מקורות ודרכי הובלה של חומר גרגרי דק לים המלח: תמונת מצב בהווה ובמשך הרביעון

The Dead Sea watershed situated at the fringe of the Desert belt accumulates fine detritus- desert dust that was transported mainly under two synoptic conditions. The Red Sea Trough and the Cold Depression which predominately derive dust from Arabia and the Sahara deserts, respectively. The allochthonous fine detritus (i.e. the dust) is deposited on the mountains and valleys in the watershed where it can undergo pedogenesis, forming various types of surface cover. Seasonal flash floods wash the surface cover, or freshly deposited fine detritus, to the Dead Sea forming the detritus layers of the lacustrine formations of the late Quaternary Dead Sea. During the Quaternary global climate has periodically changed from glacial to interglacial conditions, also manifested in latitudinal changes of the boundary between the Desert belt and the Mediterranean climate zone. Such changes would affect the patterns of atmospheric circulation, that governs dust mobilization and precipitation. In order to reconstruct wind and precipitation patterns in the Dead Sea watershed I used modern and Quaternary archives of fine detritus. I sampled dust storms, flood and surface cover sediments and compared them to sediments recovered from the DSDDP core drilled at the deepest floor of the Dead Sea and KL cores drilled at the floor of the Red Sea. This work focuses on the fine detritus deposited during the final part of the penultimate glacial, Marine Isotope Stage 6, and the last interglacial peak, Marine Isotope Stage 5e, between ~180 to 116 ka. I used grain size distribution, mineralogy, chemical composition and Nd-Sr isotope ratios to characterize and compare the various data sets.

There are two contemporary sources of fine detritus to the Dead Sea: (1) "fresh" settled dust that is rapidly mobilized to the lake; and (2) surface cover, mainly loess and gypsic-loess soils, that accumulate in valleys such as the Hurkanya valley (termed “Valley loess”). Both sources have different geochemical characterizations that can be traced in the DSDDP core, and hence, enabling the reconstruction of the regional hydro-climatic conditions during the desired time intervals. Between ~180 and 135 ka (glacial interval), the Dead Sea and the northern Red Sea received fine detritus mainly from north Sahara deserts probably by the activity of the Cold Depressions that also dictated the rain regime in the Dead Sea watershed (allowing precipitation and flash floods). Then, during the termination period (135 to 130 ka) along with deposition of salt and lake retreat in the Dead Sea, the fine detritus was mainly remobilized to the Dead Sea from the accumulated “Valley loess”, while the northern  Red Sea continued accumulating dust from the Sahara. This configuration suggests a weakening of the Mediterranean rain activity but persistence of dust transport from the Sahara. The peak of the interglacial (129 to 121 ka) is characterized by enhanced floods that transported fine detritus from the entire Red Sea and Dead Sea watersheds. At the end of the interglacial peak (121 to 116 ka) the effect of the floods diminished and more “Valley loess” detritus was remobilized to the Dead Sea while the Red Sea received Sahara and Nile Valley dusts. Thus, the termination period is overall characterized by hyper-arid conditions along the entire Red Sea-Dead Sea transect and expansion of the desert belt northward, while the peak of the previous interglacial was characterized by regional floods and contraction of the desert belt


תאריך 29/01/2017 10:30 12:00
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