Experimental seawater-basalt interaction at 300°C, 500 bars,chemical exchange,secondary mineral formation and implications for the transport of heavy metals     

W.E Seyfried  J.L Bischoff 《Geochimica et cosmochimica acta》1981,45(2):135-147

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Lithospheric structure in the area of Crete constrained by receiver functions and dispersion analysis of Rayleigh phase velocities   总被引：2，自引：0，他引：2  

B. Endrun  T. Meier  M. Bischoff  H.-P. Harjes 《Geophysical Journal International》2004,158(2):592-608

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A procedure for testing the assumption of homoscedasticity in least squares residuals: a case study of GPS carrier-phase observations     

W. Bischoff  B. Heck  J. Howind  A. Teusch 《Journal of Geodesy》2005,78(7-8):397-404

Due to increased demands on the quality of the results of Global Positioning System (GPS) evaluations, various authors have studied improvements of the stochastic model of GPS carrier-phase observations. These improvements are based on the reasonable assumption that the commonly used stochastic model with independent and homoscedastic (i.e. equal variance) errors is unrealistic. However, this has not been proved rigorously so far. A statistical test procedure based on uncorrelated least–squares residuals, which allows verification of the hypothesis of a heterogeneous variance, is provided. The statistical test procedure is of interest in its own right, and is independent of the practical problem considered. The presented technique is applied to GPS carrier-phase observations. Results show that the variances of the investigated observations are far from homogeneous. It is indicated that the error variances of the presented data increase with decreasing GPS satellite elevation. These results confirm the assumption that the commonly used stochastic model of GPS observations is inadequate and has to be improved.  相似文献   

A Procedure for Estimating the Variance Function of Linear Models and for Checking the Appropriateness of Estimated Variances: A Case Study of GPS Carrier-phase Observations     

W. Bischoff  B. Heck  J. Howind  A. Teusch 《Journal of Geodesy》2006,79(12):694-704

In many applications of linear model theory, homogeneous variances are assumed. In practice, however, the variances are frequently heterogeneous. Therefore, to improve the results, the unknown variances have to be estimated. The appropriateness of the estimated variances has then to be checked by a suitable statistical test procedure. Such a procedure is also useful to study models of global positioning system (GPS) carrier-phase observations. While the functional model of GPS carrier-phase observations is widely accepted, the stochastic model is still under development. As well as the neglected correlations of GPS observations, a homogenous variance function is frequently assumed. In Bischoff et al. (J Geod 78:397–404, 2005), we showed by statistical testing that the assumption of constant variances is not appropriate. In this paper, we give a procedure to estimate an individual variance function for a pair of satellites and a procedure to check the appropriateness of the estimated variances. As an example, the approach is applied to double-differenced carrier-phase GPS observations.  相似文献   

Dissolution Enthalpies of Magnesian Calcites     

William D. Bischoff 《Aquatic Geochemistry》1998,4(3-4):321-336

A series of synthetic and biogenicmagnesian calcites was dissolved in weak acetic acidsolutions to measure the enthalpies of dissolution at25°C. For the synthetic phases, heat releasedwas 33.5 kJ/mol for calcite, decreasing to 33 kJ/molfor a phase of 2 mol % MgCO3, and increasing to35 kJ/mol for a phase of 15 mol %. Values of excessenthalpies, ΔHxs, calculated using calciteand magnesite end-members, average about -1 kJ/mol forthe synthetic phases. Total entropies of solidsolution formation, ΔSSS, also werecalculated using available data on Gibbs free energiesof formation and these excess enthalpies. Values ofΔSSS range from -2 J/(mol-K) at 2 mol % to-5 J/(mol-K) at 15 mol % MgCO3. These negativevalues of ΔHxs and ΔSSSsuggest that some form of ordering (cation?) isobtained in the synthetic phases, and that vibrationalentropies of the solid solution are diminished incomparison to the end-members. In contrast, biogenic samples generally have positivevalues of ΔHxs, increasing from +1 kJ/molat 5 mol % to +3 kJ/mol at 20 mol % MgCO3. Mostvalues of ΔSSS are equal (within errors)to values expected from configurational enthalpyalone. Thus, in most biogenic materials cationordering probably is not obtained, and most phases aremore typical of equivalent-site solid solutions.  相似文献   

The critical point and two-phase boundary of seawater, 200–500°C     

James L. Bischoff  Robert J. Rosenbauer 《Earth and Planetary Science Letters》1984,68(1):172-180

The two-phase boundary of seawater was determined by isothermal decompression of fully condensed seawater in the range of 200–500°C. The pressure at which phase separation occurred for each isotherm was determined by a comparison of the refractive index of fluid removed from the top and bottom of the reaction vessel. The critical point was determined to be in the range of 403–406°C, 285–302 bar and was located by the inflection in the two-phase boundary and by the relative volume of fluid and vapor as a function of temperature. The two-phase boundary of 3.2% NaCl solution was found to coincide exactly with that of seawater over the range tested in the present study. The boundary for both is described by a single seventh-order polynomial equation. The two-phase boundary defines the maximum temperature of seawater circulating at depth in the oceanic crust. Thus the boundary puts a limit of about 390°C for seawater circulating near the seafloor at active ocean ridges (2.5 km water depth), and about 465°C at the top of a magma chamber occurring at 2 km below the seafloor.  相似文献   

Hydrological model parameter (in)stability – “crash testing” the HBV model under contrasting flood seasonality conditions     

Klaus Vormoor  Maik Heistermann  Axel Bronstert  Deborah Lawrence 《水文科学杂志》2018,63(7):991-1007

This paper investigates the transferability of calibrated HBV model parameters under stable and contrasting conditions in terms of flood seasonality and flood generating processes (FGP) in five Norwegian catchments with mixed snowmelt/rainfall regimes. We apply a series of generalized (differential) split-sample tests using a 6-year moving window over (i) the entire runoff observation periods, and (ii) two subsets of runoff observations distinguished by the seasonal occurrence of annual maximum floods during either spring or autumn. The results indicate a general model performance loss due to the transfer of calibrated parameters to independent validation periods of ?5 to ?17%, on average. However, there is no indication that contrasting flood seasonality exacerbates performance losses, which contradicts the assumption that optimized parameter sets for snowmelt-dominated floods (during spring) perform particularly poorly on validation periods with rainfall-dominated floods (during autumn) and vice versa.  相似文献   

Methane in the southern North Sea: Sources, spatial distribution and budgets   总被引：3，自引：0，他引：3  

Maik Grunwald  Olaf Dellwig  Melanie Beck  Joachim W. Dippner  Jan A. Freund  Cora Kohlmeier  Bernhard Schnetger  Hans-Jürgen Brumsack 《Estuarine, Coastal and Shelf Science》2009,81(4):445-456

Measurements of methane (CH₄) so far have always shown supersaturation in the entire North Sea relative to the atmospheric partial pressure and the distribution of surface CH₄ reveals a distinct increase towards the shore. Since North Sea sediments presumably are an insignificant source for CH₄ the coastal contribution via rivers and tidal flats gains in importance.In this work, CH₄ data from the River Weser, the back barrier tidal flats of Spiekeroog Island (NW Germany), and the German Bight are presented. Results from the River Weser are compared to other rivers draining into the German Bight. Measurements in the tidal flat area of Spiekeroog Island highlight this ecosystem as an additional contributor to the overall CH₄ budget of the southern North Sea. A tidally driven CH₄ pattern is observed for the water column with maximum values during low tide. Tidal flat sediments turn out to be the dominating source because pore waters discharged during low tide are highly enriched in CH₄. In contrast, the freshwater contribution to the tidal flats by small coastal tributaries has almost no impact on water column CH₄ concentrations. The CH₄ level seems to be disturbed irregularly by wind forcing due to elevated degassing and prevention of advective flow when tidal flats remain covered by water.Based on our data, two model calculations were used to estimate the impact of tidal flats on the CH₄ budget in the German Bight. Our results demonstrate that the back barrier tidal flats of the east Frisian Wadden Sea contribute CH₄ in an order of magnitude between the Wash estuary and River Elbe and thus have to be considered in budget calculations.  相似文献   

Itawa Bhopji (L3–5) chondrite regolith breccia: Fall,classification, and cosmogenic records     

N. Bhandari  S. V. S. Murty  P. N. Shukla  A. D. Shukla  R. R. Mahajan  M. M. Sarin  G. Srinivasan  K. M. Suthar  M. S. Sisodia  S. Jha  A. Bischoff 《Meteoritics & planetary science》2002,37(4):549-563

Abstract— A stony meteorite fell at Itawa Bhopji, Rajasthan, India on 2000 May 30. This is the fifth recorded fall in a small area of Rajasthan during the past decade. The meteorite is an ordinary chondrite with light clasts in a dark matrix, consisting of a mixture of equilibrated (mainly type 5) and unequilibrated components. Olivine is Fa_24–26 and pyroxene Fs_20–22 but, within the unequilibrated components, olivine (Fa_5–29) and low calcium pyroxene (Fs_5–37) are highly variable. Based on petrographic studies and chemical analyses, it is classified as L(3–5) regolith breccia. Studies of various cosmogenic records, including several gamma‐emitting radionuclides varying in half‐life from 5.6 day ⁵²Mn to 0.73 Ma ²⁶Al, tracks and rare gases have been carried out. The exposure age of the meteorite is estimated from cosmogenic components of rare gases to be 19.6 Ma. The track density varies by a factor of ?3 (from 4 to 12 times 10⁶/cm²) within the meteorite, indicating a preatmospheric body of ?9 cm radius (corresponding to a meteoroid mass of ?11 kg) and small ablation (1.5 to 3.6 cm). Trapped components in various rare gases are high and the solar component is present in the dark portion of the meteorite. Large excess of neutron‐produced ⁸²Kr and ¹²⁸Xe in both the light and the dark lithology but very low ⁶⁰Co, indicating low neutron fluxes received by the meteoroid in the interplanetary space, are clear signatures of an additional irradiation on the parent body.  相似文献   

Lava bubble-wall fragments formed by submarine hydrovolcanic explosions on Lō'ihi Seamount and Kīlauea Volcano     

David A. Clague  Alice S. Davis  James L. Bischoff  Jacqueline E. Dixon  Renee Geyer 《Bulletin of Volcanology》2000,61(7):437-449

 Glassy bubble-wall fragments, morphologically similar to littoral limu o Pele, have been found in volcanic sands erupted on Lō'ihi Seamount and along the submarine east rift zone of Kīlauea Volcano. The limu o Pele fragments are undegassed with respect to H₂O and S and formed by mild steam explosions. Angular glass sand fragments apparently form at similar, and greater, depths by cooling-contraction granulation. The limu o Pele fragments from Lō'ihi Seamount are dominantly tholeiitic basalt containing 6.25–7.25% MgO. None of the limu o Pele samples from Lō'ihi Seamount contains less than 5.57% MgO, suggesting that higher viscosity magmas do not form lava bubbles. The dissolved CO₂ and H₂O contents of 7 of the limu o Pele fragments indicate eruption at 1200±300 m depth (120±30 bar). These pressures exceed that generally thought to limit steam explosions. We conclude that hydrovolcanic eruptions are possible, with appropriate pre-mixing conditions, at pressures as great as 120 bar. Received: 22 December 1998 / Accepted: 16 July 1999  相似文献