Downloaded from gsabulletin.gsapubs.org on January 6, 2014 Geological Society of America Bulletin Particle-size fractionation of eolian sand along the Sinai−Negev erg of Egypt and Israel Joel Roskin, Itzhak Katra and Dan G. Blumberg Geological Society of America Bulletin 2014;126, no. 1-2;47-65 doi: 10.1130/B30811.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2013 Geological Society of America Downloaded from gsabulletin.gsapubs.org on January 6, 2014 Particle-size fractionation of eolian sand along the Sinai–Negev erg of Egypt and Israel Joel Roskin†, Itzhak Katra, and Dan G. Blumberg Department of Geography and Environmental Development, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, 84105, Israel ABSTRACT of the very fi ne sand component within the and soils in the central United States (Olson dunes and probably further downwind. Our et al., 1997) and northern Africa (McTainsh, Eolian sand fractions along the west-east results suggest that particle-size distribution 1984) but has not been fully described for transport system of the northern Sinai Pen- can elucidate much about erg history over dunes. At a smaller temporal and spatial scale, insula–northwestern Negev erg of Egypt and time scales of a glacial-interglacial cycle, es- sand plumes of several hundred meters extend- Israel were analyzed in this study with re- pecially in cases where the sand provenance ing downwind of an abandoned sandy agricul- gard to source, dune geomorphology, eolian is of a single dominant source. tural fi eld in the Mojave Desert were found to transport, and paleoclimate. The studied erg exhibit fractionation, with smaller effective par- is composed of active linear (seif) dunes in INTRODUCTION ticle sizes toward the toe of the plume (Okin and northern Sinai (western part), and stabilized Painter, 2004). vegetated linear dunes in the NW Negev dune Particle-Size Changes in Eolian Deposits Particle-size-distribution changes in sand fi eld (eastern part). Linear seif dunes differ along a transport path of mobilized dunes can from vegetated linear dunes in their vegeta- Particle-size distributions of dune sands are result from eolian abrasion of sand grains due tion cover, linearity, internal structure, and an important factor in understanding the mor- to grain collision and erosion of the substrate dynamics. Sand samples were analyzed for phology and dynamic processes of dunes (e.g., underlying the dunes, as well as winnowing sand-grain morphology and particle-size dis- Bagnold, 1937; Tsoar, 1986; Lancaster, 1995; out of coarser eolian particle sizes. The relative tribution. Although both dune types are con- Pye and Tsoar, 2009). Theoretical and empiri- importance of these different processes is still tinuous landforms with similar orientations cal studies have shown that particle-size distri- unclear for most dune bodies. Theoretically, and sand-grain roundness values, the linear butions of eolian sand deposits change along given a particular amount of wind power, the dunes of Sinai are coarser grained than the their transport paths (McLaren, 1981; McLaren particle-size distribution of loose source sedi- Negev vegetated linear dunes. The vegetated and Bowles, 1985). Some dune fi elds, com- ment, which includes sand, silt, and clay, should linear dunes have a variable but higher pro- posed mainly of parabolic dunes, are reported be differentiated (fractionated) along the trans- portion of very fi ne sand (50–125 μm) con- to undergo a gradual and slight decrease in port pathway as different particle-size fractions tent and a varying but lower sand fi ning ratio particle-size means with downwind distance, have different threshold values for entrainment (defi ned as the ratio of fi ne sand percentage interpreted to be partially due to winnowing of and transport. Silt and clay particles (<50 μm), to very fi ne sand percentage). From these ob- coarse sands (Muhs et al., 1999; Muhs and Hol- carried primarily in suspension (Tsoar and Pye, servations, we infer that fractionation of sand liday, 2001). Particle-size distribution modes 1987), should attain transport distances well occurred along the studied eolian transport have been mapped at various scales for active beyond the region of sand and dune mobiliza- path during periods of enhanced windiness. linear dune bodies, but signifi cant trends have tion (Pye and Tsoar, 1987; Pye, 1995) in accor- Very fi ne sands are suggested to have been not been identifi ed (Lancaster, 1995, and refer- dance to the duration and velocity of the winds. transported by saltation and low suspension ences within). from source deposits and sand sheets. We Particle-size fractionation acts on sediment in Very Fine Sand suggest that a signifi cant proportion of the transport that undergoes selective deposition due very fi ne sand fraction of Nile Delta sands to decreasing energy of the transporting process. Unimodal particle-size distributions of 125– has been transported downwind through As the transport energy decreases, coarser sedi- 250 μm modes (fi ne sand) are common in active northern Sinai during the late Pleistocene, ment has a greater probability of being depos- dune sands, as this particle size is optimal for especially when linear dunes reached the NW ited than fi ner particle sizes (McLaren, 1981). transport by saltation. However, the transport Negev due to last-glacial period windiness Particle-size fractionation of sand and silt has dynamics of very fi ne sand are different than and probably larger sediment supply. Gener- been shown for different arid environments, fi ne sand. Very fi ne sand, being dynamically ally decreasing wind velocities and increas- mainly with regard to accreting soils and eolian transitional between saltation and low, modi- ing precipitation along the west-east dune mantles that are formed by a mixture of sus- fi ed suspension in strong storms (Bagnold, transport path enhanced vegetative cover in pended and saltating particles (McTainsh, 1984; 1941; Chepil, 1951; Pye and Tsoar, 1987; Xu the northern Negev and enabled deposition Holliday, 1989). The process of fractionation of et al., 2007), makes this size fraction poten- sand-grain fractions along transport paths has tially and more uniquely sensitive to current and †E-mail: [email protected] been reported for fi ner-grained eolian deposits past changes in wind velocities. Very fi ne sand GSA Bulletin; January/February 2014; v. 126; no. 1/2; p. 47–65; doi:10.1130/B30811.1; 13 fi gures; 2 tables. For permission to copy, contact [email protected] 47 © 2013 Geological Society of America Downloaded from gsabulletin.gsapubs.org on January 6, 2014 Roskin et al. transport by suspension over longer distances Study Outline and Goals et al., 2001; Mohamed, 2012). At Gebel Maghara is inferred from small amounts of very fi ne in the west, mean annual rainfall is ~50 mm sand, interpreted to be eolian, found in offshore The study area, the northern Sinai Peninsula– (Ahmed, 2010), and, along the Mediterranean cores of the Mediterranean Sea (Hamann et al., northwestern Negev erg (Sinai-Negev erg), strad- coast, annual averages are 67 mm at Port Said 2008; Mulitza et al., 2010), on Mediterranean dles the Egypt-Israel border (Figs. 1 and 2). The (Tsoar, 1995), 104 mm at El-Arish, and 200 mm islands (Sevink and Kummer, 1984), and off studied erg is composed of active linear (seif) at Rafah (Abdel Galil et al., 2000) (Fig. 1B). the North African coast (McGee et al., 2013). dunes in northern Sinai (western part) and sta- The northern Sinai dune fi eld east of the Nile The very fi ne sand fraction, defi ned here as the bilized vegetated linear dunes in the NW Negev Delta is spatially continuous over substantial range of 50–125 μm, is a slight extension of the dune fi eld (eastern part) of Israel. The Sinai areas, with dune heights exceeding 30 m (Gad, Krumbein phi (φ) scale for the very fi ne sand dune sands are the immediate source of the 2004). The Sinai sands are mainly sparsely veg- fraction of 63–125 μm, and includes the tail of Negev dunes. etated to bare and uncrusted seif linear dunes (coarse) silt (50–63 μm). The 50 μm cutoff is The Sinai-Negev erg is an ideal “fi eld labora- (Tsoar, 1989; Abdel Galil et al., 2000; Rabie in accordance with the lower cutoff of the U.S. tory” to study eolian particle-size trends along a et al., 2000), and complex braided linear dunes Department of Agriculture (USDA) fi ne sand sand transport system for several reasons. The erg (Tsoar, 1989, 1995) (Figs. 3A and 3B).