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Determination of Antarctic Ice Sheet stability over the last ∼500 ka through a study of iceberg-rafted debris
No full textWe have analyzed ice‐rafted debris (IRD) from the South Atlantic Ocean (∼43°S, 9°E) in order to investigate Antarctic Ice Sheet history during the late Pleistocene; the cores examined for this study include piston core TN057‐6‐PC4 and Ocean Drilling Program Leg 177 drill core Site 1090 (177‐1090). Over the last 500 ka at this distal location, IRD arrived during both glacials and interglacials. IRD is present even during warmer intervals, is greatest during colder intervals, and is absent only during terminations and a few other brief intervals. Four different methods are used to normalize the IRD counts, which are then compared to support our interpretation. Several other high‐quality climate proxies from this location also aid our interpretations. We conclude that sea surface temperatures are the primary control on the delivery of IRD to this site. During cold times more icebergs survived to reach this distal location. During warm times only a few of the largest icebergs could travel this far. Garnets found in these sediments suggest a likely East Antarctic origin for the IRD; the presence of garnets even during warm intervals further strongly supports that the iceberg source must be the East Antarctic Ice Sheet (EAIS). Therefore, the EAIS must have continued to reach the ocean at least in some part of its margin throughout the last 500 ka. On the other hand, we cannot specifically trace any IRD to the West Antarctic Ice Sheet (WAIS), so WAIS persistence cannot be tested. A particular radiolarian, identified as Dictyocoryne profunda (Ehrenberg) (sensu Boltovskoy (1998)), shows up in the examined size fraction generally only during warm phases. We suggest that D. profunda is a sensitive indicator of warm water temperatures and that it deserves further study
Uncharted seismic risk
No full textThe potential for earthquakes along the plate boundaries has been mapped with reasonable success. Our attention should now focus on the threat posed by unanticipated quakes located in the continental interiors.
The 11 March 2011, Mw 9 Tōhoku earthquake contains a remarkable story of resilience. Neither the continuing hazard posed by the after-effects of the shaking and tsunami nor criticism of the methods for long-term forecasting of earthquakes in Japan1 should blind us to the fact that the death rate in this disaster was impressively low. The earthquake exposed more than six million people to shaking of magnitude VIII or greater on the Mercalli intensity scale. Of those, approximately 25,000, or about 0.4%, died or are still missing, mostly as a result of the tsunami. In striking contrast, death rates in earthquakes within continental interiors have often exceeded 5%, and can be as high as 30%. We argue that the knowledge that underpins earthquake resilience in Japan or California must be transferred to countries in which earthquakes regularly inflict huge death tolls, often far from the media spotlight, as discussed at a workshop on earthquake vulnerability2 in Oxford in January 2011
Multivesicular bodies in the enigmatic amoeboflagellate Breviata anathema and the evolution of ESCRT
No full textEndosomal sorting complexes required for transport (ESCRTs) are heteromeric protein complexes required for multivesicular body (MVB) morphogenesis. ESCRTs I, II, III and III-associated are ubiquitous in eukaryotes and presumably ancient in origin. ESCRT 0 recruits cargo to the MVB and appears to be opisthokont-specific, bringing into question aspects of the current model of ESCRT mechanism. One caveat to the restricted distribution of ESCRT 0 was the previous limited availability of amoebozoan genomes, the supergroup closest to opisthokonts. Here, we significantly expand the sampling of ESCRTs in Amoebozoa. Our electron micrographic and bioinformatics evidence confirm the presence of MVBs in the amoeboflagellate Breviata anathema. Searches of genomic databases of amoebozoans confirm the ubiquitous nature of ESCRTs I-III-associated and the restriction of ESCRT 0 to opisthokonts. Recently, an alternate ESCRT 0 complex, centering on Tom1 proteins, has been proposed. We determine the distribution of Tom1 family proteins across eukaryotes and show that the Tom1, Tom1L1 and Tom1L2 proteins are a vertebrate-specific expansion of the single Tom1 family ancestor, which has indeed been identified in at least one member of each of the major eukaryotic supergroups. This implies a more widely conserved and ancient role for the Tom1 family in endocytosis than previously suspected
A Partial Record of Mixing of Mantle Melts Preserved in Icelandic Phenocrysts
No full textThe record of mixing of mantle melts in magma chambers has previously been observed in the compositions of olivine-hosted melt inclusions from Borgarhraun, a primitive basalt flow from the Theistareykir volcanic system, northern Iceland. Borgarhraun also contains high Mg-number (85–92) clinopyroxenes, which exist in polycrystalline nodules and as phenocrysts. Coincident major and trace element analyses were made in compositional zones of these clinopyroxenes, and Ce/Yb ratios of the melts in chemical equilibrium with each of the clinopyroxene zones were calculated using carefully selected crystal–melt partition coefficients. These calculations allow direct comparison of clinopyroxene compositions with existing melt inclusion data. The range of Ce/Yb ratios in the crystals and in the equilibrium melts cannot be accounted for by crystallization alone, requiring simultaneous mixing and crystallization of compositionally variable mantle melts. However, the range in Ce/Yb for melts in equilibrium with these high Mg-number clinopyroxenes is smaller than that of melt inclusions hosted by olivines with equivalent Fo contents. Also, the mean composition of the melts from which clinopyroxene grew has significantly lower Ce/Yb than the olivine-hosted melt inclusions. The record of mantle melt variability in clinopyroxenes is thus biased towards more depleted (low Ce/Yb) melt compositions. This bias can be understood if the trace element variation in the Borgarhraun parental melts is coupled to major element variation, as expected from petrological parameterizations of mantle melting. The major element variation influences the phase relationships and controls the appearance of liquidus phases during fractional crystallization in near-Moho magma chambers. Small-degree, deep melts, formed in the presence of garnet, have high Ce/Yb ratios. On cooling, these melts have a longer olivine-only crystallization path than melts derived from the shallow mantle. When these deep-sourced melts eventually become clinopyroxene saturated, they have too low Mg-numbers to crystallize high Mg-number clinopyroxenes such as are found in Borgarhraun. In contrast, shallow, depleted melts saturate in clinopyroxene at high Mg-number. The delayed onset of clinopyroxene crystallization in the enriched melts, coupled with concurrent mixing and crystallization of melts generated at a range of depths in the mantle, can account for the difference in the distribution of the trace element composition of high Mg-number melts saturated in olivine and clinopyroxene. The trace element compositions of high Mg-number clinopyroxenes in Borgarhraun therefore provide only a partial and biased record of the mixing of mantle melts. As well as showing that melt mixing may be preserved in phenocryst compositions, the results illustrate that trace element disequilibrium between crystals and carrier melt can be a consequence of magma mixing, rather than necessitating a xenocrystic origin for the crystals. Furthermore, care must be taken when using clinopyroxene separates from primitive basalts to examine compositional heterogeneity, as they provide a record of the chemical evolution of the magmatic system that is biased towards depleted compositions and therefore incomplete
Earthquake depth distributions in central Asia, and their relations with lithosphere thickness, shortening and extension
No full textThis paper examines the relationship between seismogenic thickness, lithosphere structure and rheology in central and northeastern Asia. We accurately determine earthquake depth distributions which reveal important rheological variations in the lower crust. These variations exert a fundamental control on the active tectonics and the morphological evolution of the continents. We consider 323 earthquakes across the Tibetan Plateau, the Tien Shan and their forelands as well as the Baikal Rift, NE Siberia and the Laptev Sea and present the source parameters of 94 of these here for the first time. These parameters have been determined through body wave inversion, the identification of depth phases or the modelling of regional waveforms. Lower crustal earthquakes are found to be restricted to the forelands in areas undergoing shortening, and to locations where rifting coincides with abrupt changes in lithosphere thickness, such as the NE Baikal Rift and W Laptev Sea. The lower crust in these areas is seismogenic at temperatures that may be as high as 600°C, suggesting that it is anhydrous, and is likely to have great long-term strength. Lower crustal earthquakes are therefore a useful proxy indicating strong lithosphere in places that are too small in areal extent for this to be confirmed independently by estimating effective elastic thickness from gravity–topography relations. The variation in crustal rheology indicated by the distribution of lower-crustal earthquakes has many implications ranging from the support of mountain belts and the formation of steep mountain fronts, to the localization and orientation of rifting. In combination, these processes can also be responsible for the separation of the front of the thin-skinned mountain belts from their hinterlands when continents separate
High pressure ferroelastic phase transition in SrTiO <sub>3</sub>
No full textHigh pressure measurements of the ferroelastic phase transition of SrTiO3 (Guennou et al 2010 Phys. Rev. B 81 054115) showed a linear pressure dependence of the transition temperature between the cubic and tetragonal phase. Furthermore, the pressure induced transition becomes second order while the temperature dependent transition is near a tricritical point. The phase transition mechanism is characterized by the elongation and tilt of the TiO6 octahedra in the tetragonal phase, which leads to strongly nonlinear couplings between the structural order parameter, the volume strain and the applied pressure. The phase diagram is derived from the Clausius–Clapeyron relationship and is directly related to a pressure dependent Landau potential. The nonlinearities of the pressure dependent strains lead to an increase of the fourth order Landau coefficient with increasing pressure and, hence, to a tricritical–second order crossover. This behaviour is reminiscent of the doping related crossover in isostructural KMnF3
Thermally activated avalanches: Jamming and the progression of needle domains
No full textLarge-scale computer simulations of a simple model with a square-lattice topology, a small shear deformation (4° shear angle), and open (free) boundary conditions show that domain boundary movements under adiabatic strain deformation lead to Vogel-Fulcher behavior at high temperatures. The activation energy is independent of temperature and details of the twin patterns. Below the Vogel-Fulcher temperature, no thermal activation was found and the time evolution of the domain pattern becomes athermal. The movement of domain boundaries is now dominated by the nucleation and growth of needle domains. Their movement occurs in fast jerks. The probability to observe jerks follows a power-law spectrum with energy exponents close to α≈2. At even lower temperatures, the boundary kinetics becomes erratic even in our large (106atoms) system. The lateral movement of twin walls is found for our thin twin walls (w=3 layers) to operate by kinks which propagate along the twin wall. The needle domains nucleate either from the surface or from other existing twin walls. Intersections of twin walls constitute pinning centers which impede the free movement of the kinks in the walls. These intersection points then act as a pattern of intrinsic, self-induced defects which lead ultimately to the power-law distribution of the crackling noise of the domain walls
Dynamics of cementation in response to oil charge: Evidence from a Cretaceous carbonate field, U.A.E.
No full textOil charge is thought to inhibit the growth of cements within subsurface pore systems. We explore this phenomenon in a giant Cretaceous carbonate field from U.A.E., where the oil-filled crest porosity ranges from 10 to 50% and permeability from 0.08 to 830 mD but coeval water leg porosity is reduced to 10 to 23% and permeability to 0.1 to 4 mD. Only 5% of primary interparticle pores (> 30 [mu]m diameter) in the crest are fully cemented, compared to 99% of pores in the water leg.Syntaxial calcite burial cements (> 10 [mu]m diameter) in the oil leg show 12 cathodoluminescence zones with oil inclusions (n = 27) occurring in four of the five final zones. Mean in-situ ion microprobe [delta]18OVPDB data from the oil leg cements range from -1.2[per mille sign] in the oldest zone decreasing to -10.3[per mille sign] in zone 11, returning to -7.7[per mille sign] in the final zone. The oldest distinguishable cement zone in the water leg shows highly variable [delta]18O from -3.6[per mille sign] to -9.3[per mille sign] with a mean of -7.3[per mille sign], and with subsequent zones decreasing to a mean value of -9.4[per mille sign] for the youngest cement zone. Decreasing [delta]18O values are interpreted as indicating increasing temperature reflecting burial and the evolution of pore water composition: broadly similar trends in the oil and water legs suggest precipitation under the same general conditions.Unlike the oil leg cements, the final zone in the water leg occludes nearly all remaining pore space. The [delta]18OVPDB of bulk micrite from the water leg shows an average of -7.4[per mille sign] (n = 9) compared to -6.2[per mille sign] (n = 10) from the oil leg, suggesting the precipitation of further micrite cement at greater burial depths. We infer that burial cementation slowed in the presence of oil due to a reduction of potential nucleation sites as well as porewater and solute movement within weakly oil-wet pores, whereas continued flow and solute movement through all pores including the micropores (< 10 [mu]m diameter) enabled extensive cementation in the water leg
Spin orientation in a natural Ti-bearing hematite: Evidence for an out-of-plane component
No full textThe orientation of spins in a natural sample of Ti-bearing hematite (Fe2O3) has been measured from 2-300 K using time-of-flight neutron powder diffraction. It is shown that the antiferromagnetic alignment vector is tilted out of the basal plane by an average angle of 30{degrees}, independent of temperature, contrary to the normal expectation that all spins lie in the basal plane due to the suppression of the Morin transition by Ti. This unusual result is related to the non-uniform spatial distribution of Ti in this sample, which takes the form of ~1 nm exsolution lamellae of ilmenite (FeTiO3), observed using transmission electron microscopy. It is suggested that the exsolution lamellae lead to a localization of Fe2+ species within the lamellar interfaces, which cause tilting of some spins toward the crystallographic c axis. The presence of an out-of-plane component of spin at room temperature reconciles experimental and computational attempts to explain the phenomenon of "giant exchange bias" that appears when this sample is zero-field cooled below the ilmenite Neel temperature
The fluid mechanics of the natural ventilation of a narrow-cavity double-skin facade
No full textThis paper investigates the natural ventilation of a double-skin facade connected to a room in a multi-storey building. The room and the facade are connected to the exterior through vents at different levels. The room contains a horizontally distributed heat source analogous to occupants in an open-plan office or an underfloor heating system. The facade cavity contains a vertically distributed heat source analogous to a shading blind/louvers heated by solar radiation. These two sources of heat combine to provide buoyancy driving the ventilation. Two basic modes of facade operation are proposed and investigated. These two modes of operation should be alternated according to exterior climatic conditions. In colder seasons, the room draws air from the portion of the facade which extends one floor below the room, and solar radiation on the facade preheats supply air into the room. In warmer seasons, the room vents to the exterior through the portion of the facade which extends one floor above the room, and solar radiation on the facade enhances the ventilation and prevents overheating in the system. A quantitative model is developed to describe the fluid mechanics of the ventilation in these two modes of operation. The model is successfully tested with laboratory experiments. It shows how the height of the facade and the size of the openings can be adjusted to maximise the preheating of the room in colder seasons, and to prevent overheating in the room and the facade in warmer seasons. The model is used to explore the principles for design and control in different climatic and occupancy conditions