Early diagenetic processes and sulphur speciation in pore waters and sediments of the hypersaline Tyro and Bannock basins, eastern Mediterranean

Abstract

Anoxic hypersaline basins have been found in two different tectonic environments in the eastern Mediterranean. Within the Tyro area (the western Strabo Trench) there are three pull apart basins: the Tyro Basin, presently filled with anoxic hypersaline bottomwater, and the Poseidon and Kretheus Basins, thought to have been anoxic and filled with hypersaline water in the past. The Mediterranean Ridge shows a "cobblestone topography" (Camerlenghi, 1990). The Bannock Basin is the largest depression found within this area and is divided into nine sub-basins. Tectonic deformations of Messinian evaporites are thought to be the major processes involved in the formation of these anoxic hypersaline basins. The interstitial water chemistry of sediments from these basins have been studied. The fact that the Poseidon and Kretheus Basins have been filled with hypersaline bottomwater is reflected by an increase of the CI and Na concentration in the pore waters downcore. These brines are thought to have been similar to that of the Tyro Basin. A diffusionadvection model has been applied to the CI profiles. From time calculations it was possible to estimate the time elapsed since hypersaline conditions in the Poseidon and Kretheus Basins changed into normal saline conditions. A renewal time between 2000 - 3000 years was found. Organic matter reaching the sediments is decomposed by sulphate reducing bacteria. Despite the high salinity (260 %0) in the Tyro and Bannock Basins, sulphate reduction rates are higher than in the sediments from the Poseidon and Kretheus Basins. These higher sulphate reduction rates could be related to a better "preservation" of the organic matter in the anoxic brine. The release of inorganic metabolites, such as ammonium, alkalinity, phosphate and sulphide leads to the formation of authigenic minerals in these sediments such as dolomite, gypsum and sulphides. The sulphur chemistry in the anoxic hypersaline sediments from the Tyro and Bannock Basins have been studied in detail. Despite the difference in major element chemistry of the brines (De Lange et aI., 1990a) their sulphur chemistry in the sediments is very similar. The following sulphur species have been determined and quantified: elemental sulphur, Acid Volatile Sulphur (AVS), organic polysulphides, humic sulphur (0.5 M NaOH extractable) and pyritic sulphur. Pyritic sulphur was found to be the main phase of the inorganic reduced sulphur (50 - 80% of the total sulphur pool) and was at the same level (250 pmoles per gram dry weight) in both cores. Remarkably, humic sulphur was found to account for 17 to 28% of the total sulphur pool in the Tyro Basin and for 10 - 43 % in the Bannock Basin. Sulphur isotope data show negative 634S values for both pyritic sulphur (-16 to -40 %0) and humic sulphur (-16 to -30 %0). Pyritic and humic sulphur are thought to be mainly formed at the interface of oxic seawater and anoxic hypersaline bottom water. At this interface material is captured due to the large density difference. Higher amounts of particulate and dissolved organic carbon have been observed at these interfaces as well as sulphate reduction and sulphide oxidation processes. Turbiditic deposition transports pyritic and humic sulphur compounds to the sediments in the Tyro and Bannock Basins.