Pierre Rognon

Origins of peri-Saharan dust deposits traced by their Nd and Sr isotopic composition

Abstract Isotopic ratios of Sr and Nd can be used as very precise tracers— in conjunction with mineralogical, geochemical tracers and climatic models—, to identify the origins of dust deposit and to quantify their fluxes, both today and throughout the last climati cycles. This technique is useful for constraining aeolian transport and atmospheric circulation trajectories from desert sources and over the oceans. The present work concerns dust deposits surrounding the Saharan desert. On the Canary Islands, the African fraction of the dust deposits displays isotopic values which are typical of Moroccan origin. On the other hand, Tunisian loesses have been derived from the northern part of the Sahara. In this case, tracers are sensitive enough to distinguish between center Saharan shield origin, and peri-Saharan secondary sources, allowing us to distinguish between transport due to the Trade Winds, vs the Harmattan or “Mediterranean” wind regimes.

Reconstitution of dune morphologies and palaeowind regimes in the Proto-Namib since the Miocene

In the Namib Desert, indurated fossil dunes are found between the active sands. Establishment of a biostratigraphic scale based on associated mammal faunas and ratite eggshells allows the study of palaeowind fluctuations that have controlled these palaeodunes since the Middle Miocene with a temporal resolution of between 1 and 3 Ma. In the southern Namib, the Proto-Namib aeolian systems consist of crescent dunes (barkhan type). In the eastern part of the Namib Desert, the aeolianite cross-bedding corresponds to longitudinal or pyramidal dunes. These aeolian structures are also controlled by local winds, resulting from high relief and the southern trade winds : this regime is identical to that which currently sweeps, through the Namib Desert, and is related to the presence of anticyclonic conditions in the Atlantic Ocean. A regime of seasonal winds from the northeastern sector is also recorded starting in the late Miocene.

Relationships between the Characteristics of Soils, the Wind Energy and Dust Near the Ground, in the Western Sandsea (N.W. Sahara)

In and around a sandsea of 80,000 km2, 120 dust samples have been collected by air filtration during a wind still period, a windy period and several dust storms between March 1985 and March 1986. In relation to these different meteorological conditions, Si concentration in dust increases from 0.5 to 70 μg/m3, correlated with the increasing wind energy and has been used as an indicator of the wind erosion intensity. As Si concentration increases, the Ca/Si ratio and the amount of calcite and gypsum’decrease. Inversely, the amounts of fine particles (<2μm) and of quartz and clay minerals increase. These results are in good agreement with the “behaviour” of the surface soils towards the deflation. About 60 soil samples have been sampled around the sites and classified according to the their ability to be eroded by the wind. For instance the most erodible soils are the carbonated powdery ones, which yield their directely available amount of calcite even when the wind intensity is low.

Aeolian Dust and Dust Deposits

In Earth science history, numerous subjects, such as pollen and freshwater diatoms have been studied from first a geological point of view, then in relation to the presentday environment. This is the case for dust; loess deposit studies began as far back as 1880 and included many international publications on the subject, but scientific works concerning the eolian dust only started toward the middle of the 20th century. Nevertheless, investigators of eolian dust have made great strides, as it is pointed out in the interesting book by Kenneth Pye, which puts together the present studies on dust and its history since the Quaternary and Cenozoic epochs. In addition to loess, the studies also cover the contribution of dust to deep-ocean sediments, ice, and soils.