crystalline Fe minerals on limestone surface.     

Identification and Quantification of Base Flow Using Carbon Isotopes

Six surface water samples from locations along Otter Creek in Southeastern Montana and a groundwater sample from a nearby monitoring well completed in the Knobloch coal were analyzed for stable carbon isotope ratios. Along the length of its perennial reach, between the towns of Otter and Ashland, Otter Creek crosses several coal outcrops, including the Knobloch coal zone. The carbon isotope ratio of the creek becomes progressively more similar to that of the Knobloch coal aquifer groundwater in samples collected downgradient from the town of Otter. The isotope ratio of the stream changes from ?10.5 to ?8.9‰ reflecting the influence of the coal‐aquifer base flow contribution, as represented by Knobloch coal groundwater, which has a carbon isotope value of +3.9‰. The dissolved inorganic carbon concentrations of the groundwater and surface water are similar (～100 mg/L), which allowed the use of the simplified, first‐order, two‐end‐member mixing equation. Using carbon isotope ratios, calculations of the fraction of water contributed by coal aquifers indicate that approximately 11% of the surface water in Otter Creek at its mouth near Ashland was supplied by groundwater from the coal aquifers that crop out between Otter and Ashland. This study was conducted in December, when Otter Creek is at low flow. At times of higher surface flow, the contribution from groundwater base flow will be correspondingly smaller. This study illustrates that carbon isotopes can be an effective, low‐cost tool in base flow studies.     

Enhanced Acoustic Backscatter Due to High Abundance of Sand Dollars,Dendraster excentricus

Abstract

Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity of Humboldt Bay, California. During these surveys, a band of enhanced acoustic backscatter was observed offshore from the bay entrance, approximately parallel to the isobaths, in water depths ranging from 16–24 m. In order to assess the cause of the increase in backscatter levels, a more comprehensive study was conducted in August and September 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwater video recording. New observations indicated that a dense population of sand dollars (Dendraster excentricus) coincided with the enhanced backscatter band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the band shifted eastward by 160 m, possibly resulting from a change in the biological or physical factors which influence the location and breadth of sand dollars.

The relationship between high sand dollar abundance and enhanced acoustic backscatter was further verified in the near shore region off Samoa Beach California, where a dense, banded population of sand dollars was previously observed. Video footage confirmed the presence of a band of sand dollars, also nominally parallel to the isobaths, in water depths of 8–15 m. A band of enhanced backscatter coincided with the dense sand dollar population. The identification of dense aggregations of sand dollars through enhanced acoustic backscatter could lead to the use of acoustic techniques to study sand dollar distributions and abundance.     

An X-ray and optical study of the new SMC X-ray binary pulsar system SXP7.92 and its probable optical counterpart, AzV285

Optical and X-ray observations are presented here of a newly reported X-ray transient system in the Small Magellanic Cloud (SMC) – SXP7.92. A detailed analysis of the X-ray data reveal a coherent period of 7.9 s. A search through earlier X-ray observations of the SMC reveal a previously unknown earlier detection of this system. Follow-up X-ray observations identified a new transient source within the error circle of the previous observations. An optical counterpart, AzV285, is proposed which reveals clear evidence for a 36.8 d binary period.     

Influence of reach geomorphology on hyporheic communities in a gravel‐bed stream

The surface sediment, subsurface sediment, and hyporheos were sampled by freeze‐coring at three sites of contrasting geomorphology in a New Zealand gravel‐bed stream. No differences in size class composition were detected among the subsurface sediments of the three sites but the forced pool‐riffle (a stretch of stream where the sequence of pool and riffles is controlled by in‐channel bedrock outcrops) had significantly finer surface sediments than the other sites. The hyporheos of all sites was dominated numerically by insects, including nymphs of Deleatidium and Oniscigaster, and chironomid larvae. Of the non‐insect taxa, Isopoda, Oligochaeta, and Acari were most common. No significant differences existed between total invertebrate density or taxon richness at the sites. Aoteapsyche was found only in the plane bed site, whereas Nesameletus was found only in the forced pool‐riffle. Psilochorema and Nematoda occurred at significantly lower densities in the forced pool‐riffle than in the plane bed or floodplain sites. For most taxa, local variation within a site was greater than that between sites. Total invertebrate density was highest near the sediment surface but no significant differences were found between the three sites in this or any other depth layer. Significant site‐depth interactions were obtained for three taxa (Zelandobius, Ostracoda, and Oligochaeta) indicating that local geomorphology may result in differential depth distributions of hyporheic species.     

Near-future changes in storm surges along the Atlantic Iberian coast

Natural Hazards - Decadal predictions bridge the gap between the short-term weather/seasonal forecasts and the long-term climate projections. They target the reproduction of large-scale weather...