We performed detailed rock magnetic and geochemical analyses on a previously published Early Jurassic magnetostratigraphic section. The results improve our understanding of acquisition and preservation processes of magnetization in a series of alternating claystones and limestones. The main carrier of magnetization is magnetite. Anhysteretic remanent magnetization (ARM) varies by a factor of 40 and the ARM variations are linked to magnetite grain size. Comparison of magnetic and geochemical data shows that when carbonate content is high (>30%) and Δ13C≈0?, magnetite is characterized by small grain sizes, whereas when carbonate content is low (<20%), magnetite is coarse-grained. It appears that the oxidation of organic matter by sulfate reduction controls both Δ13C and magnetite grain size. H2S produced during sulfate reduction causes partial dissolution of magnetite grains, with the finest magnetite grains (those that best record the magnetic signal) being dissolved first. Despite this partial dissolution, both the direction and polarity of the original remanent magnetization are preserved. ¿ 2000 American Geophysical Union