变化中的地球所面临的挑战:全球变化的科学理解(英文)

简要介绍了变化中的地球所面临的挑战 ,并在描述地球系统科学近年来所取得成就的基础上给出了全球变化的科学理解。同时也简要介绍了世界正在面临的主要环境问题 ;描述了气候系统研究所达成的共识、面临的挑战及所取得的研究进展 ;并从大气、海洋和陆地间的CO2 交换、转换、不确定性等方面阐述了碳循环 ;然后 ,简单描述了全球水循环研究进展 ;最后 ,展望了在变化的地球中生存所要面临的挑战与机遇。     

BLENDING HEIGHTS FOR WINDS OBLIQUE TO CHECKERBOARDS

Analysis of the two-dimensional checkerboardproblem of many alternating surfaces with different properties andvarious scales and aspect ratios is extended to the general case ofwinds oblique to the surface pattern. Relevant periodic solutions ofa three-dimensional atmospheric convection-diffusion equation arefound, and used to estimate blending heights for both concentrationand flux.Illustrative results show negligible directional effect on blendingheight for X1* (the smaller pattern wavelength) of 10 m, but a ratio of maximum/minimum heights of about 3 to 5 for X1* = 100 m. For X1* = 1000 mthe directional variation is strongly peaked with a maximum/minimum ratio of 8 to 14. At fixed X1* the maximum increases with check aspect ratio. It is associated withthe case where the wind blows along one check diagonal. In thatsingular situation, damping of the dominant harmonic varying normalto the diagonal depends purely on transverse and vertical diffusion:horizontal convection and wind shear play no part.     

Sediment suspension under waves and currents: time scales and vertical structure

Field measurements of the vertical structure of near-bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time-dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near-bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind-wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near-bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near-bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid.     

ENHANCED MIGRATION OF SEISMIC DATA1

The so-called ‘enhanced migration’ which uses diffraction tomography as the ‘repair tool’ for correction of amplitudes (reflection coefficients) of migrated sections is discussed. As with any linearized procedure, diffraction tomography requires knowledge of the initial model. It is suggested that the initial model is taken as the migrated image. It will be demonstrated that diffraction tomography applied to the data residuals improves the amplitudes of the migrated images. Migration is redefined as the reconstruction of the wavefront sets of distributions (reflection interfaces), and the inversion process as tomographic correction of migrated images.     

Geochemistry of meteoric calcite cements in some Pleistocene limestones

In this study, the stable isotope and trace element geochemistries of meteoric cements in Pleistocene limestones from Enewetak Atoll (western Pacific Ocean), Cat Island (Bahamas), and Yucatan were characterized to help interpret similar cements in ancient rocks. Meteoric calcite cements have a narrow range of δ¹⁸O values and a broad range of δ¹³C values in each geographical province. These Pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location. Enewetak calcite cements have a mean δ¹⁸O composition of ?6.5%₀ (PDB) and δ¹³C values ranging from ?9.6 to +0.4%0 (PDB). Sparry calcite cements from Cat Island have a mean δ¹⁸O composition of ?4.1%₀ and δ¹³C values ranging from ?6.3 to + 1.1%₀. Sparry cements from Yucatan have a mean δ¹⁸O composition of ?5.7%₀ and δ¹³C values of ?8.0 to ?2.7%₀. The mean δ¹⁸O values of these Pleistocene meteoric calcite cements vary by 2.4%₀ due to climatic variations not related directly to latitude. The δ¹³C compositions of meteoric cements are distinctly lower than those of the depositional sediments. Variations in δ¹³C are not simply a function of distance below an exposure surface. Meteoric phreatic cements often have δ¹³C compositions of less than —4.0%₀, which suggests that soil-derived CO₂ and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements. Concentrations of strontium and magnesium are quite variable within and between the three geographical provinces. Mean strontium concentrations for sparry calcite cements are, for Enewetak Atoll, 620 ppm (σ= 510 ppm); for Cat Island, 1200 ppm (σ= 980 ppm); and for Yucatan, 700 ppm (σ= 390 ppm). Equant cements, intraskeletal cements, and Bahamian cements have higher mean strontium concentrations than other cements. Equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments. Bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements. Magnesium concentrations in Pleistocene meteoric cements are similar in samples from Enewetak Atoll (mean =1.00 mol% MgCO₃; σ= 0.60 mol% MgCO₃) and Cat Island (mean = 0.84 mol% MgCO₃; σ= 0.52mol% MgCO₃) but Yucatan samples have higher magnesium concentrations (mean = 2.20 mol% MgCO₃: σ= 0.84mol% MgCO₃). Higher magnesium concentrations in some Yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water.     

Metamorphic History of a High-Grade Blueschist Exotic Block from the Franciscan Complex, California

A high-grade blueschist tectonic block from the Franciscan Complexof the northeast Diablo Range shows evidence of three episodesof retrograde blueschist facies metamorphism ± deformationdeveloped under progressively declining P-T conditions. Thefirst retrograde metamorphism involved formation of an outerrind of actinolite + chlorite + rutile ± phengite, andthe growth of coarse-grained chlorite + pumpellyite within theblock. During the second event the rind and outer edge of theblueschist were folded, sheared and fractured, and primary glaucophanewithin the blueschist was replaced by albite, medium-grainedchlorite, and glaucophane-crossite. The third retrograde metamorphismwas marked by the pseudomorphic replacement of rind actinoliteby aragonite and quartz. Aragonite also crystallized extensivelywithin th block, accompanied by lawsonite, chlorite, jadeiticpyroxene, and crossite; this last mineral assemblage is identicalto that of the surrounding Franciscan metasedimentary rocks.Features characteristic of the first and second retrograde metamorphicevents are readily observed in other high-grade tectonic blocksof the Franciscan Complex and the correlative Otter Point Formationof Oregon. In contrast, evidence of a third retrograde metamorphism,matching that of presently associated Franciscan and Otter Pointrocks, has been found in some but not all blocks examined sofar. The high P/T conditions of prograde metamorphism and the availablemetamorphic age determinations suggest that the tectonic blocksoriginally formed in a pre-Franciscan subduction zone setting.Fragments of blueschist and eclogite from this metamorphic terrainwere tectonically incorporated in a serpentinite diapir, andthey developed alteration rinds through interaction with theenclosing ultramafic rock. The available data suggest that theexamined exotic block and at least some others were transferredto the Franciscan as detritus from a body of serpentinite thatreached the earth's surface. Such blocks were then resubductedand metamorphosed along with their presently associated sedimentarysequences.     

Pressure--Temperature--Time Paths of Regional Metamorphism I. Heat Transfer during the Evolution of Regions of Thickened Continental Crust

The development of regional metamorphism in areas of thickenedcontinental crust is investigated in terms of the major controlson regional-scale thermal regimes. These are: the total radiogenicheat supply within the thickened crust, the supply of heat fromthe mantle, the thermal conductivity of the medium and the lengthand time scales of erosion of the continental crust. The orogenicepisode is regarded as consisting of a relatively rapid phaseof crustal thickening, during which little temperature changeoccurs in individual rocks, followed by a lengthier phase oferosion, at the end of which the crust is at its original thickness.The principal features of pressure—temperature—time(PTt) paths followed by rocks in this environment are a periodof thermal relaxation, during which the temperature rises towardsthe higher geotherm that would be supported by the thickenedcrust, followed by a period of cooling as the rock approachesthe cold land surface. The temperature increase that occursis governed by the degree of thickening of the crust, its conductivityand the time that elapses before the rock is exhumed sufficientlyto be affected by the proximity of the cold upper boundary.For much of the parameter range considered, the heating phaseencompasses a considerable portion of the exhumation (decompression)part of the PTt path. In addition to the detailed calculationof PTt paths we present an idealized model of the thickeningand exhumation process, which may be used to make simple calculationsof the amount of heating to be expected during a given thickeningand exhumation episode and of the depth at which a rock willstart to cool on its ascent path. An important feature of thesePTt paths is that most of them lie within 50 °C of the maximumtemperature attained for one third or more of the total durationof their burial and uplift, and for a geologically plausiblerange of erosion rates the rocks do not begin to cool untilthey have completed 20 to 40 per cent of the total uplift theyexperience. Considerable melting of the continental crust isa likely consequence of thickening of crust with an averagecontinental geotherm. A companion paper discusses these resultsin the context of attempts to use metamorphic petrology datato give information on tectonic processes.     

Pressure--Temperature--Time Paths of Regional Metamorphism II. Their Inference and Interpretation using Mineral Assemblages in Metamorphic Rocks

A companion paper (England & Thompson, 1984a) investigatesthe pressure-temperature-time (PTt) paths followed by rocksundergoing burial metamorphism in continental thickening events.This paper discusses problems involved in inferring such pathsfrom the petrological data available in metamorphic rocks and—oncesuch paths are determined—how they may be interpretedin terms of the thermal budgets of metamorphism. Each of theprincipal facies series (glaucophane-jadeite, andalusite- sillimaniteand kyanite-sillimanite) may be encountered by rocks involvedin the thickening and erosion of continental crust in a regimeof average continental heat flow. The inference of a minimumthermal budget required for a given metamorphism depends stronglyon a knowledge of the PTt paths followed by rocks during themetamorphism. Discrimination between possible thermal regimesis greatly enhanced if portions of PTt paths, rather than singlePT points, are available, and additional constraint is possibleif these paths are supplemented by geochronological, structuraland heat flow data.     

INVERSION OF AN UNMIGRATED STACKED SECTION TO DETERMINE AN INTERVAL VELOCITY MODEL*

A seismic inversion procedure is developed that inverts data available from an unmigrated stacked section to produce an interval velocity model. It attempts to overcome some of the limitations of existing methods by using a generalized linear inversion technique. The inversion process incorporates several features: (i) Lateral interval velocity variations are permitted, (ii) A fast accurate forward model was developed, (iii) Input data is weighted according to the accuracy with which it has been acquired. The procedure is applied to seismic data from the Gippsland Basin, an area offshore South-East Australia.