Search

International audience

International audience; Mineral dust is an active component of the climate system. Dust can absorb and/or scatter solar and terrestrial radiations and may act as condensation or ice nuclei in the atmosphere. Eolian mineral particles therefore have an impact on the radiative balance of the earth surface-atmosphere system and on the hydrological cycle. Dust plays an additional role as a fertilizer by supplying iron and phosphorous, among other micronutrients, to oceanic and terrestrial ecosystems, potentially boosting the biological productivity in some regions. Besides, mineral dust has also an impact on air quality. The optical and hygroscopic properties of the dust, as well as its biogeochemical and health impact, depend on its mineralogical and chemical composition as well as on its grain size. The Sahara/Sahel is by far the most important dust source region in the world ; it is estimated that several hundreds of Tg of Saharan dust is injected in the atmosphere every year, most of which in the direction of the North-eastern Tropical Atlantic Ocean (Yu et al. 2015). Considering the vastness of the Saharan desert and the intermittency of dust emission, however, Saharan dust geochemical composition is poorly constrained. The main objective of this work is to improve our knowledge of the mineralogical and geochemical composition of the Saharan dust sources, particularly those responsible for long-range transport over the Atlantic Ocean (see joint presentation by Aloys Bory et al.).This study combines remote sensing data and field measurements. First, we use the IDDI (Infrared Difference Dust Index) satellite product to identify the geographical location of Saharan dust sources (Legrand, Plana-Fattori, and N’doumé 2001). This satellite product takes advantage of Meteosat Infrared imagery and is based on the impact of aerosols on thermic infrared radiance. The difference of brightness temperature is calculated using a reference value given by the maximal brightness temperature during a period of 15 days centred on the day of interest. Indeed, ground brightness temperature can be up to 15-to-20°C lower with dust than without. A database of 24 years of IDDI images (1982-2006) is available for this study. This time series enables us to establish a new mapping of dust emission in the Sahara-Sahel region and its temporal variability throughout the year. IDDI monthly means will be discussed in the light of other available satellite products such as TOMS/OMI (Prospero 2002) or BMDI/DSA (Schepanski et al. 2007).The mineralogical and geochemical characterization of Saharan dust is based on a time series of dust samples collected at M’bour in Senegal (80 km south of Dakar) since 2006 on the premises of the Institut de la Recherche et du Développement (IRD) ecological center. Total dust deposition has been collected using a Capyr-type reversed pyramid-shaped PVC passive collector installed at about 12 m above ground on a sampling tower facing the Atlantic Ocean. We will present results from a selection of samples collected during major Saharan dust outbreaks originating from various sectors of West Africa. These samples are currently being characterized mineralogically (clay mineral assemblages) and geochemically (major and trace elements, including REE, as well as Sr and Nd isotopes). The observed mineralogical and geochemical differences observed at M’bour will be tracked as precisely as possible to the likely sources, on the basis of back trajectories computed with the HYSPLIT model and the IDDI source map. This study complements earlier work from Skoniecnzy et al. (Skonieczny et al. 2013) and should enable us to provide a preliminary characterization of the mineralogical and chemical signatures of several important sources in West Africa.Keywords: mineral dust, Sahara, Sahel, source regions, remote sensing, clay mineralogy, geochemistry, Sr and Nd isotopes.AcknowledgementsWe gratefully thank Kristin Schepanski for grating us access to her BMDI/DSA data. ReferencesLegrand, M., A. Plana-Fattori, and C. N’doumé. 2001. “Satellite Detection of Dust Using the IR Imagery of Meteosat: 1. Infrared Difference Dust Index.” Journal of Geophysical Research: Atmospheres 106 (D16): 18251–74. https://doi.org/10.1029/2000JD900749.Prospero, Joseph M. 2002. “Environmental Characterization of Global Sources of Atmospheric Soil Dust Identified with the NIMBUS 7 Total Ozone Mapping Spectrometer (TOMS) Absorbing Aerosol Product.” Reviews of Geophysics 40 (1). https://doi.org/10.1029/2000RG000095.Schepanski, K., I. Tegen, B. Laurent, B. Heinold, and A. Macke. 2007. “A New Saharan Dust Source Activation Frequency Map Derived from MSG-SEVIRI IR-Channels.” Geophysical Research Letters 34 (18). https://doi.org/10.1029/2007GL030168.Skonieczny, C., A. Bory, V. Bout-Roumazeilles, W. Abouchami, S.J.G. Galer, X. Crosta, A. Diallo, and T. Ndiaye. 2013. “A Three-Year Time Series of Mineral Dust Deposits on the West African Margin: Sedimentological and Geochemical Signatures and Implications for Interpretation of Marine Paleo-Dust Records.” Earth and Planetary Science Letters 364 (February): 145–56. https://doi.org/10.1016/j.epsl.2012.12.039.Yu, Hongbin, Mian Chin, Huisheng Bian, Tianle Yuan, Joseph M. Prospero, Ali H. Omar, Lorraine A. Remer, et al. 2015. “Quantification of Trans-Atlantic Dust Transport from Seven-Year (2007–2013) Record of CALIPSO Lidar Measurements.” Remote Sensing of Environment 159 (March): 232–49. https://doi.org/10.1016/j.rse.2014.12.010.

The Gollum Channel System is the only major downslope sediment supplying system on the Irish Atlantic margin. A decade ago, its structure and development was still poorly understood, compared with its counterparts on the Celtic and Armorican margins. A variety of data, collected from 1999 to 2006, has shed a new light on the upper reaches of this system. These data allowed two different channel settings to be distinguished clearly. The main Gollum channel system is characterized by several deeply incised canyons with numerous slide scars on their flanks. Their pathways seem to be influenced by a structural control, creating a bayonet-shaped course. Upstream of this structural feature, the channel floor deposits are characterized by thick acoustically transparent units, suggesting ponded turbidites or mass-wasting deposits. A long piston core, however, only yields a small number of fine-grained turbidites in a muddy hemipelagic host sediment. This implies that this system has known a relatively low activity during Quaternary times. Moreover, at the abyssal end of the slightly sinuous Gollum channel, only a weakly developed deep-sea fan is found, confirming a low-sediment supply. In the northern part of the system, on the other hand, the Kings channels show an entirely different situation. Here the channels are broad and smooth with relatively gentle flanks. Along the longitudinal axis of the northernmost channel, an intra-channel levee is observed, suggesting a relatively higher bottom current activity compared with the southern Gollum channels. The most remarkable feature, however, is a large field of mass-wasting deposits and escarpments. This calls for a dramatic phase of slope instability within the Neogene. Until now, little evidence is found for the cause of this event. The evidence of pockmarks north of this area could invoke the mediation of fluid migration.