Microbially induced smectite to illite transformation in natural sediments during laboratory compression

Definitive evidence for the microbially induced smectite-to-illite (S-I) reaction has previously been shown using culture experiments with pure clay minerals, whereas recognition in nature remains difficult. Here, we investigated the microbially induced S-I reaction in natural sediments during laboratory compression and applied new and previously used techniques that can positively identify the products of this reaction. We performed resedimentation experiments without (control experiment) and with the Fe reducing bacteria Shewanella oneidensis MR-1 (microbially amended experiment) added to natural sediments collected from the Ursa and Brazos-Trinity regions in the Gulf of Mexico during Integrated Ocean Drilling Program Expedition 308. Following these experiments, subsamples were collected and analyzed with bulk and clay fraction X-ray diffraction, micro X-ray fluorescence, electron microprobe elemental mapping, and energy dispersive spectroscopy (EDS) spot analyses at the µm-scale. We found in the microbially amended experiments of both sediment samples that (1) clay fraction XRD scans and quantitative analyses revealed layer collapse, permanent K fixation, and decreased expandability in mixed layered illite-smectite indicating progressive illite formation, (2) electron microprobe mapping and EDS spot analyses both showed a decrease in Si, increases in Al/Si and K, and no change in Al, and (3) layer charge calculated using EDS spot data increased relative to the control experiments. The control and amended experiments of both sediments displayed little to no change in bulk elemental compositions. Our results indicate that the microbially induced S-I reaction occurred to a relatively small, yet systematic, degree in the amended experiments of both sediment samples and that electron microprobe elemental mapping, EDS spot analyses at the µm-scale, and clay fraction mineralogy can be used to infer this reaction in natural sediments, whereas bulk elemental compositions may not. This research will help provide a path forward in recognizing the microbially induced S-I reaction in natural settings and assist in understanding elemental cycling during early diagenesis, sediment pore fluid overpressures, and fault zone behavior.