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Macroevolutionary turnover through the Ediacaran transition: ecological and biogeochemical implications
No full textEcological and evolutionary principles are often context-dependent, particularly where the context is biologically defined. Organ-grade animals (eumetazoans) are particularly powerful contextual agents, with a unique capacity to drive escalatory co-evolution and build multi-tiered food-webs. The evolution of eumetazoans through the Ediacaran and early Cambrian fundamentally altered macroecological and macroevolutionary dynamics, including the structure and function of the marine carbon cycle. Pelagic eumetazoans can be held responsible for driving the evolution of relatively large eukaryotic phytoplankton, thereby shifting the system from a turbid, stratified, cyanobacteria-dominated stable state to the clear-water, well-oxygenated, algae-dominated condition typical of the Phanerozoic. Intermittent return to the pre-Ediacaran state during Phanerozoic extinctions and oceanic anoxic events suggests that the widespread anoxia detected in pre-Ediacaran deep-marine sequences may be a consequence of this alternate biological pump rather than a reflection of fundamentally lower levels of atmospheric oxygen. The transition between the pre- and post-Ediacaran states is also associated with the oldest commercially exploitable hydrocarbons, a possible by-product of invading animals and their top-down impact on the biological pump
Sulfuric acid as an agent of carbonate weathering constrained by [delta]13CDIC: Examples from Southwest China
No full textAbstract
Rock weathering by carbonic acid is thought to play an important role in the global carbon cycle because it can geologically sequestrate atmospheric CO2. Current model of carbon cycle evolution usually assumes that carbonic acid is the major weathering agent and that other acids are not important. Here, we use carbon isotopic evidence and water chemistry of springs and rivers from the Beipanjiang River basin (Guizhou Province, Southwest China) to demonstrate that sulfuric acid is also an important agent of rock weathering. The [delta]13C of dissolved inorganic carbon (DIC) in the water samples ranges from - 13.1[per mille sign] to - 2.4[per mille sign], and correlates negatively to [HCO3-]/([Ca2+] + [Mg2+]) ratios and positively to [SO42-]/([Ca2+] + [Mg2+]) ratios. These relationships are interpreted as mixing diagrams between two reactions of carbonate weathering, using carbonic acid and sulfuric acid as a proton donor, respectively. Mixing proportions show that around 42% of the divalent cations in the spring water from Guizhou are originated from the interaction between carbonate minerals and sulfuric acid. It is shown that 40% of this sulfuric acid is derived from the atmosphere and has an anthropogenic origin. The remaining 60% are derived from the oxidative weathering of sulfide minerals in sedimentary rocks. Our results show the positive action of sulfuric acid on the chemical weathering of carbonate. Particularly, we show that sulfuric acid generated by coal combustion has increased by almost 20% the weathering rates of carbonate in Southwest China. This is a clear evidence that human activities are changing the weathering rates of rocks and demonstrates a negative feedback on the acidification of the ocean by greenhouse gases. Because of the involvement of sulfuric acid in weathering reactions, 63% of the alkalinity exported by rivers is derived from carbonate, instead of 50% when atmospheric CO2 is the only acid involved in chemical weathering of carbonate. In the Guizhou Province, the weathering of carbonate is thus, at least transiently, a net source of CO2 to the atmosphere. When extrapolated at global scale, sulfuric acid-induced carbonate weathering could counterbalance a significant part of the CO2 consumed by silicate weathering. This paper highlights the competition between silicate weathering by carbonic acid and carbonate weathering by sulfuric acid for the regulation of the atmospheric CO2 level
Dinosaurs of Dorset: Part I, the carnivorous dinosaurs (Saurischia, Theropoda)
No full textTheropod dinosaurs in Dorset are found in the Lower Jurassic (Blue Lias Formation), the Middle Jurassic (Inferior Oolite and Forest Marble Formations), the Late Jurassic (Kimmeridge Clay and Portland Stone) and the Lower Cretaceous (Purbeck Limestone Group and ‘Wealden Beds’). They include representatives of several major theropod groups: the megalosaurids Duriavenator and Magnosaurus, the sinraptorid Metriacanthosaurus, the basal tyrannosauroid Stokesosaurus and the dromaeosaurid Nuthetes. Theropod fossils from Dorset have been reported since 1835 and the record from the county is one of the oldest globally. It continues to yield new data on dinosaur evolution and biogeography and will contribute to our understanding and insight of theropod evolution into the foreseeable future
Chlorine variations in the magma of Soufrière Hills Volcano, Montserrat: Insights from Cl in hornblende and melt inclusions
No full textThe Soufrière Hills Volcano in Montserrat erupts a Cl-rich, porphyritic andesite. HCl degassing accompanies eruption and is dependent on the growth rate of the lava dome. The magma contains hornblende phenocrysts that show repetitive zoning in most elements, including Cl. On the basis of the zoning data, (Cl/OH) ratios in the melt, calculated from partitioning data, increase rimward through each zone, indicating that the phenocrysts formed under conditions of varying (Cl/OH)m. An empirical relationship between A-site occupancy in the hornblende and temperature implies that crystallisation of each zone is also accompanied by increasing temperature. Each zone ends at a resorption horizon, and crystallisation recommences at lower temperature and (Cl/OH)m. Melt inclusion H2O and Cl contents for the 8th January 2007 explosive eruption can be explained by closed-system degassing with DClfl-m between 5 and 30, or by open-system degassing accompanied by a small amount of crystallisation. However, neither simple closed-system degassing nor convective circulation of magma can explain the positive correlation of (Cl/OH)m with temperature. We suggest that the zoning can be caused by accumulation of CO2-rich vapour in the andesite, probably as a result of mafic magma injection into the chamber. Decreasing H2O fugacity and/or increasing Clm result in increasing (Cl/OH)m while heat transferred with the volatiles causes the rise in temperature. Intermittently, the accumulated fluid is lost to the surface, possibly as a result of renewed eruptive activity. This model requires the CO2-rich fluid to be decoupled from the magma, consistent with previous observations of continuous CO2 emissions at the surface
Oxide superlattices with alternating p and n interfaces
No full textThe physics of oxide superlattices is considered for pristine (001) multilayers of the band insulators LaAlO3 and SrTiO3 with alternating p and n interfaces. A model of charged capacitor plates offers a simple paradigm to understand their dielectric properties and the insulator to metal transition (IMT) at interfaces with increasing layer thicknesses. The model is supported by first-principles results based on density-functional theory. The charge at insulating interfaces is argued and found to be as predicted from the formal ionic charges, not populations. Different relative layer thicknesses produce a spontaneous polarization of the system, and allow manipulation of the interfacial electron gas. Large piezoresistance effects can be obtained from the sensitivity of the IMT to lateral strain. Carrier densities are found to be ideal for exciton condensation
An introduction to atomistic simulation methods
No full textVarious methods for simulating materials and minerals at an atomic level are reviewed, including lattice energy relaxation, lattice dynamics, molecular dynamics and Monte
Carlo methods, and including also the use of empirical interatomic potentials and ab-initio quantum mechanical methods. A small number of diverse applications are
described. Approaches to job and data management are also discussed
Understanding the problem of glass transition on the basis of elastic waves in a liquid
No full textWe propose that the properties of a glass transition can be understood
on the basis of elastic waves. Elastic waves originating from atomic
jumps in a liquid propagate local expansion due to the anharmonicity
of the interatomic potential. This creates dynamic compressive stress,
which increases the activation barrier for other events in a liquid.
The non-trivial point is that the range of propagation of
high-frequency elastic waves, del, increases with liquid relaxation
time t. A self-consistent calculation shows that this increase gives
the Vogel-Fulcher-Tammann (VFT) law. In the proposed theory, we
discuss the origin of two dynamic crossovers in a liquid: (1) the
crossover from exponential to non-exponential and from Arrhenius to
VFT relaxation at high temperature and (2) the crossover from the VFT
to a more Arrhenius-like relaxation at low temperature. The
corresponding values of t at the two crossovers are in quantitative
parameter-free agreement with experiments. The origin of the second
crossover allows us to reconcile the ongoing controversy surrounding
the possible divergence of t. The crossover to Arrhenius relaxation
universally takes place when del reaches system size, thus avoiding
divergence and associated theoretical complications such as
identifying the nature of the phase transition and the second phase
itself. Finally, we discuss the effect of volume on t and the origin
of liquid fragility
Uplift Histories from River Profiles
No full textLongitudinal river profiles, where elevation of a river bed is plotted as a function of distance along the river bed, contain information about uplift rate. When a region adjacent to a reference level (e.g., sea level) is uplifted, a rapid change in gradient occurs near the river mouth. The erosional process causes this change in gradient to migrate upstream. Thus a river profile is effectively a ‘tape recording’ of the uplift rate history, provided that the erosional process can be adequately parameterized. Here, we use a non-linear equation to relate the shape of a river profile, z(x), to uplift rate history, U(t). If erosion is assumed to be dominated by knickpoint retreat, an inverse model can be formulated and used to calculate uplift rate histories. Our model builds upon standard stream profile analysis, which focuses on the relationship between profile slope and drainage area. We have applied this analytical approach to river profiles from the Bié Dome, Angola. Calculated uplift rate histories agree with independent geologic estimates
Juvenile chemical sediments and the long term persistence of water at the surface of Mars
No full textChemical sediments and the aqueous alteration products of volcanic rocks clearly indicate the presence of water, at least episodically, at the Martian surface. Compared to similar materials formed on the early Earth, however, Martian deposits are juvenile, or diagenetically under-developed. Here we examine the role of water in facilitating various diagenetic reactions and evaluate the predicted effects of time and temperature for aqueous diagenesis on Mars. Using kinetic formulations based on terrestrial sedimentary geology, we quantify the integrated effects of time and temperature for a range of possible burial and thermal histories of precipitated minerals on Mars. From this, we estimate thresholds beyond which these precipitates should have been converted to the point of non-detection in the presence of water. Surface water has been shown to be at least episodically present in recent times. Nonetheless, the integrated duration of aqueous activity recorded over geologically long intervals by hydrated amorphous silica, smectite clays and Fe-sulfate minerals suggests that where these minerals occur water did not persist much beyond their initial deposition. This geochemical conclusion converges with geomorphologic studies that suggest water limitation during the late Noachian-Hesperian peak of valley formation and a still more limited footprint of water since that time. In addition to documenting the presence of water and its chemical properties, a complete assessment of potentially habitable environments on Mars should address the timescales on which liquid water has persisted and the timing of aqueous episodes relative to major planetary events. B) 2009 Elsevier B.V. All rights reserved