ESC Publications - Cambridge Univesity
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Discharge, discharge variability, and the bedrock channel profile
No full textLong-term bedrock incision is driven by daily discharge events of variable magnitude and frequency, with ineffective events below an incision threshold. We explore theoretically how this short-term stochastic behavior controls long-term steady state incision rates and bedrock channel profiles, combining a realistic frequency-magnitude distribution of discharge with a deterministic, detachment-limited incision model in which incision rate is a power function of basal shear stress above a critical shear stress. Our model predicts a power law relationship between steady state slope and drainage area consistent with observations. The exponent of this power law is independent of discharge mean and variability, while the amplitude factor, which controls mountain belt relief, is a power law function of mean runoff (with an exponent of -0.5) and a complex function of runoff variability. In accordance with evidence that incision occurs between 6 and 20% of time in rapidly incising rivers (>1 mm/yr) our model predicts that channel steepness is virtually insensitive to runoff variability. Runoff variability can only decrease channel steepness for very slow incision rates and/or weak lithologies. The relationship between channel steepness and incision rate is always a power law whose exponent depends on the channel cross-sectional geometry and runoff variability. This contradicts models neglecting discharge stochasticity in which the steepness-incision scaling is set by the incision law exponent. Our results suggest that changes in climate variability cannot explain an increase in bedrock incision rates during the Late Cenozoic within the context of a detachment limited model
Cooling of the lower oceanic crust
No full textThermal models of mid-ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust. These models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast-spreading ridges. The rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros. Successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge, with ~50–70 kW of hydrothermal cooling for every 1 m of ridge axis at crustal ages of <0.1–0.4 Ma. These timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges
Thermal structure of oceanic and continental lithosphere
No full textRecent studies of the focal depths of earthquakes in old continental lithosphere have shown that they are almost entirely confined to the crust. Except where recent subduction of oceanic lithosphere is likely to have occurred, no earthquakes with a magnitude of > 5.5 have yet been located beneath the Moho. In contrast, in oceanic lithosphere earthquakes commonly occur within the mantle. The principal control on whether or not deformation occurs by brittle failure has long been believed to be temperature. We re-examine the thermal models of both oceans and shields. Taking account of the temperature dependence of the thermal conductivity lowers the temperature within the oceanic lithosphere. Except beneath the outer rises of trenches, where the strain rates are large, intraplate oceanic earthquakes are confined to regions cooler than 600 °C. In continental regions most earthquakes occur in the mobile belts that surround Archaean cratons, where the crust is as thick as 50–60 km. Recent studies, of the Canadian Shield in particular, have shown that radiogenic heating is not as concentrated at shallow depths as was previously believed. Taking account of both these effects and the temperature dependence of the thermal conductivity increases the Moho temperatures, which can exceed 600 °C, and produces geotherms that agree well with pressure and temperature estimates from nodule suites from kimberlites. Therefore the mechanical behaviour of oceanic and continental upper mantle appears to depend on temperature alone, and there is as yet no convincing evidence for any compositional effects
Temperature dependence of the cation distribution in CuAl2O4.
No full textThe temperature dependence of the cation distribution in CuAl2O4 spinel has been determined from 600 to 1100°C on
quenched specimens by powder X-ray diffraction, and from room temperature to 1000°C by in-situ high-temperature powder
neutron diffraction. The results, in agreement with earlier work, show that CuAl2O4 is a largely normal spinel, which is already
highly disordered at 600°C, the lowest temperature at which the cation distribution is inferred to be in equilibrium, with an inversion
parameter, x, at this temperature of 0.35 ± 0.005 from the neutron diffraction experiments. Increasing temperature to 1000°C
only causes a modest increase in x, to 0.40 ± 0.005. This somewhat unusual behaviour may be ascribed to local distortions from
the Jahn-Teller effect in both octahedrally and tetrahedrally coordinated Cu2+. Thermodynamically, the ordering may be described
either with a large entropy of disordering, or alternatively, with a large and positive quadratic term in the enthalpy of disordering.
The Jahn-Teller effect produces no long-range distortion of the structure, which has cubic symmetry (space group Fd–3m) under
all investigated condition
Gas-phase rate coefficients for the reactions of nitrate radicals with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene at room temperature
No full textRate coefficients for reactions of nitrate radicals (NO3) with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene were determined to be (6.55 +/- 0.78)x 10(-13) cm3 molecule(-1) s(-1), (3.78 +/- 0.45)x 10(-13) cm3 molecule(-1) s(-1), (5.30 +/- 0.73)x 10(-13) cm(3) molecule(-1) s(-1), (3.83 +/- 0.47)x 10(-13) cm(3) molecule(-1) s(-1), (4.37 +/- 0.49)x 10(-13) cm(3) molecule(-1) s(-1), (3.61 +/- 0.40)x 10(-13) cm3 molecule(-1) s(-1) and (8.9 +/- 1.5)x 10(-12) cm3 molecule(-1) s(-1), respectively. We performed kinetic experiments at room temperature and atmospheric pressure using a relative-rate technique with GC-FID analysis. The experimental results demonstrate a surprisingly large cis-trans(Z-E) effect, particularly in the case of the pent-2-enes, where the ratio of rate coefficients is ca. 1.7. Rate coefficients are discussed in terms of electronic and steric influences, and our results give some insight into the effects of chain length and position of the double bond on the reaction of NO3 with unsaturated hydrocarbons. Atmospheric lifetimes were calculated with respect to important oxidants in the troposphere for the alkenes studied, and NO3-initiated oxidation is found to be the dominant degradation route for (Z)-pent-2-ene, (Z)-hex-3-ene and (E)-3-methylpent-2-ene
Evidence for late Oligocene establishment of the Antarctic Circumpolar Current.
No full textRemoval of the last impediment to circum-Antarctic flow – opening of the Drake Passage between Antarctica and South
America – is essential to establishment of the ACC, and has been placed in both the earliest Oligocene and earliest Miocene on
the basis of paleomagnetic reconstructions. The Tasman Gateway between Antarctica and Tasmania notably deepened
following ~33.5 Ma. Completion of the ACC circuit has been seen as the key to major growth of the East Antarctic Ice
Sheet by thermal isolation of the continent. An alternative argument attributes ice sheet growth to changes in atmospheric
pCO2. We present evidence on current strength and water-mass properties suggesting a clear increase in flow speed and
homogeneity of Southern Ocean water masses in the latest Oligocene after about 23.95 Ma, marking establishment of the ACC,
attributable to deep opening of Drake Passage. The Mi-1 glaciation may be a consequence. Establishment of a permanent, low
altitude ice sheet does not appear to take place prior to mid-Miocene cooling
Chelicerate Arthropods, including the Oldest Phalangiotardbid Arachnid, from the Early Devonian (Siegenian) of the Rhenish Massif, Germany
No full textABSTRACT—A relatively diverse chelicerate fauna has been detected in Early Devonian, Siegenian strata of the Westerwald area,Rhineland-Palatinate, Germany. The arachnids, comprising trigonotarbids and the oldest phalangiotarbids, are described and figured
here along with the chasmataspidids. To accomodate the phalangiotarbid a new genus and species in the family Architarbidae, Devonotarbus hombachensis, is raised. Devonotarbus n. gen. is characterized by an approximately straight posterior carapace margin, abbreviated
and undivided anterior tergites, a large sixth tergite, and fused posterior tergites. The chasmataspidid closely resembles Diploaspis casteri from the Emsian assemblage of Alken an der Mosel, but is readily discernible as a new species, D. muelleri, by a strong tuberculation of its dorsal integument. With only fragmentary opisthosomal remains available, the trigonotarbids cannot be placed in
known taxa with any certainty at this tim
Using finite-element analysis to investigate suture morphology: a case study using large carnivorous dinosaurs.
No full textFinite-element analysis (FEA) can be used to investigate the mechanical significance of
sutures and regions of intracranial flexibility in skulls. By modeling the stress response to
feeding forces in a finite-element skull model (with appropriate boundary conditions), one can
compare the axis of distortion and orientation of stress and strain in the model to the degree
of movement at actual sutural contacts in the real skull. Hypotheses detailing the effect of
introducing patency or flexibility on mechanical performance can be constructed and subsequently
tested. In this study, the correlation between stress environment, cranial strength,
and sutural morphology and mobility is investigated in the cranium of the large theropod
dinosaur Allosaurus fragilis. Theropods are an especially interesting model system as their
skulls were massive (over 100 cm in some cases), may have generated extremely large bite
forces, yet patent sutures persisted between many of the facial bones. In this analysis, it was
discovered that Allosaurus cranial sutures appear generally capable of accommodating stress
and strain patterns generated during biting. This study highlights the potential of FEA in
devising and testing hypotheses of form and function and argues that useful information can
be obtained from finite-element models of extinct animals, providing that adequate assumptions
are made and appropriate questions asked
Crossover from classical to 3d-Ising critical behaviour near the antiferrodistortive phase transition of lawsonite.
No full textResults of X-ray scattering intensities and peak profiles of lawsonite single crystals are presented in a wide temperature range including the antiferrodistortive phase transition Cmcm - (Tc 270 K) → Pmcn. In the integrated intensities of the superlattice reflections above Tc pronounced pretransitional tails similar to those detected recently in birefringence [P. Sondergeld et al., Phys. Rev. B62, 6143 (2000)] and in excess entropy data [S. A. Hayward et al., Eur. J. Min. 14, 1145 (2002)] are observed. These tails correspond to diffuse Lorentzian shaped scattering peaks. The temperature variation of the diffuse scattering intensity and profile can be well described within a Landau-Ginzburg model. In combination with entropy data we obtain a complete set of free energy parameters. Using these coefficients we are able to fit the spontaneous part and the precursor tails of the integrated intensities, the excess birefringence and entropy in a broad temperature range outside a temperature interval of about ±3 K around Tc. In the vicinity of Tc the experimental data are in excellent agreement with the predictions of a crossover model, the effective critical exponent αeff(τ) varying from the classical (αeff = 0.5) to the 3d Ising limit (αeff = 0.11). Lawsonite therefore represents an ideal system to study the crossover from classical to critical behavior
Thermally-induced structural modification of dental enamel apatite: decomposition and transformation of carbonate groups.
No full textDental enamel ismainly composed of non-stoichiometric hydroxyapatite with A- and B-type carbonate groups in OH and
phosphate sites, respectively. Structural and chemical modifications of dental enamel apatite were studied using FTIR and XRD
techniques after heat treatment in air for 1h from 300 to 1193 K. Both IR and XRD results show a high degree of crystallinity of
apatite that is enhanced with increasing temperature. The loss of B-type and A-type carbonate was studied; the amount of B-type
carbonate and the total carbonate content decrease on heating while the amount of A-type carbonate first decreases up to 573 K and
then increases from 573 to 973 K. Almost 50 % of the carbonate ions were released from dental enamel with the formation of q -
tricalcium phosphate phase ( q -TCP) after heat treatment at 973 K for 1 h. The incorporation of CO2 and cyanate species in dental
enamel was observed in the temperature range of 273–973 K and 673–1073 K, respectively. The content of CO2 in dental enamel
increases from 473 K to a maximum near 773 K and decreases thereafter. The mechanism of the decomposition and transformation
of carbonate groups at different sites in enamel apatite structure is discussed