Myriam KARS: "Magnetic mineral authigenesis and diagenesis: examples from gas-bearing sedimentary rocks"
In marine and lacustrine sediments, many (bio)geochemical processes occurring during burial cause alteration, dissolution and formation of magnetic minerals. Especially microbial and/or abiotic iron reduction, and liberation of hydrogen sulfide into pore water that is related to sulfate reduction, have a strong impact on the primary magnetic mineral assemblage and therefore on the preservation of original magnetic properties. Additional diagenetic stages (e.g. deep hot fluid and gas migration, hydrocarbon formation) can lead to subsequent magnetic mineralogical changes and pervasive remagnetizations.
For instance, ferrimagnetic iron sulfide minerals (greigite and monoclinic pyrrhotite) form in association with methane. Methane migration through the sediments, methane production (biogenic or thermogenic) or gas hydrate occurrence can all produce non steady state environmental conditions favoring the preservation of ferrimagnetic iron sulfides in the sediments.
Therefore, understanding magnetic mineral diagenesis is of major importance when studying sediments and using the assumed detrital magnetic mineral assemblage for reconstructing environmental changes, paleoclimatic variations (e.g. glacial/interglacial cycle) or determining the paleointensity variations of the Earth’s magnetic field.