The geochemistry of lead in rivers,estuaries and the continental shelf of the Southeastern United States

Transport by southeastern rivers has insignificant influence on the lead concentration of southeastern shelf waters. If the rate of mobilization and transport of lead by these rivers is representative of uncontaminated fluvial transport during the Pleistocene, only about 5% of the prehistoric output of lead to the North Atlantic can be accounted for by river input.Lead concentrations in southeastern shelf waters are similar to those of North Atlantic Water from the upper 1000 m, which is probably similar to the intrusion source water for the shelf. Atmospheric inputs to the shelf of the same magnitude as observed for the Western North Atlantic are difficult to reconcile given the residence time of shelf waters and their lead concentration unless the rate of loss of lead to shelf sediments is about the same as the atmospheric flux.     

Crustal evolution of the early earth: The role of major impacts

Impact cratering was an important — even dominant — process affecting the crustal evolution of the small terrestrial planets. The fundamental highlands/maria dichotomy of the Moon's surface can be traced to a late heavy bombardment by basin-forming, asteroid-sized bodies which produced not only a topographic division in the lunar crust but also localized the later eruptions of mare basalts. Major impact basins with diameters in excess of 200 km are recognized throughout the inner solar system from Mars to Mercury. Similar craters must have formed on the Earth prior to 4 Ga ago, and the minimum number of such basin-forming impacts can be calculated by scaling from the observed (minimum) number preserved on the Moon. When allowance is made for differences in impact velocity, gravitational cross-section and the effects of gravity on crater diameter, it is found that at least 50% of a presumed global sialic crust would have been converted into impact basins by 4 Ga ago. Among the effects resulting from the impact of an asteroidal object on the early crust were: (a) establishment of a topographic dichotmy of 3–4 km (after isostatic adjustment), (b) pressure-release partial melting of the upper mantle and rapid flooding of the basin floor by basalt, and (c) enhancement of thermal gradients in the sub-basin lithosphere and upper asthenosphere. Comparative planetary data such as impact scaling can be used as important constraints on models of the early terrestrial crust. For example, the topography resulting from impact bombardment produced discrete oceans and dry land by 4 Ga ago, making unreasonable models of a globe-encircling ocean on the Earth after that time.     

Investigation of NaCl deliquescence in porous substrates using RH-XRD

We report on the use of X-ray diffractometry under controlled conditions of temperature and relative humidity (RH-XRD) for the investigation of NaCl deliquescence in the pore space of glass filter frits, which were used as model substrates. The study confirms that RH-XRD is an appropriate experimental technique for the in situ observation of phase transformation in porous materials. It is used for an investigation of both the deliquescence kinetics and the deliquescence humidity within pores of different median pore diameter. Several major influences affecting deliquescence rates in the pore space close to the surface of a porous material are discussed. It appears that quite short-term variation of ambient relative humidity, e.g., typical daily fluctuations, might induce damaging deliquescence–crystallization cycles within the pore space of building materials. In agreement with theoretical considerations it was found that confinement of NaCl crystals in pores with median diameters in the range 1.4–70 μm does not affect the deliquescence humidity of the salt.     

Spatially distributed hydrotope‐based modelling of evapotranspiration and runoff in mountainous basins

River basins in mountainous regions are characterized by strong variations in topography, vegetation, soils, climatic conditions and snow cover conditions, and all are strongly related to altitude. The high spatial variation needs to be considered when modelling hydrological processes in such catchments. A complex hydrological model, with a great potential to account for spatial variability, was developed and applied for the hourly simulation of evapotranspiration, soil moisture, water balance and the runoff components for the period 1993 and 1994 in 12 subcatchments of the alpine/pre‐alpine basin of the River Thur (area 1703 km²). The basin is located in the north‐east of the Swiss part of the Rhine Basin and has an elevation range from 350 to 2500 m a.s.l. A considerable part of the Thur Basin is high mountain area, some of it above the tree‐line and a great part of the basin is snow covered during the winter season. In the distributed hydrological model, the 12 sub‐basins of the Thur catchment were spatially subdivided into sub‐areas (hydrologically similar response units—HRUs or hydrotopes) using a GIS. Within the HRUs a hydrologically similar behaviour was assumed. Spatial interpolations of the meteorological input variables wereemployed for each altitudinal zone. The structure of the model components for snow accumulation and melt, interception, soil water storage and uptake by evapotranspiration, runoff generation and flow routing are briefly outlined. The results of the simulated potential evapotranspiration reflect the dominant role of altitudinal change in radiation and albedo of exposure, followed by the influence of slope. The actual evapotranspiration shows, in comparison with the potential evapotranspiration, a greater variability in the lower and medium altitudinal zones and a smaller variability in the upper elevation zones, which was associated with limitations of available moisture in soil and surface depression storages as well as with the evaporative demand of the local vegetation. The higher altitudinal dependency and variability of runoff results from the strong increase in precipitation and the decrease in evaporation with increased altitude. An increasing influence of snow cover on runoff as well as evapotranspiration with altitude is obvious. The computed actual evapotranspiration and runoff were evaluated against the observed values of a weighting lysimeter and against runoff hydrographs. Copyright © 1999 John Wiley & Sons, Ltd.     

Forstmeteorologische Untersuchungen an der Wald- und Baumgrenze in den Zentralalpen

Zusammenfassung Bei vergleichsweisen Registrierungen der Lufttemperatur und relativen Luftfeuchtigkeit, sowie bei Verdunstungseinzel-messungen, die im Bereiche der jetzigen (künstlichen) Wald- und Baumgrenze 10 km südwestlich von Innsbruck (Tirol) in Seehöhen von 1400 bis 1900 m in Bestand, Horst und Freiland durchgeführt wurden, ergaben sich deutliche Stationsunterschide. Trotz stärkerer Einstrahlung und höheren Windgeschwindigkeiten, wie sie die Höhenlage neben geringerer Bestockungsdichte bedingt, ergab sich im Bestandesinneren ein, gegenüber dem Freiland stark gemäßigtes Klima, das die Aushagerung des Bodens verhindert und vor allem der jungen Holzpflanze eine nicht zu übersehende Feuchtigkeitsreserve bietet. Bei extrem auftretenden Außeneinflüssen erfolgt allerdings eine raschere Angleichung an die Freilandsverhältnisse als in tieferen Lagen. Hoch- und Tieflagen unterscheiden sich demnach nur graduell und soweit, als es die lichtere Bestockung und größere Außenbeeinflussung der Hochlagen bedingen. Der Wirkungsgrad der Außeneinflüsse wurde auch von der Exposition bestimmt, die im Zusammenwirken mit der vorherrschenden Windrichtung das Auftreten der einzelnen Holzarten beeinflußte. Der stärkere Einfluß der Luftbewegung drückt sich auch im unsteten Momentancharakter der relativen Luftfeuchtigkeit aus.Der Einfluß der Vegetationsform auf die lokalen Kleinklimaverhältnisse ergab sich auch aus den Ergebnissen der Verdunstungsmessungen, die ein nach Holzart und Bestockungsform spezifisches Mosaikklima deutlich erkennen lie\en.

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Summary Distinctly perceptible differences between the stations in thicket and amount of timber as well as in open air resulted from comparing registrations of air temperature and relative humidity, and from measurements of evaporation which were made in range of the present (unnatural) timber line 10 km. south-west of Innsbruck (Tyrol) in altitudes of 1400–1900 m. (4600–6300 ft) above sea-level. In spite of the more intensive insolation and increased wind-velocity—corresponding to altitude and less density of wood—a very temperate climate in comparison with that of open field was evident within amount of timber, preventing impoverishment of soil and-first of all—offering a considerable reserve of moisture to the young tree. Extremely appearing external influences, however, effect a rapid assimilation to the conditions of open field likely to lower levels. The efficacy of the external influences also was fixed by the exposure which influences—together with the prevailing directions of wind—the occurences of the different species of timber. The greater effect of air-movement is also expressed by the unsteady momentary-character of the relative humidity.The influence of different plant communities upon the local climates followed from the results of evaporation measurements (evaporimeter Piche) which made recognizable a tesselated, mosaic-climate specifically to species and formation of timber, unless extreme external influences operated levelling.

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Résumé Des enregistrements comparatifs de la température de l'air et de l'humidité relative ainsi que des mesures d'évaporation effectués à la limite de l'atmosphère forestière ont mis en évidence de nettes différences entre les stations d'essais. Ces expériences ont été faites à 10 km. au sudouest d'Innsbruck (Tirol) dans les bois et la région non boisée, à une altitude de 1400 à 1900 m. Malgré l'insolation plus forte et les vitesses de vent plus grandes, dûes à l'altitude et à la densité plus faible de la forêt, on a constaté à l'intérieur du bois un climat très modéré par opposition à la région non boisée. Ce climat modéré s'oppose au desséchement du sol et assure aux jeunes pousses une humidité suffisante. Si les effets de la température sont extrêmes, l'adaptation aux conditions de la région non boisée est pourtant plus rapide qu'aux altitudes plus basses. Par conséquent les zones supérieures et inférieures ne se distinguent que graduellement, en fonction des forêts moins denses et des influences temporaires plus accusées. Le rôle du climat local est nettement déterminé par l'exposition qui, selon la direction dominante du vent, a une répercussion sur l'apparition des différentes essences de bois. La circulation active de l'air se traduit aussi par la variabilité instantaneé de l'humidité relative de l'air.Les mesures faites avec l'évaporimètre de Piche ont montré l'effet du bioclimat local sur la végétation. Ces travaux font nettement apparaître un climat «mozaïque» spécifique dépendant des différentes essences et de la densité de la population végétale.

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Mit 4 Textabbildungen.     

Quartz as indicator mineral in the Central Swiss Alps: the quartz recrystallization isograd in the rock series of the northern Aar massif

Earlier observations on the deformation of rock-forming quartz during Alpine metamorphism that were made by Voll (1976) along a traverse following the Reuss valley (Aar massif, Central Swiss Alps) are still relevant but can be explained in more modern term. Voll found that a few kilometres south of the contact between the Aar massif and its sedimentary cover quartz has partly undergone a combination of dynamic recrystallization and nucleation along grain boundaries and fractures at the expense of the deformed parent grains. Further to the south, increasing grain growth of quartz under nearly static conditions is observed, with polygonal grain shapes typical of annealed microfabrics. Comparable stages of recrystallization could be found in several N–S traverses through the Aar massif so that “points of first occurrence” of newly formed quartz could be connected to define a quartz recrystallization isograd. This isograd extents over 90 km through the northern Aar massif, following a course parallel to the sanidine/microcline isograd but at a distance of 10–15 km further to the north. The evolution of the quartz microfabric is discussed with respect to recent estimates of the T-t path of the Aar massif.     

A source for hyperbolic cosmic dust particles

Earlier analyses of the Pioneer 8 and 9 experimental meteoroid data have shown that the detectors on these two spacecraft are intercepting meteoroids with hyperbolic orbital parameters. It is shown in this paper that these results are entirely consistent with and, indeed, to be expected from other observations of the interplanetary meteoroid complex. Collisional breakup of meteoroids and post-collision radiation pressure modification of their orbits is found to be a sufficient cause for the observed results. Details of the calculations as well as of the results are presented.     

Solar wind induction in Mercury: Constraints on the formation of a magnetosphere

The discovery of Mercury's magnetosphere by Mariner 10 was surprising since the conventional view of regenerative planetary dynamos had been that the spin requirement would likely have been in excess of the observed spin rate of Mercury. Also internal fluid motions were not expected to be sufficiently large. This paper explores the alternative model of the formation of Mercury's magnetosphere via electromagnetic induction forced by the solar wind. It is shown, however, that the constraints are so severe as to limit severely the applicability of such a model. Although induction is easily observed on the Moon, the modification of the magnetic boundary condition associated with a plasma magnetosphere on Mercury rules out its formation via induction except for interplanetary driving fields which are decreasing in amplitude. That model is explored but retains the difficulty that induced magnetospheres tend to be of small radial and temporal extent compared to that inferred by Ness et al. for Mercury.     

Cadmium release caused by the die-back of the saltmarsh cord grass Spartina anglica in Poole Harbour (UK)

As in many other areas, the perennial cord grass Spartina anglica C.E. Hubbard has been extremely successful at colonising the English and Irish south coasts, but it is dying back for reasons that are not completely understood. The present study considers the relationship between die-back and metal concentrations in estuarine sediment. No obvious impact of the metal contamination on the S. anglica growth/die-back could be detected in the study zone, Poole Harbour, although the die-back did seem to have substantially influenced the metal concentrations in the sediments of the estuary. Based on core profiles, the remaining patches of S. anglica still retain elevated concentrations and the overall cadmium concentrations in the sediments have risen since 1925. However, considerable quantities of the cadmium stored in the sediment by S. anglica appear to have been washed out rapidly in the die-back zones. Should serious erosion of the saltmarsh occur, triggered for example by sea-level rise, then sudden and high levels of cadmium release may cause harmful effects to marine biota.     

High oxidation state during formation of Martian nakhlites

Abstract– The oxygen fugacities recorded in the nakhlites Nakhla, Yamato‐000593 (Y‐000593), Lafayette, and NWA998 were studied by applying the Fe,Ti‐oxide oxybarometer. Oxygen fugacities obtained cluster closely around the FMQ (Fayalite–Magnetite–Quartz) buffer (NWA998 = FMQ ? 0.8; Y‐000593 = FMQ ? 0.7; Nakhla = FMQ; Lafayette = FMQ + 0.1). The corresponding equilibration temperatures are 810 °C for Nakhla and Y‐000593, 780 °C for Lafayette and 710 °C for NWA998. All nakhlites record oxygen fugacities significantly higher and with a tighter range than those determined for Martian basalts, i.e., shergottites whose oxygen fugacities vary from FMQ ? 1 to FMQ ? 4. It has been known for some time that nakhlites are different from other Martian meteorites in chemistry, mineralogy, and crystallization age. The present study adds oxygen fugacity to this list of differences. The comparatively large variation in fO₂ recorded by shergottites was interpreted by Herd et al. (2002) as reflecting variable degrees of contamination with crustal fluids that would also carry a light rare earth element (REE)‐enriched component. The high oxygen fugacities and the large light REE enrichment of nakhlites fit qualitatively in this model. In detail, however, it is found that the inferred contaminating phase in nakhlites must have been different from those in shergottites. This is supported by unique ¹⁸²W/¹⁸⁴W and ¹⁴²Nd/¹⁴⁴Nd ratios in nakhlites, which are distinct from other Martian meteorites. It is likely that the differences in fO₂ between nakhlites and other Martian meteorites were established very early in the history of Mars. Parental trace element rich and trace element poor regions (reservoirs) of Mars mantle ( Brandon et al. 2000 ) must have been kept isolated throughout Martian history. Our results further show significant differences in closure temperature among the different nakhlites. The observed range in equilibration temperatures together with similar fO₂ values is attributable to crystallization of nakhlites in the same cumulate pile or lava layer at different burial depths from 0.5 to 30 m below the Martian surface in agreement with Mikouchi et al. (2003) and is further confirmed by similar crystallization ages of about 1.3 Ga ago (e.g., Misawa et al. 2003 ).