Iodine-xenon analysis of ordinary chondrite halide: implications for early solar system water     

A Busfield  J.D Gilmour  G Turner 《Geochimica et cosmochimica acta》2004,68(1):195-202

We report the results of iodine-xenon analyses of irradiated halide grains extracted from the H-chondrite Monahans (1998) and compare them with those from Zag (Whitby et al., 2000) to address the timing of aqueous processing on the H-chondrite parent body. Xe isotopic analyses were carried out using the RELAX mass spectrometer with laser stepped heating. The initial ¹²⁹I/¹²⁷I ratio in the Monahans halide was determined to be (9.37 ± 0.06) × 10⁻⁵ with an iodine concentration of ∼400 ppb. Significant scatter, especially in the Zag data, indicates that a simple interpretation as a formation age is unreliable. Instead we propose a model whereby halide minerals in both meteorites formed ∼5 Ma after the enstatite achondrite Shallowater (at an absolute age of 4559 Ma). This age is in agreement with the timing of aqueous alteration on the carbonaceous chondrite parent bodies and ordinary chondrite metamorphism and is consistent with the decay of ²⁶Al as a heat source for heating and mobilisation of brines on the H-chondrite parent body. Post accretion surface impact events may have also contributed to the heat source.  相似文献   

Doing groups: Situating knowledge and creating stories     

Andrew McGregor 《The Australian geographer》2004,35(2):141-149

This paper presents group research as a potentially powerful research tool for cultural geographers interested in the ways knowledge is produced at the interpersonal level. I argue that groups can create unique research spaces that incorporate 'sociality', the everyday social norms, logics and languages that guide ordinary conversations, into the research agenda. By concentrating on these discursive norms, insights can be obtained into the ways that knowledge is negotiated, produced and constrained in different places. To demonstrate the value of group research, a case study involving Australian environmentalists is analysed, to show how forests are constructed, contested and naturalised within this particular subculture. The article concludes by emphasising the importance of group research to critical geographers interested in social empowerment and change.  相似文献   

The Elusive AD 1826 Tsunami, South Westland, New Zealand   总被引：1，自引：0，他引：1  

James R. Gofr Andrew Wells  Catherine Chagu‐Goff  Scott L. Nichol  Robert J.N. Devoy 《New Zealand geographer》2004,60(2):28-39

In AD 1826 sealers reported earthquake and tsunami activity in Fiordland, although, contemporary or near‐contemporary accounts of tsunami inundation at the time are elusive. A detailed analysis of recent sediments from Okarito Lagoon builds on contextual evidence provided by earlier research concerning past tsunami inundation. Sedimentological, geochemical, micropalaeontological and geochronological data are used to determine palaeoenvironments before, during and after what was most probably tsunami inundation in AD 1826. The most compelling chronological control is provided fry a young cohort of trees growing on a raised shoreline bench stranded fry a drop in the lagoon water level following tsunami inundation.  相似文献   

Sedimentary rocks at Meridiani Planum: Origin, diagenesis, and implications for life on Mars     

Steven W. Squyres  Andrew H. Knoll 《Earth and Planetary Science Letters》2005,240(1):1-10

The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe3D3 (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference.  相似文献   

A review of paired catchment studies for determining changes in water yield resulting from alterations in vegetation   总被引：21，自引：0，他引：21  

Alice E. Brown  Lu Zhang  Thomas A. McMahon  Andrew W. Western  Robert A. Vertessy   《Journal of Hydrology》2005,310(1-4):28-61

Paired catchment studies have been widely used as a means of determining the magnitude of water yield changes resulting from changes in vegetation. This review focuses on the use of paired catchment studies for determining the changes in water yield at various time scales resulting from permanent changes in vegetation. The review considers long term annual changes, adjustment time scales, the seasonal pattern of flows and changes in both annual and seasonal flow duration curves. The paired catchment studies reported in the literature have been divided into four broad categories: afforestation experiments, deforestation experiments, regrowth experiments and forest conversion experiments. Comparisons between paired catchment results and a mean annual water balance model are presented and show good agreement between the two methodologies. The results highlight the potential underestimation of water yield changes if regrowth experiments are used to predict the likely impact of permanent alterations to a catchment's vegetation. An analysis of annual water yield changes from afforestation, deforestation and regrowth experiments demonstrates that the time taken to reach a new equilibrium under permanent land use change varies considerably. Deforestation experiments reach a new equilibrium more quickly than afforestation experiments. The review of papers reporting seasonal changes in water yield highlights the proportionally larger impact on low flows. Flow duration curve comparison provides a potential means of gaining a greater understanding of the impact of vegetation on the distribution of daily flows.  相似文献   

Channel geometry of mountain streams in New Zealand     

Ellen E. Wohl  Andrew Wilcox 《Journal of Hydrology》2005,300(1-4):252-266

A dataset of 21 study reaches in the Porter and Kowai rivers (eastern side of the South Island), and 13 study reaches in Camp Creek and adjacent catchments (western side of the South Island) was used to examine downstream hydraulic geometry of mountain streams in New Zealand. Streams in the eastern and western regions both exhibit well-developed downstream hydraulic geometry, as indicated by strong correlations between channel top width, bankfull depth, mean velocity, and bankfull discharge. Exponents for the hydraulic geometry relations are similar to average values for rivers worldwide. Factors such as colluvial sediment input to the channels, colluvial processes along the channels, tectonic uplift, and discontinuous bedrock exposure along the channels might be expected to complicate adjustment of channel geometry to downstream increases in discharge. The presence of well-developed downstream hydraulic geometry relations despite these complicating factors is interpreted to indicate that the ratio of hydraulic driving forces to substrate resisting forces is sufficiently large to permit channel adjustment to relatively frequent discharges.  相似文献   

Geomagnetism and Schmidt quasi-normalization     

D. E. Winch  D. J. Ivers  J. P. R. Turner  R. J. Stening 《Geophysical Journal International》2005,160(2):487-504

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Re-examination of the magnetic lineations of the Gascoyne and Cuvier Abyssal Plains, off NW Australia     

Masako S. Robb  Brian Taylor  Andrew M. Goodliffe 《Geophysical Journal International》2005,163(1):42-55

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Iodine-xenon analysis of Chainpur (LL3.4) chondrules     

G. Holland  J.C. Bridges  A. Busfield  G. Turner 《Geochimica et cosmochimica acta》2005,69(1):189-200

We present I-Xe analyses of ten chondrules from Chainpur LL3.4 by IR laser-stepped heating. Five chondrules provided isochrons of varying quality, giving a range of ages from 0.5 Ma before Shallowater to 17.8 after Shallowater. This confirms the extended range of Chainpur chondrule ages determined by previous data. We discuss evidence for fluid alteration, shock, and thermal events in explaining the chondrule ages and suggest that chondrule remelting events, presumably from bombardment of the parent body surface, are responsible for resetting the I-Xe chronometer. Previous data show a negative correlation between ¹³²Xe/¹²⁹Xe of the trapped Xe component and ¹²⁷I/¹²⁹I of an initial iodine component. This behaviour that requires the presence of a component with trapped ¹²⁹Xe/¹³²Xe lower than the planetary value has been cited as evidence for closed system evolution of the I-Xe system. We find no evidence of an unambiguous trapped component lower than planetary and no evidence of a negative correlation in our data. Therefore we suggest that open system behaviour more suitably explains the I-Xe systematics of Chainpur chondrules.  相似文献   

Negative δO values in Allan Hills 84001 carbonate: Possible evidence for water precipitation on mars     

G. Holland  J.M. Saxton  G. Turner 《Geochimica et cosmochimica acta》2005,69(5):1359-1370

The Martian meteorite ALH84001 contains ∼1% by weight of carbonate formed by secondary processes on the Martian surface or in the shallow subsurface. The major form of this carbonate is chemically and isotopically zoned rosettes which have been well documented elsewhere. This study concentrates upon carbonate regions ∼200 μm across which possess previously unobserved magnesium rich inner cores, interpreted here as rosette fragments, surrounded by a later stage cement containing rare Ca-rich carbonates (up to Ca₈₁Mg₀₇Fe₀₄Mn₀₇) intimately associated with feldspar. High spatial resolution ion probe analyses of Ca-rich carbonate surrounding rosette fragments have δ¹⁸O_V-SMOW values as low as −10‰. These values are not compatible with deposition from a global Martian atmosphere invoked to explain ALH84001 rosettes. The range of δ¹⁸O values are also incompatible with a fluid that has equilibrated with the Martian crust at high temperature or from remobilisation of carbonate of rosette isotopic composition. At Martian atmospheric temperatures, the small CO₂(gas)-CO₂(ice) fractionation makes meteoric CO₂ an unlikely source for −10‰ carbonates. In contrast, closed system Rayleigh fractionation of H₂O can generate δ¹⁸O_H2O −30‰, as observed at high latitudes on Earth. We suggest that atmospheric transport and precipitation of H₂O in a similar fashion to that on Earth provides a source of suitably ¹⁸O depleted water for generation of carbonate with δ¹⁸O_V-SMOW = −10‰.  相似文献