Gelatinous pellicles in deep anoxic hypersaline basins from the Eastern Mediterranean

Abstract

Gelatinous pellicles have been observed in three out of ten cores recovered from the Bannock Basin (or Bacino Bannock) and in two dredges from its flanks during a cruise of the R.V. Bannock in the Eastern Mediterranean. Similar pellicles have also been detected in two cores recovered from the Tyro Basin. The Bannock and Tyro basins are brine-filled depressions which are presently characterized by anoxic sedimentation; in the Bannock Basin also gypsum precipitation occurs. The gelatinous pellicles, some millimeters thick and dark grey to greenish in color, are always found in very dark colored sediments of Quaternary age. These pellicles have been investigated by optical microscopy, by scanning electron and transmission electron microscopy, and by flash pyrolysis-gas chromatography and flash pyrolysis-gas chromatography-mass spectrometry to decipher their nature. Abundant skeletal tests of siliceous and calcareous planktonic organisms (radiolaria, diatoms, silicoflagellates, formainifera and nannofossils) are observed, together with amorphous organic matter. The pellicles enclosing these remains consist of organic matter; they foliate into submillimetric undulating and anastomosing microlaminae. Cellular structures are never observed, neither in the pellicles nor in the microlaminae. Organic geochemical analyses point to a high contribution of terrestrial material. The organic matter of the pellicles investigated consists of a “coal-like” material. It is suggested that the pellicles are originally formed at the brine-seawater interface, but subsequently sink to the bottom due to increasing load of entrapped biogenic and non-biogenic detritus.