Jean-Louis Le Mouël

The age of the inner core

Abstract The energy conservation law, when applied to the Earth’s core and integrated between the onset of the crystallization of the inner core and the present time, gives an equation for the age of the inner core. In this equation, all the terms can be expressed theoretically and, given values and uncertainties of all relevant physical parameters, the age of the inner core can be obtained as a function of the heat flux at the core–mantle boundary and the concentrations in radioactive elements. It is found that in absence of radioactive elements in the core, the age of the inner core cannot exceed 2.5 Ga and is most likely around 1 Ga. In addition, to have an inner core as old as the Earth, concentrations in radioactive elements needed in the core are too high to be acceptable on geochemical grounds.

On cooling of the Earth's core

Abstract We have constructed a self-consistent model for cooling of the Earths core in which the thermal history of the core is computed as a function of the time evolution of the heat flux delivered to the mantle across the core-mantle boundary. The temperature profile in the convecting core is first assumed to be adiabatic, and its evolution in time is calculated with the only constraint that energy be globally conserved. When the temperature at the centre drops below the freezing point of the core alloy, the inner core starts growing and cools by conduction; it is found that it cannot have reached its present size in more than 1.7 billion years. If the heat flux delivered to the mantle becomes less than that conducted down the adiabat, the temperature profile becomes subadiabatic in a shell at the top of the core, through which heat is evacuated by conduction. Although it is stable against thermal convection, this shell is not necessarily stagnant and may be the seat of motions owing to compositional convection.

Does infiltration of core material into the lower mantle affect the observed geomagnetic field

Abstract A quantitative estimate of the depth of penetration of liquid iron from the core into the base of the lower mantle has been obtained, from laboratory data and theoretical models. The thickness of the infiltrated layer is found to be of the order of 1–100 m depending on the grain size of the mantle material. Even with the assumption that convection in the D″ layer entrains the electrically conducting core fluid over 100 km, the effective conductivity of the layer is hardly modified and the perturbation caused to the secular variation field is negligible. The same conclusion obtains in the improbable limiting case where all the infiltrated core fluid is gathered in a single mass.

GEOMAGNETIC FIELD DIRECTION IN PARIS SINCE THE MID-SIXTEENTH CENTURY

Abstract The geomagnetic field experienced at any point on the globe is essentially generated by dynamo processes in the Earths core. This main geomagnetic field is subject to change in its direction and strength on time-scales ranging from years to centuries, known as secular variation. Studies of secular variation can reveal much about the physics of the Earths core, but for this we must have homogeneous, high-quality measurements of the geomagnetic field covering as long a time span as possible. In this paper we present measurements of declination (for the years 1541–1883) and inclination (for the years 1660–1883) made in the vicinity of Paris. The data have been gathered from a variety of sources, and some analysis has been carried out to determine corrections for certain sub-series of measurements, so that the resulting time-series may be regarded as having been obtained from a single location—that of the Paris Observatory. To conduct this analysis we have had to consider the methods and instruments used by the early observers, and the likely errors in the measurements. Once compiled, the edited data from the Paris Observatory van be combined with those for the present-day observatory at Chambon-la-Foret, covering the years 1883–1994. The resulting time-series records changes in direction of the geomagnetic field spanning more than 450 years for declination and more than 320 years for inclination. The similarities and differences with the only other comparable time-series, i.e. that of London (Malin and Bullard, 1981, Philos. Trans. R. Soc. London, 299: 357–422) should be the focus of promising further work.

On the characteristics of successive geomagnetic jerks

Spherical harmonic models of the 1969, 1979 and 1992 geomagnetic jerks are computed using data from about 160 worldwide geomagnetic observatories. The dominance of the internal origin part with respect to the external one confirms again the internal origin of these events. A change of sign is observed between two successive jerks (1969–1979, 1979–1992). The acceleration jump of the fluid flow at the core mantle boundary (CMB) generating the three jerks is computed. Striking similarities between the three acceleration maps are observed (within the sign change mentioned above). These results suggest some long time scale memory in the processes that are responsible for the jerks. These processes remain to be elucidated.

The Earth's magnetic field: Which geometry?

Were it not for the presence of a solar wind, the intrinsic magnetic field of the Earth—if observed from far enough out in space—would appear to be almost perfectly dipolar, with the axis of the dipole presently tilted by some 10° with respect to the rotation axis. At the Earths surface, the axial dipolar term is dominant, which serves among other uses as a basis for both orientation with a compass and plate-tectonic applications of paleomagnetism. One can ask whether this dipolar dominance would hold were the field observed from just above its source at the core-mantle boundary. The answer is that indeed it does (Figure 1), although of course the dipolar part is reduced in amplitude relative to the higher-order (shorter wavelength) terms. These higher-order terms are compatible with a flat, white-noise-like, power spectrum. This implies that there are similar amounts of energy stored in the various harmonic terms (see, for example,Constable and Parker [1988]), although a “pink” spectrum (that is, slightly more power at the longer wavelengths) is as plausible [Hulot et al., 1992]. Terms beyond degree and order 13 (that is, wavelengths shorter than 1500 km) are contaminated by crustal and lithospheric (that is, surficial static) fields.

Time evolution of the fluid flow at the top of the core. Geomagnetic jerks

The knowledge of the geomagnetic field and its secular variation allows us to compute the fluid flow at the core surface. The poloidal and toroidal components of the fluid flow at the core-mantle boundary (CMB) have been calculated every year from the Bloxham and Jackson model (1992) and plotted at 50 year intervals over the last three centuries. The flow patterns conserve some broad features over this whole time-span. The time constant of the degree 1 component of the motion is larger than the time constant of the rest of the flow. The average motion over 300 years appears to be in large part symmetrical with respect to the equator. This average flow can be represented by the sum of a few geostrophic vectors. The acceleration fields corresponding to the well documented jerks of 1969, 1979, 1992 have also been computed. The geometry of these acceleration fields is the same, within a change of sign, for the three events. Moreover, this geometry has close connections with the geometry of the flow itself. The spatial and temporal variations of the flow field can be simply described, in a first approximation; it is possible to give an analytical schematic representation of the flow field during the last three decades. Some characteristics of the decadal length-of-day variations follow if the coupling torque between core and mantle is topographic.

The volcanic activity of La Soufrière of Guadeloupe (lesser antilles): structural and tectonic implications

Abstract Among observations made on La Soufriere volcano (Guadeloupe) to monitor its its activity are temperature measurements: temperature of fumaroles, hot springs, ground surface, and water inside two boreholes 80 and 90 m deep. Those measurements proved to be of prime importance in understanding mechanisms working in the volcano and governing its activity. They lead with other observations, such as self-potential measurements and geochemical analysis of fumaroles and hot springs, to a possible interpretation of phreatic activity rather different from the one retained earlier which was essentially based on seismic data. In this model the magma reservoir is only in a stable state and, in particular, supplies heat at a constant rate to the overlying geological structures. The cycle of volcanic activity and period of quiescence is interpreted as due to a cycle of clogging and reopening of the system of fractures transferring heat from a lower aquifer to an upper one, and eventually to the atmosphere. The model is discussed in regard of other geophysical observations, in particular seismicity. Some inferences are made concerning a new phase of activity and suitable observations to predicit it.

Electrokinetic and magnetic fields generated by flow through a fractured zone: A Sensitivity study for La Fournaise Volcano

A number of electric and magnetic signals have been observed on La Fournaise volcano, attributed to electrokinetic effects. A simple model is proposed here to check this hypothesis. The volcano is idealized as a 2d heterogeneous structure composed of a fractured zone located between two porous zones through which meteoritic waters flow downwards; flow occurs predominantly in the fractured zone and induces electromagnetic fields. Correct orders of magnitude are obtained for the measured surface fields. The importance of the heterogeneous character of the underground medium is demonstrated; local measurements of various quantities are recommended.

Field characterization of the relationship between electrical potential gradients and soil water flux

Electrical potential differences between electrodes installed vertically at four depths (0.3, 0.5, 0.7 and 0.8 m) were monitored continuously during a 2-month period in a measurement site under natural fallow. Simultaneously, changes in soil water content and in hydraulic head were measured on a daily basis at different depths of the soil profile at the same site. They were analysed to obtain daily values of the soil water flux at the depth z = 0.4 m. This was in particular carried out over a 10-day period following a rainfall event. At that depth the water flux was first oriented downwards (infiltration), then shifted progressively upwards (evaporation). It is clearly shown that there exists a very significant linear correlation between the electric potential gradient at that level and the value of the flux. Owing probably to electrode potential problems, there is a residual value when the flux is null. If the relationship is legitimately forced through the origin, it becomes clear that electrode measurements could be used to infer water circulation in the soil in terms of direction and amount of flow.