Retention of ion-implanted-xenon in olivine: Dependence on implantation dose

The diffusion of Xe in olivine, a major mineral in both meteorites and lunar samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500°C. The fraction of implanted Xe retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100% occurred for an implantion dose of 3 × 10¹⁵ Xe ions/cm². Retention was less at lower doses, with ≥ 50% loss at 1 × 10¹⁴ Xe ions/cm². Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 μm layer for 10⁷ years was calculated as a function of metamorphic temperature. For example, an activation energy of 50 kcal/mole implies Xe retention may be possible for metamorphic temperatures below 500°C.     

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind,Pallasites and Trace Element Measurements

Electron probe microanalyser measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~ 100 µg g^?1 level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In order to minimise electron beam current density, a rastered 50 × 100 µm electron beam was adequate and minimised sensitivity to small Al‐rich contaminants. Reproducible analyses of eleven SIMS‐cleaned spots on San Carlos olivine agreed at 69.3 ± 1.0 µg g^?1. The known Al mass fraction was used to calibrate an Al implant into San Carlos. Accurate measurements of Al were made for olivines in the pallasites: Imilac, Eagle Station and Springwater. Our focus was on Al in olivine, but our technique could be refined to give accurate electron probe measurements for other contamination‐sensitive trace elements. For solar wind, it is projected that the Al/Mg abundance ratio can be determined to 6%, a factor of 2 more precise than the solar spectroscopic ratio.     

The future of Genesis science

Solar abundances are important to planetary science since the prevalent model assumes that the composition of the solar photosphere is that of the solar nebula from which planetary materials formed. Thus, solar abundances are a baseline for planetary science. Previously, solar abundances have only been available through spectroscopy or by proxy (CI). The Genesis spacecraft collected and returned samples of the solar wind for laboratory analyses. Elemental and isotopic abundances in solar wind from Genesis samples have been successfully measured despite the crash of the re‐entry capsule. Here we present science rationales for a set of 12 important (and feasible postcrash) Science and Measurement Objectives as goals for the future (Table 1). We also review progress in Genesis sample analyses since the last major review (Burnett 2013 ). Considerable progress has been made toward understanding elemental fractionation during the extraction of the solar wind from the photosphere, a necessary step in determining true solar abundances from solar wind composition. The suitability of Genesis collectors for specific analyses is also assessed. Thus far, the prevalent model remains viable despite large isotopic variations in a number of volatile elements, but its validity and limitations can be further checked by several Objectives.     

Evaluating groundwater discharge to tidal rivers based on a Rn-222 time-series approach

The natural flux of groundwater into coastal water bodies has recently been shown to contribute significant quantities of nutrients and trace metals to the coastal ocean. Groundwater discharge and hyporheic exchange to estuaries and rivers, however, is frequently overlooked though it often carries a distinctly different chemical signature than surface waters. Most studies that attempt to quantify this input to rivers use multiple geochemical tracers. However, these studies are often limited in their spatial and temporal extents because of the labor-intensive nature of integrating multiple measurement techniques. We describe here a method of using a single tracer, ²²²Rn, to rapidly characterize groundwater discharge into tidally-influenced rivers and streams. In less than one week of fieldwork, we determined that of six streams that empty into the Indian River Lagoon (IRL), Florida, three (Eau Gallie River, Turkey Creek, and Main Canal) did not receive substantial groundwater inputs, one canal (C-25 Canal) was dominated by groundwater exchange, and the remaining two (Sebastian River system and Crane Creek) fell somewhere in between. For more detailed discharge assessments, we focused on the Sebastian River system, a stratified tidal river estuary, during a relatively dry period (June) and a wet period (July) in 2008. Using time-series ²²²Rn and current velocity measurements we found that groundwater discharge into all three branches of the Sebastian River increased by 1–2 orders of magnitude during the wetter period. The estimated groundwater flow rates were higher than those reported into the adjacent IRL, suggesting that discharge into these rivers can be more important than direct discharge into the IRL. The techniques employed here should work equally well in other river/stream systems that experience significant groundwater discharge. Such assessments would allow area managers to quickly assess the distribution and magnitude of groundwater discharge nature into rivers over large spatial ranges.     

Radon and radium isotopes as tracers of submarine groundwater discharge – Results from the Ubatuba,Brazil SGD assessment intercomparison

We determined groundwater flow rates shortly after the wet season into an embayment near Ubatuba, Brazil as part of an international intercomparison experiment for submarine groundwater discharge (SGD) assessment techniques. Our estimated rates were determined by the combined use of continuous radon measurements and assessment of radium isotope patterns. The spatial distribution of the short-lived radium isotopes (²²³Ra and ²²⁴Ra) provided the means for independent evaluations of radon losses by mixing and atmospheric evasion. We were thus able to construct a well-constrained mass balance for radon that included a groundwater flux term. Our results showed that the groundwater discharge into this embayment from the fractured crystalline rock aquifer is not steady-state but varies with tidal modulation and rain-induced forcing. Tidally modulated and rain-induced flow rates were comparable during this period. The SGD rates estimated from radon ranged from 1 cm/day to 29 cm/day (cm³/cm² day) with a mean and standard deviation of 13 ± 6 cm/day. These estimates were mostly similar to a dye-dilution automatic seepage meter (15 ± 19 cm/day) and were within the broad ranges estimated by manual and continuous heat seepage meters but lower than indicated by an artificial tracer test performed nearshore.     

Carbon isotopic composition and lattice-bound carbonate of Peru-Chile margin phosphorites

New light-stable carbonate-carbon isotope and lattice-bound CO₂ data from Quaternary Peru-Chile margin phosphatic nodules, crusts and pelletal grains, and from associated dolomicritic concretions, are presented, which provide constraints on the timing and mechanisms of growth of these phases in organic carbon-rich sediments. Comparison of δ¹³C values from carbonate fluorapatite (CFA) nodules and pelletal grains (−4.8 to 0.0‰ and −2.9 to +1.0‰ PDB, respectively) with pore-water total dissolved δ¹³C values from these sediments suggests early authigenic CFA precipitation from pore waters within a few centimeters of the sediment-water interface in association with suboxic to perhaps anoxic microbial degradation of organic matter. In contrast, the dolomicritic cores of nodules recovered from about 12°S display both strongly negative to positive δ¹³C values (−10.8 to +6.1‰) characteristic of formation deeper in the sediments in association with methanogenic and perhaps sulfate reduction microbial processes.

The amount of structural carbonate in CFA suggests that carbonate substitution generally increases as δ¹³C in CFA decreases, a probable consequence of increasing carbonate and accompanying charge-balancing substitutions in the CFA lattice in response to increasing pore-water carbonate ion concentrations with depth below the sediment-water interface. In one buried upward-growing nodule, decreasing CFA δ¹³C and increasing structural CO₂ also correspond to decreasing CFA growth rates. These data suggest that in addition to other constraints such as pore-water phosphorus and fluoride availability, the lower limit of CFA precipitation in suboxic to anoxic sediments may be controlled by lattice poisoning due to excessive dissolved carbonate ion concentrations. In organic-rich Peru-Chile margin sediments this depth threshold appears to be at approximately 5–10 cm below the sediment-water interface where maximum CFA CO₂ contents of about 6 Wt.% occur; in less organic-rich settings, greater depths of precipitation of CFA may be anticipated. Below this relatively shallow depth of CFA precipitation on the Peru shelf, high pore-water alkalinity and associated elevated total dissolved carbon and carbonate ion concentration apparently favor the precipitation of authigenic carbonates.     

Commodifying poverty: gentrification and consumption in Vancouver’s Downtown Eastside

The Downtown Eastside of Vancouver, British Columbia, remains one of the poorest neighborhoods in Canada, yet is also a site of rapid gentrification. Both new and revitalized restaurants have created new spaces of consumption, transforming the neighborhood into a dining destination. Site visits and an analysis of the discourses used in newspaper articles and magazine features, as well as on blogs and in other online spaces, indicate that the presence of poor and marginalized residents of the Downtown Eastside is one of the reasons for some consumers’ decisions to visit new upscale establishments in the area. Analysis of advertisements and other primary documents indicates that the presence of poor and marginalized residents has become a competitive niche for the promotion of distinctive and authentic culinary adventures. This trend toward poverty tourism signals a shift from the simple displacement of low-income residents to a more complex form of gentrification in which residents face spatial management and control while their poverty is commodified.     

REGIONAL-SCALE TROUGHING OVER THE SOUTHWESTERN UNITED STATES: TEMPORAL CLIMATOLOGY,TELECONNECTIONS, AND CLIMATIC IMPACT

Regional-scale middle- and upper-tropospheric troughing over the southwestern United States represents a departure from the modal circulation pattern for western North America. Once developed, southwestern troughs often are associated with positive vorticity advection aloft, surface cyclone formation, and moisture advection over areas of the western Great Plains and Intermountain West. These trough systems may play an important role in the precipitation climatology of the western and central United States. However, very little work has focused on the temporal climatology, developmental characteristics, or climatic impacts of southwestern troughs. This study provides a detailed climatology of southwestern troughing that focuses on: (1) the temporal frequency of these events; (2) the teleconnective circulation changes that are associated with their development; and (3) the importance of these systems in the precipitation climatology of the western and central United States.

The temporal climatology of southwestern troughs reveals that most of these systems occur during spring and autumn, with somewhat fewer events in winter and very few events in summer. An examination of 500-mb geopotential height and 24-hr height change composites during trough development shows that much of the wave-train activity that accompanies trough onset is limited to the North Pacific and North American regions. These changes are characterized by the amplification and eastward movement of a ridge/trough couplet over the eastern Pacific, which is preceded by synoptic-scale transient wave activity over the western and central Pacific. While southwestern troughs occur less than 30% of the time, southwestern trough-derived precipitation comprises over 60% of the monthly totals for some sites.