Rare earth elements in the modern sediments of the Bay of Biscay (France)

Abstract

Particulate rare earth elements (REEs) were measured in modern sediment from the Bay of Biscay. Vertical profiles of total and reactive (ascorbate reducible) REEs have been studied in four contrasted sites ranging at water depth from 150 to 2800 m. The focus is on the coupling between the diagenetic conditions and the behavior of particulate REE. Total REEs fraction appears to be affected by the presence of metal sulphides that may act as an important carrier phase for solid REEs. In sediments where sulphate reduction is weak, the transfer from an unknown previous phase, stable in oxic conditions, to another one associated with sulphides seems to be direct, without mobilization in porewaters. In sediments where greater sulphate reduction occurs, we note the formation of an additional “authigenic” phase. Consequently, a threshold level of dissolved sulphide is a prerequisite for the formation of an additional “authigenic” REEs fraction. No direct relationship between the REE asc profiles and authigenic metal oxides have been observed, indicating that authigenic Fe and Mn oxides do not act as an efficient traps in the sediment. Despite particulate REE are mobile in these sediments, no fractionation of the lanthanide series is observed in relation to the redox conditions.