Climate and vegetation: Simulating the African humid period

The outputs of the climate simulated by two General Circulation Models (GCMs), (IPSL and UGAMP) have been used to force a vegetation model (LPJ-GUESS) to analyze the Holocene African humid period (AHP) and related vegetation changes over the 18°W-35°E, 5°S-25°N region. At the continental scale, simulations with the two models confirm the intensified African monsoon during the Holocene as compared to now, and the early but gradual termination of the AHP in eastern regions as compared to western regions. At the regional scale, the two GCMs results present important differences in the timing of the AHP, its spatial extent and the summer rainfall amplitude. Consequently, the vegetation model simulates changes that are globally in agreement with pollen data, but with large differences according to the region and the model considered. During the AHP, the IPSL climate induced proper vegetation changes in the eastern Sahara and in the Sahel, whereas the UGAMP climate induced correct changes in the western Sahara and in the equatorial zone.     

Estimate of recharge of a rising water table in semiarid Niger from 3H and 14C modeling

A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O, 2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.     

A phytolith index as a proxy of tree cover density in tropical areas: calibration with Leaf Area Index along a forest–savanna transect in southeastern Cameroon

The aim of the study is to calibrate the phytolith index of tree cover density, D/P (the ratio of ligneous dicotyledons phytoliths (D) over Poaceae phytoliths (P)) with Leaf Area Index (LAI) measurements. LAI is the vertically integrated surface of leaves per unit of ground area (m² leaves/m² ground). Modern soil samples from southeastern Cameroon, collected along a continuous forest–savanna transect, have been analyzed for phytoliths. Phytolith assemblages and D/P index clearly record the physiognomy of the forest and savanna communities and of the transition between both of them. A highly significant relationship was obtained between D/P and LAI. The relationship between phytolith data and the vegetation transect is also discussed and compared with existing palynological results obtained along the same transect.     

The X-ray spectrum and variability of the Seyfert 2 galaxy NGC 7172

We present evidence of flux variability, on both short (hours) and long (months) time-scales, of the Seyfert 2 galaxy NGC 7172. These results are based on the ASCA observation of NGC 7172 performed in 1996 May. The source was detected at a rather low flux level, about 3 times fainter than its usual state (including 1 yr before, when it was also observed by ASCA ).   The source also varied by about 30 per cent during the observation, confirming the presence of a type 1 nucleus in its centre. However, its spectrum appears to be flatter than the typical Seyfert 1 spectrum (in agreement with findings on other Seyfert 2s), posing problems for the unification model unless complex absorption is invoked.     

Experimental detection and reception performance for uplink underwater acoustic communication using a remote, in-air, acousto-optic sensor

Covert communications between underwater and aerial platforms would increase the flexibility of surface and air vehicles engaged in undersea warfare by providing a new netcentric warfare communications capability and could have a variety of commercial and oceanographic applications. Research into an acousto-optic sensor shows promise as a means for detecting acoustic data projected toward the water surface from a submerged platform. The laser-based sensor probes the water surface to detect perturbations caused by an impinging acoustic pressure field. Experimental studies were conducted to demonstrate acousto-optic sensor feasibility for obtaining accurate phase preserved recordings of communication signals across the air-water interface. The recorded surface velocity signals were transferred to an acoustic communications receiver that used conventional acoustic telemetry algorithms such as adaptive equalization to decode the signal. The detected, equalized, and decoded bit error rate performance is presented for hydrostatic and more realistic, hydrodynamic water surface conditions.     

Seismic response of Beirut (Lebanon) buildings: instrumental results from ambient vibrations

Resonance period is a key parameter in the seismic design of a structure, thus dynamic parameters of buildings in Beirut (Lebanon) were investigated based on ambient vibration method for risk and vulnerability assessment. Lebanon is facing high seismic hazard due to its major faults, combined to a high seismic risk caused by dense urbanization in addition to the lack of a seismic design code implementation. For this study, ambient vibration recordings have been performed on 330 RC buildings, period parameters extracted and statistically analyzed to identify correlations with physical building parameters (height, horizontal dimensions, age) and site characteristics (rock sites or soft sites). The study shows that (1) the building height or number of floors (N) is the primary statistically robust parameter for the estimation of the fundamental period T; (2) the correlation between T and N is linear and site dependent: T ≈ N/23 for rock sites and N/18 for soft sites; (3) the measured damping is inversely proportional to the period: the taller the building the lower is the damping; (4) a significant overestimation of the period exists in current building codes. However part of the large discrepancy with building code recommendations may be due to the very low level of loading.     

TIMS measurements of Ra and Ra in the Gulf of Lion, an attempt to quantify submarine groundwater discharge

Submarine groundwater discharge (SGD) is now recognized as an important pathway for water and chemical species fluxes to the coastal ocean. In order to determinate SGD to the Gulf of Lion (France), we measured the activities of ²²⁶Ra and ²²⁸Ra by thermal ionization mass spectrometry (TIMS) in coastal waters and in the deep aquifer waters of the Rhone deltaic plain after pre-concentration of radium by MnO₂. Compared to conventional counting techniques, TIMS requires lower quantities of water for the analyses, and leads to higher analytical precision. Radium isotopes were thus measured on 0.25–2 L water samples containing as little as 20 fg of ²²⁶Ra and 0.2–0.4 fg of ²²⁸Ra with precision equal to 2%. We demonstrate that coastal surface waters samples are enriched in ²²⁶Ra and ²²⁸Ra compared to the samples further offshore. The high precision radium measurements display a small but significant ²²⁶Ra and ²²⁸Ra enrichment within a strip of circa 30 km from the coast. Radium activities decrease beyond this region, entrained in the northern current along the shelf break or controlled by eddy diffusion. The radium excess in the first 30 km cannot be accounted for by the river nor by the early diagenesis. The primary source of the radium enrichment must therefore be ascribed to the discharge of submarine groundwater. Using a mass-balance model, we estimated the advective fluxes of ²²⁶Ra and ²²⁸Ra through SGD to be 5.2 × 10¹⁰ and 21 × 10¹⁰ dpm/d respectively. The ²²⁶Ra activities measured in the groundwater from the Rhone deltaic plain aquifer are comparable to those from other coastal groundwater studies throughout the world. By contrast, ²²⁸Ra activities are higher by up to one order of magnitude. Taking those groundwater radium activities as typical of the submarine groundwater end-member, a minimum volume of 0.24–4.5 × 10¹⁰ l/d is required to support the excess radium isotopes on the inner shelf. This has to be compared with the average rivers water runoff of 15.4 × 10¹⁰ l/d during the study period (1.6 to 29% of the river flow).