Computer simulations of iron in magnesium silicate perovskite

We use atomistic computer simulation techniques to investigate the site partitioning of iron in (Mg,Fe)SiO_{3} perovskites. Our calculations predict that the most energetically favourable reaction for iron substitution will be a direct exchange of Fe^{2+} for Mg^{2+}. Substitution of Fe into the octahedral site and Si into the 8–12 fold coordinated site, as proposed by Jackson et al. [1987], is predicted to be extremely unlikely

Research collaboration: Maximising the UK’s research strengths

This briefing proposes that research policy and funding should support greater collaboration between institutions as a necessary response to the significant challenges that universities face, in particular by establishing a dedicated fund to support collaborative partnerships. We believe that this would ensure that the concentration of research funding on excellence can be balanced with sustaining the dynamism of the UK research base

The Wisdom Agenda: Research-intensive universities in the 21st century

Leading universities have the potential to transform themselves into wisdom universities and thus – in partnership with government, commerce and society – offer robust solutions to aspects of the world’s major problems

Thermal expansion of mantle minerals at high pressures - A theoretical study

Recent experimental work has shown that the pressure dependence of the thermal expansion coefficient can be expressed as: ( α / α 0 ) = ( ρ / ρ 0 ( - δ T   ( 1 ) where δ_{T}, the Anderson‐Gruneisen parameter, is assumed to be independent of pressure, and for the materials studied has a value that lies between 4 and 6. Calculation of δ_{T} from seismic data, however, appears to suggest a contradictory value of between 2 and 3 for mantle‐forming phases. Using an atomistic model based on our previously successful many‐body interatomic potential set (THBl), we have performed calculations to obtain values of δ_{T} for four major mantle‐forming minerals. Our model results are in excellent agreement with experimental data, yielding values of between 4 and 6 for forsterite and MgO, and values in the same range for MgSiO_{3-}perovskite and γ‐Mg_{2}SiO_{4}. Moreover, the calculations confirm that δ_{T} is indeed constant with pressure up to the core‐mantle boundary. The apparent conflict between the values of δ_{T} predicted from seismic data and those obtained from experiment, and now from theory, is discussed

CaSiO3 perovskite at lower mantle pressures

We investigate by first-principles the structural behavior of CaSiO3 perovskite up to lower mantle pressures. We confirm that the cubic perovskite modification is unstable at all pressures. The zero Kelvin structure is stabilized by SiO6 octahedral rotations that lower the symmetry to tetragonal, orthorhombic, rhombohedral, or to a cubic supercell. The resulting structures have comparable energies and equation of state parameters. This suggests that relatively small deviatoric/ shear stresses might induce phase transformations between these various structures softening some elastic moduli, primarily the shear modulus. The seismic signature accompanying a local increase in CaSiO3 content should be a positive density anomaly and a negative V-S anomaly

High temperature elastic anisotropy of the perovskite and post-perovskite polymorphs of Al2O3

Finite temperature ab initio molecular dynamics calculations were performed to determine the high temperature elastic and seismic properties of the perovskite and post-perovskite phases of pure end-member Al2O3. The post-perovskite phase exhibits very large degrees of shear-wave splitting. The incorporation of a few mole percent of Al2O3 into MgSiO3 is predicted to have little effect on the perovskite to post-perovskite phase transition pressure and seismic properties of the post-perovskite phase; although a small difference in shear-wave splitting may be observable

Comparison between the lattice dynamics and molecular dynamics methods: Calculation results for MgSiO3 perovskite

The lattice dynamics (LD) and molecular dynamics (MD) methods have been used to calculate the structure, bulk modulus, and volume thermal expansivity of MgSiO3 perovskite, in order to investigate the reliability of the two simulation techniques over a wide range of temperature and pressure conditions. At an intermediate temperature of 500 K and zero pressure, the LD and MD values are in exellent agreement for both the structure and bulk modulus of MgSiO3 perovskite. At high temperatures and zero pressure, however, the LD method, which is based on the quasi-harmonic approximation, increasingly overestimates the molar volume of MgSiO3 perovskite because of the neglect of higher-order anharmonic terms. At the high temperatures and high pressures prevailing in the lower mantle, the errors in the LD values for both the molar volume and bulk modulus, relative to the MD values, are generally small or negligible. However, since anharmonicity decreases substantially with pressure but increases rapidly with temperature, the error in the LD simulated volume thermal expansivity is serious, especially in the lower pressure region

Subsequent biotic crises delayed marine recovery following the late Permian mass extinction event in northern Italy

The late Permian mass extinction event was the largest biotic crisis of the Phanerozoic and has the longest recovery interval of any extinction event. It has been hypothesised that subsequent carbon isotope perturbations during the Early Triassic are associated with biotic crises that impeded benthic recovery. We test this hypothesis by undertaking the highest-resolution study yet made of the rock and fossil records of the entire Werfen Formation, Italy. Here, we show that elevated extinction rates were recorded not only in the Dienerian, as previously recognised, but also around the Smithian/Spathian boundary. Functional richness increases across the Smithian/Spathian boundary associated with elevated origination rates in the lower Spathian. The taxonomic and functional composition of benthic faunas only recorded two significant changes: (1) reduced heterogeneity in the Dienerian, and (2) and a faunal turnover across the Smithian/Spathian boundary. The elevated extinctions and compositional shifts in the Dienerian and across the Smithian/Spathian boundary are associated with a negative and positive isotope excursion, respectively, which supports the hypothesis that subsequent biotic crises are associated with carbon isotope shifts. The Spathian fauna represents a more advanced ecological state, not recognised in the previous members of the Werfen Formation, with increased habitat differentiation, a shift in the dominant modes of life, appearance of stenohaline taxa and the occupation of the erect and infaunal tiers. In addition to subsequent biotic crises delaying the recovery, therefore, persistent environmental stress limited the ecological complexity of benthic recovery prior to the Spathian

The phylogenetic and palaeogeographic evolution of the miogypsinid larger benthic foraminifera

Access to new material from South Africa, Corsica, Cyprus, Syria and Sumatra has allowed a systematic biostratigraphic comparison and correlation of the miogypsinids from the Mediterranean–West Africa and the Indo-Pacific provinces, and for the first time from South Africa. Twelve new species have been identified: Neorotalia tethyana, Miogypsinella bornea, Miogypsina subiensis, M. niasiensis, M. regularia, M. samuelia, Miolepidocyclina banneri, Miogypsinella cyprea, Miogypsina mcmillania, M. africana, M. ianmacmilliana and M. southernia. The palaeogeographical evolution of the miogypsinids started with a trans-Atlantic migration of Neorotalia from the Americas, where miogypsinids had originated. The eastward migration followed two paths: one to the south towards South Africa, where a distinct phylogenetic lineage, but similar to that found in America, developed but went extinct in the Burdigalian; the other to the north, through the Mediterranean corridor. During the Chattian and Aquitanian significant miogypsinid forms evolved in the Mediterranean from the morphologically distinct Mediterranean Neorotalia and migrated, within a few million years of their first appearance, into the Indo-Pacific, where they diversified further. The tectonically driven closure of the seaway between the Mediterranean and the Indo-Pacific in the Burdigalian triggered the extinction of Mediterranean miogypsinids in the Langhian. Miogypsinids survived in the Indo-Pacific into the Serravallian

ISOTOPIC ANALYSIS OF JURASSIC (CALLOVIAN) MOLLUSKS FROM THE CHRISTIAN MALFORD LAGERSTATTE (UK): IMPLICATIONS FOR OCEAN WATER TEMPERATURE ESTIMATES BASED ON BELEMNOIDS

Isotopic data (C and O) derived from Callovian (Middle Jurassic) mollusks (bivalves, ammonites and belemnoids, including true belemnites and Belemnotheutis) are presented from a narrow stratigraphic interval in the Christian Malford Lagerstätte, UK. The exceptionally well-preserved mollusks include aragonite-calcite pairs precipitated by individual belemnite animals that enable an assessment of possible “vital” effects and the reliability of using belemnite calcite to determine ocean water compositions. The oxygen isotope data derived from the calcitic rostra of the belemnites (Cylindroteuthis) show modest variability, ranging from −1.2 to 0.9‰ (V-PDB), while their accompanying aragonitic phragmocones range from −1.4 to 0.0‰. Data derived from the ammonite Kosmoceras show some scatter, with oxygen isotope values varying from −3.6 to −0.2‰. The aragonite data from Cylindroteuthis, Kosmoceras and Belemnotheutis all overlap, suggesting they inhabited similar (surface) water depths. However, the corresponding data from the calcitic rostra of the Cylindroteuthis specimens suggest temperatures ∼ 5°C cooler. As we have analyzed aragonite-calcite pairs, the discrepancy cannot be explained by environmental effects. Though clearly a vital effect, it is difficult to resolve whether the temperatures derived from the aragonite (phragmocone) are too warm or from the calcite (rostrum) are too cool. Consequently, the applicability of standard paleotemperature equations to Cylindroteuthid belemnite rostra remains unproven. Sequentially sampled ontogenetic isotope data derived from Belemnotheutis phragmocones reveal only modest δ18O variation, consistent with limited movement between warmer (shallower) and cooler (deeper) waters. A coincidental systematic pattern of δ13C enrichment may signal changes in metabolic activity associated with a shift in ecology or feeding with age