Meireles R.P.,University of The Azores |
Quartau R.,Instituto Portugues do Mar e da Atmosfera |
Ramalho R.S.,Institute For Geophysik |
Ramalho R.S.,University of Bristol |
And 5 more authors.
Sedimentology | Year: 2013
Oceanic islands - such as the Azores in the mid-North Atlantic - are periodically exposed to large storms that often remobilize and transport marine sediments along coastlines, and into deeper environments. Such disruptive events create deposits - denominated tempestites - whose characteristics reflect the highly dynamic environment in which they were formed. Tempestites from oceanic islands, however, are seldom described in the literature and little is known about storm-related sediment dynamics affecting oceanic island shelves. Therefore, the geological record of tempestite deposits at oceanic islands can provide invaluable information on the processes of sediment remobilization, transport and deposition taking place on insular shelves during and after major storms. In Santa Maria Island (Azores), a sequence of Neogene tempestite deposits was incorporated in the island edifice by the ongoing volcanic activity (thus preserved) and later exposed through uplift and erosion. Because it was overlain by a contemporary coastal lava delta, the water depth at the time of deposition could be inferred, constituting an excellent case-study to gain insight on the still enigmatic processes of insular shelf deposition. Sedimentological, palaeontological, petrographic and palaeo-water depth information allowed the reconstruction of the depositional environment of these sediments. The sequence typifies the characteristics of a tempestite (or successive tempestites) formed at ca 50 m depth, in a steep, energetic open insular shelf, and with evidence for massive sediment remobilization from the nearshore to the middle or outer shelf. The authors claim that cross-shelf transport induced by storm events is the main process of sediment deposition acting on steep and narrow shelves subjected to high-energetic environments, such as the insular shelves of open-sea volcanic islands. © 2013 International Association of Sedimentologists.
Leblanc F.,University Pierre and Marie Curie |
Chassefiere E.,University Paris - Sud |
Chassefiere E.,French National Center for Scientific Research |
Gillmann C.,Institute For Geophysik |
Breuer D.,German Aerospace Center
Icarus | Year: 2012
Noble gas 40Ar may be used as a tracer of the past evolution of volatiles in Mars' crust, mantle and atmosphere. 40Ar is formed by the radioactive decay of 40K in the mantle and in the crust and is released from the mantle to the atmosphere due to volcanism and from the crust by erosion such as eolian and hydrothermal erosion. Furthermore, 40Ar can escape from the atmosphere into space via atmospheric escape mechanisms. The evolution of the atmospheric abundance of 40Ar thus depends on these three processes whose efficiencies vary with time.In the present study we reconsider atmospheric escape mechanism efficiencies and describe various possible scenarios of the evolution of 40Ar with a model describing the three main reservoirs of 40Ar, the mantle, crust and atmosphere. First, we show that atmospheric escape, which is stronger in the early evolution, does not significantly influence the present abundance of the atmospheric 40Ar. In the early evolution the atmospheric concentration of 40Ar is very low as the outgassing of 40Ar from the mantle occurs relatively late in the martian evolution. Thus, the atmospheric 40Ar concentration is essentially a tracer of Mars' outgassing history and not of the escape processes. Second, using the results of the most recent published crustal formation models, the calculated present 40Ar atmospheric abundance is smaller than its observed value. This discrepancy may be explained by a significant 40Ar supply from the crust by erosion (16-30% of the 40Ar content of the upper first 10km of crust). The knowledge of the fraction of crustal 40Ar outgassed to the atmosphere is an important constraint for any future global modelling of past Mars' hydrothermal activity aiming at better characterizing the role of subsurface aqueous alteration processes in Mars climate evolution. One of the main sources of the uncertainty of these results is the present uncertainty in the measured atmospheric 40Ar value (±20%). More precise measurements of 40Ar and 36Ar in the martian atmosphere are therefore required to better constrain the model. © 2012 Elsevier Inc.
Durand S.,University Claude Bernard Lyon 1 |
Durand S.,Institute For Geophysik |
Debayle E.,University Claude Bernard Lyon 1 |
Ricard Y.,University Claude Bernard Lyon 1 |
Lambotte S.,University of Strasbourg
Geophysical Research Letters | Year: 2016
We present SEISGLOB1, a pure SV tomographic model of Earth's mantle based on Rayleigh phase velocities and normal mode self- and cross-coupling data. SEISGLOB1 is the first model that incorporates the cross-coupling of normal modes since the pioneering work of Resovsky and Ritzwoller (1999). The simultaneous inversion of new cross-coupling normal modes and self-coupling of high-order normal modes measured by Deuss et al. (2013) and Stoneley modes measured by Koelemeijer et al. (2013) allows us to show that the velocity structure at the base of the mantle is more complex than that expected from a dominant spherical harmonic degree 2 and that the relative strength of odd degrees has previously been underestimated. Near the core-mantle boundary, the large low-shear-velocity provinces are less homogeneous than in previous studies, and various local maxima, often potentially associated with hot spot sources, are observed. ©2016. American Geophysical Union. All Rights Reserved.
Cheng J.S.,University of California at Los Angeles |
Stellmach S.,Institute For Geophysik |
Ribeiro A.,University of California at Los Angeles |
Grannan A.,University of California at Los Angeles |
And 2 more authors.
Geophysical Journal International | Year: 2015
We present laboratory and numerical models investigating the behavioural regimes of rapidly rotating convection in high-latitude planetary core-style settings. Our combined laboratorynumerical approach, utilizing simplified geometries, can access more extreme parameters (e.g. Rayleigh numbers Ra ≲ 1013; Nusselt numbers Nu ≲ 103; Ekman numbers E ≳ 3 × 10-8) than current global-scale dynamo simulations. Using flow visualizations and heat transfer measurements, we study the axialized flows that exist near the onset of rotating convection, as well as the 3-D flows that develop with stronger forcing. With water as the working fluid (Prandtl number Pr ≲ 7), we find a steep scaling trend for rapidly rotating convective heat transfer, Nu~(Ra/RaC)3.6, that is associated with the existence of coherent, axialized columns. This rapidly rotating trend is steeper than the trends found at moderate values of the Ekman number, and continues a trend of ever-steepening scalings as the rotation rate of the system is increased. In contrast, in more strongly forced or lower rotation rate cases, the heat transfer scaling consistently follows a shallower slope equivalent to that of non-rotating convection systems. The steep heat transfer scaling in the columnar convection regime, corroborated by our laboratory flow visualizations, imply that coherent, axial columns have a relatively narrow range of stability. Thus, we hypothesize that coherent convection columns are not stable in planetary core settings,where the Ekman number is estimated to be~10-15. As a consequence, convective motions in the core may not be related to the columnar motions found in presentday global-scale models. Instead, we hypothesize that turbulent rotating convection cascades energy upwards from 3-D motions to large-scale quasi-2-D flow structures that are capable of efficiently generating planetary-scale magnetic fields. We argue that the turbulent regimes of rapidly rotating convection are essential aspects of core dynamics and will be necessary components of robust, next-generation and multiscale dynamo models. © The Authors 2015.
Markovic S.B.,University of Novi Sad |
Hambach U.,University of Bayreuth |
Stevens T.,Royal Holloway, University of London |
Kukla G.J.,Lamont Doherty Earth Observatory |
And 5 more authors.
Quaternary Science Reviews | Year: 2011
The Stari Slankamen loess-palaeosol section is located on the northeastern part of the Srem Loess Plateau (Vojvodina region, North Serbia). The c. 40-m thick cliff comprises loess intercalated with 9 major palaeo pedocomplexes and can be considered to be one of the most important Quaternary sections in the Carpathian (Panonnian) basin. Here we present new magnetostratigraphic and aminostratigraphic evidence that demonstrates the importance of the site in terms of its age and the long-term palaeoclimatic record it preserves.Directional palaeomagnetic data, obtained through alternating field demagnetization demonstrates the presence of reversed polarity below a profile depth of 36 m indicating a Matuyama chron age of this interval. This interpretation is confirmed by new high resolution palaeomagnetic investigations (434 oriented samples) from the lower part of the profile. The new magnetic susceptibility record and aminostratigraphy indicate a missing pedocomplex (V-S2), with an erosional unconformity represented by a distinct gravel layer. The combined new magnetostratigraphic and aminostratigraphic based age model requires a significant revision of hitherto published chronostratigraphic subdivisions at the site.The relative completeness and long time frame covered by the section is unusual in European loess sequences. Hence, the sequence could form the basis of a continental scale stratigraphic scheme that would alleviate much current chronostratigraphic uncertainty and enable more broad-scale climatic reconstructions. The section also provides a rare opportunity to investigate detailed and long-term climatic change over the Middle Pleistocene in a region influenced by air masses originating from high and middle latitudes, as well as the North Atlantic and Mediterranean. The changing relative importance of these air masses through time provides insight into local and regional atmospheric systems and their evolution through the last c.1 Ma. The section can thus be considered as one of the key climatic archives in the Europe. © 2011 Elsevier Ltd.
Colombi A.,ETH Zurich |
Colombi A.,Joseph Fourier University |
Nissen-Meyer T.,ETH Zurich |
Nissen-Meyer T.,University of Oxford |
And 2 more authors.
Geophysical Journal International | Year: 2014
The topography of the core-mantle boundary (CMB) is directly linked to the dynamics of both the mantle and the outer core, although it is poorly constrained and understood. Recent studies have produced topography models with mutual agreement up to degree 2. A broad-band waveform inversion strategy is introduced and applied here, with relatively low computational cost and based on a first-order Born approximation. Its performance is validated using synthetic waveforms calculated in theoretical earth models that include different topography patterns with varying lateral wavelengths, from 600 to 2500 km, and magnitudes (∼10 km peak-to-peak). The source-receiver geometry focuses mainly on the Pdiff, PKP, PcP and ScS phases. The results show that PKP branches, PcP and ScS generally perform well and in a similar fashion, while Pdiff yields unsatisfactory results. We investigate also how 3-D mantle correction influences the output models, and find that despite the disturbance introduced, the models recovered do not appear to be biased, provided that the 3-D model is correct. Using cross-correlated traveltimes, we derive new topography models from both P and S waves. The static corrections used to remove the mantle effect are likely to affect the inversion, compromising the agreement between models derived from P and S data. By modelling traveltime residuals starting from sensitivity kernels, we show how the simultaneous use of volumetric and boundary kernels can reduce the bias coming from mantle structures. The joint inversion approach should be the only reliable method to invert for CMB topography using absolute cross-correlation traveltimes. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.
Mandea M.,CNRS Paris Institute of Global Physics |
Holme R.,University of Liverpool |
Pais A.,University of Coimbra |
Pinheiro K.,National Observatory |
And 2 more authors.
Space Science Reviews | Year: 2010
The secular variation of the core field is generally characterized by smooth variations, sometimes interrupted by abrupt changes, named geomagnetic jerks. The origin of these events, observed and investigated for over three decades, is still not fully understood. Many fundamental features of geomagnetic jerks have been the subject of debate, including their origin internal or external to the Earth, their occurrence dates, their duration and their global or regional character. Specific tools have been developed to detect them in geomagnetic field or secular variation time series. Recently, their investigation has been advanced by the availability of a decade of high-quality satellite measurements. Moreover, advances in the modelling of the core field and its variations have brought new perspectives on the fluid motion at the top of the core, and opened new avenues in our search for the origin of geomagnetic jerks. Correlations have been proposed between geomagnetic jerks and some other geophysical observables, indicating the substantial interest in this topic in our scientific community. This paper summarizes the recent advances in our understanding and interpretation of geomagnetic jerks. © 2010 Springer Science+Business Media B.V.
Auer L.,Institute For Geophysik |
Boschi L.,CNRS Paris Institute of Earth Sciences |
Becker T.W.,University of Southern California |
Nissen-Meyer T.,University of Oxford |
Giardini D.,Institute For Geophysik
Journal of Geophysical Research: Solid Earth | Year: 2014
We present a tomographic model of radially anisotropic shear velocity variations in the Earth's mantle based on a new compilation of previously published data sets and a variable block parameterization, adapted to local raypath density. We employ ray-theoretical sensitivity functions to relate surface wave and body wave data with radially anisotropic velocity perturbations. Our database includes surface wave phase delays from fundamental modes up to the sixth overtone, measured at periods between 25 and 350 s, as well as cross-correlation traveltimes of major body wave phases. Before inversion, we apply crustal corrections using the crustal model CRUST2.0, and we account for azimuthal anisotropy in the upper mantle using ray-theoretical corrections based on a global model of azimuthal anisotropy. While being well correlated with earlier models at long spatial wavelength, our preferred solution, savani, additionally delineates a number of previously unidentified structures due to its improved resolution in areas of dense coverage. This is because the density of the inverse grid ranges between 1.25 in well-sampled and 5 in poorly sampled regions, allowing us to resolve regional structure better than it is typically the case in global S wave tomography. Our model highlights (i) a distinct ocean-continent anisotropic signature in the uppermost mantle, (ii) an oceanic peak in above average ξ<1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness, and (iii) a long-wavelength pattern of ξ<1 associated with the large low-shear velocity provinces in the lowermost mantle. ©2014. American Geophysical Union. All Rights Reserved.
Waszek L.,University of Cambridge |
Thomas C.,Institute For Geophysik |
Deuss A.,University Utrecht
Geophysical Research Letters | Year: 2015
Precursors to the core phase PKP are generated by scattering of seismic energy from heterogeneities in the mantle. Here we examine a large global data set of PKP precursors in individual seismograms and array data, to better understand scattering locations. The precursor amplitudes from individual seismograms are analyzed with respect to the inner core phase PKIKP and mantle phase PP. We find and correct for a hemispherical asymmetry in the precursor/PKIKP amplitudes, resulting from inner core structure. Using ray tracing, we locate scatterers in our array data and use these to infer scattering locations in the individual data. The scattering strength displays regional variation; however, we find no relationship with long-scale core-mantle boundary velocity structure. Scattering is observed in all regions of data coverage, as are paths with no precursors. This indicates that scattering occurs from various small-scale heterogeneities, including but not limited to ultralow velocity zones or partial melt, and slabs. ©2015. American Geophysical Union. All Rights Reserved.
King E.M.,University of California at Berkeley |
Stellmach S.,Institute For Geophysik |
Buffett B.,University of California at Berkeley
Journal of Fluid Mechanics | Year: 2013
Rotating Rayleigh-Bénard convection provides a simplified dynamical analogue for many planetary and stellar fluid systems. Here, we use numerical simulations of rotating Rayleigh-Bénard convection to investigate the scaling behaviour of five quantities over a range of Rayleigh (103 Ra 109), Prandtl (1≤Pr100) and Ekman (10-6E) numbers. The five quantities of interest are the viscous and thermal boundary layer thicknesses, δv and δ T , mean temperature gradients, β , characteristic horizontal length scales, ℓ , and flow speeds, Pe}. Three parameter regimes in which different scalings apply are quantified: non-rotating, weakly rotating and rotationally constrained. In the rotationally constrained regime, all five quantities are affected by rotation. In the weakly rotating regime, δT, β and Pe, roughly conform to their non-rotating behaviour, but δv and ℓ are still strongly affected by the Coriolis force. A summary of scaling results is given in table 2. © 2013 Cambridge University Press.