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

摘要

Sediments from a boxcore in the previously anoxicbrine-filled Piseidon Basin, eastern Mediterranean, have been studiedand compared to sediments deposited in a ‘normal’ easternMediterrean environment. The boxcore can be devided into threemain sedimentary intervals based on AMS-radiocarbon ages,foraminifearl and geochemical zonations. From the base of the coreupwards these are: (1) 12.3-31.2 cm, organic-rich sedimentsredeposited from within the brine; (2) 6.6-12.3 cm, sedimentcontaining a ‘cold’ foraminiferal fauna redeposited from abouvethe brine into the basin while the brine was still present; (3) 0-6.6 cm,oxic pelagic sediment accumulated since the reoxygenation ofPoseidon Basin which occurred ~ 1800 yrs BP. Near the base of thelatter unit, a Mn-oxide peak has formed and it marks the presentboundary between oxic and suboxic environments. A progressivedownward oxidation front, which is ususally found in ‘normal’sapropel S1 sediments, has never formed in Piseidon Basin sediments.This has resulted in the preservation of the relationship betweenorganic carbon and organic-related trace elements, e.g. Se, in theorganic-rich sediments of Poseidon Basin, whereas such arelationshsip has been obliterated in ‘normal’sapropel S1 sediments.On the basis of the carbonate content as well as the Sr/Ca ratio,preservation of carbonated appears to be better in thebrine sapropelsediments of BC15 than it is in ‘normal’sapropel S1 sediments. Thehigh opal content of BC15 shows that biogenic opal is also muchbetter preserved. The overall lower Corg/Ba ratio in BC15 suggests abetter preservation of barite relative to that of organic carbon inshallow brine sediments, but is as yet inconclusive for the organiccarbon preservation potential of brine relative to ‘normal’unoxidised sediments.