Dead Sea sediments: GFZ

Dead Sea sediments

Bathymetry (mbsf) of the North Basin of the Dead Sea, Israel and coring sites on the western shore (left). Sediment composition and age model based on radiocarbon dating for the Ein Gedi profile (right). *(GFZ)*

During a drilling campaign in 1997 sediment cores with lengths between 0.6 and 19 m were recovered from four different sites on the western shore of the Dead Sea. The sediments in all cores are composed of alternating fine layers of detrital material and authigenic aragonite with intercalated sand as well as salt layers of up to 2 m thickness. Thin dark layers that are present in nearly all sediment sections point to the presence of Fe-sulfides, during periods of anoxic lake bottom conditions. The depth-age models, based on AMS 14C dating on plant remains, yielded an age of 10.04 ka cal. BP for the base of the longest profile from site Ein Gedi, a continuous lacustrine sediment sequence.

Backer: DFG, German Israel Foundation (GIF)

Detailed paleo- and rock magnetic investigations included the analysis of laboratory induced magnetizations and high temperature runs of the saturation magnetization. Ti-magnetite and greigite were identified as the main magnetic carrier minerals, reflecting a detrital as well as an authigenic phase in the Dead Sea sediments. The variations in concentration, grain size and coercitivity depended parameters reflect the varying amount of greigite with respect to Ti-magnetite. Samples with a high greigite concentration are characterized by S-ratios close to 1 in combination with low JARM/JSIRM ratios. The results of a hierarchical agglomerative cluster analysis of the rock magnetic data presented in form of scatter plots allow for a qualitative identification of greigite-indicative parameters / parameter ratios and thus semi-quantitative estimation of the greigite content. The samples from the Dead Sea are distributed along a mixing line between the end members pure greigite and low Ti-magnetite.

Identification of greigite and magnetite by temperature dependent measurements of the saturation magnetization in samples from cores DSEn-C/A. Red diamonds denote the heating curves, blue diamonds the cooling curves.

Estimating the portions of Ti-magnetite (T) and greigite (G) in samples from DSF-B, DSEn-C/A and DSZ-A by combining different rock magnetic parameters indicative for coercitivity and grain size in a series of scatter plots. The different colored symbols correspond to the subdivision of the samples into four clusters as explained in Frank et al. (2007b). Green crosses: G>>T, blue squares: G ≥ T, red diamonds: G ≤ T, orange stars: G << T.

Directions of the characteristic remanent magnetization were obtained from stepwise AF-demagnetization of the natural remanent magnetization and subsequent principle component analysis. Comparison of the inclination and declination records from all Dead Sea cores and nearby sited lakes as well as archaeomagnetic records indicates that the low frequency variations documented in the Dead Sea sediments are likely of geomagnetic origin.

Comparison of the ChRM-inclinations and ChRM-declinations obtained from the sediments of Ein Gedi with those from Bulgaria (Kovacheva et al., 1998, Surveys in Geophysics, 19, 431-460)

Microscopical analysis of the Ein Gedi sediment cores revealed the existence of sediment sequences that were deformed by tectonic activity along the Dead Sea transform. These mixed layers or seimites ( Marco et al., 1996, J. Geophys. Res., 101, 6179-6191) could be ascribed to major earthquakes in the region by comparing and correlating the historical earthquake record with the Ein Gedi chronology based on radiocarbon dating and varve counting. In combination with the results by Ken-Tor et al. (2001, J. Geophys. Res. 106, 2221-2234) there is now a high resolution record of seismic activity in the Dead Sea region during the last 6000 years available.

Five typical examples for mixed layers in the Ein Gedi sediments as could be seen in a core photo (left) .- Comparison and correlation of the Ein Gedi depth/age model with the record of historical earthquakes (right).

The major changes in the lithology of the sediment profiles from Ein Gedi, Ein Feshka and Ze’elim (laminated authigenic aragonite, gypsum, halite and clastic material from the catchment area) were interpreted in terms of lake level changes of the Dead Sea. These changes are supposed to reflect the variations in the precipitation in the Dead Sea drainage area. Based on the given chronologies, lake level high (low) stands were interpreted as wet (dry) phases, reflecting the variabillity of the eastern Mediterranean paleolimate during the Holocene.