DIAGENETIC EVOLUTION OF THE DOP FACIES FROM THE ATLANTIS-II-DEEP (RED-SEA) - EVIDENCE OF EARLY HYDROTHERMAL ACTIVITY

The 23,000 year old sediments of the Atlantis II Deep rest on a basaltic substratum and are overlaid by a dense and hot brine pool. The facies of the lower sequence of the West basin consists essentially of bioclastic and detrital particles (DOP facies). The primary calcite and aragonite of planktonic tests of the DOP facies have been transformed into Ca-, Mg-, Mn-, and Fe-carbonate solid solutions. Pyrite amounts to about 1-2% (wt) at the bottom of the sedimentary series, and as much as 20% (wt) in the upper part of the sequence. These high iron - and manganese - earing minerals are the product of early diagenetic processes in an environment favourable to bacterial activity, to which biogenic and detrital particles, organic matter, and iron and manganese oxides were supplied. The high proportion of pyrite and secondary carbonates indicates that the environment of the W basin was different from the Red Sea pelagic environment during the deposition of the DOP facies, which take place between 23,000 and 11,000 years B.P. The Sr-87/Sr-86 ratios, approaching 0.708, and the conservation of siliceous tests indicate the occurrence of an initial brine pool in the Atlantis II Deep. The Deep was therefore hydrothermally active prior to 11,000 years B.P., i.e. before the deposition of the purely metalliferous facies that overlie the DOP facies.