One year measurements of aerosol optical properties over an urban coastal site: Effect on local direct radiative forcing

We present results of direct aerosol radiative forcing over a French Mediterranean coastal zone based on one year of continuous observations of aerosol optical properties during 2005–2006. Monthly-mean aerosol optical depth at 440 nm ranged between 0.1 and 0.34, with high Angstrom coefficient (α > 1.2). The single scattering albedo (at 525 nm) estimated at the surface ranged between 0.7 and 0.8, indicating significant absorption. The presence of aerosols over the Mediterranean zone during summer decreases the shortwave radiation reaching the surface by as much as 26 ± 3.9 W m^− 2, and increases the top of the atmosphere reflected radiation by as much as 5.2 ± 1.0 W m^− 2. The shortwave atmospheric absorption translates to an atmospheric heating of 2.5 to 4.6 K day^− 1. Concerted efforts are needed for investigating the possible impact of the increase in heating rate on the maintenance of heat-waves frequently occurring over this coastal region during summer time.     

Chad Basin: Paleoenvironments of the Sahara since the Late Miocene

Since the mid 1990s, the Mission paléoanthropologique francotchadienne (MPFT) conducts yearly paleontological field investigations of the Miocene-Pliocene of the Chad Basin. This article synthesizes some of the results of the MPFT, with focus on the Chad Basin development during the Neogene. We propose an overview of the depositional paleoenvironments of this part of Africa at different scales of time and space, based on a multidisciplinary approach (sedimentary geology, geomorphology, geophysic, numerical simulations and geochronology). The Miocene-Pliocene paleoenvironments are examined through the sedimentary archives of the early hominids levels and the Holocene Lake Mega-Chad episode illustrates the last major paleoenvironmental change in this area. The sedimentary record of the Chad Basin since the Late Miocene can be schematized as the result of recurrent interactions from lake to desert environments.     

Fragmentation of magma during Plinian volcanic eruptions

 The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V _G /V _L ) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V _G /V _L in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between pumices at a given level in a deposit. These variations in V _G /V _L do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary evolution. On the timescale of Plinian eruptions, pumices with viscosities <10⁹ Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity. Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <10⁵ Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching. After accounting for these effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is a relatively constant minimum value of V _G /V _L of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >10⁵ Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes. Received: 19 April 1995 / Accepted: 3 April 1996     

Impact of imperfect rainfall knowledge on the efficiency and the parameters of watershed models

It is crucial to analyze the sensitivity of watershed (rainfall-runoff) models to imperfect knowledge of rainfall input, in order to judge whether or not they are reliable and robust, especially if they are to be used for operational purposes. In this paper, a new approach to sensitivity analysis is proposed, based on a comparison between the efficiency ratings and parameter values of the models and the quality of rainfall input estimate (GORE and BALANCE indexes, assessing the quality of rainfall time distribution and the total depth respectively). Data from three watersheds of increasing size (71, 1120, and 10700 km²), are used to test three watershed models of varying complexity (three-parameter GR3J model and six-parameter modified versions of TOPMODEL and IHACRES).

These models are able to cope with imperfect rainfall input estimates, and react to improvements in rainfall input accuracy by better performance and reduced variability of efficiency. Two different types of model behavior were identified: the models either benefit from improved rainfall data by producing more consistent parameter values, or they are unable to take advantage of the improvements. Although the watershed size seems to be immaterial, the smaller watersheds appear to need more precise areal rainfall estimates (a higher concentration of raingages) to ensure good modeling results.     

Single-zircon Pb-evaporation and 40Ar/39Ar dating of the metamorphic and granitic rocks in north-west Spitsbergen

North-west Spitsbergen consists of a complex of Caledonian and Grenvillian crystalline rocks, situated at the north-west corner of the Barents Shelf. The aim of this study is to understand the extent of pre-Caledonian basement rocks and their protoliths. Micas and zircon grains from six rocks from north-west Spitsbergen have been dated by the ⁴⁰Ar/³⁹Ar and single-zircon Pb-evaporation methods. Two grey granites yielded Late Caledonian mica ⁴⁰Ar/³⁹Ar and zircon ages of ca. 420-430 My, with inherited zircon grains as old as 1725 My. Zircon grains from a gneissose granite xenolith in a grey granites gave crystallization ages of ca. 960 My; some grains from a migmatite neosome show similar ages. Zircon grains yielding Archean and late Palaeoproterozoic ages (1600-1800 My) are interpreted as xenocrysts of detrital origin. The youngest ages obtained from detrital zircon grains from a greenschist facies quartzite of the Signehamna unit are ca. 1800 My. Similar schists are included as xenoliths in the 960 My old gneissose granite; therefore, the sedimentary protoliths of the unit are Mesoproterozoic. The dating results suggest a significant tectonothermal event during Grenvillian time; subsequent Caledonian events had less extensive thermal effects. However, it is still a matter of debate whether Grenvillian or Caledonian metamorphism produced the majority of the migmatites. A large population of zircon grains with Late Palaeoproterozoic ages suggests a wide surface exposure of rocks of this age in the source area, with some Archean zircons.     

Cosmogenic neon in ultramafic nodules from Asia and in quartzite from Antarctica

We have performed systematic analyses of both cosmogenic ³He (³He_c) and cosmogenic ²¹Ne (²¹Ne_c) in ultramafic xenoliths from Central Asia and in a quartz sample from Antarctica. Five xenoliths, which show no or insignificant ²¹Ne_c excesses, were used to estimate the initial ⁴He/³He ratio of 90,470 in the subcontinental lithospheric mantle under the Baikal extension zone. Seven xenoliths show large ²¹Ne/²²Ne anomalies ranging up to 0.204 and ⁴He/³He down to 31,000, due to the presence of cosmogenic ²¹Ne and ³He. The (³He/²¹Ne)_c ratio is 1.41 ± 0.22 in the xenoliths and 2.76 in the quartzite. This difference is due to the dependence of the ²¹Ne_c production rate on the elemental composition of the target material. We estimated the ³He_c and ²¹Ne_c production rates at different locations worldwide and calculated the ³He_c and ²¹Ne_c exposure ages. These ages range between 7100 and 28,000 years for the xenoliths, and we determined their relative positions within the volcanic tuff layer. The mean ³He_c and ²¹Ne_c exposure ages of the quartz sample are 1.35 ± 0.07 and 2.21 ± 0.12 Ma, respectively. This difference is most probably related to ³He_c diffusive losses from the quartz mineral grains, even at low temperatures, due to the relatively high diffusion coefficient for cosmogenic ³He.     

The ReOs systematics of the Ronda Ultramafic Complex of southern Spain

The Os isotopic compositions of twelve ultramafic and six mafic layer samples from the Ronda Ultramafic Complex of southern Spain have been determined. Among the ultramafic rocks, ¹⁸⁷Os/¹⁸⁶Os varies from 0.98 to 1.12. A weak correlation is observed between ¹⁸⁷Os/¹⁸⁶Os and Re/Os. A much stronger correlation exists between Os isotopic ratio and Mg#, suggesting that the Re/Os ratios have been perturbed to some extent. Two alternatives are proposed to explain the relationship between Os composition and Mg#: (1) Continuous processes in the convecting mantle; (2) Radiogenic ingrowth since an ancient melt depletion event. No relationship is observed between ¹⁸⁷Os/¹⁸⁶Os and ¹⁴³Nd/¹⁴⁴Nd. This is probably because the Nd systematics were strongly affected by a recent metasomatic event, which apparently had little effect on the Os isotopic compositions.The Os isotopic ratios of the mafic layers range from 1.7 to 47.9. Within a single thick layer, the ratios vary from 16.5 to 47.9. These high ratios demonstrate that the layers are ancient features. Among the mafic samples, Os isotopic ratio is found to decrease strongly with increasing Os concentration, which ranges from 0.009 ppb to 1.16 ppb. One layer, which had a SmNd model age of less than 200 Ma, yielded a ReOs model age of about 2 Ga. This implies that neither system can be trusted to give accurate information about the time of mafic layer formation.