Potential control of climatic changes on flood events in the Yangtze Deltaduring 1100-2002

Wide collection on the historic records of the climatic changes and flood events is performed in the Yangtze Delta. Man-Kendall (MK) method is applied to explore the changing trends of the time series of the flood discharge and the maximum high summer temperature. The research results indicate that the flood magnitudes increased during the transition from the medieval warm interval into the early Little Ice Age. Fluctuating climate changes of the Little Ice Age characterized by arid climate events followed by the humid and cold climate conditions give rise to the frequent flood hazards. Low-lying terrain made the study region prone to the flood hazards, storm tide and typhoon. MK analysis reveals that the jumping point of the time series of the flood discharge changes occurred in the mid-1960s, that of the maximum summer temperature changes in the mid-1990s, and the exact jump point in 1993. The flood discharge changes are on negative trend before the 1990s, they are on positive tendency after the 1990s; the maximum high summer temperature changes are on negative trend before the 1990s and on positive tendency after the 1990s. These results indicate that the trend of flood discharge matches that of the maximum high summer temperature in the Yangtze Delta. The occurrence probability of the maximum high summer temperature will be increasing under the climatic warming scenario and which will in turn increase the occurrence probability of the flood events. More active solar action epochs and the higher sea surface temperature index (SST index) of the south Pacific Ocean area lying between 4oN-4oS and 150oW-90oW correspond to increased annual precipitation, flood discharge and occurrence frequency of floods in the Yangtze Delta. This is partly because the intensified solar activities and the higher SST index give rise to accelerated hydrological circulation from ocean surface to the continent, resulting in increased precipitation on the continent.     

Interpreting photometry of regolith-like surfaces with different topographies: shadowing and multiple scattering

The effects of various types of topography on the shadow-hiding effect and multiple scattering in particulate surfaces are studied. Two bounding cases were examined: (1) the characteristic scale of the topography is much larger than the surface particle size, and (2) the characteristic scale of the topography is comparable to the surface particle size. A Monte Carlo ray-tracing method (i.e., geometric optics approximation) was used to simulate light scattering. The computer modeling shows that rocky topographies generated by randomly distributed stones over a flat surface reveal much steeper phase curves than surface with random topography generated from Gaussian statistics of heights and slopes. This is because rocks may have surface slopes greater than 90°. Consideration of rocky topography is important for interpreting rover observations. We show the roughness parameter in the Hapke model to be slightly underestimated for bright planetary surfaces, as the model neglects multiple scattering on large-scale topographies. The multiple scattering effect also explains the weak spectral dependences of the roughness parameter in Hapke's model found by some authors. Multiple scattering between different parts of a rough surface suppresses the effect of shadowing, thus the effects produced by increases in albedo on the photometric behavior of a surface can be compensated for with the proper decreases in surface roughness. This defines an effective (photometric) roughness for a surface. The interchangeability of albedo and roughness is shown to be possible with fairly high accuracy for large-scale random topography. For planetary surfaces that have a hierarchically arranged large-scale random topography, predictions made with the Hapke model can significantly differ from real values of roughness. Particulate media with surface borders complicated by Gaussian or clumpy random topographies with characteristic scale comparable to the particle size reveal different photometric behaviors in comparison with particulate surfaces that are flat or the scale of their topographies is much larger than the particle size.     

Démantèlement des paysages cuirassés anciens en zones forestières tropicales d'Afrique centrale : formation d'accumulations ferrugineuses actuelles en bas de versantOld ferricrete landscape dismantling in Central Africa rain forest zone: formation of the present downslope iron accumulations

Present downslope iron accumulations were investigated in the rainforest zone in southern Cameroon. Six clay and Fe-hydroxide dominated patterns have been identified and occur on the lower part of hill slopes. They can be subdivided in three different sequences, related to gentle, moderate or steep slopes. They are discontinuous with respect to the dismantling zone of the old ferricrete cap formed at Cretaceous period. They show a gradual development from a soft Fe-crust (carapace) to a vesicular facies that will, with time, cover the whole landscape again. To cite this article: É. Temgoua et al., C. R. Geoscience 334 (2002) 537–543.     

Neogene tectonostratigraphic history of the southern Neuquén basin (39°–40°30′S,Argentina): implications for foreland basin evolution

Although the Neuquén basin in Argentina forms a key transitional domain between the south‐central Andes and the Patagonian Andes, its Cenozoic history is poorly documented. We focus on the sedimentologic and tectonic evolution of the southern part of this basin, at 39–40°30′S, based on study of 14 sedimentary sections. We provide evidence that this basin underwent alternating erosion and deposition of reworked volcaniclastic material in continental and fluvial settings during the Neogene. In particular, basement uplift of the Sañico Massif, due to Late Miocene–Pliocene intensification of tectonic activity, led to sediment partitioning in the basin. During this interval, sedimentation was restricted to the internal domain and the Collon Cura basin evolved towards an endorheic intermontane basin. From stratigraphic interpretation, this basin remained isolated 7–11 Myr. Nevertheless, ephemeral gateways seem to have existed, because we observe a thin succession downstream of the Sañico Massif contemporaneous with the Collon Cura basin‐fill sequence. Comparisons of stratigraphic, paleoenvironmental and tectonic features of the southern Neuquén basin with other foreland basins of South America allow us to classify it as a broken foreland with the development of an intermontane basin from Late Miocene to Late Pliocene. This implies a thick‐skinned structural style for this basin, with reactivation of basement faults responsible for exhumation of the Sañico Massif. Comparison of several broken forelands of South America allows us to propose two categories of intermontane basins according to their structural setting: subsiding or uplifted basins, which has strong implications on their excavation histories.     

Deformation processes at fast to ultra-fast oceanic spreading axes: mechanical approach

The morphology of fast to ultra-fast oceanic spreading ridges such as the East Pacific Rise (EPR) is characterized by an axial dome, 5–10 km wide, culminating at 300–500 m above the surrounding seafloor. This dome is bounded by lateral grabens that develop systematically 2 to 6 km apart from the spreading axis. A large summit trough, 200 m to 2 km wide, locally notches the axial high, only where the dome is inflated, indicative of a time-average robust magma supply. This summit trough is thought to represent an elongated axial summit caldera (ASC) created as a result of the subsidence of the top of the axial magma chamber (AMC). Such subsidence is likely caused by a temporary decrease in melt supply into the shallow magma reservoir suffering continuous regional extension. Analog experiments using small-scale modeling have been performed in order to better constrain the tectonic evolution of the axial region. The experimental apparatus includes an elongated balloon filled with water as an analog of the magma reservoir set in a central groove in a table. It is capped with a silicone layer representing hot rocks below the brittle–ductile transition and is covered by a sand layer representing the brittle crust. The experiments integrate withdrawal of the balloon and extension at the boundary of the model by the mean of two mobile walls. Three experimental setups allowed us to study independently the mechanical parameters controlling the axial tectonic evolution: extension without withdrawal, withdrawal without extension, withdrawal and synchronous extension. We show that the morphology of the EPR axis can be considered as the result of both horizontal and vertical movements. Two symmetrical lateral grabens develop on both sides of a non-deformed axial dome when single extension is applied to a model with a thin silicone layer. Normal faults of the lateral grabens are rooted on two divergent velocity zones (DVZs) located on the edges of the groove. This situation is regarded as an analog of the natural case where the top of the AMC acts as a stress-free boundary that fails to transmit the extensional stresses to the upper brittle layer. An important deflation of the balloon without extension results in the creation of a central collapse trough limited by reverse faults. During synchronous extension and withdrawal, the initiation of the lateral grabens is favored by a balloon deflation, even if such deflation is unable to generate a superficial collapse. This last case is considered as representative of the evolution of EPR segments showing little variations in melt supply into the AMC. Higher deflation rates under continuous extension correspond to EPR segments undergoing strong variations in melt supply. In such experiments, the lateral grabens are created together with a central collapse trough developing in a way similar to that of experiments involving only balloon deflation. Finally, we show that DVZs located at the brittle–ductile boundary are the key mechanical elements which may explain the structural evolution of the axial region of fast to ultra-fast spreading ridges. The distance from axis and the width of the DVZs directly control the location and the distribution of the lateral grabens.     

IR band intensities of DC3N and HC3N: Implication for observations of Titan's atmosphere

We have obtained the infrared spectra and the corresponding absolute band intensities for two HC₃N isotopomers: DC₃N and HC₃¹⁵N. Our results for DC₃N are in good agreement with previous measurements except for the ν₂ and ν₃ stretching modes. For HC₃¹⁵N, this study is the first including intensity measurements.We have also studied the possible detection of these isotopomers in Titan's atmosphere using the CIRS spectrograph onboard the Cassini spacecraft. Our simulation of the expected spectra shows that for a signal-to-noise ratio better than 100, the ¹⁵N isotopomer of HC₃N could be detected. But, further study of HC₃N hot bands are needed since some of them overlap the HC₃¹⁵N Q-branch.     

Ocean change around Greenland under a warming climate

Climate Dynamics - The impact of climate warming on the ocean near Greenland is investigated with a high resolution coupled global climate model. The ocean around Greenland exhibits a strong...     

Cloud optical thickness and albedo retrievals from bidirectional reflectance measurements of POLDER instruments during ACE‐2

The POLDER instrument is devoted to global observations of the solar radiation reflected by the Earth–atmosphere system. The airborne version of the instrument was operated during the ACE‐2 experiment, more particularly as a component of the CLOUDYCOLUMN project of ACE‐2 that was conducted in summer 1997 over the subtropical northeastern Atlantic ocean. CLOUDYCOLUMN is a coordinated project specifically dedicated to the study of the indirect effect of aerosols. In this context, the airborne POLDER was assigned to remote measurements of the cloud optical and radiative properties, namely the cloud optical thickness and the cloud albedo. This paper presents the retrievals of those 2 cloud parameters for 2 golden days of the campaign 26 June and 9 July 1997. Coincident spaceborne ADEOS‐POLDER data from 2 orbits over the ACE‐2 area on 26 June are also analyzed. 26 June corresponds to a pure air marine case and 9 July is a polluted air case. The multidirectional viewing capability of airborne POLDER is here demonstrated to be very useful to estimate the effective radius of cloud droplet that characterizes the observed stratocumulus clouds. A 12 μm cloud droplet size distribution appears to be a suitable cloud droplet model in the pure marine cloud case study. For the polluted case the mean retrieved effective droplet radius is of the order of 6–10 μm. This only preliminary result can be interpreted as a confirmation of the indirect effect of aerosols. It is consistent with the significant increase in droplet concentration measured in polluted marine clouds compared to clean marine ones. Further investigations and comparisons to in‐situ microphysical measurements are now needed.     

Cenozoic denudation in the Marrakech High Atlas,Morocco: insight from apatite fission‐track thermochronology

In North Africa, the High Atlas belt culminates at more than 4000 m. In Morocco, recent work shows that a lithospheric thinning explains about 1000 m of the mean topography, the remaining topography being related to crustal shortening. We combine regional geology with new apatite fission‐track (AFT) ages to constrain the timing of these events in the Marrakech High Atlas (MHA). In the inner belt, 10 AFT ages are comprised between 9 ± 1 and 27 ± 3 Ma. These Neogene ages indicate that the MHA underwent significant denudation during that time. In the southern foreland domain of the belt, three samples give scattered AFT ages between 27 ± 2 and 87 ±5 Ma. Geological evidences allow us to constrain the age of a major denudation event during Middle Miocene age. We propose that it is linked to the thermal doming highlighted in the whole Moroccan Atlas domain.