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51.
Gordon R.M. Bromley Brenda L. Hall Joerg M. Schaefer Gisela Winckler Claire E. Todd Kurt M. Rademaker 《第四纪科学杂志》2011,26(1):37-43
The occurrence of pronounced climate reversals during the last glacial termination has long been recognised in palaeoclimate records from both hemispheres and from high to low latitudes. Accurate constraint of both the timing and magnitude of events, such as the Younger Dryas and Antarctic Cold Reversal, is vital in order to test different hypotheses for the causes and propagation of abrupt climate change. However, in contrast to higher‐latitude regions, well‐dated records from the Tropics are rare and the structure of late‐glacial tropical climate remains uncertain. As a step toward addressing this problem, we present an in situ cosmogenic 3He surface exposure chronology from Nevado Coropuna, southern Peru, documenting a significant fluctuation of the ice margin during the late‐glacial period. Ten tightly clustered ages from a pair of moraines located halfway between the modern glacier and the Last Glacial Maximum terminus range from 11.9 to 13.9 ka and give an arithmetic mean age of 12.8 ± 0.7 ka (1σ). These data constitute direct evidence for a readvance, or prolonged stillstand, of glaciers in the arid Andes of southwestern Peru. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
52.
Three methods for calculation of geocenter motion are intruduced. 相似文献
53.
Using model studies, the total gradient (TG) of the Z-component magnetic field is shown to be a useful quantity for delineating sources of satellite-altitude magnetic anomalies; this field is used to constrain the location and lateral boundaries of sources of high amplitude magnetic anomalies of southern highlands of Mars. The TG field suggests two parallel linear and oppositely magnetized sources of 1000 and 1800 km length separated by 1000 km of region of intervening non-parallel sources. The simplest interpretation of the long, linear features is that they are zones of multitudinous crustal scale dikes formed in separate episodes of rifting, and not features associated with the mechanism of seafloor spreading. Forward modeling with uniformly magnetized sources suggests that magnetizations of the order of 10-50 A/m (40 km thickness) over ∼100 km width in the case of the southern source and of 12.5-27.5 A/m (40 km thickness) and ∼200 km width for the northern source are necessary to explain the Z-component amplitudes and features of the TG field. If the crustal magnetization on Mars were to be distributed fractally as on Earth, magnetizations matching the largest amplitude features on Mars may be spatially correlated from a 50-100 km distance range (β ∼ 3) to approaching nearly uniform magnetization (β ∼ 5) values. To keep magnetization intensity as small as possible, the higher end of β values are preferred, whereas, small amplitude anomaly features could be generated from sources with β ∼ 3. Many of the Mars anomaly features could be coalescence effects similar to the coalescence of anomaly features observed on http://icarus.cornell.edu/information/keywords.html.Earth. 相似文献
54.
A. Määttänen F. Montmessin F. Scholten F. González-Galindo A. Spiga E. Hauber J.-P. Bibring 《Icarus》2010,209(2):452-469
This study presents the latest results on the mesospheric CO2 clouds in the martian atmosphere based on observations by OMEGA and HRSC onboard Mars Express. We have mapped the mesospheric CO2 clouds during nearly three martian years of OMEGA data yielding a cloud dataset of ∼60 occurrences. The global mapping shows that the equatorial clouds are mainly observed in a distinct longitudinal corridor, at seasons Ls = 0-60° and again at and after Ls = 90°. A recent observation shows that the equatorial CO2 cloud season may start as early as at Ls = 330°. Three cases of mesospheric midlatitude autumn clouds have been observed. Two cloud shadow observations enabled the mapping of the cloud optical depth (τ = 0.01-0.6 with median values of 0.13-0.2 at λ = 1 μm) and the effective radii (mainly 1-3 μm with median values of 2.0-2.3 μm) of the cloud crystals. The HRSC dataset of 28 high-altitude cloud observations shows that the observed clouds reside mainly in the altitude range ∼60-85 km and their east-west speeds range from 15 to 107 m/s. Two clouds at southern midlatitudes were observed at an altitude range of 53-62 km. The speed of one of these southern midlatitude clouds was measured, and it exhibited west-east oriented speeds between 5 and 42 m/s. The seasonal and geographical distribution as well as the observed altitudes are mostly in line with previous work. The LMD Mars Global Climate Model shows that at the cloud altitude range (65-85 km) the temperatures exhibit significant daily variability (caused by the thermal tides) with the coldest temperatures towards the end of the afternoon. The GCM predicts the coldest temperatures of this altitude range and the season Ls = 0-30° in the longitudinal corridor where most of the cloud observations have been made. However, the model does not predict supersaturation, but the GCM-predicted winds are in fair agreement with the HRSC-measured cloud speeds. The clouds exhibit variable morphologies, but mainly cirrus-type, filamented clouds are observed (nearly all HRSC observations and most of OMEGA observations). In ∼15% of OMEGA observations, clumpy, round cloud structures are observed, but very few clouds in the HRSC dataset show similar morphology. These observations of clumpy, cumuliform-type clouds raise questions on the possibility of mesospheric convection on Mars, and we discuss this hypothesis based on Convective Available Potential Energy calculations. 相似文献
55.
The latest measurements from the two encounters of the MESSENGER spacecraft in year 2008 have discovered several interesting features of the magnetosphere of Mercury. We have performed high-resolution 3D hybrid model calculations to simulate the solar wind interaction with the Hermean magnetosphere during the first two Mercury encounters of the MESSENGER spacecraft in 2008. It is found that the global structure of the Hermean magnetosphere is significantly controlled by the direction of the interplanetary magnetic field. The bow shock size and shape and the magnetotail configuration have very large differences in these two encounters with northward-pointing and southward-pointing interplanetary magnetic field, respectively. Comparisons are also given with the observed magnetic field profiles and the computational results. In general, good agreement can be found including the interesting feature of the relatively thick magnetopause current layer at outbound measurements. Our work shows that 3D hybrid simulation is a promising method to study in detail the Hermean magnetosphere in parallel with the post-MOI observations of the MESSENGER spacecraft and the Bepi-Colombo mission in future. 相似文献
56.
We present results from a suite of N-body simulations that follow the formation and accretion history of the terrestrial planets using a new parallel treecode that we have developed. We initially place 2000 equal size planetesimals between 0.5 and 4.0 AU and the collisional growth is followed until the completion of planetary accretion (>100 Myr). A total of 64 simulations were carried out to explore sensitivity to the key parameters and initial conditions. All the important effect of gas in laminar disks are taken into account: the aerodynamic gas drag, the disk-planet interaction including Type I migration, and the global disk potential which causes inward migration of secular resonances as the gas dissipates. We vary the initial total mass and spatial distribution of the planetesimals, the time scale of dissipation of nebular gas (which dissipates uniformly in space and exponentially in time), and orbits of Jupiter and Saturn. We end up with 1-5 planets in the terrestrial region. In order to maintain sufficient mass in this region in the presence of Type I migration, the time scale of gas dissipation needs to be 1-2 Myr. The final configurations and collisional histories strongly depend on the orbital eccentricity of Jupiter. If today’s eccentricity of Jupiter is used, then most of bodies in the asteroidal region are swept up within the terrestrial region owing to the inward migration of the secular resonance, and giant impacts between protoplanets occur most commonly around 10 Myr. If the orbital eccentricity of Jupiter is close to zero, as suggested in the Nice model, the effect of the secular resonance is negligible and a large amount of mass stays for a long period of time in the asteroidal region. With a circular orbit for Jupiter, giant impacts usually occur around 100 Myr, consistent with the accretion time scale indicated from isotope records. However, we inevitably have an Earth size planet at around 2 AU in this case. It is very difficult to obtain spatially concentrated terrestrial planets together with very late giant impacts, as long as we include all the above effects of gas and assume initial disks similar to the minimum mass solar nebular. 相似文献
57.
Chemistry of atmospheres formed during accretion of the Earth and other terrestrial planets 总被引:1,自引:0,他引:1
We used chemical equilibrium and chemical kinetic calculations to model chemistry of the volatiles released by heating different types of carbonaceous, ordinary and enstatite chondritic material as a function of temperature and pressure. Our results predict the composition of atmospheres formed by outgassing during accretion of the Earth and other terrestrial planets. Outgassing of CI and CM carbonaceous chondritic material produces H2O-rich (steam) atmospheres in agreement with the results of impact experiments. However, outgassing of other types of chondritic material produces atmospheres dominated by other gases. Outgassing of ordinary (H, L, LL) and high iron enstatite (EH) chondritic material yields H2-rich atmospheres with CO and H2O being the second and third most abundant gases. Outgassing of low iron enstatite (EL) chondritic material gives a CO-rich atmosphere with H2, CO2, and H2O being the next most abundant gases. Outgassing of CV carbonaceous chondritic material gives a CO2-rich atmosphere with H2O being the second most abundant gas. Our results predict that the atmospheres formed during accretion of the Earth and Mars were probably H2-rich unless the accreted material was dominantly CI and CM carbonaceous chondritic material. We also predict significant amounts of S, P, Cl, F, Na, and K in accretionary atmospheres at high temperatures (1500-2500 K). Finally, our results may be useful for interpreting spectroscopic observations of accreting extrasolar terrestrial planets. 相似文献
58.
59.
Sediment samples (213 sites) collected from the tectonic-active continental margin, offshore southwestern Taiwan were analyzed for grain sizes, organic carbon, nitrogen and carbon isotopic composition to obtain mass accumulation rate of terrestrial organic carbon and carbon budget to evaluate fate of terrestrial organic carbon from small mountainous rivers on the continental margin offshore southwestern Taiwan. Terrestrial organic carbon accumulation rates range from 0.29 to 45.6 g C m−2 yr−1 with a total accumulation budget of 0.063 Mt yr−1, which accounts for less than 13% of total river particulate organic carbon loads exported from the adjacent rivers, the Gaoping (a.k.a., Kaoping), Erhjen and Tsengwen rivers. This low burial efficiency of terrestrial organic carbon demonstrated that a majority of river-borne particles together with organic materials was moved away from the study area.For the river-borne particles from the Gaoping river, a pair of depocenters in the upper slope flanking the Gaoping submarine canyon are the locations where the maximum TCorg accumulation rate were observed which hold up to 45% (0.016 Mt yr−1) of the calculated accumulation found in the study region. On the other hand, the occurrence of higher-fraction terrestrial organic carbon in the upper and middle Gaoping submarine canyon suggests that a majority of particulate organic carbon of the Gaoping river was transported directly into the deep-sea basin through the Gaoping submarine canyon. Our results demonstrated that active margin with narrow shelf and slope is not an efficient sink for the large amount of terrigenous organic carbon supplied by the small rivers, but, a transient environment for these river derived particles. 相似文献
60.
陆生植物氮同位素组成与气候环境变化研究进展 总被引:3,自引:0,他引:3
近年来,由于植物氮同位素组成(δ15N)记载了气候环境变化的信息,因而被广泛应用于全球变化研究中,成为古气候环境再造和了解现代气候环境变化信息的有力工具。然而,人们对气候环境引起的δ15N变化及其指示的气候环境意义并不完全清楚,这就有可能限制植物δ15N在古气候环境变化等领域研究中的应用。在概述植物氮同位素分馏和植物不同氮源的氮同位素分布的基础上,分析了温度、降水、大气CO2浓度和海拔高度等气候环境因子对陆生植物δ15N的影响以及它们的关系。指出了目前研究中存在的问题及其研究前景,认为在全球变化研究中利用植物氮同位素技术不仅可以重建古气候环境(如重建大气CO2浓度变化),揭示历史时期温度、降水的变化,而且还可以在一定的时间和空间上综合反映生态系统氮循环的特征。 相似文献