Atlas and Data of Solid-Solution Equilibria of Marine Evaporites
Evaporites, particularly halite, mineral precipitation permits the passage of photosynthetically active radiation and acts as UV-light scatterers so they can provide protection from cosmic radiation and allow certain life forms to survive in salt fluid inclusions for more than million years [39,40].
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Moreover, the interior of halite crusts seems to have unique microhabitats whose microenvironmental conditions cannot be found in soils or other lithic substrates . This particular microhabitat is determined essentially by its hygroscopic nature which enhances the moisture conditions.
Sea-water solubility phase diagram. Application to an extractive process
A study by Wierzchos et al. In some cases however, they are endoevaporitic and found only within the halite rocks other than colonizing the surface of quartz grains. Also cyanobacteria can carry out their metabolic activities under the stressed conditions of high salt and even low water . EPS could act as a shield, slowing down desiccation and ameliorating the extreme external conditions . Rothschild et al. The survival of archaea halophiles in dry salt over geological time scales has been reported by McGenity et al. Modern coastal sabkhas are widely colonized by microbial mats and biofilms.
The abundance of a biofilm of EPS reflects the behavior developed by microbial mats living in such hypersaline systems. EPS layers allow cyanobacteria to increase their fossilization potential through early diagenesis of crystallization and cementation by gypsum and halite of these soft shaped layers.
Preservation of EPS and biofilms depends on their rapid lithification before degradation . Once lithified, these gel layers retain their biologic related morphology, which can be recognized in the fossil record.
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Furthermore, in carbonate environment the formation and persistence of ancient stromatolites depend on the binding and stabilizing of sediments by microbial mats and biofilms . The initial biogenic stabilization of depositional systems may be an essential requirement to allow or enhance future lithification of the sediments .
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The preservation potential of microbial mats with ancient evaporites has been documented by many authors. Barbieri et al. These biosignatures include gypsified microfibers formed after the partial degradation of bacterial mucilaginous secretions. Recently, Noffke et al.
The microbial mats generate a plethora of well-preserved MISS arising from interaction with sedimentary processes such as erosion or evaporite crystal growth. Upon comparison of fossil and modern biogenic structures and their facies-related distribution in sabkha settings, they strongly addressed the presence of microbial communities in the Paleoarchean of such hypersaline and extreme ecosystem. Sabkhas, with its easier recognition and sampling is a good model not only for evaporites deposition in shallow marine environments, but also for preserving potential of traces of life within chemical precipitates.
The climate of Ras Gemsa is hot and dry and solar radiation is intensive. These conditions favor the deposition of halite, which serves as a good conduit for light, reduces the effect of intensive harmful solar radiation, and provides protection from high cosmic radiation, which allows microbial mats to survive and flourish. The area is characterized by a low sedimentation rate, little wave action, lack of bioturbation, and is protected by vegetated patches of sea grasses, which results in an optimal development of microbial mats and biofilms.
The microbial mats of Ras Gemsa sabkha produce distinctive sedimentary structures such as frozen multidirected ripples, salt-encrusted crinkle mats, jelly roll structures and petee structures. Most of these structures were found to be encrusted with halite. Thus within the halite rich rocks there will continually exist conditions suitable for the survival of microbes, particularly cyanobacteria. If the right balance is met between absence of surface hydrological cycles and rapid sealing provided by precipitating evaporite minerals, an ultimate lithification process with good preservation of microbially induced sedimentary structures can form.
Intensive researches on microbial processes occurring in modern coastal sabkhas, which are extensively inhabited by microbial mats and biofilms, are needed to open the path of fully and better understanding of ancient microbial biota. This article does not contain any studies with human or animal subjects. The author wishes to express her thanks to the anonymous referees for their constructive criticisms of an earlier version of the manuscript. Phillip, Cairo University, is highly acknowledged for his critical reading of the manuscript.
Abdel-Motelib and Dr. Wagdi, Cairo University, are highly acknowledged for their help in the field work. Peer review under responsibility of Cairo University. National Center for Biotechnology Information , U. Journal List J Adv Res v. J Adv Res. Published online Aug 2.
Amany G. Author information Article notes Copyright and License information Disclaimer. Taher: moc. Abstract The coastal sabkha in Ras Gemsa, Red Sea coast with its colonizing microbial mats and biofilms was investigated. Introduction Many studies on microbial mats, the oldest and most successful microorganisms, showed that metabolic activity of cyanobacteria and heterotrophic bacteria in carbonate marine environments induces the precipitation of carbonates, which in turn form a microbial buildup named stromatolites [1—3].
Open in a separate window. Material and methods The material used in the present study was collected during June and December and October Climatic setting The study area could be regarded as semiarid. Results and discussion Grain size distribution The grain size distribution and related textural classification of the surface sediments in the sabkha are summarized in Table 1.
Site No. Depositional subenvironments in Ras Gemsa sabkha Geomorphological and Sedimentological features enable identification of four well-defined zones located at different topographical levels 1 perennial saline pools; 2 ephemeral saline pools; 3 dry and capillary mud flat; and 4 supratidal flat of efflorescent halite crusts. Perennial saline pools Perennial saline pools extend along the deepest part of the sabkha area Fig.
Ephemeral saline pools Ephemeral saline pools are mainly restricted to the southwestern part of the area. Dry sand and capillary mud flats The shallow hypersaline pool surface water is widely surrounded by air-exposed saline flats with a general scarcity of water bodies Fig. Microbially induced surface sedimentary structures Because the studied site is relatively isolated anthropogenic activity is low, and in the absence of metazoans grazing, microbial mats can grow freely without being disturbed. Preservation potential of microbes with evaporites It is highly accepted that evaporites could be preserved over geological times where surface hydrological cycles are absent.
Conclusions Sabkhas, with its easier recognition and sampling is a good model not only for evaporites deposition in shallow marine environments, but also for preserving potential of traces of life within chemical precipitates. Conflict of interest The author has declared no conflict of interest. Compliance with Ethics Requirements This article does not contain any studies with human or animal subjects. Acknowledgments The author wishes to express her thanks to the anonymous referees for their constructive criticisms of an earlier version of the manuscript.
Footnotes Peer review under responsibility of Cairo University. References 1. Gerdes G. Structural diversity of biogenic carbonate particles in microbial mats. Olveri E. Carbonate stromatolites from a Messinian hypersaline setting in the Caltanissetta Basin, Sicily: petrographic evidence of microbial activity and related stable isotope and rare earth element signatures. Spadafora A.
Microbial biomineralization processes forming modern Ca:Mg carbonate stromatolites. Schieber J. Microbial mats in terrigenous clastics: the challenge of identification in the rock record. Unexplored microbial worlds. Palaios ; Noffke N.