Internal and forced variability along a section between Greenland and Portugal in the CLIPPER Atlantic model

Numerical models are used to estimate the meridional overturning and transports along the paths of two hydrographic cruises, carried out in 1997 and 2002 from Greenland to Portugal. We have examined the influence of the different paths of the two cruises and found that it could explain 0.4 to 2 Sv of difference in overturning (the precise value is model-dependent). Models show a decrease in the overturning circulation between 1997 and 2002, with different amplitudes. The CLIPPER ATL6 model reproduces well the observed weakening of the overturning in density coordinates between the cruises; in the model, the change is due to the combination of interannual and high-frequency forcing and internal variability associated with eddies and meanders. Examination of the -coordinate overturning reveals model–data discrepancies: the vertical structure in the models does not change as much as the observed one. The East Greenland current variability is mainly wind-forced in the ATL6 model, while fluctuations due to eddies and instabilities explain a large part of the North Atlantic Current variability. The time-residual transport of dense water and heat due to eddy correlations between currents and properties is small across this section, which is normal to the direction of the main current.     

Extinction, survival and recovery of corals from the Triassic to Middle Jurassic time

Recognizing extinction events and determining their cause at the Triassic/Jurassic (T/J) transition and near the Pliensbachian–Toarcian (Lower Jurassic) boundary is a field of growing interest. We provide arguments for these events through a literature based new evaluation of coral diversity from Triassic to Dogger and a new palaeobiogeographical map. The T/J extinction of corals is clearly related to the breakdown of reef environments. Origination curves show that Hettangian (the lowest Jurassic stage) was not only a survival phase but already rather a recovery phase. Post-extinction evolution of reefs and their survival only in the northernmost margin of the Tethys support the hothouse hypothesis for the T/J extinction event. During Pliensbachian, many new taxa appear, but mostly solitary corals, not really framebuilders. Many of these taxa do not occur anymore during the following stages. The new increase in diversity is related to the development of Bajocian (Middle Jurassic) reefs.     

Surface-rupturing history of the Bree fault scarp, Roer Valley graben: Evidence for six events since the late Pleistocene

Since 1996 paleoseismological investigations have been used to develop the surface- rupturing history of the Bree fault scarp, the morphologically best-defined segment of the southwestern border fault of the Roer Valley graben in northeastern Belgium. The first studies determined that the escarpment is associated with a surface fault, and they exposed evidence for three surface displacements since about 40 ka BP. The most recent eventprobably occurred between 1000 and 1350 yr cal BP. Geophysical and trenching studies at a new site near the southeastern end of the fault scarp reconfirmed the coincidence of the frontal escarpment with a shallow normal fault, which displaces the Middle Pleistocene `Main Terrace' of the Maas River, as well as overlying coversands of Saalian to late Weichselian age. Different amounts of displacement shown by the two youngest coversand units indicate two discrete faulting events, but primary evidence for the coseismic nature of these events is sparse. Radiocarbon and optically stimulated luminescence dating constrainthe age of these events to the Holocene and between 14.0 ± 2.3 ka BP and 15.8 ± 2.9 ka BP, respectively. In addition, four older surface-rupturing events are inferred from the presence of four wedge-shaped units of reworked Main Terrace deposits that are interbedded with coversand units in the hanging wall of the trench and in shallow boreholes. These wedges are interpreted as colluvial wedges, produced by accelerated slope processes in response torejuvenation of the fault scarp, most probably in a periglacial environment. Luminescence dating indicates that five out of a total of six identified faulting events are younger than 136.6 ± 17.6 ka. The antepenultimate event was the largest faulting event, associated with a total fault displacement in excess of 1 m. Thus, the newly investigated trench site represents the longest and most complete record of surface rupturing recovered so far along the Bree fault scarp. This study also demonstrates the viability of the paleoseismological approach to identify past large earthquakes in areas of present-day moderate to low seismic activity.     

An Application of Multivariate Simulation in the Cement Industry

The profitability of a cement plant depends largely on its capacity to produce homogeneous cement with chemical composition close to specified targets for the cement type produced. One crucial step is the mixing of limestone with other raw materials in proportions calculated to meet these targets. Major design and operation decisions depend on the efficiency of this homogenizing step. The adequate modeling of the mixing process requires simulation of representative cross-correlated time series of chemical compositions of the raw materials involved. The chemical composition signals are obtained by multivariate geostatistical simulation using an LU (Cholesky) decomposition of the covariance matrix. Modifications to the usual LU method are presented. First, the effect on the raw covariance matrix of the closure property of chemical analysis is imposed. Second, the problem of memory space limitations in the LU method is tackled by using overlapping sliding neighbourhoods. The simulation algorithm is applied to the Joppa cement plant owned by Lafarge North America. The simulated raw material input streams are fed into the quality mix control (QMC), a proprietary software that models and controls the mixing operation to produce an output stream with cement characteristics as close as possible to desired targets. Two signal series are studied, one autocorrelated with a moderate temporal range and one with no autocorrelation. The QMC produces C₃S output signals having comparable short scale periodic variograms except that the variance of the uncorrelated signal is four times greater than those of the autocorrelated signal and the real Joppa data. The raw material feeder variograms have the same sill for both the white noise and the autocorrelated signals. However, the autocorrelated signal feeder variogram presents lower short term dispersion variance, a characteristic feature of Joppa operations. Our results show the importance of simulating the right temporal structure of the raw materials to realistically forecast the behavior of the output signals. We also discuss some practical implications of these findings for the design and operation of a cement plant.     

Compressibility of Ca3Al2Si3O12 perovskite up to 55 GPa

Compressibility of perovskite-structured Ca₃Al₂Si₃O₁₂ grossular (GrPv) was investigated at high pressure and high temperature by means of angle-dispersive powder X-ray diffraction using a laser-heated diamond anvil cell. We observed the Pbnm orthorhombic distortion for the pure phase above 50 GPa, whereas below this pressure, Al-bearing CaSiO₃ perovskite coexists with an excess of corundum. GrPv has a bulk modulus (K ₀ = 229 ± 5 GPa; \(K_{0}^{{\prime }}\) fixed to 4) almost similar to that reported for pure CaSiO₃ perovskite. Its unit-cell volume extrapolated to ambient conditions (V ₀ = 187.1 ± 0.4 Å³) is found to be ~2.5 % larger than for the Al-free phase. We observe an increasing unit-cell anisotropy with increasing pressure, which could have implications for the shear properties of Ca-bearing perovskite in cold slabs subducted into the Earth’s mantle.     

Rotating micro-structures in Antarctic cold basal ice: implications for glacier flow and its interpretation

Structural analyses were conducted in the basal zone of an Antarctic glacier. The studied basal ice sequence was retrieved from a 20-m-long subglacial tunnel dug at the margin of the glacier and is at the temperature of −17°C. For the first time, rotating clast systems embedded within debris-rich ice were thin-sectioned using specially designed cutting techniques. The observed structures reflect the occurrence of pervasive shearing at the base of the glacier, and can be used as shear sense indicators. In addition, some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the ice. It is showed here that cautious analysis of deformation structures present in debris-bearing ice may bring insights not only into the flow dynamics of the embedding matrix, but also into the behaviour of the interstitial fluid network at the base of cold glaciers and ice sheets.     

Numerical convergence of the dynamics of a GCM

 Atmospheric general circulation models (GCMs) are characterized by many features but especially by: (1) the manner of discretizing the governing equations and of representing the variables involved at a given resolution, and (2) the manner of parameterizing unresolved physical processes in terms of those resolved variables. These two aspects of model formulation are not independent and it is difficult to untangle their intertwined effects when assessing model performance. The attempt here is to separate these aspects of GCM behaviour and to ask, “Given a perfect parameterization of the physical processes in a model, what resolution is needed to capture the dominant dynamical aspects of the atmospheric climate?” Alternatively, “At what resolution do the dynamics of a GCM converge”? The perfect parameterization approach assumes that the calculation of the physical terms returns the “correct” result at all resolutions. In the idealized case, a time-independent forcing is one of the simplest that satisfies this condition. However, experiments show that it is difficult for the dynamics of a GCM to balance a time-independent forcing with atmosphere-like flows and structures. The model requires, and the atmosphere presumably includes, physical feedback mechanisms which act so as to maintain the kinds of flows and structures that are observed. Resolution experiments are performed with a simplified forcing function for the thermodynamic equation which combines a dominant time-independent specified forcing with a weak linear relaxation feedback. These experiments show that the dynamics of the GCM have essentially converged at T32 and certainly by T63 which is the next resolution considered. This is shown by the constancy of structures, variances, covariances, transports and energy budgets with increasing resolution. Experiments with an alternative forcing proposed by Held and Suarez, which has the form of a linear relaxation, show somewhat less evidence of convergence at these resolutions. In both cases the “physics” are known by assumption. However, the form and nature of the forcing is different, as is the behaviour with resolution. The implication for the real system is that the resolution required for simulating the dynamical aspects of climate is rather modest. The simulated climate does, however, apparently depend on the ability to correctly and consistently parameterize the physical processes in a GCM, involving both forcing and feedback mechanisms, as a function of resolution. Received 19 January 1996/Accepted 22 August 1996