Magnetic signatures of biogeochemical processes at a hydrocarbon contaminated site

Abstract:

Magnetic properties have been considered a promising tool for proxying biogeochemical processes involving Fe-cycling by microbes and the environment, forming new magnetic minerals and resulting in changes in magnetic minerals content. The magnetic particles produced in these processes are responsible for increasing magnetic susceptibility (MS) and often have a broad size distribution lying within the superparamagnetic (SP) range. In this work, we seek for changes of magnetic properties with depth. The limiting frequency effect (LFE) estimated from MS measurements using alternating low fields reflects changes in the SP content. MS changes with sample temperature (thermomagnetic curves) indicate the magnetic minerals composition. Measurements of frequency dependence susceptibility (FDS) and of the susceptibility changes with temperature were conducted on sediment core samples from an area contaminated by accidental creosote spills in São Paulo, Brazil. Samples were collected from a background region (no contamination) and from locations where contaminant free phase was observed. At the contaminated region two sediment cores were sample: one being sampled in regular intervals with depth and the second sampled based on observations of UV light reflection, to guarantee contaminated sediments were sampled. MS depth profiles showed a similar pattern for contaminated and non-contaminated sites. LFE, on the other hand, revealed significant changes around the depth of the water table (WT) only in the contaminated sites. The increase in SP magnetic particles around the WT fluctuation zone where the aerobic / anaerobic conditions are dynamic suggests the improvement of the biologically induced mineralization processes in that zone, which can be considered a biogeochemical hot zone. The results of the thermomagnetic curves made it possible to identify the occurrence of mineral phase transformations and magnetite as the main mineral present. This result suggests including FDS measurements when investigating hydrocarbon biodegradation. While MS is a bulk property of all magnetic minerals, FDS isolates the SP response, which is the distribution range expected to result from microbial activity. We conclude that recording changes of the SP content is a promising approach to study biodegradation processes.