The role of iron minerals in laminae formation in Late Pleistocene sediments of the Caspian Sea
Résumé
The 995 cm long sediment core was collected from the Central basin of the Caspian Sea. The sedimentary sequence, which covers the Late Pleistocene and Holocene, is marked in its Late Pleistocene section by well-developed black and grey/brownish laminae, which promptly disappear under oxidising conditions.Studies of granulometry, major elements, carbonate and organic matter content, and magnetic parameters, were conducted at laminae scale. In non-magnetic parameters, only sulphur clearly changes between black and grey/brownish laminae. Magnetic parameters indicate the presence of high contents of iron sulphide (greigite) in black laminae, while low contents of iron oxide (magnetite) are detected in grey/brownish laminae. The lamination is due to early diagenesis in sediment just below the sea bed, related to the oxygenation state of the bottom water. Under poorly oxygenated conditions, greigite isformed in anoxic sediment, with increased oxygenation, detrital magnetite is preserved. Broadly similar processes were described from the Southern Caspian Sea basin. During the Late Pleistocene, both basins were subjected to the same, rhythmic ventilation of bottom waters. Efficient ventilation is most probablyrelated to the harshness of the winter (cold/mild winters) and thus production of more or less cold, dense descending waters, and the content of fresh water supply from the catchment area. These two processes can act annually, as well as on a longer time scale.