Crustal thickness along the western Galápagos Spreading Center and the compensation of the Galápagos hotspot swell

Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Galápagos Spreading Center (GSC) between 97°W and 91°25′W thickens by 2.3 km as the Galápagos plume is approached from the west. This crustal thickening can account for ∼52% of the 700 m amplitude of the Galápagos swell. After correcting for changes in crustal thickness, the residual mantle Bouguer gravity anomaly associated with the Galápagos swell shows a minimum of −25 mGal near 92°15′W, the area where the GSC is intersected by the Wolf-Darwin volcanic lineament (WDL). The remaining depth and gravity anomalies indicate an eastward reduction of mantle density, estimated to be most prominent above a compensation depth of 50-100 km. Melting calculations assuming adiabatic, passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of ∼30°C. The associated thermal expansion and increase in melt depletion reduce mantle densities, but to a degree that is insufficient to explain the geophysical observations. The largest density anomalies appear at the intersection of the GSC and the WDL. Our results therefore require the existence of compositionally buoyant mantle beneath the GSC near the Galápagos plume. Possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the Galápagos Islands that is later transported to the GSC. Our estimate for the buoyancy flux of the Galápagos plume (700 kg s⁻¹) is lower than previous estimates, while the total crustal production rate of the Galápagos plume (5.5 m³s⁻¹) is comparable to that of the Icelandic and Hawaiian plumes.     

Evaluation of the Johnson and Ettinger Model for Prediction of Indoor Air Quality

Screening level models are now commonly used to estimate vapor intrusion for subsurface volatile organic compounds (VQCs). Significant uncertainty is associated with processes and models and, to date, there has been only limited field-based evaluation of models for this pathway. To address these limitations, a comprehensive evaluation of the Johnson and Ettinger (J&E) model is provided through sensitivity analysis, comparisons of model-predicted to measured vapor intrusion for 11 petroleum hydrocarbon and chlorinated solvent sites, and review of radon and flux chamber studies. Significant intrusion was measured at five of 12 sites with measured vapor attenuation ratios (α_m's) (indoor air/source vapor) ranging from ∼1 × 10⁻⁶ to 1 × 10⁻⁴. Higher attenuation ratios were measured for studies using radon, inert tracers, and flux chambers; however, these ratios are conservative owing to boundary conditions and tracer properties that are different than those at most VOC-contaminated sites. Reasonable predictions were obtained using the J&E model with comparisons indicating that model-predicted vapor attenuation ratios (α_p's) were on the same order, or less than the α_m's. For several sites, the α_m were approximately two orders of magnitude less than the α_p's indicating that the J&E model is conservative in these cases. The model comparisons highlight the importance in using appropriate input parameters for the J&E model. The regulatory implications associated with use of the J&E model to derive screening criteria are also discussed.     

From berries to blocks: carbon stock quantification of a California vineyard

Background

Quantifying terrestrial carbon (C) stocks in vineyards represents an important opportunity for estimating C sequestration in perennial cropping systems. Considering 7.2 M ha are dedicated to winegrape production globally, the potential for annual C capture and storage in this crop is of interest to mitigate greenhouse gas emissions. In this study, we used destructive sampling to measure C stocks in the woody biomass of 15-year-old Cabernet Sauvignon vines from a vineyard in California’s northern San Joaquin Valley. We characterize C stocks in terms of allometric variation between biomass fractions of roots, aboveground wood, canes, leaves and fruits, and then test correlations between easy-to-measure variables such as trunk diameter, pruning weights and harvest weight to vine biomass fractions. Carbon stocks at the vineyard block scale were validated from biomass mounds generated during vineyard removal.

Results

Total vine C was estimated at 12.3 Mg C ha^?1, of which 8.9 Mg C ha^?1 came from perennial vine biomass. Annual biomass was estimated at 1.7 Mg C ha^?1 from leaves and canes and 1.7 Mg C ha^?1 from fruit. Strong, positive correlations were found between the diameter of the trunk and overall woody C stocks (R² = 0.85), pruning weights and leaf and fruit C stocks (R² = 0.93), and between fruit weight and annual C stocks (R² = 0.96).

Conclusions

Vineyard C partitioning obtained in this study provides detailed C storage estimations in order to understand the spatial and temporal distribution of winegrape C. Allometric equations based on simple and practical biomass and biometric measurements could enable winegrape growers to more easily estimate existing and future C stocks by scaling up from berries and vines to vineyard blocks.

     

Continental magnetic anomalies and the evolution of the Scotia arc

Linear belts 50–100 km in width of long-wavelength positive magnetic anomalies exceeding 500 nT are observed on continental blocks of the Scotia arc. The most developed is the West Coast Magnetic Anomaly which may be traced for more than 1300 km along the Antarctic Peninsula. Comparison of magnetic profile data after low-pass filtering and reduction to pole reveals a striking similarity between the individual anomaly belts. Correlation of the anomalies with positive gravity anomalies, seismic refraction data and geology indicates that the sources are linear batholiths intruded during Mesozoic/Cenozoic subduction. The anomaly belts are truncated at the block margins reflecting Cenozoic fragmentation of a cuspate convergent margin. An early Cenozoic reconstruction, based on the assumption that the batholiths once formed a continuous curvilinear feature, shows a good alignment of Mesozoic strato-tectonic terrains and is compatible with the known history of Scotia Sea opening.     

Lithophile elements and exploration using centre-lake bottom sediments from the East Kemptville area, Southern Nova Scotia, Canada

The East Kemptville Sn deposit in southwestern Nova Scotia was discovered in 1978 by Shell Canada Resources, Limited using regional till exploration procedures. In 1977 and 1978, southern Nova Scotia was covered by a government-funded regional centre-lake bottom sediment survey (1 site per 5 km²) for Cu-Pb-Zn-Ag-Ni-Co-Fe-Mn-Ca-Mg-Mo-Hg-As-U-LOI. Detailed lithogeochemical studies on drill core from East Kemptville have demonstrated significant roles for the elements Zn-Cu-Th-Li-F-Sn-Cl-Rb in the hydrothermal mineralization event. Most of these significant mineralizing elements were not analyzed as part of the original lake sediment survey.Later, in 1985, a subset of 55 lake sediment samples from the deposit area was analyzed for Sn, Rb, F and Cl. The pilot study was aimed to test the applicability of these lithophile elements to regional centre-lake bottom sediment geochemical programs. Anomalous values of F, Rb and Sn form a distinct halo of elevated levels southwest, southeast and northeast of the East Kemptville deposit, which appears to lie in an area of background values. The detailed till sampling undertaken by Shell Canada at East Kemptville provides a check, especially for Sn as to probable sources for these elements found in the centre-lake bottom sediments.Cassiterite is the main economic mineral at East Kemptville. Trace wolframite is found along with subsidiary chalcopyrite and sphalerite. Zn and Cu are recovered as by-products at the mine. Heavy minerals were separated from a bulk lake sediment sample from Moosefly Lake (the site of the highest Sn value from the regional survey subset). Spot analysis by scanning electron microscope indicates the presence of angular and discrete grains of cassiterite in the centre-lake bottom sediment, in addition to numerous grains of zircon, monazite and magnetite.The Sn anomaly in the centre-lake bottom sediment is due, at least in part, to the presence of discrete cassiterite grains. Their lack of abrasion would seem to indicate derivation from the nearby known cassiterite-rich tills. The lack of correlation of Sn data with organic material (as LOI) and the other elements probably indicates the dominance of mechanical over chemical dispersion processes in Sn distribution from the tills to the lakes.A common dispersion model for centre-bottom lake sediment surveys is one of initial glacial clastic dispersion, followed by subsequent hydromorphic dispersion into the lake basins. Although the dominance of hydromorphic processes is recognized, it is clear that clastic dispersion models must also be considered. Centre-lake bottom surveys appear to have greater application than previously presumed in the search for lithophile elements commonly dispersed as refractory grains.     

Internal wave breaking and microstructure (The chicken and the egg)

This review emphasizes the space-time scales of internal waves in the sea, and the complex set of interrelations with microstructure. M ixing by breaking internal waves is briefly considered. A curious asymmetry is noted between the methods of observing microstructure in the oceans and atmosphere.[/p]     

Happy Jack Uraninite: A New Reference Material for High Spatial Resolution Analysis of U‐Rich Matrices

There is currently a lack of well‐characterised matrix‐matched reference materials (RMs) for forensic analysis of U‐rich materials at high spatial resolution. This study reports a detailed characterisation of uraninite (nominally UO_2+x) from the Happy Jack Mine (UT, USA). The Happy Jack uraninite can be used as a RM for the determination of rare earth element (REE) mass fractions in nuclear materials, which provide critical information for source attribution purposes. This investigation includes powder X‐ray diffraction (pXRD) data, as well as major, minor and trace element abundances determined using a variety of micro‐analytical techniques. The chemical signature of the uraninite was investigated at the macro (cm)‐scale with micro‐X‐ray fluorescence (µXRF) mapping and at high spatial resolution (tens of micrometre scale) using electron probe microanalysis (EPMA) and laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analyses. Based on EPMA results, the uraninite is characterised by homogeneous UO₂ and CaO contents of 91.57 ± 1.49% m/m (2s uncertainty) and 2.70 ± 0.38% m/m (2s), respectively. Therefore, CaO abundances were used as the internal standard when conducting LA‐ICP‐MS analyses. Overall, the major element and REE compositions are homogeneous at both the centimetre and micrometre scales, allowing this material to be used as a RM for high spatial resolution analysis of U‐rich samples.     

Changes in the angular separation of the lensed images PKS 1830-211 NE & SW

We present 8 epochs of 7 mm dual-polarization VLBA observations of the gravitational lens system PKS 1830-211 made over the course of 14 weeks. Clear changes in the relative positions of the cores of up to 80 μarcseconds (μas) were observed between epochs (each separated by ≈2 weeks). A comparison with previous 7 mm VLBA maps shows that the separation of the cores has changed by almost 280 μas over 12 months. This leads us to conclude that changes in the brightness distribution of the mm-VLBI core of the background source must be occurring rapidly. This is the first clear observation of significant radio source evolution in a gravitational lens system. It is also the first time that changes in source structure have been detected in a distant extra-galactic source on such short time-scales. This is partly accounted for by the magnification provided by the lens system.