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1.
J.-M. Gherardi L. Labeyrie J.F. McManus R. Francois L.C. Skinner E. Cortijo 《Earth and Planetary Science Letters》2005,240(3-4):710-723
A first study from the subtropical western Atlantic, using 231Pa/230Th ratios as a kinematic proxy for deep water circulation, provided compelling evidence for a strong link between climate and the rate of Meridional Overturning Circulation (MOC) over the last deglaciation. However, these results warrant confirmation from additional locations and water depths because the interpretation of the sedimentary 231Pa/230Th ratio in terms of circulation vigor can be biased by variations in particle flux and composition. We have measured 231Pa/230Th in a core from the Iberian margin, in the Northeastern Atlantic basin, and have compared these new results to the data from the western Atlantic basin. We find that the reduction in the circulation during H1 and YD and the subsequent increases first recognized in the sediment deposited on Bermuda Rise are also evident in the eastern basin, in a totally different sedimentary regime, confirming that sedimentary 231Pa/230Th ratios record basin-wide changes in deep water circulation. However, some differences between the eastern and western records are also recognized, providing preliminary evidence to differentiate between renewal rates in the two North Atlantic basins and between shallower and deeper overturning. Our results suggest the possible existence of two sources of Glacial North Atlantic Intermediate Deep Water (GNAIW), one in the south Labrador Sea and another west of Rockall Plateau. Both sources contributed to the meridional overturning but the two had different sensitivity to meltwater from the Laurentide and the Fennoscandian ice sheets during the deglaciation. These results indicate that additional information on the geometry and strength of the ventilation of the deep Atlantic can be obtained by contrasting the evolution of sediment 231Pa/230Th in different sections of the Atlantic Ocean. 相似文献
2.
干密度是评价软土固化质量的重要指标,一般通过室内击实试验获得。以氯氧镁水泥(MOC)为结合料,砾石土为固化对象,研究了原料配比对MOC固化砾石土击实性能的影响。通过室内击实试验,考察了轻烧镁粉掺量、卤水浓度、卤水掺量和活性MgO/MgCl_2物质的量比(Mg/Cl比)影响混合料干密度的规律。结果表明:MOC固化砾石土击实料的干密度随卤水掺量增加先增加后降低,随轻烧镁粉掺量及Mg/Cl比的增加而先增加后降低,随卤水浓度及轻烧镁粉掺量的增加而增加,随卤水浓度增加及Mg/Cl比降低而增加。 相似文献
3.
The scaling law linking the strength of the meridional overturning to the surface meridional temperature gradient is re-examined in two ocean general circulation models at coarse resolution in an idealised single-hemisphere setting. Two sets of results are presented, where the surface meridional temperature gradient is decreased either by increasing the northernmost temperature and keeping the equator temperature fixed, or by decreasing the equator temperature and keeping the northernmost temperature fixed. The maximum of the meridional overturning first increases and then decreases when the northernmost temperature is gradually increased, whereas the maximum overturning decreases monotonically when the equator temperature is decreased. No scaling law can be derived when the northernmost temperature is increased, whereas a 2/3 power law is found when the temperature is decreased at the equator. The behaviour of the overturning is strongly influenced by the vigour and, particularly, the spatial patterns of convection, which vary substantially between the two sets and which control the horizontal and vertical density gradients at high latitudes. 相似文献
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5.
The relation between the size and velocity of impact crater ejecta has been studied by both laboratory experiments and numerical modeling. An alternative method, used here, is to analyze the record of past impact events, such as the distribution of secondary craters on planetary surfaces, as described by Vickery (Icarus 67 (1986) 224; Geophys. Res. Lett. 14 (1987) 726). We first applied the method to lunar images taken by the CLEMENTINE mission, which revealed that the size-velocity relations of ejecta from craters 32 and 40 km in diameter were similar to those derived by Vickery for a crater 39 km in diameter. Next, we studied the distribution of small craters in the vicinity of kilometer-sized craters on three images from the Mars Orbiter Camera (MOC) on board the Mars Global Surveyor (MGS). If these small craters are assumed to be secondaries ejected from the kilometer-sized crater in each image, the ejection velocities are of hundreds of meters per second. These data fill a gap between the previous results of Vickery and those of laboratory studies. 相似文献
6.
将氯氧镁水泥(MOC)创新性地引入淤泥固化,通过无侧限抗压强度、固化体含水率、扫描电子显微镜(SEM)和X射线能谱测定(EDS)等试验,研究了初始含水率、MgO/MgCl2摩尔比、养护龄期和MgO活性等复杂因素对淤泥固化效能影响及其驱动机制。结果表明:初始含水率越高,试样强度越低,中等含水率时MOC固化淤泥中出现针杆状的3相或5相晶体;随MgO/MgCl2摩尔比升高,试样抗压强度随之增加,水化产物从无定形凝胶逐渐转化为3相、5相和Mg(OH)2晶体;随养护龄期延长,固化淤泥强度总体呈上升趋势,28d前强度增长相对较快,28d后强度趋于稳定;养护后期,MgO/MgCl2摩尔比较高的试样表面易出现泛霜现象;MgO活性提高使得MOC固化淤泥试样含有更多有效活性组分,无侧限抗压强度更高,但MgO活性高低对水化产物演变规律并无显著影响。研究成果可为绿色、低碳MOC基胶凝材料研发及其在淤泥固化等土体加固领域中应用提供理论支撑。 相似文献
7.
Observed variations in the Deep Western Boundary Current (DWBC) at 26.5°N, which carries the deep limb of the Atlantic Meridional Overturning Circulation (MOC), have been shown to greatly exceed in magnitude the variations of the overall basin-wide MOC, with strong variability at a range of time scales from weeks to multiple-months. Attribution of these strong DWBC variations will be crucial for understanding variations in the MOC itself. Nevertheless, despite many years of moored observations of the DWBC at 26.5°N, understanding of these variations has been elusive. Two years of observations from a high horizontal resolution array of pressure-equipped inverted echo sounders are used together with output from a modern high-resolution numerical model to investigate the mechanisms behind these ±20×106 m3 s−1 volume transport variations. The model and observational results together suggest that the strongest variations cannot be explained solely via either of the two most commonly proposed mechanisms – meandering or pulsation of the DWBC. The dominant mechanism appears to be propagation of Rossby Wave-like structures into the region from the east, and it is the impact of these features in the region that yield the largest transport anomalies. These waves have been observed and discussed in the past – however their key role as the dominant source of DWBC variability has not previously been recognized. The implications of these results are also discussed in the context of future observing systems for the DWBC. 相似文献
8.
以MgCl2溶液模拟氯氧镁水泥孔隙液,研究了A3钢在MgCl2溶液中的腐蚀行为和腐蚀速率,并利用电化学测试仪对A3钢腐蚀过程的电化学特征进行了研究。实验结果表明,随着MgCl2浓度的减小以及在MgCl2溶液中浸泡时间的延长,A3钢的腐蚀失重速率逐渐变小;极化曲线测试与腐蚀失重试验结果相吻合;阻抗曲线(EIS)研究发现,随着A3钢在MgCl2溶液中浸泡时间的延长钝化膜的电阻变大;扫描电镜观察表明,经MgCl2溶液腐蚀后,在A3钢样品表面产生了许多腐蚀产物,XRD分析证实主要产物为Fe3O4和MgFe2O4。 相似文献
9.
Recently, a particular statistical method - spatial point pattern analysis (SPPA) - has been introduced as an effective means by which qualitative, observable variations in polygonal terrain network arrangements on Earth and Mars can be quantified. A number of ground- and aerial-based techniques are available from which to derive the required input data: the spatial (x-y) coordinates of all polygon trough intersections within the site. However, each of the data collection methods may contain some level of error. Thus, the overarching question addressed by this research is: “how are the results of SPPA affected by the method by which the input data were generated?” At two polygonal terrain sites in the Canadian High Arctic, we performed ground-based surveys using differential and non-differential Global Positioning Systems (GPS) as well as photogrammetric analysis of aerial and satellite images of varying resolution to determine the trough intersection coordinates. It was found that the most robust statistical results were produced when using data from a combination of differential GPS surveys and high-resolution (∼0.25 m/pixel) aerial images. Images of pixel size ≥1 m were found to be unsuitable for this type of analysis. With respect to the investigation of similar Martian landforms, HiRISE and MOC images of polygonal terrain sites in southwestern Utopia Planitia were analyzed. Our results show that it is strongly preferable to perform SPPA using HiRISE images, though an empirical model is outlined that could be used to correct for errors arising from the reduced resolution inherent to MOC images. 相似文献
10.
Atlantic multidecadal oceanic variability and its influence on the atmosphere in a climate model 总被引：3，自引：3，他引：0
The mechanisms controlling the decadal to multidecadal variability of the Atlantic Meridional Overturning Circulation (MOC)
and its influence on the atmosphere are investigated using a control simulation with the IPSL-CM4 climate model. The multidecadal
fluctuations of the MOC are mostly driven by deep convection in the subpolar gyre, which occurs south of Iceland in the model.
The latter is primarily influenced by the anomalous advection of salinity due to changes in the East Atlantic Pattern (EAP),
which is the second mode of atmospheric variability in the North Atlantic region. The North Atlantic Oscillation is the dominant
mode, but it plays a secondary role in the MOC fluctuations. During summer, the MOC variability is shown to have a significant
impact on the atmosphere in the North Atlantic–European sector. The MOC influence is due to an interhemispheric sea surface
temperature (SST) anomaly with opposite signs in the two hemispheres but largest amplitude in the northern one. The SST pattern
driven by the MOC mostly resembles the model Atlantic Multidecadal Oscillation (AMO) and bears some similarity with the observed
one. It is shown that the AMO reflects both the MOC influence and the local atmospheric forcing. Hence, the MOC influence
on climate is best detected using lagged relations between climatic fields. The atmospheric response resembles the EAP, in
a phase that might induce a weak positive feedback on the MOC. 相似文献