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41.
D.J. Stevenson 《Physics of the Earth and Planetary Interiors》1980,22(1):42-52
By use of the modern theory of liquids and some guidance from the hard-sphere model of liquid structure, the following new results have been derived for application to the Earth's outer core. (1) dK//K, where K is the incompressibility and P the pressure. This is valid for a high-pressure liquid near its melting point, provided that the pressure is derived primarily from a strongly repulsive pair potential φ. This result is consistent with seismic data, except possibly in the lowermost region of the outer core, and demonstrates the approximate universality of dK/dP proposed by Birch (1939) and Bullen (1949). (2) dlnTM/dlnρ = (γCV ? 1)/( is the melting point, ρ the density, γ the atomic thermodynamic Grüneisen parameter and CV the atomic contribution to the specific heat in units of Boltzmann's constant per atom. This reduces to Lindemann's law for CV = 3 and provides further support for the approximate validity of this law. (3) It follows that the “core paradox” of Higgins and Kennedy can only occur if . However, it is shown that , which cannot be achieved for any strongly repulsive pair potential φ and the corresponding pair distribution function g. It is concluded that and that the core paradox is almost certainly impossible for any conceivable core composition. Approximate calculations suggest that γ ~ 1.3–1.5 in the core. Further work on the thermodynamics of the liquid core must await development of a physically realistic pair potential, since existing pair potentials may be unsatisfactory. 相似文献
42.
Thomas?R.?FisherEmail author Anne?B.?Gustafson Gregory?M.?Radcliffe Karen?L.?Sundberg J.?Court?Stevenson 《Estuaries and Coasts》2003,26(6):1450-1460
Photosynthetically available radiation (PAR; 400–700 nm, E m−2 d−1) is the fraction of the total solar energy (Mjoules m−2 d−1) that is used by organisms for photosynthesis and vision. We present a statistical summary of a 17-yr time series of PAR
data (1982–1998) collected near Chesapeake Bay as well as a second set of data on PAR and total solar energy gathered over
a shorter time span (1997–1998). The time series data (5,126 daily totals) varied between 1–67 E m−2 d−1 and were used to estimate the minimum and maximum values of PAR as a function of day of the year. In monthly frequency distributions
of the PAR data, three modes were observed corresponding to sunny, partly cloudy, and overcast days. The second set of PAR
and total solar energy data were used to examine the ratio of PAR to total solar energy, which was 2.04 E Mjoule−1 for PAR between 10 and 70 E m−2 d−1. On overcast days, the ratio increased to as high as 3 E Mjoule−1 as PAR increased in importance as a fraction of the total solar energy. These values were consistent with others in the literature,
and the relationships reported here can be used to predict the climatology of PAR and total solar energy within the Chesapeake
region. The PAR data were also combined with reported minimum values of PAR for net primary production in the surface mixed
layer of the water column of aquatic systems to estimate the combinations of mixed layer depth and diffuse attenuation coefficient
(number of optical depths) under which light limitation of phytoplankton primary production is expected to occur. 相似文献
43.
Trace element partitioning during high-P partial melting and melt-rock interaction; an example from northern Fiordland, New Zealand 总被引:2,自引:0,他引:2
F. C. Schröter J. A. Stevenson N. R. Daczko G. L. Clarke N. J. Pearson K. A. Klepeis 《Journal of Metamorphic Geology》2004,22(5):443-457
Pods of granulite facies dioritic gneiss in the Pembroke Valley, Milford Sound, New Zealand, preserve peritectic garnet surrounded by trondhjemitic leucosome and vein networks, that are evidence of high‐P partial melting. Garnet‐bearing trondhjemitic veins extend into host gabbroic gneiss, where they are spatially linked with the recrystallization of comparatively low‐P two‐pyroxene‐hornblende granulite to fine‐grained high‐P garnet granulite assemblages in garnet reaction zones. New data acquired using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA‐ICPMS) for minerals in various textural settings indicate differences in the partitioning of trace elements in the transition of the two rock types to garnet granulite, mostly due to the presence or absence of clinozoisite. Garnet in the garnet reaction zone (gabbroic gneiss) has a distinct trace element pattern, inherited from reactant gabbroic gneiss hornblende. Peritectic garnet in the dioritic gneiss and garnet in trondhjemitic veins from the Pembroke Granulite have trace element patterns inherited from the melt‐producing reaction in the dioritic gneiss. The distinct trace element patterns of garnet link the trondhjemitic veins geochemically to sites of partial melting in the dioritic gneiss. 相似文献
44.
Habitat requirements for submerged aquatic vegetation in Chesapeake Bay: Water quality, light regime, and physical-chemical factors 总被引:1,自引:0,他引:1
W. Michael Kemp Richard Batleson Peter Bergstrom Virginia Carter Charles L. Gallegos William Hunley Lee Karrh Evamaria W. Koch Jurate M. Landwehr Kenneth A. Moore Laura Murray Michael Naylor Nancy B. Rybicki J. Court Stevenson David J. Wilcox 《Estuaries and Coasts》2004,27(3):363-377
We developed an algorithm for calculating habitat suitability for seagrasses and related submerged aquatic vegetation (SAV) at coastal sites where monitoring data are available for five water quality variables that govern light availability at the leaf surface. We developed independent estimates of the minimum light required for SAV survival both as a percentage of surface light passing though the water column to the depth of SAV growth (PLW min) and as a percentage of light reaching reaching leaves through the epiphyte layer (PLL min). Value were computed by applying, as inputs to this algorithm, statistically dervived values for water quality variables that correspond to thresholds for SAV presence in Chesapeake Bay. These estimates ofPLW min andPLL min compared well with the values established from a literature review. Calcultations account for tidal range, and total light attenuation is partitioned into water column and epiphyte contributions. Water column attenuation is further partitioned into effects of chlorophylla (chla), total suspended solids (TSS) and other substances. We used this algorithm to predict potential SAV presence throughout the Bay where calculated light available at plant leaves exceededPLL min. Predictions closely matched results of aerial photographic monitoring surveys of SAV distribution. Correspondence between predictions and observations was particularly strong in the mesohaline and polythaline regions, which contain 75–80% of all potential SAV sites in this estuary. The method also allows for independent assessment of effects of physical and chemical factors other than light in limiting SAV growth and survival. Although this algorithm was developed with data from Chesapeake Bay, its general structure allows it to be calibrated and used as a quantitative tool for applying water quality data to define suitability of specific sites as habitats for SAV survival in diverse coastal environments worldwide. 相似文献
45.
46.
Science-based management of shallow-water habitats is limited by information on the spatial distribution of properties of sediments. This limitation in part stems from the lack of an adequate model or system to classify and delineate subaqueous soil types (sediments). Present classification systems are inadequate because the existing paradigm does not actually consider them as “soils” but merely as “sediments”. Field observations suggest that these sediments could be better understood as “soils”, and the present paradigm could be modified to incorporate a new one—a pedological paradigm. We propose the application of a pedological paradigm for subqueous soils of subtidal habitats to develop ecological interpretations of subaqueous soil types and apply an inventory of subaqueous soil resources for management of estuarine shallow-water habitats. *** DIRECT SUPPORT *** A01BY074 00009 相似文献
47.
A tensile, flexural model for the initiation of subduction 总被引:2,自引:0,他引:2
48.
49.
Estuaries and Coasts - Effects of the herbicide, atrazine, on the submersed vascular plant,Potamogeton perfoliatus, were monitored for 4 wk in 700 l microcosms containing water, sediments and... 相似文献
50.
David J. Stevenson 《Comptes Rendus Geoscience》2003,335(1):99-111
Mantle convection is the method of heat elimination for silicate mantles in terrestrial bodies, provided they are not too small or too hot. Bodies that are small (~Moon or smaller, possibly even Mercury) may rely largely on conduction or melt migration, and bodies that are very hot (Io, very early Earth) may use massive melt migration (magma oceans) and heat pipes. In the standard, simple picture, we can use scaling laws to determine the secular cooling of a planet, likelihood and form of volcanism, and the possibility of a core dynamo. Contrary to popular belief, small planets do not cool faster than larger planets (provided they convect) but they do tend to have a slightly lower internal temperature at all times and thus may cease to be volcanically active at an earlier epoch. On the other hand, a larger volume fraction of a small planet may be involved in melt generation. However, our understanding of heat transfer by mantle convection is limited by three very important, largely unsolved problems: The complexities of rheology, the effects of compositional gradients, and the effects of phase transitions, especially melting. The most striking manifestation of the role of rheology lies in the difference between a mobile lid mode (plate tectonics for Earth) and a stagnant lid mode (other large terrestrial bodies). This difference may arise because of the role of water, but perhaps also because of melting, or size (gravity), or the vagaries of history. It has profound effects for the differences in history of Earth, Venus and Mars, including their surface geology, volatile reservoirs and magnetic fields. Since thermal convection is driven by small density differences, it can also be greatly altered or limited by compositional or phase effects. Melt migration introduces additional complications to the heat transport as well as being a source for the irreversible differentiation that might promote layering. Our limited understanding and ability to model these processes continues to limit the development of a predictive framework for the differences among the terrestrial planets. 相似文献