Progress towards an improved Precambrian seawater 87Sr/86Sr curve

The secular trend of seawater strontium isotope ratio (87Sr/86Sr) reflects changes in the relative contributions of continental versus mantle reservoirs to ocean composition, and informs global tectonic events, weathering rates and biogeochemical cycling through Earth history. However, the Precambrian seawater 87Sr/86Sr curve is known in far less detail than its Phanerozoic counterpart. For this study, we compiled 2249 strontium isotope ratios of Precambrian marine sedimentary rocks published since 2002, alongside previously compiled older data. Here we evaluate the uncertainty of all published data for constraining coeval seawater 87Sr/86Sr using four criteria (depositional environment, diagenetic alteration, age constraint and dissolution method). The resultant seawater 87Sr/86Sr curve uses mainly ‘high certainty’ data and shows an overall increasing trend from ~0.7005 at c. 3.5 Ga to ≥0.7089 towards the end of the Ediacaran Period. The improved curve shows an earlier deviation of seawater 87Sr/86Sr from the contemporaneous mantle by c. 3.5 Ga, which might reflect the first significant emergence of evolved continental crust related to nascent tectonics. Additionally, the updated curve records two major rises at 2.5-2.2 Ga and 1.9-1.7 Ga in addition to a well-established event at 0.8-0.5 Ga. Despite the relative scarcity of high-certainty data, these two increases are consistent with enhanced continental weathering following the onset of oxidative weathering and assembly of the supercontinent Nuna, respectively. Although confirmation of these two events awaits more high-certainty data, Precambrian seawater 87Sr/86Sr experienced stronger oscillations and better correspondence with supercontinent cycles than previously shown

Microstructure and superconducting properties of hot isostatically pressed MgB2

Bulk samples of MgB2 have been formed by hot isostatic pressing (HIPping) of commercial powder at 100MPa and 950=B0C. The resulting material is 100% dense with a sharp superconducting transition at 37.5K. Microstructural studies have indicated the presence of small amounts of second phases within the material, namely MgO and B rich compositions, probably MgB4. Magnetisation measurements performed at 20K have revealed values of Jc=1.3 x 106A/cm2 at zero field, and 9.3 x 105A/cm2 at 1T. Magneto optical (MO) studies have shown direct evidence for the superconducting homogeneity and strong intergranular current flow in the material.Comment: 3 pages, 6 figures, text updated, new references included and discussed. Submitted to Superconductor Science and Technolog

Termination of Cryogenian ironstone deposition by deep ocean euxinia

Widespread deposition of iron-rich sedimentary rocks (ironstones) occurred during the Sturtian ice age, the earlier of two Cryogenian ‘Snowball Earth’ glaciations. However, the reasons for the termination of Cryogenian Iron Formation (CIF) deposition remain poorly understood. Here we report a multi-isotope (Fe-C-S) study of the Xinyu CIF in South China that can directly address this question. The isotopic compositions of these multivalent elements exhibit remarkable covariance at the top of the Xinyu CIF that reflects progressive redox stratification during iron oxide deposition. Iron oxide deposition ended abruptly due to the titration of ferrous iron as pyrite following a large influx of riverine sulfate. Although termination of CIF deposition is commonly attributed to oxygenation, our data show that at least in some Cryogenian ocean basins, CIF deposition ended with the spread of euxinic rather than oxygenated waters

Free carrier effects in gallium nitride epilayers: the valence band dispersion

The dispersion of the A-valence-band in GaN has been deduced from the observation of high-index magneto-excitonic states in polarised interband magneto-reflectivity and is found to be strongly non-parabolic with a mass in the range 1.2-1.8 m_{e}. It matches the theory of Kim et al. [Phys. Rev. B 56, 7363 (1997)] extremely well, which also gives a strong k-dependent A-valence-band mass. A strong phonon coupling leads to quenching of the observed transitions at an LO-phonon energy above the band gap and a strong non-parabolicity. The valence band was deduced from subtracting from the reduced dispersion the electron contribution with a model that includes a full treatment of the electron-phonon interaction.Comment: Revtex, 4 pages, 5 figure

Long-lived charged multiple-exciton complexes in strong magnetic fields

We consider the charged exciton complexes of an ideal two-dimensional electron-hole system in the limit of strong magnetic fields. A series of charged multiple-exciton states is identified and variational and finite-size exact diagonalization calculations are used to estimate their binding energies. We find that, because of a hidden symmetry, bound states of excitons and an additional electron cannot be created by direct optical absorption and, once created, have an infinite optical recombination lifetime. We also estimate the optical recombination rates when electron and hole layers are displaced and the hidden symmetry is violated.Comment: 12 pages + 2 PostScript figures, Revtex, Submitted to Phys. Rev. Let

Thermal breakage of window glass in room fires conditions - Analysis of some important parameters

In a compartment fire, the breakage and possible fallout of a window glass has a significant impact on the fire dynamics. The thermal breakage of glass depends on various parameters such as glass type, edge shading, edges conditions and constraints on the glass. The purpose of the present study is to investigate some of the key parameters affecting the thermal breakage of window glass in fire conditions using a recently developed and validated computer tool. Fallout is not within the scope of this study. Different boundary conditions of the glass pane (unconstrained and constrained) subjected to fire radiant heat are investigated. The analysis shows that to prevent glass thermal breakage, it is important to provide enough spacing between the frame and glass pane to accommodate the thermal expansion, and constraints on the glass structure should be avoided. The zones where the glass is likely to crack first are shown. The study also quantifies the effects of glass edge conditions on its thermal breakage in fire conditions; such analysis has not been reported in the literature due to its complexity and the statistical nature of edge flaws. The results show that an ordinary float glass mostly used in windows, with the “as-cut” edge condition would break later and is stronger than a ground edge or polished edge glass for the scenarios investigated. The study demonstrates how a predictive tool could be employed for a better understanding of thermal breakage of window glass in fires and for design guidance

Long-term evolution of terrestrial inputs from the Ediacaran to early Cambrian: Clues from Nd isotopes in shallow-marine carbonates, South China

The emergence and radiation of metazoans have been widely attributed to a progressively more oxidizing surface environment through the Ediacaran–Cambrian transition interval. However, the root causes for atmospheric and oceanic oxygenation are still disputed. Long-term tectonic changes could possibly have led to atmospheric oxygenation but geochemical evidence for this linkage remains elusive. In this study, we analyzed the radiogenic Nd isotopic compositions (εNd) of shallow-marine carbonates from South China in order to track secular variations in terrestrial inputs from Ediacaran to early Cambrian time. Compared with most other geochemical indices, the Nd isotope system in carbonates is less susceptible to early diagenetic exchange and can thus act as a robust proxy for continental materials undergoing weathering. We interpret an abrupt excursion to lower εNd values during the middle Ediacaran as due to rapid exchange of different water masses. However, the more gradual trend towards lower Nd isotope values from the Ediacaran to early Cambrian, accompanied by increasing 87Sr/86Sr ratios of the studied carbonates, likely indicates the enhanced weathering of old continental rock following the assembly of Gondwana. Increased net accumulation of atmospheric oxygen as a result of enhanced organic carbon burial may have benefited from intense continental denudation

The Growth of Black Holes and Their Host Spheroids in (Sub)mm-loud QSOs at High Redshift

We study the growth of black holes and stellar population in spheroids at high redshift using several (sub)mm-loud QSO samples. Applying the same criteria established in an earlier work, we find that, similar to IR QSOs at low redshift, the far-infrared emission of these (sub)mm-loud QSOs mainly originates from dust heated by starbursts. By combining low-z IR QSOs and high-z (sub)mm-loud QSOs, we find a trend that the star formation rate (\Mstardot) increases with the accretion rate (\Mdot). We compare the values of \Mstardot/\Mdot for submm emitting galaxies (SMGs), far-infrared ultraluminous/hyperluminous QSOs and typical QSOs, and construct a likely evolution scenario for these objects. The (sub)mm-loud QSO transition phase has both high \Mdot and \Mstardot and hence is important for establishing the correlation between the masses of black holes and spheroids.Comment: 19 pages,3 figures,submitted to Chin. J. Astron. Astrophys. This paper was first prepared for publication on August 10th, 200

A short-lived oxidation event during the early Ediacaran and delayed oxygenation of the Proterozoic ocean

The Ediacaran Period was characterised by major carbon isotope perturbations. The most extreme of these, the ∼570 Ma Shuram/DOUNCE (Doushantuo Negative Carbon isotope Excursion) anomaly, coincided with early radiations of benthic macrofauna linked to a temporary expansion in the extent of oxygenated seawater. Here we document an earlier negative excursion (the ∼610 Ma WANCE (Weng'An Negative Carbon isotope Excursion)) anomaly in the Yangtze Gorges area, South China, that reached equally extreme carbon isotope values and was associated with a similar degree of environmental perturbation. Specifically, new uranium isotope data evidence a significant, but transient, shift towards more oxygenated conditions in tandem with decreasing carbon isotope values, while strontium and sulfur isotope data support an increase in continental weathering through the excursion. We utilize a biogeochemical modelling approach to demonstrate that the influx of such a weathering pulse into an organically-laden, largely anoxic ocean, fully reproduces each of these distinct isotopic trends. Our study directly supports the hypothesis that a large dissolved marine organic pool effectively buffered against widespread oxygenation of the marine environment through the Proterozoic Eon, and in doing so, substantially delayed the radiation of complex aerobic life on Earth

Cosmic Hydrogen Was Significantly Neutral a Billion Years After the Big Bang

The ionization fraction of cosmic hydrogen, left over from the big bang, provides crucial fossil evidence for when the first stars and quasar black holes formed in the infant universe. Spectra of the two most distant quasars known show nearly complete absorption of photons with wavelengths shorter than the Ly-alpha transition of neutral hydrogen, indicating that hydrogen in the intergalactic medium (IGM) had not been completely ionized at a redshift z~6.3, about a billion years after the big bang. Here we show that the radii of influence of ionizing radiation from these quasars imply that the surrounding IGM had a neutral hydrogen fraction of tens of percent prior to the quasar activity, much higher than previous lower limits of ~0.1%. When combined with the recent inference of a large cumulative optical depth to electron scattering after cosmological recombination from the WMAP data, our result suggests the existence of a second peak in the mean ionization history, potentially due to an early formation episode of the first stars.Comment: 14 Pages, 2 Figures. Accepted for publication in Nature. Press embargo until publishe