摘要:

Compared to the mantles of the Moon and perhaps Mars, the Earth's mantle is much less differentiated chemically. Both the Moon and Mars appear to have undergone a major differentiation accompanying planet formation. The only clear signature of a similar event on the Earth is core formation. While this might imply that the Earth did not experience extensive melting and differentiation during planet formation, the higher pressures and temperatures present in the Earth could have led to a distinctly different chemical evolution for this initial differentiation. The most significant potential outcome of early differentiation on the Earth's mantle is formation of chemically distinct upper and lower mantles distinguished by Mg/Si higher and lower than chondritic, respectively. Plate tectonics on Earth provides a continuing mechanism for planet differentiation that forms crust at the expense of chemical differentiation of the mantle. Plate tectonics, however, also offers a mechanism to return the chemically distinct materials of the crust back into the mantle. Mixing of subducted crustal material into the mantle through the stirring provided by mantle convection can serve to negate the effects of crust formation on the chemical composition of the mantle. Similarly, mixing within the mantle could serve to destroy evidence of early differentiation, if such differentiation occurred on Earth. Completely efficient operation of the plate tectonic cycle would result in remixing of crust and differentiated mantle, with the end result being a homogenous mantle with composition identical to that of the bulk earth minus the materials segregated into the core. In part, this may explain the relatively undifferentiated nature of the Earth's mantle. Plate tectonics has not been completely efficient on Earth, however. Both oceanic and continental crust exist, and there is widespread evidence for chemical variability in the mantle. At least four chemically and isotopically distinct components are observed in mantle‐derived rocks. The nature of these components points to the importance of crust formation and recycling in determining the chemical variability of the mantle. Mapping of the surface expression of chemical heterogeneity in the mantle is providing new views of the chemical structure of the mantle and the geodynamic processes that operate in the Earth's interio

ISSN:8755-1209    

DOI:10.1029/94RG01874

年代:1994

数据来源: WILEY

摘要:

If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depthh, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values ath. This nonlocal “Kprofile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind‐driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double‐diffusion density ratio,Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long‐Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than

ISSN:8755-1209    

DOI:10.1029/94RG01872

年代:1994

数据来源: WILEY

摘要:

Noble gas elemental and isotopic data on mantle‐derived materials such as mid‐ocean ridge basalt (MORB), hotspot volcanics, diamonds, and mantle xenoliths published up to August 1993 are reviewed critically. Characteristic features of the mantle‐derived materials, which are important in evaluating the significance of the data, are discussed. We choose MORB glasses, hotspot volcanics, mantle xenoliths, and diamonds as the sources to infer the noble gas state in the mantle: MORB and hotspot volcanics to represent the modern depleted and the less degassed mantle, and diamonds to represent the ancient mantle. Because of various fractionation processes, the elemental abundance data obtained from the mantle‐derived materials is unlikely to constrain the noble gas composition of the mantle, except for the systematic enrichment of the heavier noble gases relative to air. On the other hand, apart from the secondary addition of radiogenic components such as4He,21Ne,40Ar,129Xe, and131–136Xe, the noble gas isotopic compositions deduced from the mantle‐derived materials can provide useful information as to the values in the mantle; a best estimate of the mantle noble gas state i

ISSN:8755-1209    

DOI:10.1029/94RG01875

年代:1994

数据来源: WILEY

摘要:

Earth is unique among the planets of the solar system in possessing a full hydrological cycle. The role of water in the evolution of planetary atmospheres is discussed. As the atmospheres of the planets developed and modified the early climates of the planets, only the climate trajectory of Earth intercepted the water phase transitions near the triple point of water, thus allowing the full gamut of water forms to coexist. As a result, transitions between the water phases pervade the entire system and probably are responsible for the creation of a unique climate state. The interactions between the components of the climate system are enriched by the nonlinearity of the water phase transitions. The nonlinear character of the phase transitions of water suggests that the climate should be particularly sensitive to hydrological processes, especially in the tropics. Signatures of the nonlinearity are found in both the structure of the oceans and the atmosphere. Specific processes that determine the character of ocean‐atmosphere interaction, including the role of ambient water vapor and clouds, the selective absorption of radiation by the ocean, the distribution of total heating in the ocean‐atmosphere system, and the role of the flux of freshwater, are discussed in detail. Models of the ocean and atmospheric and oceanic data and models of the coupled system are used to perform systematic analyses of hydrological processes and their role in system interaction. The analysis is extended to consider the role of hydrological processes in the basic dynamics and thermodynamics of oceanic and atmospheric systems. The role hydrological processes play in determining the scale of the major atmospheric circulation patterns is investigated. Explanations are offered as to why large‐scale convection in the tropical atmosphere is constrained to lie within the 28°C sea surface temperature contour and how hydrological processes are involved in interannual climate variability. The relative roles of thermal and haline forcing of the oceanic thermohaline circulation are discussed. Hydrological processes are considered in a global context by the development of a conceptual model of a simple planetary system. The constancy and maintenance of the very warm tropical sea surface temperatures are seen to be critical for the stability of climate. However, within the confines of the simplicity of the theory, the climate system, dominated by hydrological processes, conspires to maintain the temperature of the tropical warm

ISSN:8755-1209    

DOI:10.1029/94RG01873

年代:1994

数据来源: WILEY