A 16,000 C yr B.P. packrat midden series from the USA–Mexico Borderlands

A new packrat midden chronology from Playas Valley, southwestern New Mexico, is the first installment of an ongoing effort to reconstruct paleovegetation and paleoclimate in the U.S.A.–Mexico Borderlands. Playas Valley and neighboring basins supported pluvial lakes during full and/or late glacial times. Plant macrofossil and pollen assemblages from nine middens in the Playas Valley allow comparisons of two time intervals: 16,000–10,000 and 4000–0 ¹⁴C yr B.P. Vegetation along pluvial lake margins consisted of open pinyon–juniper communities dominated by Pinus edulis, Juniperus scopulorum, Juniperus cf. coahuilensis, and a rich understory of C₄ annuals and grasses. This summer-flowering understory is also characteristic of modern desert grassland in the Borderlands and indicates at least moderate summer precipitation. P. edulis and J. scopulorum disappeared or were rare in the midden record by 10,670 ¹⁴C yr B.P. The late Holocene is marked by the arrival of Chihuahuan desert scrub elements and few departures as the vegetation gradually became modern in character. Larrea tridentata appears as late as 2190 ¹⁴C yr B.P. based on macrofossils, but may have been present as early as 4095 ¹⁴C yr B.P. based on pollen. Fouquieria splendens, one of the dominant desert species present at the site today, makes its first appearance only in the last millennium. The midden pollen assemblages are difficult to interpret; they lack modern analogs in surface pollen assemblages from stock tanks at different elevations in the Borderlands.     

Impurities and nonstoichiometry in the bulk and on the (101?4) surface of dolomite

Atomistic computer simulation methods have been used to model the nature of nonstoichiometry and impurity defects in the bulk and at the (101?4) surface of dolomite (MgCa(CO₃)₂). Calcium and Mg in the bulk and at the surface have been replaced with divalent Ni, Co, Zn, Fe, Mn, and Cd. The results of these calculations indicate that in the bulk, these impurities will prefer to substitute at the Ca site rather than the Mg site. Ca excess in dolomite is most likely incorporated as basal stacking faults; this nonstoichiometry can influence the site distribution of impurities. The calculated surface segregation energies suggest that of all the impurities studied, only Cd will show a strong preference for the (101?4) surface of dolomite.     

Solar Magnetograms at 12 μm Using the Celeste Spectrograph

We present the first solar vector magnetogram constructed from measurements of infra-red Mg I 12.32-μm line spectra. Observations were made at the McMath-Pierce Telescope using the Celeste spectrometer/polarimeter. Zeeman-split Stokes line spectra were fitted with Seares profiles to obtain the magnetic field parameters. Maps of absolute field strength, line-of-sight angle, and azimuth are presented. Analysis shows that the variation in field strength within a spatial resolution element, 2 arcseconds, is greatest in the sunspot penumbra and that this is most likely caused by vertical field strength gradients, rather than horizontal image smearing. Widths of the Zeeman-split σ components, assuming a formation layer thickness of 200 km, indicate that vertical field strength gradients can be as large as 6.5 G/km in a penumbra.     

The Axis of Evil revisited

In light of the three-year data release from the Wilkinson Microwave Anisotropy Probe , we re-examine the evidence for the 'Axis of Evil' (AoE). We discover that previous statistics are not robust with respect to the data sets available and different treatments of the Galactic plane. We identify the cause of the instability and implement an alternative 'model selection' approach. A comparison to Gaussian isotropic simulations finds the features significant at the 94–98 per cent level, depending on the particular AoE model. The Bayesian evidence finds lower significance, ranging from 'substantial' at  Δ(ln  E ) ∼ 1.4  to no evidence for the most general AoE model.     

Natural hazard impacts in small island developing states: A review of current knowledge and future research needs

Small island developing states (or SIDS) are exposed to a large number of natural hazards and many characteristics of small island developing states make them particularly vulnerable to the impacts of natural hazards. In spite of this acknowledged vulnerability, there are relatively few studies which focus on the impacts of natural hazards in these countries. This paper presents a review of our current state of knowledge of impacts in small island developing states and highlights a number of research needs. Central to these is the need to integrate natural hazards research within a sustainable development context and the need to exploit existing procedures such as government coordinated disaster impact assessments to generate a detailed understanding of natural hazards impacts.     

Arsenic sequestration by sorption processes in high-iron sediments

High-iron sediments in North Haiwee Reservoir (Olancha, CA), resulting from water treatment for removal of elevated dissolved arsenic in the Los Angeles Aqueduct system, were studied to examine arsenic partitioning between solid phases and porewaters undergoing shallow burial. To reduce arsenic in drinking water supplies, ferric chloride and a cationic polymer coagulant are added to the aqueduct upstream of Haiwee Reservoir, forming an iron-rich floc that scavenges arsenic from the water. Analysis by synchrotron X-ray absorption spectroscopy (XAS) showed that the aqueduct precipitate is an amorphous hydrous ferric oxide (HFO) similar to ferrihydrite, and that arsenic is associated with the floc as adsorbed and/or coprecipitated As(V). Arsenic-rich floc and sediments are deposited along the inlet channel as aqueduct waters enter the reservoir. Sediment core samples were collected in two consecutive years from the edge of the reservoir along the inlet channel using 30- or 90-cm push cores. Cores were analyzed for total and extractable arsenic and iron concentrations. Arsenic and iron speciation and mineralogy in sediments were examined at selected depths by synchrotron XAS and X-ray diffraction (XRD). Sediment-porewater measurements were made adjacent to the core sample sites using polyacrylamide gel probe samplers. Results showed that sediment As(V) is reduced to As(III) in all cores at or near the sediment-water interface (0-4 cm), and only As(III) was observed in deeper sediments. Analyses of EXAFS spectra indicated that arsenic is present in the sediments mostly as a bidentate-binuclear, inner-sphere sorption complex with local atomic geometries similar to those found in laboratory studies. Below about 10 cm depth, XAS indicated that the HFO floc had been reduced to a mixed Fe(II, III) solid with a local structure similar to that of synthetic green rust (GR) but with a slightly contracted average interatomic Fe-Fe distance in the hydroxide layer. There was no evidence from XRD for the formation of a crystalline GR phase. The release of dissolved iron (presumably Fe²⁺) and arsenic to solution, as monitored by in situ gel probes, was variable but, in general, occurred at greater depths than arsenic reduction in the sediments by spectroscopic observations and appears to be near or below the depth at which sediment GR was identified. These data point to reductive dissolution of the sorbent iron phase as the primary mechanism of release of sorbed arsenic to solution.     

MTBE and gasoline hydrocarbons in ground water of the United States

The occurrence of methyl tert-butyl ether (MTBE) and gasoline hydrocarbons was examined in three types of studies of ground water conducted by the U.S. Geological Survey: major aquifer surveys, urban land-use studies, and agricultural land-use studies. The detection frequency of MTBE was dependent on the study type, with the highest detection frequency in urban land-use studies. Only 13 ground water samples from all study types, or 0.3%, had concentrations of MTBE that exceeded the lower limit of the U.S. EPA's Drinking-Water Advisory. The detection frequency of MTBE was highest in monitoring wells located in urban areas and in public supply wells. The detection frequency of any gasoline hydrocarbon also was dependent on study type and generally was less than the detection frequency of MTBE. The probability of detecting MTBE in ground water was strongly associated with population density, use of MTBE in gasoline, and recharge. Ground water in areas with high population density, in areas where MTBE is used as a gasoline oxygenate, and in areas with high recharge rates had a greater probability of MTBE occurrence. Also, ground water from public supply wells and shallow ground water underlying urban land-use areas had a greater probability of MTBE occurrence compared to ground water from domestic wells and ground water underlying rural land-use areas. The probability of detecting MTBE in ground water was weakly associated with the density of leaking underground storage tanks, soil permeability, and aquifer consolidation, and only concentrations of MTBE >0.5 microg/L were associated with dissolved oxygen.     

The oxidation states of copper and iron in mineral sulfides, and the oxides formed on initial exposure of chalcopyrite and bornite to air

Metal L_2,3, sulfur K and oxygen K near-edge X-ray absorption fine structure (NEXAFS) spectra for chalcopyrite, bornite, chalcocite, covellite, pyrrhotite and pyrite have been determined from single-piece natural mineral specimens in order to assess claims that chalcopyrite should be regarded as Cu^IIFe^IIS₂ rather than Cu^IFe^IIIS₂, and that copper oxide species are the principal initial oxidation products on chalcopyrite and bornite exposed to air. Spectra were obtained using both fluorescence and electron yields to obtain information representative of the bulk as well as the surface. Where appropriate, NEXAFS spectra have been interpreted by comparison with the densities of unfilled states and simulated spectra derived from ab initio calculations using primarily the FEFF8 code and to a lesser extent WIEN2k. Metal 2p and S 2p photoelectron spectra excited by monochromatised Al K_α X-rays were determined for each of the surfaces characterised by NEXAFS spectroscopy. The X-ray excited Cu LMM Auger spectrum was also determined for each copper-containing sulfide. FEFF8 calculations were able to simulate the experimental NEXAFS spectra quite well in most cases. For covellite and chalcocite, it was found that FEFF8 did not provide a good simulation of the Cu L₃-edge spectra, but WIEN2k simulations were in close agreement with the experimental spectra. Largely on the basis of these simulations, it was concluded that there was no convincing evidence for chalcopyrite to be represented as Cu^IIFe^IIS₂, and no strong argument for some of the Cu in either bornite or covellite to be regarded as Cu(II). The ab initio calculations for chalcopyrite and bornite indicated that the density of Cu d-states immediately above the Fermi level was sufficient to account for the Cu L₃-edge absorption spectrum, however these incompletely filled Cu d-states should not be interpreted as indicating some Cu(II) in the sulfide structure. It was also concluded that the X-ray absorption spectra were quite consistent with the initial oxidation products on chalcopyrite and bornite surfaces being iron oxide species, and inconsistent with the concomitant formation of copper-oxygen species.