Ultrafine magnetic particles characterization as a proxy of biogeochemical processes at a brownfield site

Biogas formation on brownfield sites due to either anthropogenic or geogenic processes is the result of microbial activity degrading organic matter. Organic material confined in anaerobic environments (e.g., landfills) is biodegraded through a series of fermentation and methanogenesis steps that generate carbon dioxide and methane as major products. In this work we present magnetic properties of sediment samples from a brownfield site in São Paulo – Brazil. Mineralogy, concentration and grain size variations of the magnetic carriers in the samples were investigated using Isothermal Magnetization and Demagnetization and Frequency/Temperature Dependent Susceptibility methods. The increase of particles in the Superparamagnetic (SP) - Stable Single Domain (SSD) range were observed at depths were methane was detected. The distribution of SP concentration correlates well with the detected methanogens, suggesting that the microbial activity producing methane is synergistically producing these ultrafine particles, different from the magnetic particles produced at other depths. The results presented here support the use of environmental magnetism techniques to investigate biogeochemical processes of anaerobic microbial activity.