Turbulence in an almond orchard: Spatial variations in spectra and coherence

The spatial variability of turbulence in a fully-leafed almond orchard was studied. Two three-dimensional sonic anemometers were used to measure turbulence spectra and coherence at different vertical and lateral separations inside the canopy. Peak frequencies of the horizontal velocity components, normalized by local horizontal wind speed, are greater in the canopy crown than in the trunkspace. Peak-normalized frequencies for the vertical velocity power spectra are similar in the canopy crown and in the subcanopy trunkspace. Spectral slopes in the inertial subrange are more negative than those predicted with Kolmogorov's -2/3 theory. It is thought that the foliage elements act to short-circuit the eddy cascade. Lateral separation of the instruments in the subcanopy trunkspace has little effect on the shape of the velocity spectra. On the other hand, lateral and vertical velocity coherences between spatially separated sensors are low inside the canopy. These low coherences are due to the Eulerian length scales being of the same order of magnitude as the separation distances of the anemometers. Phase angles between velocity components are about zero for small separation distances. When the two instruments are separated by 9 m and one instrument is positioned in a row while the other is between two rows, vertical velocities are about 180 deg out of phase and the streamwise velocities are about 40 to 60 deg out of phase. These data support the contention that preferred differences occur between within- and between-row wind flow regimes.     

Atrazine Used as a Tracer of Induced Recharge

The city of Lincoln draws water from a well field along the banks of the Platte River in east-central Nebraska. We have been able to follow the infusion of atrazine into this well field under induced recharge. Samples of water from the river, several monitoring wells and a production well were analyzed by GC/MS and solid phase extraction and found to change in concentration over the range 0.1 and 5.0 ppb of atrazine through the spring, summer, and fall 1989. Increases in the concentration of atrazine relating to precipitation events in the Platte River Basin were observable in the well water samples and ultimately in Lincoln municipal tap water. Atrazine from the river was seen to move via induced recharge and into well field ground water and away from the river at an observable rate. The concentration of atrazine in the river is dependent upon time of year and precipitation in the river basin.     

The characterisation and origin of graphite in cratonic lithospheric mantle: a petrological carbon isotope and Raman spectroscopic study

Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ¹³ C_PDB = ? 12.3 to ? ?3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO₂) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.     

The quaternary stratigraphy of mount Etna,sicily: the effects of differing palaeoenvironments on styles of volcanism

Using recently published radiometric dates of important horizons within the Etna succession, correlations are proposed with the Late Quaternary biostratigraphies of the Italian mainland and the Mediterranean sea floor. This enables palaeoclimatic, palaeohydrological and palaeogeographical conditions on Etna to be established at stages in the volcano's evolution. It is argued that several major volcaniclastic suites were formed through the operation of processes that were under complex environmental, rather than simple volcanological control alone. In particular it is suggested that certain episodes of phreatomagmatic and laharic activity cannot be related to episodes of more evolved magma production, but rather to times during which the environment, height and configuration of Etna favoured the eruption and emplacement of these deposits. The authors reiterate their explanation of the formation of the Valle del Bove through processes of massive slope failure and indicate that conditions favouring this occurred at an appropriate stage in the history of the volcano. It is concluded that on active basaltic volcanoes, especially those like Etna which are also high mountains, the explanation of more violent phases requires the consideration of both internal volcanological and external environmental and morphological variables and that this is particularly important when its development has been set within the context of Late Quaternary environmental change. Finally the implications of these findings for the assessment of hazard are considered.

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Resumé Utilisant récemment des publications sur les datations radiométriques de couches importantes à l'intérieur de la succession de l'Etna, nous proposons d'établir des corrélations entre les biostratigraphies du Quaternaire Récent de la Péninsule Italienne et due fond marin Méditerranéen. Par ce procédé, nous pouvons établir les conditions paléoclimatiques, paléohydrologiques et paléogéographiques par stades selon l'évolution du volcan, capendant il faut tenir compte du fait que plusieurs suites volcaniclastiques majeures se sont formées à partir d'opérations de processus qui etaient eux-mêmes sous une complexe influence écologique plutôt que volcanologique seulement. Il est suggéré, en particulier, que certains épisodes d'activités phreatomagmatiquer et laharique ne puissent être reliés à des épisodes de production de magma plus elaborée, mais plutôt à des périodes pendant lesquelles le milieu, la hauteur et la configuration de l'Etna ont favorisé l'éruption et la localisation de ces dépôts. Les auteurs réitèrant leur explication sur la formation de la Vallée del Bove selon un processus de rupture radicale de pente et indiquent que les conditions favorables à ce dernier sont survenues à un stade opportun dans l'histoire du volcan. Ceci amènè à conclure que, sur des volcans basaltiques actifs comme l'Etna en particulier qui sont egalement des montagnes élevées, l'explication de phases plus violentes demande la consideration de facteurs volcanologiques internes d'une part, et morphologiques et écologiques externes d'autre part et que ceci est essential lorsque son développement a été fixé dans le contexte du changement de milieu au Quaternaire Recent. Enfin, il faut tenir compte de l'implication de ces résultats face aux incidences de risques.

     

Provincialism and correlations between some properties of vitrinites

Optical and chemical properties for hand-picked samples of vitrinite from a number of British coalfields are correlated and the correlations compared with previously published data. The form of the relationship found for the properties of British vitrinites is generally similar to that exhibited by a number of other sets of analyses, but some significant differences also exist in several of the correlations. For maximum reflectance as a function of carbon content, the present data indicate that at medium and low ranks, maximum reflectance is lower than the level suggested by most previous studies. The relationships of volatile-matter yield to reflectance and carbon content suggest that at low ranks, volatile-matter yield is strongly dependent upon the nature of the coalification history of the vitrinite. Furthermore, these correlations are likely to show provincialism, in that correlations which hold for one coalfield may not be representative of the relationship in other coalfields. Similarly, major differences in relationships involving bireflectance are associated with the tectonic setting at the time of effective coalification. The data presented here indicate that for low-rank coals at least, correlations between properties of vitrinites must take account of provincialism if they are to be sufficiently reliable to be useful. The measurement of a number of rank-sensitive variables can yield additional information about rank, as compared with the use of a single rank-sensitive variable.     

使用微机的自动气象站

本文综述了应用微型计算机作为自动气象站的核心部分应考虑的要点。阅读本文并不要求读者掌握微机的基本知识。本文所描述的原理涉及来自气象感应元件的信号处理、计算机硬件性能的选择、编制程序的语言和技术、最终结果的输出方法等。作为一个实例,本文介绍了一个用于向公众展示当天气象要素及气候统计资料的自动气象站。     

Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics

The Comores Islands together with the Tertiary volcanic province of northern Madagascar form a sublinear trend of alkali olivine basalt shield volcanoes across the northern entrance of the Mozambique Channel. Potassium-argon dating of shield-building lavas confirms an eastward increase in age of volcanism along the chain, consistent with a hotspot origin for the lineament. The velocity of the Somali plate over the mantle magma source is 50 mm/yr.

We use the distribution of ages along the Comores-Madagascar chain in conjunction with existing age data for the Reunion-Mascarene Plateau hotspot track to model the absolute motion of the Somali plate for the last 10 m.y. We calculate the relative motion across the East African Rift by subtracting the Somali plate absolute motion from African plate absolute motion during this period. The model predicts 320 km of total separation across the East African Rift during the past 10 m.y. which is greater than has been estimated from surface geological evidence

The geometry of older portions of the Comores and Reunion trends indicates that there was no significant relative motion between the African and Somali plates prior to about 10 m.y. ago.