Hf‐isotope data of >1100 detrital zircon grains from the Palaeozoic, south‐central Andean Gondwana margin record the complete crustal evolution of South America, which was the predominant source. The oldest grains, with crustal residence ages of 3.8–4.0 Ga, are consistent with complete recycling of existing continental crust around 4 Ga. We confirm three major Archaean, Palaeoproterozoic (Transamazonian) and late Mesoproterozoic to early Neoproterozoic crust‐addition phases as well as six igneous phases during Proterozoic to Palaeozoic time involving mixing of juvenile and crustally reworked material. A late Mesoproterozoic to early Neoproterozoic, Grenville‐age igneous belt can be postulated along the palaeo‐margin of South America. This belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages. Crustal reworking likely dominated over juvenile addition during the Palaeozoic era, and Proterozoic and Archaean zircon was mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc. 相似文献
Estimating the magnitude of Agulhas leakage, the volume flux of water from the Indian to the Atlantic Ocean, is difficult because of the presence of other circulation systems in the Agulhas region. Indian Ocean water in the Atlantic Ocean is vigorously mixed and diluted in the Cape Basin. Eulerian integration methods, where the velocity field perpendicular to a section is integrated to yield a flux, have to be calibrated so that only the flux by Agulhas leakage is sampled. Two Eulerian methods for estimating the magnitude of Agulhas leakage are tested within a high-resolution two-way nested model with the goal to devise a mooring-based measurement strategy. At the GoodHope line, a section halfway through the Cape Basin, the integrated velocity perpendicular to that line is compared to the magnitude of Agulhas leakage as determined from the transport carried by numerical Lagrangian floats. In the first method, integration is limited to the flux of water warmer and more saline than specific threshold values. These threshold values are determined by maximizing the correlation with the float-determined time series. By using the threshold values, approximately half of the leakage can directly be measured. The total amount of Agulhas leakage can be estimated using a linear regression, within a 90% confidence band of 12 Sv. In the second method, a subregion of the GoodHope line is sought so that integration over that subregion yields an Eulerian flux as close to the float-determined leakage as possible. It appears that when integration is limited within the model to the upper 300 m of the water column within 900 km of the African coast the time series have the smallest root-mean-square difference. This method yields a root-mean-square error of only 5.2 Sv but the 90% confidence band of the estimate is 20 Sv. It is concluded that the optimum thermohaline threshold method leads to more accurate estimates even though the directly measured transport is a factor of two lower than the actual magnitude of Agulhas leakage in this model. 相似文献
Several multi-century and multi-millennia simulations have been performed with a complex Earth System Model (ESM) for different
anthropogenic climate change scenarios in order to study the long-term evolution of sea level and the impact of ice sheet
changes on the climate system. The core of the ESM is a coupled coarse-resolution Atmosphere–Ocean General Circulation Model
(AOGCM). Ocean biogeochemistry, land vegetation and ice sheets are included as components of the ESM. The Greenland Ice Sheet
(GrIS) decays in all simulations, while the Antarctic ice sheet contributes negatively to sea level rise, due to enhanced
storage of water caused by larger snowfall rates. Freshwater flux increases from Greenland are one order of magnitude smaller
than total freshwater flux increases into the North Atlantic basin (the sum of the contribution from changes in precipitation,
evaporation, run-off and Greenland meltwater) and do not play an important role in changes in the strength of the North Atlantic
Meridional Overturning Circulation (NAMOC). The regional climate change associated with weakening/collapse of the NAMOC drastically
reduces the decay rate of the GrIS. The dynamical changes due to GrIS topography modification driven by mass balance changes
act first as a negative feedback for the decay of the ice sheet, but accelerate the decay at a later stage. The increase of
surface temperature due to reduced topographic heights causes a strong acceleration of the decay of the ice sheet in the long
term. Other feedbacks between ice sheet and atmosphere are not important for the mass balance of the GrIS until it is reduced
to 3/4 of the original size. From then, the reduction in the albedo of Greenland strongly accelerates the decay of the ice
sheet. 相似文献
Seaweed and urchins accumulate Fe and Zn to higher concentrations near a lead/zinc ore body than further away. The concentration of three metals (Cu, Fe and Zn) in seaweed increase with age of tissue. As and Cd are lowest in tissue of intermediate age, suggesting a reversible bonding mechanism for the two elements.Concentrations of Fe in whole urchins, and Zn in the gonads, are higher than concentrations in Fucus tips, which represent a lower level in the food web. Fe is higher in whole sea urchins than in their gonads. For As and Zn the relationship is reversed. 相似文献
The boundary value problem of physical geodesy has been solved with the use of a harmonic reduction down to an internal sphere using a discrete procedure. (For gravity cf. Bjerhammar 1964 and for the potential cf. Bjerhammar 1968). This was a finite-dimensional approach mostly with one-to-one correspondence between observations and unknowns on the sphere. Earlier studies were made with the use of surface elements (on the sphere) with constantgravity. Integration over the surface elements was replaced by a discrete approach with the use of the distance to a point in the centre of the surface element. See Bjerhammar (1968) and (1969). This approach was later presented as a “reflexive prediction” technique for a weakly stationary stochastic process. Bjerhammar (1974, 1976). Krarup (1969) minimized the L2-norm of the potential on the internal sphere. It will here be proved that the two solutions are identical for a proper choice of the radii of the internal spheres. The proof is given for a spherical earth with selected choice of “carrier points”. The convergence problem is discussed. The L2-norm solution is found convergent for the fully harmonic case. Uniform convergence is obtained in the non-harmonic case with the use of the original procedure applied in accordance with the theorems of Keldych-Lavrentieff and Yamabe. 相似文献
The distribution of iron and magnesium between coexisting garnet and clinopyroxene is expressed by the distribution coefficient KDga-cpx. This coefficient has been experimentally determined as a function of temperature and pressure, and is used to determine the temperature of equilibration of natural eclogites.
The presence of relict zoning in both garnet and clinopyroxene in low to medium temperature eclogites permits evaluation of the P,T path followed by these rocks during prograde metamorphism.
The average P,T path for eclogites of blueschist terranes (type C eclogites) is suggested to be 40°C/Kb during prograde metamorphism. The Tasmanian eclogite (type B) records crystallization along a different P,T path (lower pressure at given temperature) from those of the type C eclogites. Eclogites from the migmatite-gneiss terrances of Poland and Norway do not preserve evidence of prograde metamorphism in mineral zoning but indicate higher temperatures at the metamorphic maximum; pressures were similar to those attained by type C eclogites. 相似文献
Natural background levels of trace metals in marine organisms from a West Greenland inlet were studied during the summer of 1972 and 1973. Dry weight arsenic levels ranged from 11.1 to 307 ppm in fish fillets and from 7.6 to 512 ppm in fish livers. Prawns had arsenic levels ranging up to 80.2 ppm, contrasting with 6.0 ppm in planktonic copepods. The results suggest that in certain marine organisms organo-arsenics predominate and are less toxic than inorganic arsenic compounds. 相似文献