Self-reversal explanation for the Laschamp/Olby geomagnetic field excursion     

Friedrich Heller  Nikolai Petersen 《Physics of the Earth and Planetary Interiors》1982,30(4):358-372

The reversed natural remanent magnetisation (NRM) of the volcanic features at Laschamp and Olby (Chaîne des Puys, Auvergne, France) is commonly believed to document the youngest excursion of the geomagnetic field during the present Brunhes epoch. Recent radiometric dating determines the age of these lavas to be 35000–45000 y. Continuous thermal demagnetisation indicates that under laboratory conditions the NRM of many Olby samples undergoes complete or partial self-reversal; to a lesser extent this is also true for the Laschamp material. Thus self-reversal may be another possible explanation of the reversed NRM directions. Magnetic, optical and microprobe analyses suggest that the self-reversal mechanism is based upon magnetostatic interaction between differently oxidised titanomagnetite phases.  相似文献   

Biogeochemistry and bacteriology of ferrous iron oxidation in geothermal habitats     

Thomas D. Brock  Susan Cook  Sandra Petersen  J.L. Mosser 《Geochimica et cosmochimica acta》1976,40(5):493-500

A survey of hot, acid springs in Yellowstone Park has shown that high concentrations of ferrous and ferric iron are often present. Total ionic iron concentrations in different springs ranged from less than 1 ppm to greater than 200 ppm, and up to 50% of the ionic iron was in the ferrous form. Some of these springs also have high concentrations of reduced sulfur species (S^2? and S⁰). Significant populations of the bacterium Sulfolobus, acidocaldarius, an autotrophic organism able to live and oxidize sulfur compounds at low pH and high temperature, were present in most of these springs. The role of this organism in the oxidation of ferrous iron was investigated by incubating natural samples of water and assaying for disappearance of ferrous iron. Controls in which bacterial activity was inhibited by addition of 10% NaCl were also run. Bacterial oxidation of ferrous iron occurred in most but not all of the spring waters. The temperature optimum for oxidation varied from spring to spring, but significant oxidation occurred at temperatures of 80–85°C, but not at 90°C. Thus, 85–90°C is the upper temperature at which bacterial iron oxidation occurs; a similar upper limit has previously been reported for sulfur oxidation in the same kinds of springs. The steady-state concentrations of ferrous and ferric iron are determined by the rate at which these ions move into the spring pools with the ground water (flow rate), by the rate at which ferric iron is reduced to the ferrous state by sulfide, and by the rate of bacterial oxidation. The bacterial oxidation rate is faster than the flow rate, so that the rate of reduction of ferric iron is probably the rate-controlling reaction. In several springs, no decrease in ferrous iron occurred, even though high bacterial populations were present. It was shown that in these springs, ferrous iron oxidation occurred but the ferric iron formed was reduced back to the ferrous state again. These springs were all high in suspended sediment and the reductant was shown to be present in the sediment. X-ray diffraction revealed that the sediment contained three major ingredients, elemental sulfur, natroalunite, and quartz. Chemical analyses showed a small amount of sulfide, too little to reduce the ferric iron. Elemental sulfur itself did not reduce ferric iron but when elemental sulfur was removed from the sediment by CS₂ extraction, the activity of the sediment was abolished. It is hypothesized that the sulfide present in the sediment (possibly bound to natroalunite) reacts with elemental sulfur to form a reductant for ferric iron. The results show that bacteria can have a profound influence on the ferrous/ferric ratios of geothermal systems, but that temperature and mineral composition of the water may significantly influence the overall result.  相似文献   

The Blosseville Coast basalts of East Greenland: Their occurrence,composition and temporal variations   总被引：1，自引：0，他引：1  

C. K. Brooks  T. F. D. Nielsen  T. S. Petersen 《Contributions to Mineralogy and Petrology》1976,58(3):279-292

In the system CaO-MgO-A1₂O₃-SiO₂ the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatite_ss+sillimanite pyrope-grossular_ss+quartz (3), anorthite+enstatite_ss pyrope-grossular_ss+diopside_ss+quartz (2), anorthite+pyrope-grossular_ss+ quartz diopside_ss+kyanite (4) and anorthite+diopside_ss grossular-pyrope_ss +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En_0.5Fs_0.5). Enstatite and plagioclase (An_0.5Ab_0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.  相似文献   

The solar hafnium abundance     

T. Andersen  P. Petersen  Ö. Hauge 《Solar physics》1976,49(2):211-215

The solar Hf abundance is determined using nine Hf ii lines in the photospheric spectrum. The transition probabilities were obtained from lifetime measurements performed by the beam-foil technique. The abundance derived from synthetic spectrum calculations is A(Hf) = 0.88 ± 0.08 in the logarithmic A(H) = 12.00 scale.  相似文献   

Seismic investigation of a bottom simulating reflector and quantification of gas hydrate in the Black Sea   总被引：1，自引：0，他引：1  

M. Zillmer  E. R. Flueh  J. Petersen 《Geophysical Journal International》2005,161(3):662-678

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Sensitivity analysis of seismic hazard for the northwestern portion of the state of Gujarat, India   总被引：1，自引：0，他引：1  

Mark D. Petersen  B.K. Rastogi  Eugene S. Schweig  Stephen C. Harmsen  Joan S. Gomberg 《Tectonophysics》2004,390(1-4):105-115

We test the sensitivity of seismic hazard to three fault source models for the northwestern portion of Gujarat, India. The models incorporate different characteristic earthquake magnitudes on three faults with individual recurrence intervals of either 800 or 1600 years. These recurrence intervals imply that large earthquakes occur on one of these faults every 266–533 years, similar to the rate of historic large earthquakes in this region during the past two centuries and for earthquakes in intraplate environments like the New Madrid region in the central United States. If one assumes a recurrence interval of 800 years for large earthquakes on each of three local faults, the peak ground accelerations (PGA; horizontal) and 1-Hz spectral acceleration ground motions (5% damping) are greater than 1 g over a broad region for a 2% probability of exceedance in 50 years' hazard level. These probabilistic PGAs at this hazard level are similar to median deterministic ground motions. The PGAs for 10% in 50 years' hazard level are considerably lower, generally ranging between 0.2 g and 0.7 g across northwestern Gujarat. Ground motions calculated from our models that consider fault interevent times of 800 years are considerably higher than other published models even though they imply similar recurrence intervals. These higher ground motions are mainly caused by the application of intraplate attenuation relations, which account for less severe attenuation of seismic waves when compared to the crustal interplate relations used in these previous studies. For sites in Bhuj and Ahmedabad, magnitude (M) 7 3/4 earthquakes contribute most to the PGA and the 0.2- and 1-s spectral acceleration ground motion maps at the two considered hazard levels.  相似文献   

Inverse kinematic problem for a random gradient medium in geometric optics approximation     

N. V. Petersen 《Pure and Applied Geophysics》1990,132(1-2):417-437

Scattering at random inhomogeneities in a gradient medium results in systematic deviations of the rays and travel times of refracted body waves from those corresponding to the deterministic velocity component. The character of the difference depends on the parameters of the deterministic and random velocity component. However, at great distances to the source, independently of the velocity parameters (weakly or strongly inhomogeneous medium), the most probable depth of the ray turning point is smaller than that corresponding to the deterministic velocity component, the most probable travel times also being lower. The relative uncertainty in the deterministic velocity component, derived from the mean travel times using methods developed for laterally homogeneous media (for instance, the Herglotz-Wiechert method), is systematic in character, but does not exceed the contrast of velocity inhomogeneities by magnitude. The gradient of the deterministic velocity component has a significant effect on the travel-time fluctuations. The variance at great distances to the source is mainly controlled by shallow inhomogeneities. The travel-time flucutations are studied only for weakly inhomogeneous media.  相似文献   

A warm and wet little climatic optimum and a cold and dry little ice age in the southern rocky mountains,U.S.A.   总被引：1，自引：0，他引：1  

Kenneth Lee Petersen 《Climatic change》1994,26(2-3):243-269

The zenith of Anasazi Pueblo Indian occupation in the northern Colorado Plateau region of the southwestern U.S.A. coincides with the Little Climatic Optimum or Medieval Warm Period (A.D. 900–1300), and its demise coincides with the commencement of the Little Ice Age. Indexes of winter (jet-stream derived) and summer (monsoon derived) precipitation and growing season length were developed for the La Plata Mountains region of southwestern Colorado. The results show that during the height of the Little Climatic Optimum (A.D. 1000–1100) the region was characterized by a relatively long growing season and by a potential dry farming zone or elevational belt (currently located between 2,000 m and 2,300 m elevation) that was twice as wide as present and could support Anasazi upland dry farming down to at least 1,600 m, an elevation that is quite impossible to dry farm today because of insufficient soil moisture. This expanded dry-farm belt is attributable to a more vigorous circulation regime characterized by both greater winter and summer precipitation than that of today. Between A.D. 1100 and 1300 the potential dry-farm belt narrowed and finally disappeared with the onset of a period of markedly colder and drier conditions than currently exist. Finally, when the Little Ice Age terminated in the mid A.D. 1800s and warmer, wetter conditions returned to the region, another group of farmers (modern Anglos) were able to dry farm the area.The U.S. Government right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.  相似文献   

Palaeomagnetic investigations of the Palaeozoic circum-Sowie Góry Mountains ophiolitic belt in the Sudetes, Poland     

M. Jele&#;ska  M. K&#;dzialko-Hofmokl    J. B. Edel  L. Jamrozik  N. Petersen  H. Soffel 《Geophysical Journal International》1995,122(2):658-674

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An evaluation of various forms of VAS retrievals in the analysis of a preconvective environment     

R. A. Petersen  D. A. Keyser 《Meteorology and Atmospheric Physics》1987,37(2):82-113

Summary Quantitative retrievals derived from VISSR Atmospheric Sounder (VAS) radiances are combined with conventional surface and radiosonde data to evaluate the impact of the higher time and space resolution geosynchronous satellite soundings on the diagnosis of a preconvective environment over the central United States on 20 July 1981. Retrievals of temperature, dewpoint temperature, equivalent potential temperature, total column precipitable water, and lifted index, all derived at 60 km resolution over approximately three-hourly intervals, are shown to be physically consistent in space and time and to compare well with available radiosonde data. When VAS fields are used to augment qualitative VAS imagery and analyses from conventional data sources mesoscale regions with convective instability are more clearly delineated prior to the development of convection. The analysis of the VAS retrievals identifies significant spatial gradients and temporal changes in the thermal and moisture fields, especially at times and locations between radiosonde observations. Direct retrievals of vertically integrated bulk precipitable water and lifted index are particularly useful in this case since they correspond well to features in the VAS imagery and provide a strong preconvective signature. The nature of the VAS instrument dictates that these bulk parameters should display more consistency in space and time than the level-specific parameters due to the poor vertical resolution of the VAS instrument. The detailed analyses also point to limitations in using VAS data. Even with nearly optimal conditions for passive remote sounding (generally clear skies, minimal orographic effects, and a rapidly changing moisture field), the VAS retrievals were still degraded in some regions by VAS instrument noise and calibration errors and unresolved cloud contamination. Another problem is the cloud-free nature of the instrument data set, which biases the results toward a drier environment. In spite of these and other limitations, the analyses demonstrate that the geosynchronous VAS can be used in a case study mode to produce high-resolution spatial and temporal measurements which are useful for the quantitative analysis of a cloud-free preconvective environment.With 16 Figures  相似文献