Elasticity of tetragonal end-member majorite and solid solutions in the system Mg4Si4O12-Mg3Al2Si3O12

 The adiabatic elastic moduli of polycrystalline En₅₀Py₅₀ and En₈₀Py₂₀ majorite-garnet solid solutions (where En=4MgSiO₃, Py=Mg₃Al₂Si₃O₁₂) and the end-member En₁₀₀ tetragonal majorite were determined at ambient conditions using Brillouin spectroscopy. The adiabatic bulk modulus, K, and shear modulus, μ, of En₅₀Py₅₀ were found to be K=173.1 (20) and μ=92.3 (10) GPa, and are indistinguishable from those of the end-member pyrope, Py₁₀₀. The moduli of the more majorite-rich samples are significantly lower and are virtually identical to each other (K=162.6(11) and μ=85.7(7) for En₈₀Py₂₀; K=166(5) and μ=88(2) for En₁₀₀). In combination with previously reported moduli for this system, we conclude that both K and μ are constant over the compositional range from Py₁₀₀ to a majorite content of about 70–80%, whereupon the moduli decrease substantially. For compositions ranging from En₈₀Py₂₀ to the end-member majorite, the moduli are also approximately independent of composition, but at a lower value. An alternative model with a continuous decrease in moduli with increasing majorite content cannot be excluded, within the uncertainties of existing measurements. The contrast in moduli between aluminous pyrope garnet and Al-free majorite are small compared with the modulus changes accompanying the pyroxene – majorite phase transformation. Received August 16, 1995 /Revised, accepted January 12, 1996     

Tuning a physically-based model of the air–sea gas transfer velocity

Air–sea gas transfer velocities are estimated for one year using a 1-D upper-ocean model (GOTM) and a modified version of the NOAA–COARE transfer velocity parameterization. Tuning parameters are evaluated with the aim of bringing the physically based NOAA–COARE parameterization in line with current estimates, based on simple wind-speed dependent models derived from bomb-radiocarbon inventories and deliberate tracer release experiments. We suggest that A = 1.3 and B = 1.0, for the sub-layer scaling parameter and the bubble mediated exchange, respectively, are consistent with the global average CO₂ transfer velocity k. Using these parameters and a simple 2nd order polynomial approximation, with respect to wind speed, we estimate a global annual average k for CO₂ of 16.4 ± 5.6 cm h^?1 when using global mean winds of 6.89 m s^?1 from the NCEP/NCAR Reanalysis 1 1954–2000. The tuned model can be used to predict the transfer velocity of any gas, with appropriate treatment of the dependence on molecular properties including the strong solubility dependence of bubble-mediated transfer. For example, an initial estimate of the global average transfer velocity of DMS (a relatively soluble gas) is only 11.9 cm h^?1 whilst for less soluble methane the estimate is 18.0 cm h^?1.     

Shallow seismicity, stress distribution and crustal deformation pattern in the Andaman-West Sunda arc and Andaman Sea, northeastern Indian Ocean

Shallow seismicity and available source mechanisms in the Andaman–westSunda arc and Andaman sea region suggest distinct variation in stressdistribution pattern both along and across the arc in the overriding plate.Seismotectonic regionalisation indicates that the region could be dividedinto eight broad seismogenic sources of relatively homogeneousdeformation. Crustal deformation rates have been determined for each oneof these sources based on the summation of moment tensors. The analysisshowed that the entire fore arc region is dominated by compressive stresseswith compression in a mean direction of N23^°, and the rates ofseismic deformation velocities in this belt decrease northward from 5.2± 0.65 mm/yr near Nias island off Sumatra and 1.12 ±0.13 mm/yr near Great Nicobar islands to as much as 0.4 ±0.04 mm/yr north of 8^°N along Andaman–Nicobar islandsregion. The deformation velocities indicate, extension of 0.83 ±0.05 mm/yr along N343^° and compression of 0.19 ±0.01 mm/yr along N73^° in the Andaman back arc spreadingregion, extension of 0.18 ± 0.01 mm/yr along N125^° andcompression of 0.16 ± 0.01 mm/yr along N35^° in NicobarDeep and west Andaman fault zone, compression of 0.84 ±0.12 mm/yr N341^° and extension of 0.77 ± 0.11 mm/yralong N72^° within the transverse tectonic zone in the Andamantrench, N-S compression of 3.19 ± 0.29 mm/yr and an E-Wextension of 1.24 ± 0.11 mm/yr in the Semangko fault zone ofnorth Sumatra. The vertical deformation suggests crustal thinning in theAndaman sea and crustal thickening in the fore arc and Semangko faultzones. The apparent stresses calculated for all major events range between0.1–10 bars and the values increase with increasing seismic moment.However, the apparent stress estimates neither indicate any significantvariation with faulting type nor display any variation across the arc, incontrast to the general observation that the fore arc thrust events showhigher stress levels in the shallow subduction zones. It is inferred that theoblique plate convergence, partial subduction of 90^°E Ridge innorth below the Andaman trench and the active back arc spreading are themain contributing factors for the observed stress field within the overridingplate in this region.     

Understanding the Role of Mass-Unloading in a Filament Eruption

We describe a partial filament eruption on 11 December 2011 that demonstrates that the inclusion of mass is an important next step for understanding solar eruptions. Observations from the Solar Terrestrial Relations Observatory-Behind (STEREO-B) and the Solar Dynamics Observatory (SDO) spacecraft were used to remove line-of-sight projection effects in filament motion and correlate the effect of plasma dynamics with the evolution of the filament height. Flux cancellation and nearby flux emergence are shown to have played a role in increasing the height of the filament prior to eruption. The two viewpoints allow the quantitative estimation of a large mass-unloading, the subsequent radial expansion, and the eruption of the filament to be investigated. A 1.8 to 4.1 lower-limit ratio between gravitational and magnetic-tension forces was found. We therefore conclude that following the loss-of-equilibrium of the flux-rope, the radial expansion of the flux-rope was restrained by the filamentary material until 70% of the mass had evacuated the structure through mass-unloading.     

A solution for the northern hemisphere climatic zonation during a glacial maximum

The same model previously used to deduce an acceptable first order picture of the present zonally averaged macroclimate is now solved for the climatic response to the “glacial” surface boundary conditions that prevailed at 18,000 BP in the northern hemisphere. The equilibrium solution obtained gives the distributions with latitude of the mean temperature, wind, humidity, precipitation, evaporation, heat balance, transient baroclinic eddy statistics (i.e., kinetic energy of the meridional wind and meridional flux of heat, momentum, and water vapor), and the energy integrals. In general terms, the solution shows the glacial atmosphere to be colder and drier than at present, with an intensified polar front, stronger mean zonal and poloidal winds, more intense transient baroclinic eddies (storms) transporting heat, momentum and water vapor poleward at higher rates, and reduced precipitation and evaporation. Also evident is an equatorward shift of the climatic zones (as delineated by the mean surface zonal winds, the poloidal motion, and the difference between mean evaporation and precipitation), particularly in higher latitudes. Other properties of the solution, such as the effect of zonal wind changes on the length of the day, are discussed.     

Recent precipitation trends over the polar ice sheets

Summary Meteorological and glaciological analyses are integrated to examine the precipitation trends during the last three decades over the ice sheets covering Antarctica and Greenland. For Antarctica, the best data source is provided by glaciologically-measured trends of snow accumulation, and for limited sectors of East Antarctica consistency with precipitation amounts calculated from the atmospheric water balance equation is obtained. For Greenland, precipitation rates parameterized from atmospheric analyses yield the only comprehensive depiction. The precipitation rate over Antarctica appears to have increased by about 5% over a time period spanning the accumulation means for the 1955–65 to 1965–75 periods, while over Greenland it has decreased by about 15% since 1983 with a secondary increase over the southern part of the ice sheet starting in 1977. At the end of the 10-year overlapping period, the global sea-level impact of the precipitation changes over Antarctica dominates that for Greenland and yields a net ice-sheet precipitation contribution of roughly 0.02 mm yr^–1. These changes are likely due to marked variations in the cyclonic forcing affecting the ice sheets, but are only weakly reflected in the temperature regime, consistent with the episodic nature of cyclonic precipitation. These conclusions are not founded on high quality data bases. The importance of such changes for understanding global sea-level variations argues for a modest research effort to collect simultaneous meteorological and glaciological observations in order to describe and understand the current precipitation variations over both ice sheets. Some suggestions are offered for steps that could be taken.With 8 Figures     

Late Quaternary marine terraces in the Mediterranean coastal area of Syria: Geochronology and neotectonics

In order to provide new data on the neotectonics and geodynamic properties of western Syria, studies of marine terraces have been carried out. The most attention was paid to the lower terraces in the range of 3–5 to 30–35 m above sea level, because they have more complete distributions along the shore. The lower terraces were examined along the coastal area from Tartus to Latakia, and along the carbonate cliff on Arwad Island. Seven ²³⁰Th/U dates for these terraces are in the range of 85–130 ka, suggesting the age interval of the last interglacial (MIS 5). New dates on the lower terraces provide a basis for stratigraphical and geomorphological interpretation as well as neotectonic reconstruction. According to the geomorphological data and lithological composition of those terraces, two main uplifted blocks can be established. One coincides with the Latakia block, and another corresponds to the western margin of the Banias volcanic plateau. These blocks are divided by a subsided structure corresponding to the Nahr el Kebir graben. The amplitude of neotectonic uplifting in the Latakia and Banias blocks reaches 15–20 m for the Late Pleistocene.     

Resistive wave breaking in the Earth's outer core

The equations for an electrically conducting fluid in cylindrical coordinates are linearized assuming that the inertial terms in the momentum equation can be ignored (small Rossby number), and that the ratio of the Elsasser number and magnetic Reynolds number is one. After these assumptions, the governing equations are linearized about an ambient solution which vanishes at the the equator. Upon assuming large Elsasser and magnetic Reynolds number, the solutions to the linearized equations are approximated by wave trains having very short wave length (relative to the core radius) but which vary slowly (on a scale of the core radius). The period of the waves is much longer than a day but much shorter than the period of the slow hydromagnetic oscillations. These waves are found to be trapped in a region about the equator and away from the axis of rotation. The waves break at a latitudinal wave region boundary, in the sense that the waves become exponentially large in a boundary layer, having as an exponent some positive power of the large azimuthal wave number. This behavior is amplified as the Elsasser number becomes smaller while still remaining relatively large. Waves in more Earth-like parameter regimes are discussed briefly.     

Origin and stratigraphy of phreatomagmatic deposits at the Pleistocene Sinker Butte Volcano, Western Snake River Plain, Idaho

Sinker Butte is the erosional remnant of a very large basaltic tuff cone of middle Pleistocene age located at the southern edge of the western Snake River Plain. Phreatomagmatic tephras are exposed in complete sections up to 100 m thick in the walls of the Snake River Canyon, creating an unusual opportunity to study the deposits produced by this volcano through its entire sequence of explosive eruptions. The main objectives of the study were to determine the overall evolution of the Sinker Butte volcano while focusing particularly on the tephras produced by its phreatomagmatic eruptions. Toward this end, twenty-three detailed stratigraphic sections ranging from 20 to 100 m thick were examined and measured in canyon walls exposing tephras deposited around 180° of the circumference of the volcano.Three main rock units are recognized in canyon walls at Sinker Butte: a lower sequence composed of numerous thin basaltic lava flows, an intermediate sequence of phreatomagmatic tephras, and a capping sequence of welded basaltic spatter and more lava flows. We subdivide the phreatomagmatic deposits into two main parts, a series of reworked, mostly subaqueously deposited tephras and a more voluminous sequence of overlying subaerial surge and fall deposits. Most of the reworked deposits are gray in color and exhibit features such as channel scour and fill, planar-stratification, high and low angle cross-stratification, trough cross-stratification, and Bouma-turbidite sequences consistent with their being deposited in shallow standing water or in braided streams. The overlying subaerial deposits are commonly brown or orange in color due to palagonitization. They display a wide variety of bedding types and sedimentary structures consistent with deposition by base surges, wet to dry pyroclastic fall events, and water saturated debris flows.Proximal sections through the subaerial tephras exhibit large regressive cross-strata, planar bedding, and bomb sags suggesting deposition by wet base surges and tephra fallout. Medial and distal deposits consist of a thick sequence of well-bedded tephras; however, the cross-stratified base-surge deposits are thinner and interbedded within the fallout deposits. The average wavelength and amplitude of the cross strata continue to decrease with distance from the vent. These bedded surge and fall deposits grade upward into dominantly fall deposits containing 75–95% juvenile vesiculated clasts and localized layers of welded spatter, indicating a greatly reduced water-melt ratio. Overlying these “dryer” deposits are massive tuff breccias that were probably deposited as water saturated debris flows (lahars). The first appearance of rounded river gravels in these massive tuff breccias indicates downward coring of the diatreme and entrainment of country rock from lower in the stratigraphic section. The “wetter” nature of these deposits suggests a renewed source of external water. The massive deposits grade upward into wet fallout tephras and the phreatomagmatic sequence ends with a dry scoria fall deposit overlain by welded spatter and lava flows.Field observations and two new ⁴⁰Ar–³⁹Ar incremental heating dates suggest the succession of lavas and tephra deposits exposed in this part of the Snake River canyon may all have been erupted from a closely related complex of vents at Sinker Butte. We propose that initial eruptions of lava flows built a small shield edifice that dammed or disrupted the flow of the ancestral Snake River. The shift from effusive to explosive eruptions occurred when the surface water or rising ground water gained access to the vent. As the river cut a new channel around the lava dam, water levels dropped and the volcano returned to an effusive style of eruption.