Deposition of sapropel S1 sediments in oxic pelagic and anoxic brine environments in the eastern Mediterranean: differences in diagenesis and preservation

Sediments from a boxcore in the previously anoxic brine-filled Poseidon Basin, eastern Mediterranean, have been studied and compared to sediments deposited in a ‘normal’ eastern Mediterranean environment. The boxcore can be divided into three main sedimentary intervals based on AMS-radiocarbon ages, foraminiferal and geochemical zonations. From the base of the core upwards these are: (1) 12.3‐31.2 cm, organic-rich sediments redeposited from within the brine; (2) 6.6‐12.3 cm, sediment containing a ‘cold’ foraminifera fauna redeposited from above the brine into the basin while the brine was still present; (3) 0‐6.6 cm, oxic pelagic sediment accumulated since the reoxygenation of Poseidon Basin which occurred1800 yrs BP. Near the base of the latter unit, a Mn-oxide peak has formed and it marks the present boundary between oxic and suboxic environments. A progressive downward oxidation front, which is usually found in ‘normal’ sapropel S1 sediments, has never formed in Poseidon Basin sediments. This has resulted in the preservation of the relationship between organic carbon and organic-related trace elements, e.g. Se, in the organic-rich sediments of Poseidon Basin, whereas such a relationship has been obliterated in ‘normal’ sapropel S1 sediments. On the basis of the carbonate content as well as the Sr=Ca ratio, preservation of carbonates appears to be better in the brine sapropel sediments of BC15 than it is in ‘normal’ sapropel S1 sediments. The high opal content of BC15 shows that biogenic opal is also much better preserved. The overall lower Corg=Ba ratio in BC15 suggests a better preservation of barite relative to that of organic carbon in shallow brine sediments, but is as yet inconclusive for the organic carbon preservation potential of brine relative to ‘normal’ unoxidised sediments. © 1999 Elsevier Science B.V. All rights reserved.