Microbial and Chemical Characterization of Underwater Fresh Water Springs in the Dead Sea

Due to its extreme salinity and high Mg concentration the Dead Sea is characterized by a very low density of cells most of which are Archaea. We discovered several underwater fresh to brackish water springs in the Dead Sea harboring dense microbial communities. We provide the first characterization of these communities, discuss their possible origin, hydrochemical environment, energetic resources and the putative biogeochemical pathways they are mediating. Pyrosequencing of the 16S rRNA gene and community fingerprinting methods showed that the spring community originates from the Dead Sea sediments and not from the aquifer. Furthermore, it suggested that there is a dense Archaeal community in the shoreline pore water of the lake. Sequences of bacterial sulfate reducers, nitrifiers iron oxidizers and iron reducers were identified as well. Analysis of white and green biofilms suggested that sulfide oxidation through chemolitotrophy and phototrophy is highly significant. Hyperspectral analysis showed a tight association between abundant green sulfur bacteria and cyanobacteria in the green biofilms. Together, our findings show that the Dead Sea floor harbors diverse microbial communities, part of which is not known from other hypersaline environments. Analysis of the water’s chemistry shows evidence of microbial activity along the path and suggests that the springs supply nitrogen, phosphorus and organic matter to the microbial communities in the Dead Sea. The underwater springs are a newly recognized water source for the Dead Sea. Their input of microorganisms and nutrients needs to be considered in the assessment of possible impact of dilution events of the lake surface waters, such as those that will occur in the future due to the intended establishment of the Red Sea−Dead Sea water conduit

Seismic Surface-wave Prospecting Methods for Sinkhole Hazard Assessment along the Dead Sea Shoreline

International audienceThe Dead Sea (DS) coastal areas have been dramatically hit by sinkhole occurrences since around 1990. It has been shown that the sinkholes along both Israeli and Jordanian shorelines are linked to evaporate karst cavities that are formed by slow salt dissolution. Both the timing and location of sinkholes suggest that: 1) the salt weakens as the result of unsaturated water circulation, thus enhancing the karstification process; and 2) sinkholes appear to be related to the decompaction of the sediments above karstified zones. The location, depth, thickness and weakening of salt layers along the DS shorelines, as well as the thickness and mechanical properties of the upper sedimentary deposits, are thus considered as controlling factors of this on-going process. The knowledge of shear-wave velocities (Vs) should add valuable insights on mechanical properties of both the salt and its overburden. We have suggested Vs estimation using surface-wave prospecting methods, based on surface-wave dispersion measurements and inversion. Two approaches have been used. Along the Israeli shoreline, Vs mapping has been performed to discriminate weak and hard zones within salt layers, after calibration of inverted Vs near boreholes. It has been shown that there is a Vs increase in the DS direction. Initially examined weak zones, located near the salt edge, associated with karstified salt, are characterized by Vs values of 760–1,050 m/s, and extend 60–100 m from the salt edge in the DS direction. Hard salt zones with velocity Vs values greater than 1,500 m/s are located at distances of more than 100–220 m from the salt edge. Finally, transition zones (1,050 < Vs < 1,500 m/s) have a 40–160 m spread. On a Jordanian site, roll-along acquisition and dispersion stacking has been performed to achieve multi-modal dispersion measurements along linear profiles. Inverted pseudo-2-D Vs sections present low Vs anomalies in the vicinity of existing sinkholes and made it possible to detect decompacted sediments associated with potential sinkhole occurrences. Moreover, Vs profiles showed a high velocity unit at 40–50 m depth that can be interpreted as a salt layer