Elasticity of polycrystalline stishovite

Ultrasonic data for the velocities of SiO₂-stishovite have been determined as a function of pressure to 10 kbar at room temperature for polycrystalline specimens hot-pressed at pressures P = 120kbar and temperatures T = 900°C. These cylindrical specimens are 2 mm in diameter and 0.9–1.4 mm long and have a grain size less than 10 μm. Compressional and shear wave velocities were measured both parallel and perpendicular to the axis of pressing and were found to be isotropic at 10 kbar with ν_p = 11.0 ± 0.2km/sec andν_s = 6.9 ± 0.3km/sec; this shear velocit is substantially higher than that of Mizutani et al. (1972) perhaps due to the presence of crack orientations in their specimen which affected ν_s but not ν_p. The Murnaghan P-V trajectories calculated from the ultrasonic data [bulk modulus K_s = 2.5 ± 0.3Mbar and assuming (?K_s/?P)_T = 6 ± 2] are consistent with recent hydrostatic compression data and with the shock wave compression data above 600 kbar. The combined evidence from the data of the ultrasonic and hydrostatic compression techniques suggests that the most probable value of the bulk modulus of stishovite at zero pressure is close to the upper limit of the uncertainty of our ultrasonically determined value, K₀ = 2.7?2.8Mbar. Elasticity data for rutile-type oxides are not compatible with normal K_s-V₀ systematics perhaps due to the neglect of non-central forces in the lattice model. These new stishovite data would make it impossible to satisfy the elasticity-density data of the lower mantle using an oxide mixture with either olivine or pyroxene stoichiometry.     

Leveraging modern climatology to increase adaptive capacity across protected area networks

Human-driven changes in the global environment pose an increasingly urgent challenge for the management of ecosystems that is made all the more difficult by the uncertain future of both environmental conditions and ecological responses. Land managers need strategies to increase regional adaptive capacity, but relevant and rapid assessment approaches are lacking. To address this need, we developed a method to assess regional protected area networks across biophysically important climatic gradients often linked to biodiversity and ecosystem function. We plot the land of the southwestern United States across axes of historical climate space, and identify landscapes that may serve as strategic additions to current protected area portfolios. Considering climate space is straightforward, and it can be applied using a variety of relevant climate parameters across differing levels of land protection status. The resulting maps identify lands that are climatically distinct from existing protected areas, and may be utilized in combination with other ecological and socio-economic information essential to collaborative landscape-scale decision-making. Alongside other strategies intended to protect species of special concern, natural resources, and other ecosystem services, the methods presented herein provide another important hedging strategy intended to increase the adaptive capacity of protected area networks.     

Mixed layer depth front and subduction of low potential vorticity water under seasonal forcings in an idealized OGCM

The mixed layer depth (MLD) front and subduction under seasonal variability are investigated using an idealized ocean general circulation model (OGCM) with simple seasonal forcings. A sharp MLD front develops and subduction occurs at the front from late winter to early spring. The position of the MLD front agrees with the curve where \({\rm D}T_{\rm s}/{\rm D}t = \partial T_{\rm s} /\partial t + {\user2{u}}_{\rm g} \cdot \nabla T_{\rm s} = 0\) is satisfied (t is time, \({\user2{u}}_{\rm g}\) is the upper-ocean geostrophic velocity, \(T_{\rm s}\) is the sea surface temperature (SST), and \(\nabla\) is the horizontal gradient operator), indicating that thick mixed-layer water is subducted there parallel to the SST contour. This is a generalization of the past result that the MLD front coincides with the curve \({\user2{u}}_{\rm g} \cdot \nabla T_{\rm s} = 0\) when the forcing is steady. Irreversible subduction at the MLD front is limited to about 1 month, where the beginning of the irreversible subduction period agrees with the first coincidence of the MLD front and \({\rm D}T_{\rm s}/{\rm D}t =0\) in late winter, and the end of the period roughly corresponds to the disappearance of the MLD front in early spring. Subduction volume at the MLD front during this period is similar to that during 1 year in the steady-forcing model. Since the cooling of the deep mixed-layer water occurs only in winter and SST can not fully catch up with the seasonally varying reference temperature of restoring, the cooling rate of SST is reduced and the zonal gradient of the SST in the northwestern subtropical gyre is a little altered in the seasonal-forcing case. These effects result in slightly lower densities of subducted water and the eastward shift of the MLD front.     

Hydrocarbons in bivalve mollusks of Port Valdez, Alaska: Consequences of five years' permitted discharge

Petroleum hydrocarbons were investigated in Mytilus edulis and Macoma balthica at six intertidal sites in Port Valdez, Alaska after three to five years of oil terminal operation. Information about the concentrations of total hydrocarbons, unresolved complex mixture, normal alkanes, isoprenoids, olefins, hopanes and polycyclic aromatic hyrocarbons shows that petroleum accumulation in these bivalve mollusks is greatest at the Valdez municipal boat harbor and a crude oil terminal which has been permitted to discharge 170 kg oil day⁻¹. Accumulation two- to ten-fold less were observed at 3 km from the terminal and boat harbor. The temporal trends in petroleum hydrocarbon concentrations in M. edulis and M. balthica reflect the effects of selective retention and depuration of different hydrocarbon classes as well as variation in hydrocarbon sources. It is not clear whether the petroleum concentrations in Port Valdez bivalves had reached steady state by 1982, the final sampling time reported here.     

Carbon Isotope Geochemistry of the Orinoco Basin

Stable carbon isotope ratios have been used to study the sources of particulate organic carbon (POC) and total dissolved inorganic carbon in the Orinoco Basin. The isotopic composition of total dissolved inorganic carbon shows a range of from -8·1 to -23·0 ppt, an indication of dominance of biological processes. The isotopic composition of POC exhibits a range of from -24·1 to -34·6 ppt with little seasonal variation. The isotopic evidence indicates that the POC is predominantly of terrestrial origin rather than a result of in situ planktonic production. The similarity of isotopic composition of POC and coastal sediments suggests that riverine organic detritus has been transported 30-50 km offshore in a direction parallel to the Orinoco river channel.     

Modelling the sustainability and productivity of pastoral systems in the communal areas of Namaqualand

We investigated the effects of rainfall and the number of animals on changes in vegetation and on the output of milk and meat from the communal areas of Namaqualand. Previously published short- and long-term models link processes that range from the levels of tissue (in, for example, the mammary gland), to the milk yields of individual animals, to the growth and survival of their young and to long-term changes in plant species populations at the ecosystem level. These models have been used to study how different factors and management strategies affect livestock productivity and vegetation composition on a 20,000 ha rangeland in Namaqualand. First, the inter-relations between rainfall, stocking rate and productivity were studied using the short-term model. This model shows that in addition to total rainfall and stocking rate, the timing of rainfall within a year also influences doe live weight and survival to the end of the year. When the long-term model is run, using recorded rainfall, predictions of small stock numbers agree closely with livestock data recorded over the same 30-year period. One thousand replicates of 100-year runs of the long-term model were then used to study the probable impact of different upper limits to stock numbers on animal performance. Off take (sales and slaughterings) are maximal when stock numbers are limited to 2000 adults. Animal numbers increase marginally as the limit is increased above this level, but the variability between years in numbers increases. Secondly, the long-term model was used to study the long-term effects of the stocking rate strategies on rangeland condition. The model predicts that although these effects are variable, when moderately degraded range is stocked with an upper limit at the recommended level it is unable to recover to less degraded states over 100 years. Thirdly, the model was used to examine the effects of reduction in stock numbers and slaughtering of kids in a drought year on goat numbers during the subsequent 5 years. Finally, the model predicts that a 10% reduction in mean annual rainfall will lead to a 35% reduction in animal numbers over 200 years.     

Bioconcentration factors for petroleum hydrocarbons, PAHs, LABs and biogenic hydrocarbons in the blue mussel

Bioconcentration factors (K_bc) for petroleum hydrocarbons, PAHs, LABs and biogenic hydrocarbons in Mytilus edulis were measured in field experiments using time-integrating water samplers. Seven deployments at five sites gave lipid weight K_bcs for total hydrocarbons ranging from 0.99 × 10⁶ to 3.1 × 10⁶ (mean 1.6 × 10⁶)—a narrower range than has been obtained previously. Bioconcentration factors for the PAHs were similar to those for total hydrocarbons where the major hydrocarbon source was oil. However, at other sites the factors for PAHs were an order of magnitude lower than those for petroleum and for hydrocarbons originating from algae. Compositional profiles for the linear alkyl benzenes (LABs) suggested that these compounds were assimilated primarily from the dissolved phase, despite their greater abundance on particles.     

Was the M<Subscript>w</Subscript> 7.5 1952 Kern County,California, earthquake induced (or triggered)?

Several recent studies have presented evidence that significant induced earthquakes occurred in a number of oil-producing regions during the early and mid-twentieth century related to either production or wastewater injection. We consider whether the 21 July 1952 M_w 7.5 Kern County earthquake might have been induced by production in the Wheeler Ridge oil field. The mainshock, which was not preceded by any significant foreshocks, occurred 98 days after the initial production of oil in Eocene strata at depths reaching 3 km, within ~1 km of the White Wolf fault (WWF). Based on this spatial and temporal proximity, we explore a potential causal relationship between the earthquake and oil production. While production would have normally be expected to have reduced pore pressure, inhibiting failure on the WWF, we present an analytical model based on industry stratigraphic data and best estimates of parameters whereby an impermeable splay fault adjacent to the main WWF could plausibly have blocked direct pore pressure effects, allowing the poroelastic stress change associated with production to destabilize the WWF, promoting initial failure. This proof-of-concept model can also account for the 98-day delay between the onset of production and the earthquake. While the earthquake clearly released stored tectonic stress, any initial perturbation on or near a major fault system can trigger a larger rupture. Our proposed mechanism provides an explanation for why significant earthquakes are not commonly induced by production in proximity to major faults.     

Analysis of Photoelectric Occultations and Development of a Digital Model of the Lunar Libration Zone

Astronomy Reports - One of the priority tasks of modern astronomy is the observation and study of transient celestial processes, which also concerns photoelectric observations of lunar occultations...