Temporal Changes in Magnetic Susceptibility Induced by Microbial Manipulation of Iron Minerals
Abstract
Sequential iron extractions were used to determine the mineral and oxidation state speciation of iron in a series of cores from an oil spill site in Bemidji, Minnesota. Cores were collected over five years, from a location where a magnetic susceptibility (MS) anomaly was initially observed in 2011. Further field and laboratory MS measurements confirmed the magnetic anomaly, yet revealed a trend to lower MS values over the measured time frame. Conventional interpretation typically dictates that MS peaks are indicative of mixed-valence oxide mineral phases (i.e. magnetite or greigite) here the diminished, but persistent MS anomaly is most closely coupled with high concentrations of the fully-oxidized iron oxides, such as hematite. Microbiological community fingerprinting analysis (PCR-DGGE) of core samples suggest that the anomaly could result from methanogenically-derived magnetite occurring during water table high stand, which was then oxidized as water levels seasonally decreased. The diminished, but persistent presence of magnetic susceptibility anomaly appears to indicate the transformation of magnetite to iron oxides mediated by microbial metabolic processes (i.e. methanogenesis). We hypothesize that the MS response is related to the biological transformations of iron in the groundwater table fluctuation zone, and that the magnitude of the anomaly evolves along with oxidation state of iron in the sediments, and may serve as a record of the microbial manipulation of iron.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMNS24A..05E
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0486 Soils/pedology;
- BIOGEOSCIENCESDE: 1830 Groundwater/surface water interaction;
- HYDROLOGYDE: 1835 Hydrogeophysics;
- HYDROLOGY