Cation disorder in garnets along the Mg3Al2Si3O12-Mg4Si4O12 join: an infrared,Raman and NMR study

We have obtained infrared and Raman spectra for garnets synthesized at high (static) pressures and temperatures along the join Mg₃Al₂Si₃O₁₂ (pyrope) — Mg₄Si₄O₁₂ (magnesium majorite). The vibrational spectra of Mg-majorite show a large number of additional weak peaks compared with the spectra of cubic pyrope garnet, consistent with tetragonal symmetry for the MgSiO₃ garnet phase. The Raman bands for this phase show no evidence for line broadening, suggesting that Mg and Si are ordered on octahedral sites in the garnet. The bands for the intermediate garnet compositions are significantly broadened compared with the end-members pyrope and Mg-majorite, indicating cation disorder in the intermediate phases. Solid state ²⁷Al NMR spectroscopy for pyrope and two intermediate compositions show that Al is present only on octahedral sites, so the cation disorder is most likely confined to Mg-Al-Si mixing on the octahedral sites. We have also obtained a Raman spectrum for a natural, shock-produced (Fe,Mg) majorite garnet. The sharp Raman peaks suggest little or no cation disorder in this sample.     

Toward an improved global network for determination of tropospheric ozone climatology and trends

An examination of typical tropospheric ozone variability on daily, monthly, annual and interannual timescales and instrumental precision indicates that the current ozonesonde network is insufficient to detect a trend in tropospheric ozone of 1% per year at the 2 level even at stations with records a decade in length. From a trend prediction analysis we conclude that in order to detect a 1% per year trend in a decade or less it will be necessary to decrease the time between observations from its present value of 3–7 days to 1 day or less. The spatial distribution of the current ozonesonde stations is also inadequate for determining the global climatology of ozone. We present a quantitative theory taking into account photochemistry, surface deposition, and wind climatology to define the effectively sampled region for an observing station which, used in conjunction with the instrumental precision and the above prediction analysis, forms the basis for defining a suitable global network for determining regional and global ozone climatology and trends. At least a doubling of the present number of stations is necessary, and the oceans, most of Asia, Africa, and South America are areas where more stations are most needed. Differential absorption lidar ozone instruments have the potential for far more frequent measurements of ozone vertical profiles and hence potentially more accurate climatology and trend determinations than feasible with ozonesondes but may produce a (fair weather) biased data set above the cloud base. A strategy for cloudy regions in which either each station utilizes both lidars and sondes or each station is in fact a doublet comprised of a near-sea-level lidar and a proximal-mountain-top lidar could serve to minimize this bias.     

The Sensitivity of Four Monitoring Well Sampling Systems to Low Concentrations of Three Volatile Organics

Four state-of-the-art ground water sampling systems were analyzed to determine their reliability in providing representative samples of the volatile chlorinated hydrocarbons trichloroethylene (TCE), perchloroethylene (PCE), and 1,1,1-trichloroethane (TCA) from a simulated monitoring well. The sampling systems studied represent four commonly used devices, including a stainless steel and Teflon® piston pump, a Teflon bailer, a Teflon bladder pump, and a PVC air-lift pump.
Controlled laboratory sampling experiments were conducted in a tank and well test chamber designed to approximate field conditions. A well purging and sampling procedure was used in the test apparatus to determine the accuracy and precision of each device for detecting low concentrations of the compounds in ground water. The compounds selected are some of the most ubiquitous hazardous contaminants found in shallow aquifers near hazardous waste sites throughout the United States.
No significant statistical difference was found among the four sampling systems in detecting the compounds.     

Improvement of ERS-1 orbits using along-track accelerations from DORIS data on SPOT2

In long-arc precise orbit determinations of altimetric satellites such as ERS-1, large errors may occur from mismodelling of aerodynamic drag and solar radiation pressure. Such surface forces for non-spherical satellites require accurate modelling of the effective area and particle-surface interactions, but the dominant source of error is neutral air density as derived from thermospheric models for aerodynamic drag. Several techniques can be employed to alleviate air-drag mismodelling but all require the solution of additional parameters from the tracking data. However, for ERS-1 the sparsity of laser range data limits the application of such empirical techniques. To overcome this, use can be made of the dense DORIS Doppler tracking for SPOT2 which is in a similar orbit to ERS-1. A recent investigation by CNES examined the use of drag scale factors from SPOT2 to constrain the ERS-1 orbit. An improvement to that methodology is to consider along-track mismodelling as observed by timing errors in the Doppler data for each pass of SPOT2. The along-track correction to the acceleration as derived from SPOT2 can then be applied to ERS-1 orbits, solving for a scale factor to absorb systematic errors - particularly that arising from the 50 km altitude difference. Results are presented of the associated improvement in ERS-1 orbits as derived from concurrent SPOT2 arcs. It will be seen that the procedure not only improves the laser range fit, but more importantly, leads to more precise radial positioning as evident in the altimeter and crossover residuals.     

The reserves of extracted resources: Historical data

The available data on known reserves of extracted resources have not previously been tabulated and graphed. In this article we compile the reserves data in one place for the first time. This serves to (1) show the trends in the reserves series, (2) make the series available to others, and (3) place the reserves data in the context of price data for which longer series have been available, and with which they can now be seen to be consistent in both showing increasing availability rather than increasing scarcity.     

Simple analytical Green's functions for ray perturbations in layered media

The total Green's function for two-point boundary-value problems can be related to the propagator for initial-value problems. A very simple expression for the Green's function is obtained when the unperturbed medium may be described by material with a constant gradient in quadratic slowness. The derivation requires a correct understanding of assumptions made in the propagator solution. Expressions are also obtained for Green's function in multilayered media.     

Variability of dissolved organic carbon in sediments of a seagrass bed and an unvegetated area within an estuary in Southern Texas

Pore-water dissolved organic carbon (PWDOC) concentrations were examined in vegetated and bare sediments of aHalodule wrightii seagrass bed, and in a mud bottom sediment of a southern Texas estuary. Temporal variability was examined at diel (dawn and noon) and bimonthly time scales. Distribution patterns of PWDOC were compared with physical, chemical, and biological factors thought to exert control on PWDOC. Concentration of PWDOC, bacterial production, and resultant PWDOC turnover times displayed statistically significant spatial and temporal variability. Concentration of PWDOC ranged from 14 mg C 1^?1 to 107 mg C 1^?1 of pore water, or 9–71 μg C cm^?3 wet sediment. PWDOC was more variable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 μg C cm^?3) and high bacterial production rates (mean = 1.92 μg C cm^?3 h^?1) were observed at the mud station, whereas PWDOC concentrations were high (mean = 24.6 μg C cm^?3) and bacterial production rates were low (mean = 0.43 μg C cm^?3 h^?1) at the bare station. PWDOC turnover times (T_t), assuming 50% bacterial growth efficiency (1–840 h) were shortest at the mud station (mean=13 h) and longest at the bare station (mean=180 h). In the overlying water column, T_t values were longer, ranging from 1,000–10,000 h. PWDOC concentrations were 25% higher in vegetated sediments than in neighboring bare sediments. This difference was probably due to inputs of labile photosynthetic excretia, since bacterial production rates in vegetated sediments displayed significant diel variability and were 4 times greater than that of bare sediments. Based upon the entire data set, PWDOC was significantly related to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxygen metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that organism activities and detrital quality are the major determinants controlling variability in PWDOC.