摘要:

Human activities have a serious impact on climate and on the natural composition of the atmosphere. Information recorded in polar ice cores over the last several hundred millennia is invaluable to studies aimed at understanding the preindustrial environmental system and anticipating the future evolution of the atmosphere. An excellent understanding of the mechanisms of the ice record formation as well as a good assessment of the present polar atmospheric composition (trace gases, aerosol) is a prerequisite to interpreting correctly the past variations of the measured parameters. This paper explains what and how atmospheric parameters are recorded. Ambient air samples are encapsulated and stored in the ice bubbled by relatively simple processes. The isotopic composition of the H2O (ice) lattice is a reliable paleothermometer. The interpretation of the chemical composition of deposited snow in terms of past atmospheric composition of deposited snow in terms of past atmospheric composition (trace gases, aerosol) is more intricate and necessitates detailed discussions. The data obtained from deep ice cores provide precise information on the ice age environmental conditions: when polar temperatures were some 10°C lower than now, atmospheric CO2and CH4contents were factors of 2 and 4 lower, respectively, than present conditions. At this time, sea salt and overall crustal dust depositions were significantly higher. The biogeochemical cycles of S and N were also disturbed according to modifications in source intensity and transport of gaseous precursors

ISSN:8755-1209    

DOI:10.1029/91RG02725

年代:1992

数据来源: WILEY

摘要:

One of the most important observations of oceanic crustal evolution is that the uppermost part of the igneous crust, layer 2A, appears to thin and eventually disappear as its seismic velocity doubles from approximately 2.2 to 4.5 km s−1in the first 40 m.y. The most likely cause is decreasing porosity in extruded basalts that compose the uppermost igneous crust by filling cracks with hydrothermally generated minerals, a by‐product of off‐axis hydrothermal circulation and alteration of basalts. New insights from two recent papers on crustal hydrothermal circulation patterns and modeling of the effects of crack geometry distributions upon seismic velocities provide guidance to relate filling of cracks within the basement to seismic velocity increases. A qualitative model of layer 2A crustal evolution is outlined that satisfies many geophysical and geochemical observations, some seemingly unrelated. Somewhat surprisingly, the crustal age at which layer 2A seismic velocities reach their maximum value is primarily controlled by the integrated permeability of the overlying sediment, which in turn, controls the hydrologic regime necessary for secondary mineral precipitation. Speculative predictions can be used to verify, refine, or invalidate this dynamic model of crustal evolution. Major scientific and technical obstacles must be overcome before a definitive model can be quantified. With such a rigorous model of crustal upper crustal evolution in hand, remote geophysical sensing tools can begin to fulfill the promise of interpreting the physical state, composition, and past history of oceanic

ISSN:8755-1209    

DOI:10.1029/91RG02811

年代:1992

数据来源: WILEY

摘要:

Stellar winds are thought to be the primary mass loss process for most stars and to play a particularly important role in mass and angular momentum transfer in newly forming stars. The physical processes that account for the acceleration of such winds are therefore of fundamental importance in astrophysics but are at present poorly understood, even for the case of the Sun. The question of just how the solar wind is accelerated to hypersonic speeds has been one of the most fundamental questions of space science since the beginning of the space age and is still far from a definitive solution. In this present review I give a general overview of some of the problems associated with trying to understand the physics of solar wind acceleration, how these problems have been approached, and the kinds of observational and theoretical efforts needed to facilitate closure between theory and observation.

ISSN:8755-1209    

DOI:10.1029/91RG02816

年代:1992

数据来源: WILEY

摘要:

The high‐temperature measurements of elastic constants and related temperature derivatives of nine minerals of interest to geophysical and geochemical theories of the Earth's interior are reviewed and discussed. A number of correlations between these parameters, which have application to geophysical problems, are also presented. Of especial interest is α, the volume coefficient of thermal expansion, and a section is devoted to this physical property. Here we show how α can be estimated at very high temperatures and how it varies with density. An estimate of α for Mg‐perovskite at deep‐mantle conditions is made. The formula for the Grüneisen ratio γ as a function ofVandTis presented, including plots of the numerical values of γ over a wideTandVrange. An example calculation of γ for MgO is made. The high‐T‐high‐Pvalues of γ calculated here agree well with results from the ab initio method of calculation for MgO. The use of the thermoelastic parameters is reviewed, showing application to the understanding of thermal pressure, thermal expansivity, enthalpy, and entropy. We review an extrapolation formula to determineKs, the adiabatic bulk modulus, at very highT. We show that the thermal pressure is quite linear withTup to high temperatures (∼1800 K), and, as a consequence, the anharmonic contribution to the Helmholtz free energy is sufficiently small, so that it can and should be ignored in thermodynamic calculations

ISSN:8755-1209    

DOI:10.1029/91RG02810

年代:1992

数据来源: WILEY

ISSN:8755-1209    

DOI:10.1029/92RG00461

年代:1992

数据来源: WILEY