Thermal histories of the core and mantle     

Frank D. Stacey  David E. Loper 《Physics of the Earth and Planetary Interiors》1984,36(2):99-115

Recognition that the cooling of the core is accomplished by conduction of heat into a thermal boundary layer (D″) at the base of the mantle, partly decouples calculations of the thermal histories of the core and mantle. Both are controlled by the temperature-dependent rheology of the mantle, but in different ways. Thermal parameters of the Earth are more tightly constrained than hitherto by demanding that they satisfy both core and mantle histories. We require evolution from an early state, in which the temperatures of the top of the core and the base of the mantle were both very close to the mantle solidus, to the present state in which a temperature increment, estimated to be ～ 800 K, has developed across D″. The thermal history is not very dependent upon the assumption of Newtonian or non-Newtonian mantle rheology. The thermal boundary layer at the base of the mantle (i.e., D″) developed within the first few hundred million years and the temperature increment across it is still increasing slowly. In our preferred model the present temperature at the top of the core is 3800 K and the mantle temperature, extrapolated to the core boundary without the thermal boundary layer, is 3000 K. The mantle solidus is 3860 K. These temperatures could be varied within quite wide limits without seriously affecting our conclusions. Core gravitational energy release is found to have been remarkably constant at ～ 3 × 10¹¹ W. nearly 20% of the core heat flux, for the past 3 × 10⁹ y, although the total terrestrial heat flux has decreased by a factor of 2 or 3 in that time. This gravitational energy can power the “chemical” dynamo in spite of a core heat flux that is less than that required by conduction down an adiabatic gradient in the outer core; part of the gravitational energy is used to redistribute the excess heat back into the core, leaving 1.8 × 10¹¹ W to drive the dynamo. At no time was the dynamo thermally driven and the present radioactive heating in the core is negligibly small. The dynamo can persist indefinitely into the future; available power 10¹⁰ y from now is estimated to be 0.3 × 10¹¹ W if linear mantle rheology is assumed or more if mantle rheology is non-linear. The assumption that the gravitational constant decreases with time imposes an implausible rate of decrease in dynamo energy. With conventional thermodynamics it also requires radiogenic heating of the mantle considerably in excess of the likely content of radioactive elements.  相似文献   

Core formation in terrestrial planets     

George H. Shaw 《Physics of the Earth and Planetary Interiors》1979,20(1):42-47

Calculations of the radial distribution of the energy released in core formation indicate that the cores of all the terrestrial planets may be expected to receive a disproportionate share of the gravitational energy released. Since the model of the process used in these calculations favors transfer of energy to the mantle, it is likely that other reasonable models of the process will result in even more energy being deposited in the cores of the early planets. The calculations also show that it is necessary for a certain amount of core phase to separate and accumulate, before the energy released by gravitational settling is sufficient to supply the latent heat of fusion of the core phase. The amount of melting required to reach this point varies according to the total mass of the planet, and mass fraction of core, but is not particularly great (<5% for the Earth to ～ 37% for the Moon). In the case of the Moon, this amount of segregation, although large, amounts to a surface layer about 260 km thick, similar to the proposed depth of early wholesale melting. Core separation in terrestrial planets appears to be a self-sustaining process even for fairly small bodies, provided that a small amount of a dense potential core phase is present. Although it seems likely to occur rapidly (within 10⁶–10⁷ years) even for small (Moon-size) bodies, detailed kinetic models will be necessary to specify the time scale.  相似文献   

Torque balance and energy budget for the precessionally driven dynamo     

David E. Loper 《Physics of the Earth and Planetary Interiors》1975,11(1):43-60

The feasibility of a precessionally driven dynamo is investigated. The relative orientation of the angular-velocity vectors of the mantle and core and the precession vector of the earth are determined from a torque balance. The core and mantle are acted upon by separate gravitational torques and mutual interaction torques resulting from pressure, viscous and magnetic stresses at the core-mantle interface. The viscous and magnetic torques are determined using the results of a detailed analysis of the Ekman-Hartmann and magnetic diffusion layers generated at the core-mantle interface by the misalignment of the mantle and core angular-velocity vectors. The dissipative torques are found to be weaker by a factor of 10^?4 than those estimated by Malkus (1968) and Stacey (1973), resulting in only 3.5 · 10⁷ W being extracted from the rotational kinetic energy of the earth by these mechanisms. Furthermore, it is found that all of this energy is dissipated in the boundary layers at the core-mantle interface and none is available to drive the geodynamo.  相似文献   

Geomagnetic polarity reversals in a turbulent core     

Peter Olson 《Physics of the Earth and Planetary Interiors》1983,33(4):260-274

The behavior of the main magnetic field components during a polarity transition is investigated using the α²-dynamo model for magnetic field generation in a turbulent core. It is shown that rapid reversals of the dipole field occur when the helicity, a measure of correlation between turbulent velocity and vorticity, changes sign. Two classes of polarity transitions are possible. Within the first class, termed component reversals, the dipole field reverses but the toroidal field does not. Within the second class, termed full reversals, both dipole and toroidal fields reverse. Component reversals result from long term fluctuations in core helicity; full reversals result from short term fluctuations. A set of time-evolution equations are derived which govern the dipole field behavior during an idealized transition. Solutions to these equations exhibit transitions in which the dipole remains axial while its intensity decays rapidly toward zero, and is regenerated with reversed polarity. Assuming an electrical conductivity of 3 × 10⁵ mho m^?1 for the fluid core, the time interval required to complete the reversal process can be as short as 7500 years. This time scale is consistent with paleomagnetic observations of the duration of reversals. A possible explanation of the cause of reversals is proposed, in which the core's net helicity fluctuates in response to fluctuations in the level of turbulence produced by two competing energy sources—thermal convection and segregation of the inner core. Symmetry considerations indicate that, in each hemisphere, helicity generated by heat loss at the core-mantle boundary may have the opposite sign of helicity generated by energy release at the inner core boundary. Random variations in rates of energy release can cause the net helicity and the α-effect to change sign occasionally, provoking a field reversal. In this model, energy release by inner core formation tends to destabilize stationary dynamo action, causing polarity reversals.  相似文献   

The cooling Earth: A reappraisal     

Frank D. Stacey 《Physics of the Earth and Planetary Interiors》1980,22(2):89-96

By treating the lithosphere as a diffusive boundary layer to mantle convection, the convective speed or mantle creep rate, $\text{?}\text{?}$, can be related to the mantle-derived heat flux, $\text{Q}\text{?}$. If cell size is independent of $\text{Q}\text{?}{}^2$ then $\text{?}\text{?}\text{∝}\text{Q}\text{?}$. (If cell size varies with $\text{Q}\text{?}$, then a different power law prevails, but the essential conclusions are unaffected.) Then the factthat for constant thermodynamic efficiency of mantle convection, the mechanical power dissipation is proportionalto $\text{Q}\text{?}$, gives convective stress $\text{σ ∝}\text{Q}\text{?}{}^{\mathrm{?1}}$, i.e. the stress increases as the convection slows. This means an increasing viscosityor stiffness of the mantle which can be identified with a cooling rate in terms of a temperature-dependent creep law. If we suppose that the mantle was at or close to its melting point within 1 or 2 × 10⁸ years of accretionof the Earth, the whole scale of cooling is fixed. The present rate of cooling is estimated to be about 4.6 × 10^?8 deg y^?1 for the average mantle temperature, assumed to be 2500 K, but this very slow cooling rate represents a loss ofresidual mantle heat of 7 × 10¹² W, about 30% of the total mantle-derived heat flux. This conclusion requires theEarth to be deficient in radioactive heat, relative to carbonaceous chondrites. A consideration of mantle outgassing and atmospheric argon leads to the conclusion that the deficiency is due to depletion of potassium, and that the K/U ratio of the mantle is only about 2500, much less than either the crustal or carbonaceous chondritic values. Thetotal terrestrial potassium is estimated to be about 6 × 10²⁰ kg. Acceptance of the cooling of the Earth removes the necessity for potassium in the core.  相似文献   

The geomagnetic dynamo-elementary energetics and thermodynamics     

F. J. Lowes 《Surveys in Geophysics》1984,7(1):91-105

This paper is a non-mathematical review, summarising the work in this field.Estimates are made of the power needed to maintain the electric currents which give the main geomagnetic field. The observed surface field needs at least 2×10⁸ W, but unobservable fields may need much more; a toroidal field of peak value 10 or 50nT would need 10¹⁰ or 2.5×10¹¹ W.Ways of obtaining this power from the Earth's rotation, particularly through precession, are considered and rejected.Thermal power sources have the disadvantages that there is inherent thermodynamic inefficiency in driving the dynamo, and that a significant fraction of the heat input will be carried away by conduction rather than convection. Radioactivity will only be important if there is a substantial amount of potassium in the core. If this is not the case the core might be cooling; cooling at 20K per 10⁹ yr would release specific heat at a rate of 10¹² W. If the cooling causes the inner core to grow by freezing from the liquid core, then an additional 10¹² W would be released from the latent heat of freezing. These heat fluxes might support a dynamo having a small toroidal field.If, as seems likely, the solid inner core is significantly denser than the liquid, such cooling would also release 0.6×10¹² W of gravitational energy, giving compositional convection which would drive the dynamo very efficiently and give a large toroidal field.  相似文献   

Evolution of non-linear α 2-dynamos and taylor's constraint     

D.R. Fearn  M.M. Rahman † 《地球物理与天体物理流体动力学》2013,107(5):385-406

A key non-linear mechanism in a strong-field geodynamo is that a finite amplitude magnetic field drives a flow through the Lorentz force in the momentum equation and this flow feeds back on the field-generation process in the magnetic induction equation, equilibrating the field. We make use of a simpler non-linear?α?²-dynamo to investigate this mechanism in a rapidly rotating fluid spherical shell. Neglecting inertia, we use a pseudo-spectral time-stepping procedure to solve the induction equation and the momentum equation with no-slip velocity boundary conditions for a finitely conducting inner core and an insulating mantle. We present calculations for Ekman numbers (E) in the range 2.5× 10^?3 to 5.0× 10^?5, for?α?=α ₀cos?θ?sin?π?(r?r_i ) (which vanishes on both inner and outer boundaries). Solutions are steady except at lower E and higher values of?α?₀. Then they are periodic with a reversing field and a characteristic rapid increase then equally rapid decrease in magnetic energy. We have investigated the mechanism for this and shown the influence of Taylor's constraint. We comment on the application of our findings to numerical hydrodynamic dynamos.  相似文献   

Geothermal observations in east Liaoning and haicheng seismic area     

Wang Andong  Ren Yuhe  Sun Wenfu  Yu Longwei  Liang Jingming  Cao Tianqing  Gu Haoding 《地震学报(英文版)》1988,1(4):60-72

We have obtained altogether heat flux data of 23 drill holes including 2 observational holes of thermal flow in the Haicheng seismic area. These data show roughly thermal structure of the crust in eastern Liaoning and in the Haicheng seismic area. The results indicate that there is higher value of heat flow in the belt north by east from Liaoyang to Xiongyue, the average thermal flux being 8.29× 10⁻²J/m²·s (2.0 hfu). The average thermal fulx in the Haicheng seismic area is 9.22×10²J/m²·s (2.2 hfu). Comparing with other known geophysical data of the Haicheng seismic area, we give a geophysical section of comprehensive interpretation. We suppose the low-velocity layer in the lower crust of the Haicheng seismic area is a result from intrusion of large-scale uper mantle substance. High temperature and low velocity mean that the layer has the nature of plastic mechanics. The focal region of the Haicheng earthquake is situated right in the upper part of that plastic layer. Obviously, this result is significant for studying the seismogenic process of the Haicheng earthquake and other intra-plate earthquakes. We attempt to emphasize that observation of heat flow is necessary for earthquake study. Gu Haoding did the actual writing.  相似文献   

Formation,history and energetics of cores in the terrestrial planets   总被引：1，自引：0，他引：1  

Sean C. Solomon 《Physics of the Earth and Planetary Interiors》1979,19(2):168-182

The cores of the terrestrial planets Earth, Moon, Mercury, Venus and Mars differ substantially in size and in history. Though no planet other than the Earth has a conclusively demonstrated core, the probable cores in Mercury and Mars and Earth's core show a decrease in relative core size with solar distance. The Moon does not fit this sequence and Venus may not. Core formation must have been early (prior to ～4 · 10⁹ yr. ago) in the Earth, by virtue of the existence of ancient rock units and ancient paleomagnetism and from UPb partitioning arguments, and in Mercury, because the consequences of core infall would have included extensional tectonic features which are not observed even on Mercury's oldest terrain. If a small core exists in the Moon, still an open question, completion of core formation may be placed several hundred million years after the end of heavy bombardment on tectonic and thermal grounds. Core formation time on Mars is loosely constrained, but may have been substantially later than for the other terrestrial planets. The magnitude and extent of early heating to drive global differentiation appear to have decreased with distance from the sun for at least the smaller bodies Mercury, Moon and Mars.Energy sources to maintain a molten state and to fuel convection and magnetic dynamos in the cores of the terrestrial planets include principally gravitational energy, heat of fusion, and long-lived radioactivity. The gravitational energy of core infall is quantifiable and substantial for all bodies but the Moon, but was likely spent too early in the history of most planets to prove a significant residual heat source to drive a present dynamo. The energy from inner core freezing in the Earth and in Mercury is at best marginally able to match even the conductive heat loss along an outer core adiabat. Radioactive decay in the core offers an attractive but unproven energy source to maintain core convection.  相似文献   

Thermal structure and heat loss at the summit crater of an active lava dome     

Steve T. Sahetapy-Engel  Andrew J. L. Harris 《Bulletin of Volcanology》2009,71(1):15-28

Forward-Looking Infrared (FLIR) nighttime thermal images were used to extract the thermal and morphological properties for the surface of a blocky-to-rubbley lava mass active within the summit crater of the Caliente vent at Santiaguito lava dome (Guatemala). Thermally the crater was characterized by three concentric regions: a hot outer annulus of loose fine material at 150–400°C, an inner cold annulus of blocky lava at 40–80°C, and a warm central core at 100–200°C comprising younger, hotter lava. Intermittent explosions resulted in thermal renewal of some surfaces, mostly across the outer annulus where loose, fine, fill material was ejected to expose hotter, underlying, material. Surface heat flux densities (radiative + free convection) were dominated by losses from the outer annulus (0.3–1.5 × 10⁴  s⁻¹m⁻²), followed by the hot central core (0.1–0.4 × 10⁴ J s⁻¹m⁻²) and cold annulus (0.04–0.1 × 10⁴ J s⁻¹m⁻²). Overall surface power output was also dominated by the outer annulus region (31–176 MJ s⁻¹), but the cold annulus contributed equal power (2.41–7.07 MJ s⁻¹) as the hot central core (2.68–6.92 MJ s⁻¹) due to its greater area. Cooled surfaces (i.e. the upper thermal boundary layer separating surface temperatures from underlying material at magmatic temperatures) across the central core and cold annulus had estimated thicknesses, based on simple conductive model, of 0.3–2.2 and 1.5–4.3 m. The stability of the thermal structure through time and between explosions indicates that it is linked to a deeper structural control likely comprising a central massive plug, feeding lava flow from the SW rim of the crater, surrounded by an arcuate, marginal fracture zone through which heat and mass can preferentially flow.  相似文献   

Axial and equatorial rotations of the Earth's cores associated with the Quaternary ice age     

Masao Nakada 《Physics of the Earth and Planetary Interiors》2006,154(2):113-147

The differential axial and equatorial rotations of both cores associated with the Quaternary glacial cycles were evaluated based on a realistic earth model in density and elastic structures. The rheological model is composed of compressible Maxwell viscoelastic mantle, inviscid outer core and incompressible Maxwell viscoelastic inner core. The present study is, however, preliminary because I assume a rigid rotation for the fluid outer core. In models with no frictional torques at the boundaries of the outer core, the maximum magnitude of the predicted axial rotations of the outer and inner cores amounts to ∼2° year⁻¹ and ∼1° year⁻¹, respectively, but that for the secular equatorial rotations of both cores is ∼0.0001° at most. However, oscillating parts with a period of ∼225 years are predicted in the equatorial rotations for both cores. Then, I evaluated the differential rotations by adopting a time-dependent electromagnetic (EM) torque as a possible coupling mechanism at the core-mantle boundary (CMB) and inner core boundary (ICB). In a realistic radial magnetic field at the CMB estimated from surface magnetic field, the axial and equatorial rotations couple through frictional torques at the CMB, although these rotations decouple for dipole magnetic field model. The differential rotations were evaluated for conductivity models with a conductance of 10⁸ S of the lowermost mantle inferred from studies of nutation and precession of the Earth and decadal variations of length of day (LOD). The secular parts of equatorial rotations are less sensitive to these parameters, but the magnitude for the axial rotations is much smaller than for frictionless model. These models, however, produce oscillating parts in the equatorial rotations of both cores and also in the axial rotations of the whole Earth and outer and inner cores. These oscillations are sensitive to both the magnitude of radial magnetic field at the CMB and the conductivity structure. No sharp isolated spectral peaks are predicted for models with a thin conductive layer (∼200 m) at the bottom of the mantle. In models with a conductive layer of ∼100 km thickness, however, sharp spectral peaks are predicted at periods of ∼225 and ∼25 years for equatorial and axial rotations, respectively, although these depend on the strength of radial magnetic field at the CMB. While the present study is preliminary in modelling the fluid outer core and coupling mechanism at the CMB, the predicted axial rotations of the whole Earth may be important in explaining the observed LOD through interaction between the equatorial and axial rotations.  相似文献   

Lateral variation of differential stress in the uppermost mantle across the island arc of southwest Japan     

Mitsuhiro Toriumi 《Earth and Planetary Science Letters》1980,48(2):371-378

The dislocation density and the subgrain size of olivine in peridotite xenoliths in southwest Japan were investigated in order to draw out the lateral variation of the differential stress in the upper mantle of the island arc. Alkali basaltic and andesitic dykes including peridotite xenoliths of Dogo, Kikuma, and Shingu are situated about 200 km behind the Nankai Trough, and those of Oki-Dogo and Takashima located at the portions 400–500 km apart from the trough. The mean dislocation densities of olivine are 2 × 10⁶ cm^?2 for Oki-Dogo, 8 × 10⁶ cm^?2 for Takashima, 1 × 10⁷ cm^?2 for Hamada, 5 × 10⁷ cm^?2 for Aratoyama, 4 × 10⁷ cm^?2 for Kikuma, 3 × 10⁷ cm^?2 for Dogo, and 5 × 10⁶ cm^?2 for Shingu peridotites.It is concluded that the differential stress is high in the uppermost mantle beneath the island arc and low in the back-arc and the mantle wedge behind the plate boundary. The lateral variation of stress may be due to the diapiric upwelling of upper mantle materials under the island arc. The size of the diapir is suggested to be 200 km in width and 60–150 km in depth.  相似文献   

Helium isotope studies of the mantle xenoliths and megacrysts from the Cenozoic basalts in the eastern China   总被引：9，自引：2，他引：7  

李延河  李金城  宋鹤彬  郭立鹤 《中国科学D辑(英文版)》2002,45(2):174-183

Helium isotope compositions of the mantle xenoliths and megacrysts in the Cenozoic basalts in the eastern China were measured. The samples were collected from Ludao of Heilongjiang, Huinan and Jiaohe of Jilin, Kuandian of Liaoning, Hannuoba of Hebei, Nüshan of Anhui, Dingan of Hainan. The³He/⁴He ratios of the mantle xenoliths and megacrysts from the most areas were about 1 × 10^-5, and were similar to those of the MORB, thus reflecting the characteristics of the MORB-typed depleted mantle. The³He/⁴He ratios of the mantle xenoliths from Jiaohe were 4.8×10^-6 and the³He/⁴He ratios of xenoliths from Hannuoba vary from 0.15× 10^-6 to 7.4 ×10^-6, obviously lower than those of the MORB, and even lower than the atmospheric helium isotope ratios, indicating that the continental mantle was strongly replaced in Jiaohe and Hannuoba areas. The helium isotope compositions of the mantle xenoliths and megacrysts in the same region vary in a very wide range. It is inferred that the mantle xenoliths and megacrysts were from different parts of the continental mantle. There were not necessary origin relations between the mantle xenoliths, megacrysts and their host basalts. An extremely high³He/⁴He ratio of garnet megacryst from Hannuoba, Hebei was found.  相似文献   

The dynamical and thermal structure of deep mantle plumes     

David E. Loper  Frank D. Stacey 《Physics of the Earth and Planetary Interiors》1983,33(4):304-317

If the interpretation of the D″ layer at the base of the mantle as a thermal boundary layer, with a temperature increment in the order of 800 K, is correct, then the formation of deep-mantle plumes to vent material from it appears inevitable. We demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long-lived chimneys that are ～ 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material. The speed of flow up the axis of the plume is correspondingly fast; 1.6 m y^?1 at the base and 4.8 m y^?1 at 670 km depth. Thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there. This convergence, which contributes only a small increment to the flux of material up the plume, also serves to throttle the flow in the chimney. The global plume mass flux necessary to transport 1.6 × 10¹² W of core heat upward through the mantle is 1.8 × 10⁶ kg s^?1. At its base, plume material is probably still significantly below its solidus or eutectic temperature, but substantial partial melting is very likely as it rises. We speculate that a small fraction of this fluid component eventually emerges at the surface in “hot spots”, with the fate of the remainder being unknown. The behaviour and properties of D″ and of plumes are closely coupled. Not only are plumes a necessary consequence of a thermal boundary layer, but their existence is impossible without that layer.  相似文献   

Inferences on the lunar temperature from gravity,stress state and flow laws     

Kurt Lambeck  Susan Pullan 《Physics of the Earth and Planetary Interiors》1980,22(1):12-28

Inferences on the lunar temperature regime are made from the inversion of gravity for density anomalies and the stress-state of the Moon's interior, and by comparing these results with flow laws and estimates of likely strain-rates.The nature of the spectrum of the lunar gravitational potential indicates that the density anomalies giving rise to the potential are mainly of near-surface origon. The average stress-differences in the lunar mantle required to support these density anomalies are of the order of a few tens of bars and have persisted for more than 3 · 10⁹ years. If current flow laws for dry olivine can be extrapolated to the conditions of the lunar mantle, and the selenotherms based on electrical conductivity models are valid, the strain rates are too high to explain the preservation of the lateral near-surface density anomalies. We suggest that the present temperatures in the Moon are relatively low, of the order of 800°C or less, at a depth of about 300 km. This compares with 1100°C based on electrical conductivity models and is near the lower limit predicted by Keihm and Langseth (1977) from lunar heat-flow observations.  相似文献   

Study of the Earth’s free core nutation by tidal gravity data recorded with international superconducting gravimeters   总被引：2，自引：0，他引：2  

Jianqiao Xu  Heping Sun  Shaocong Luo 《中国科学D辑(英文版)》2002,45(4):337-347

By stacking high-precision tidal gravity observations obtained with superconducting gravimeters at six stations in China, Japan, Belgium, France, Germany and Finland, the local systematical discrepancies in the parameter fitting, caused by atmospheric, oceanic tidal loading and the other local environmental perturbations, are eliminated effectively. As a result, the resonance parameters of the Earth’s free core nutation are accurately determined. In this study, the eigenperiod of free core nutation is given as 429.0 sidereal days, which is in agreement with those published in the previous studies. It is about 30 sidereal days less than those calculated in theoretical models (about 460 sidereal days), which confirms the real ellipticity of the fluid core of the Earth to be about 5% larger than the one expected in assumption of hydrostatic equilibrium. The quality factor (Q value) of free core nutation is given as about 9543, which, compared with those determined before based on the body tide observations, is much larger, but more close to those obtained using the VLBI observations. The complex resonance strength is also determined as (−6.10×10⁻⁴, −0.01 ×10⁻⁴)°/h, which can principally describe the deformation characteristics of an anelastic mantle.  相似文献   

Inorganic carbon and nutrient fluxes on the Arctic Shelf     

《Continental Shelf Research》2007,27(10-11):1584-1599

Historic data from the Russian-American Hydrochemical Atlas of Arctic Ocean together with data from the TRANSDRIFT II 1994 and TUNDRA 1994 cruises have been used to assess the spatial and inter-annual variability of carbon and nutrient fluxes, as well as air–sea CO₂ exchange in the Laptev and western East Siberian Seas during the summer season. Budget computations using summer data of dissolved inorganic phosphate (DIP), dissolved inorganic nitrogen (DIN) and dissolved inorganic carbon (DIC) gives that the Laptev Sea shelf is a net sink of DIP and DIN of 2.5×10⁶, 23.2×10⁶ mol d⁻¹, respectively, while it is a net source of DIC (excluding air–sea exchange) of 1249×10⁶ mol d⁻¹. In the East Siberian Seas the budget computations give 0.5×10⁶, −11.4×10⁶ and −173×10⁶ mol d⁻¹ (minus being a sink) for DIP, DIN, and DIC, respectively. In summers, the Laptev Sea Shelf is net autotrophic while the East-Siberian Sea Shelf is net heterotrophic, and both systems are weak net denitrifying. The Laptev Sea Shelf takes up 2.1 mmol CO₂ m⁻² d⁻¹ from atmosphere, whereas the western part of the East-Siberian Sea Shelf loose 0.3 mmol CO₂ m⁻² d⁻¹ to the atmosphere. The variability of DIP, DIN and DIC fluxes during summer in the different regions of the Laptev and East Siberian Seas depends on bottom topography, river runoff, exchange with surrounding seas and wind field.  相似文献   

CHANGES IN MORPHOLOGY AT THE MARGIN OF THE GREENLAND ICE SHEET (LEVERETT GLACIER), IN THE PERIOD 1943–1992: A QUANTITATIVE ANALYSIS     

FRANK G. M. VAN TATENHOVE 《地球表面变化过程与地形》1996,21(9):797-816

Changes in ice-marginal morphology near Leverett glacier, a small outlet glacier at the western margin of the Greenland ice sheet, are determined from a photogrammetrical analysis. To be able to compare two datasets from subsequent years with measurements at different coordinates, kriging was used for interpolation. In this study the kriging standard error is used to evaluate the relative accuracy of the resulting maps. Aerial photographs of 1943, 1968 and 1985 were compared. In the period 1943–1968 an area of 0.2 × 10⁶ m² was deglaciated. Approximately 1.1 × 10 m³ of material is deposited in this area. The southern part of the deglaciated area is characterized by ice-cored moraines, while moraines without ice core were formed in the north. Differences in depositional products reflect differences in meltwater activity and probably ice-marginal thermal regime. During deglaciation a small proglacial sandur decreased in altitude by 3.2 ± 0.1 m. From the early 1970s Leverett glacier advanced over a previously deglaciated area. During this advance, small ice-marginal accumulations were incorporated and eroded by the advancing glacier. Erosion products were for a substantial part stored in the proglacial sandur. About 1.2 ×10⁵ m² of the northern part of an ice-cored moraine complex decreased in altitude by −3.6 ± 0.1 m from 1943 to 1968 and over 2.7 × 10⁴ m² by −2.7 ± 0.1 m during 1968–1985. The spatial patterns of altitude change were analysed in relation to topomorphological parameters as exposition and slope angle and areas occupied by lakes. The estimated energy used to melt the subsurface ice of the ice-cored moraine is 1.4–2.2 W m² (1943–1968) and 1.0–1.6 W m² (1968–1985). These values are 30–50 times larger than the geothermal heat flux. For the expected average debris concentration of the ice core (< 10 per cent by volume) the deviation of the surface energy balance forced by climate change will be small and encompass an insignificant part of the total estimated energy used for melting.  相似文献   

Flow convergence over a broad,flat continental shelf     

L.K. Coachman 《Continental Shelf Research》1982,1(1):1-14

The broad (～500 km) southeastern Bering Sea continental shelf contains three fronts; outer (shelf break, ～170-m depth), middle (～100-m depth), and inner (～50-m depth). The shelf break and inner fronts appear to be analogous to similar fronts reported from other mid-latitude continental shelves; extensively studied examples are from the mid-Atlantic bight, off Nova Scotia, and around the British Isles. The middle front may have counterparts on the broad North Sea and East China Sea shelves.One-month current and temperature records from either side of the middle front, ～150 km landward from the shelf break, showed convergence in the layers deeper than 30 m in both the cross-shelf flow field and heat flux. The convergence was ～3 cm s^?1, so an average upwelling at ～1 × 10^?3cm s^?1 and divergence in the surface layer were required to maintain continuity. Variations in the degree of convergence arose primarily from 1 to 5-day fluctuations in sub-tidal flow across the outer shelf domain seaward of the front.Diffusive landward heat flux was dominated by tidal scales. Horizontal eddy conductivities describing the flux were ～1 ? 10⁶ on the landward side and ～5 × 10⁶cm² s^?1 on the seaward side, and were less in the layers above the bottom layer. Advective flux by the mean flow was the same order as diffusive flux, but landward in the bottom layer and seaward in the mid-water column layers, in agreement with deductions from water mass analyses. Frontal effects reduced the net cross-shelf heat flux beneath ～30 m by about 50%. The observation of a flow convergence in the middle of a broad, flat continental shelf poses an important question of dynamics.  相似文献   

The structure of 2D mantle convection and stress fields: Effects of viscosity distribution     

A. A. Baranov  A. M. Bobrov 《Izvestiya Physics of the Solid Earth》2011,47(7):575-585

Spatial fields of temperature, velocity, overlithostatic pressure, and horizontal stresses in the Earth’s mantle are studied in two-dimensional (2D) numerical Cartesian models of mantle convection with variable viscosity. The calculations are carried out for three different patterns of the viscosity distribution in the mantle: (a) an isoviscous model, (b) a four-layer viscosity model, and (c) a temperature- and pressure-dependent viscosity model. The pattern of flows, the stresses, and the surface heat flow are strongly controlled by the viscosity distribution. This is connected with the formation of a cold highly viscous layer on the surface, which is analogous to the oceanic lithosphere and impedes the heat transfer. For the Rayleigh number Ra = 10⁷, the Nusselt number, which characterizes the heat transfer, is Nu = 34, 28, and 15 in models with constant, four-layered, and p, T-dependent viscosity, respectively. In all three models, the values of overlithostatic pressure and horizontal stresses σ _xx in a vast central region of the mantle, which occupies the bulk of the entire volume of the computation domain, are approximately similar, varying within ±5 MPa (±50 bar). This follows from the fact that the dimensionless mantle viscosity averaged over volume is almost similar in all these models. In the case of temperature- and pressure-dependent viscosity, the overlithostatic pressure and stress σ _xx fields exhibit much stronger concentration towards the horizontal boundaries of the computation domain compared to the isoviscous model. This effect occurs because the upwellings and downwellings in a highly viscous region experience strong variations in both amplitude and direction of flow velocity near the horizontal boundaries. In the models considered with the parameters used, the stresses in the upper and lower mantle are approximately identical, that is, there is no denser concentration of stresses in the upper or lower mantle. In contrast to the overlithostatic pressure field, the fields of horizontal stresses σ _xx in all models do not exhibit deep roots of highly viscous downwelling flows.  相似文献