Toward a generic method for studying water renewal, with application to the epilimnion of Lake Tanganyika

We present a method, based on the concept of age and residence time, to study the water renewal in a semi-enclosed domain. We split the water of this domain into different water types. The initial water is the water initially present in the semi-enclosed domain. The renewing water is defined as the water entering the domain of interest. Several renewing water types may be considered depending on their origin. We present the equations for computing the age and the residence time of a certain water type. These timescales are of use to understand the rate at which the water renewal takes place. Computing these timescales can be achieved at an acceptable extra computer cost.The above-mentioned method is applied to study the renewal of epilimnion (i.e. the surface layer) water in Lake Tanganyika. We have built a finite element reduced-gravity model modified to take into account the water exchange between the epilimnion and the hypolimnion (i.e. the bottom layer), the water supply from precipitation and incoming rivers, and the water loss from evaporation and the only outgoing river. With our water renewal diagnoses, we show that the only significant process in the renewal of epilimnion water in Lake Tanganyika is the water exchange between the epilimnion and the hypolimnion, other phenomena being negligible.     

Experimental survey of the hydrodynamic performance of a small spar buoy

The objective in this experimental work is to evaluate the capability of several cylindrical buoys to follow and measure waves. Eleven configurations have been studied, eight among which were furnished with disks of different diameters at the waterline. The experiments took place in the test basin at ISITV. On the one hand, we have obtained a set of transfer coefficients for each frequency and each configuration in regular waves. On the other hand, we have determined the corresponding transfer functions, then we have used the latter to measure irregular waves. The time signals as measured by the buoys with and without corrections have been compared with wave gage measurements and subjected to a wave-by-wave analysis. The results allow the conclusion that the buoy with a medium size disk constitutes the best compromise. In fact, it permits the buoy to follow the free surface while minimizing parasitic motions.     

The role of organic coatings in the enrichment of marine particles with selenium. The fixation of selenite on an adsorbed amino acid

The enrichment of marine particles with selenium cannot be explained as that with metals present in cationic form by complexation with anionic functional groups of adsorbed organic matter. Physicochemical data obtained using a model system are reported. The surface of the particles is modelized by a mercury electrode whose surface charge density can be easily changed, covered by a layer of adsorbed polymers. Studies with different kinds of macromolecules and salts show a specific interaction between adsorbed polyalanine and selenite. The results can be explained by the concordance of the distances between two oxygen atoms in this oxyanion and between two amine groups of the adsorbed polypeptide. A similar mechanism could occur in marine aggregates whatever their nature, as long as they contain amine groups at their surface which result in this concordance. Some prospects derived from the results are discussed.     

Influence of submerged aquatic vegetation on size class distribution of perch (Perca fluviatilis) and roach (Rutilus rutilus) in the littoral zone of Lake Geneva (Switzerland)

The abundance of different size classes of perch and roach in the littoral zone of Lake Geneva was compared between submerged aquatic vegetation and unvegetated zones. Samples were taken with gillnets during four periods between June and October 1993. During the vegetation period (June to September), perch 9 cm and roach 10 cm were more abundant in vegetation whereas roach > 20 cm were more abundant in open water. Perch larger than 18 cm and medium roach were equally distributed in both habitats whatever the period, whereas medium perch distribution fluctuated according to the period. In October, after the decline of the vegetation, no more differences in fish distribution were observed except for small roach, which were always more abundant in the vegetated sites.     

A model of an exploding,radiating star in general relativity

In a previous paper Adams, Cary and Cohen (1994) presented a model of a supernova. In that paper the equations of General Relativity describing the evolution of a spherically symmetric, radiating star were solved analytically. The evolution of the star was determined by the application of boundary conditions at the center and at the edge. Due to lmitations in the presupernova model, only the very slow inward motion of an unstable, degenerate core could be considered. The solution was also limited by the need to exclude a runaway term, one that increased exponentially with time. Without the exclusion of the runaway, the luminosity would have increased without bound and the mass would have become negative.This paper presents a completely analytic solution to the equations of General Relativity describing the evolution of a Type II supernova. Professor S.E. Woosley kindly gave us data on the physical variables of a 12M ₀ presupernova star. In our model the core collapses within 1 s, leaving a 1.3M ₀ remnant. Shortly afterward 10.6M ₀ is ejected to infinity, and 0.17M ₀ is radiated away in the form of neutrinos. The distance of the edge from the center increases proportionally to the two-thirds power of the time. The luminosity decreases proportionally to the inverse four-thirds power.Although the runaway solution was modified by the exploding rather than a static envelope, it must still be excluded by adjusting initial conditions. Its character is changed from an exponential to a very large power (55) of time. The removal of a degree of freedom by this exclusion leads to physically non-sensical results such as negative luminosity. The inclusion of a term describing motion of the mantle due to neutrino interactions provides the additional degree of freedom necessary for physically reasonable results.     

Origin and thermal evolution of icy satellites

The paper reviews the problem of formation and evolution of the so-called regular satellites of the giant planets, and it consists of two parts: the first describes the possible origin of the satellites, the second studies their evolution, attempting to stress the relations of the present status of the satellites with their evolutionary history.The formation of regular satellite systems around giant planets is probably related to the formation of the central planet. Some characteristics of regular satellite systems are quite similar, and suggest a common origin in a disk present around the central body. This disk can originate through different mechanisms which we will describe, paying attention to the so-called accretion disk model, in which the satellite-forming material is captured. The disk phase links the formation of the primary body with the formation of satellites. The subsequent stages of the disk's evolution can lead first to the formation of intermediate size bodies, and through the collisional evolution of these bodies, to the birth of satellite embryos able to gravitationally capture smaller bodies.Given the scenario in which icy satellites may be formed by homogeneous accretion of planetesimals made of a mixtures of ice and silicates, if no melting occurs during accretion, the satellites have a homogeneous ice-rock composition. For the smaller satellites this homogeneous structure should not be substantially modified; only sporadic local events, such as large impacts, can modify the surface structure of the smaller satellites. For the larger satellites, if some degree of melting appears during accretion, a differentiation of the silicate part occurs, the amount of differentiation and hence the core size depending on the fraction of gravitational potential energy retained during the accumulation process. Melting and differentiation soon after the accretion, for the larger satellites, could also depend on the convective evolution in presence of phase transitions and generate an intermediate rock layer, considerably denser than the underlying, still homogeneous core, and unstable to overturning on a geologic time scale. Moreover the liquid water mantle could be a transient feature because the mantle would freeze over several hundred million years. For these large bodies the stable configuration is expected to be one consisting of a silicate core and a mantle of mixed rock and ice.     

Radius of convergence of Lie series for some elliptic elements

For equatorial orbits about an oblate body, we show that the Lie series for the elliptic elementse,f,l and diverge when the oblateness exceeds a critical multiple of the transformed eccentricity constant. The use of similar truncated series expansions for such elliptic elements by Brouwer accounts for the first-order errors at low eccentricity in his derived coordinates for an artificial satellite.     

Late Holocene linkages between decade–century scale climate variability and productivity at Lake Tanganyika, Africa

Microlaminated sediment cores from the Kalya slope region of Lake Tanganyika provide a near-annually resolved paleoclimate record between ∼∼2,840 and 1,420 cal. yr B.P. demonstrating strong linkages between climate variability and lacustrine productivity. Laminae couplets comprise dark, terrigenous-dominated half couplets, interpreted as low density underflows deposited from riverine sources during the rainy season, alternating with light, planktonic diatomaceous ooze, with little terrigenous component, interpreted as windy/dry season deposits. Laminated portions of the studied cores consist of conspicuous dark and light colored bundles of laminae couplets. Light and dark bundles alternate at decadal time scales. Within dark bundles, both light and dark half couplets are significantly thinner than within light bundles, implying slower sediment accumulation rates during both seasons over those intervals.Time series analyses of laminae thickness patterns demonstrate significant periodicities at interannual–centennial time scales. Longer time scale periodicities (multidecadal to centennial scale) of light and dark half couplet thicknesses are coherent and in some cases are similar to solar cycle periods on these time scales. Although laminae thickness cycles do not strongly covary with the actual Δ¹⁴C record for this same time period, two large Δ¹⁴C anomalies are associated with substantial decreases in both light and dark laminae thickness. In contrast to the multidecadal– centennial time scale, significant annual to decadal periodicities, which are broadly consistent with ENSO/PDO forcing and their impact on East African climate, are not coherent between light and dark half couplets. The coherency of light–dark couplets at decadal–centennial time scales, but not at shorter time scales, is consistent with a model of a long-term relationship between precipitation (recorded in wet season dark laminae thickness) and productivity (light laminae thickness), which is not manifest at shorter time scales. We hypothesize that this coupling results from long-term recharging of internal nutrient loading during wet periods (higher erosion of soil P) and reduced loading during drought intervals. The relationship is not expressed on short time scales during which the dominant control on productivity is wind-driven, dry season upwelling, which is uncorrelated with wet-season precipitation. Our record greatly extends the temporal record of this quasi-periodic behavior throughout the late Holocene and provides the first evidence linking decade- to century-scale episodes of enhanced productivity to enhanced precipitation levels and nutrient recharge in a productive tropical lake.     

A thermodynamic model for Fe–Mg dioctahedral K white micas using data from phase-equilibrium experiments and natural pelitic assemblages

The purpose of this study is to derive a solid-solution model for potassic white micas (KWM) encountered in rocks of various bulk compositions, over a wide range of P-T conditions. A compilation of phengite compositions lead us to propose a seven-thermodynamic-component (muscovite, Fe²⁺-Al-celadonite, Mg-Al-celadonite, annite, phlogopite, pyrophyllite and paragonite) ionic solid-solution model which accounts for the Tschermak, Fe-Mg, di/trioctahedral, pyrophyllitic and paragonitic substitutions observed in nature. A four-site mixing model with symmetric Margules parameters to model the Tschermak substitutions, asymmetric Margules parameters to model the other substitutions, and ideal intersite interaction has been adopted. In contrast to previous models, the relevant thermodynamic data and solid-solution properties are calibrated with independent sets of published experiments conducted for the KMASH, KFASH, KFMASH, and KNASH systems, as well as about 200 natural data involving KWM assemblages. The constraints span a wide range of pressure and temperature conditions (150 to 750 °C, 0.5 to 30 kbar), so that our model does not need to be extrapolated outside the calibration range to be used for P-T thermobarometric purposes. The calculated thermodynamic data are interconsistent with the TWQ thermodynamic database and solid-solution models, including that recently published for chlorites.