Iron-carbonate concretions with inverse magnetic fabrics; a record of environmental changes in the middle Eocene marine marls of the Southern Pyrenees?

Abstract

AbstractThis paper deals with the detailed analyses of magnetic fabrics, accompanied by stable isotopic composition and microscopic observations, in centimetric and metric scale authigenic carbonate concretions embedded in the Eocene flysch deposits of the Southwestern Pyrenees. Sampling was focused in the carbonate concretions, (in both metric and centimetric scale), in the marls lateral and nearby to the concretions and in the marls located several meters away from the concretions. The inverse magnetic fabrics (kmax axes cluster perpendicular to bedding plane) detected in these concretions constitute a fast methodology to uncover the presence of iron-carbonate minerals and the paleoenvironmental significance of their authigenic origin. The subfabric analyses indicate that magnetite is present in all four types of samples with normal magnetic fabric (kmin axes perpendicular to bedding). Paramagnetic fabric (low temperature anisotropy of magnetic susceptibility when magnetic susceptibility increases ~ 3.8 times the one at room temperature) overlaps the room temperature magnetic fabric. The microscope observations reveal that iron-enriched dolomite is the main carrier of the inverse fabric in the carbonate concretions at room temperature. The stable isotopic composition indicates minor differences between the Eocene marls and the carbonate concretions and, when compared with previous data, they suggest a marine pore water origin due to bacterial activity during burial. We relate the early diagenetic growth of the concretions to an enhancement in bacterial activity driven by the increased terrigenous and terrestrial organic matter supply during the Middle Eocene Climate Optimum (MECO), a period of global warming.