The magnetic parameters of sedimentary rocks can record accurately tectonic and climatic influences upon sedimentary processes, when they are not altered during diagenesis. This paper is focused on the possible alteration of the primary magnetic-susceptibility signal during early diagenesis of marine organic matter (OM).
In the late Kimmeridgian-Tithonian Argiles de Châtillon Formation (Fm.) of the Boulonnais area (northern France – lateral time equivalent of the distal organic-rich sediments of the Kimmeridge Clay Fm. of Dorset), the organic content is dominated by amorphous OM (AOM), either brown or orange, originating from selective preservation or sulphurisation, respectively. Total sulphur content correlates well with orange + brown AOM abundance, and organic S content correlates well with orange AOM abundance. The magnetic signal of these claystones and paper shales is dominantly carried by clay minerals. The magnetic susceptibility vs. brown-AOM abundance relationship shows a clear correlation. Furthermore, if the part of the magnetic-susceptibility signal linked to the lithoclastic fraction of the sediments is removed from the total signal, a positive correlation is also drawn between the brown-AOM abundance and the ‘excess’ magnetic susceptibility. The cause for this resides in the iron-sulphide abundance. The presence of both types of AOM implies that intense sulphate reduction took place and that the HS−/H2S released reacted with either organic molecules (orange AOM dominating the palynofacies) or reactive iron (brown AOM dominating). When reactive iron was abundant enough and when sulphate reduction-induced sulphide ions could react with it, iron sulphides could form and this sediment component (present now as pyrite) influences the magnetic-susceptibility signal. The type of dominating source organisms (phytoplankton) may condition the reactivity of OM towards sulphide ions. This may influence iron-sulphide formation, and in turn the sediment magnetic signal.