Effects of diagenesis on magnetic mineralogy ih a Jurassic claystone-limestone succession from the Paris Basin

We performed detailed rock magnetic and geochemical analyses on a previousl y published Early Jurassic magnetostratigraphic section. The results improv e our understanding of acquisition and preservation processes of magnetizat ion in a series of alternating claystones and limestones. The main carrier of magnetization is magnetite. Anhysteretic remanent magnetization (ARM) va ries by a factor of 40 and the ARM variations are linked to magnetite grain size. Comparison of magnetic and geochemical data shows that when carbonat e content is high (>30%) and delta(13)C approximate to 0 parts per thousand , magnetite is characterized by small grain sizes, whereas when carbonate c ontent is low (<29%), magnetite is coarse-grained. It appears that the oxid ation of organic matter by sulfate reduction controls both delta(13)C and m agnetite grain size. H2S produced during sulfate reduction causes partial d issolution of magnetite grains, with the finest magnetite grains (those tha t best record the magnetic signal) being dissolved first. Despite this part ial dissolution, both the direction and polarity of the; original remanent magnetization are preserved.