Magnetic study of Brazilian peats from São Paulo state

The magnetic properties of peats from the Paraı´ba do Sul Valley (São Paulo State) Brazil have been studied in cores up to 6.4 m long with a basal radiocarbon age of 11,080 years BP. In general, the magnetic susceptibility is low, but is consistently higher near the bottom of the profiles. Anhysteretic remanence is surprisingly strong in comparison to other magnetic properties, and is attributed to the preservation of fossil bacterial magnetosomes in the peat. Results from Mössbauer spectroscopy indicate the presence of Fe3+ and Fe2+ in the bottom of the cores, but only Fe3+ toward the top. The additional Fe2+ may be related to the higher magnetic susceptibility. Susceptometer measurements of magnetisations, in fields up to 6 T at both room temperature and 10 K, reveal predominantly paramagnetic behavior. Laboratory-induced remanences and magnetisations point to the presence of three additional magnetic phases, namely, ferrite (both low and medium stability) and a haematite or goethite component.