Geophysical investigations documenting enhanced magnetic susceptibility (MS) within the water table fluctuation zone at hydrocarbon contaminated sites suggest that MS can be used as a proxy for investigating microbial mediated iron reduction during intrinsic bioremediation. Here, we investigated the microbial community composition over a 5-year period at a hydrocarbon-contaminated site that exhibited transient elevated MS responses. Our objective was to determine the key microbial populations in zones of elevated MS. We retrieved sediment cores from the petroleum-contaminated site near Bemidji, MN, United States, and performed MS measurements on these cores. We also characterized the microbial community composition by high-throughput 16S rRNA gene amplicon sequencing from samples collected along the complete core length. Our spatial and temporal analysis revealed that the microbial community composition was generally stable throughout the period of investigation. In addition, we observed distinct vertical redox zonations extending from the upper vadose zone into the saturated zone. These distinct redox zonations were concomitant with the dominant microbial metabolic processes as follows: (1) the upper vadose zone was dominated by aerobic microbial populations; (2) the lower vadose zone was dominated by methanotrophic populations, iron reducers and iron oxidizers; (3) the smear zone was dominated by iron reducers; and (4) the free product zone was dominated by syntrophic and methanogenic populations. Although the common notion is that high MS values are caused by high magnetite concentrations that can be biotically formed through the activities of iron-reducing bacteria, here we show that the highest magnetic susceptibilities were measured in the free-phase petroleum zone, where a methanogenic community was predominant. This field study may contribute to the emerging knowledge that methanogens can switch their metabolism from methanogenesis to iron reduction with associated magnetite precipitation in hydrocarbon contaminated sediments. Thus, geophysical methods such as MS may help to identify zones where iron cycling/reduction by methanogens is occurring.
more »
« less
Microbially Induced Anaerobic Oxidation of Magnetite to Maghemite in a Hydrocarbon‐Contaminated Aquifer
Iron mineral transformations occurring in hydrocarbon‐contaminated sites are linked to the biodegradation of the hydrocarbons. At a hydrocarbon‐contaminated site near Bemidji, Minnesota, USA, measurements of magnetic susceptibility (MS) are useful for monitoring the natural attenuation of hydrocarbons related to iron cycling. However, a transient MS, previously observed at the site, remains poorly understood and the iron mineral phases acting as reactants and products associated with this MS perturbation remain largely unknown. To address these unknowns, we acquired mineral magnetism measurements, including hysteresis loops, backfield curves, and isothermal remanent magnetizations on sediment core samples retrieved from the site and magnetite‐filled mineral packets installed within the aquifer. Our data show that the core samples and magnetite packs display decreasing magnetization with time and that this loss in magnetization is accompanied by increasing bulk coercivity consistent with decreased average grain size and/or partial oxidation. Low‐temperature magnetometry on all samples displayed behavior consistent with magnetite, but samples within the plume also show evidence of maghemitization. This interpretation is supported by the occurrence of shrinkage cracks on the surface of the grains imaged via scanning electron microscopy. Magnetite transformation to maghemite typically occurs under oxic conditions, here, we propose that maghemitization occurs within the anoxic portions of the plume via microbially mediated anaerobic oxidation. Mineral dissolution also occurs within the plume. Microorganisms capable of such anaerobic oxidation have been identified within other areas at the Bemidji site, but additional microbiological studies are needed to link specific anaerobic iron oxidizers with this loss of magnetization.
more » « less- NSF-PAR ID:
- 10367792
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 127
- Issue:
- 4
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Summary Magnetic minerals form or alter in the presence of hydrocarbons, making them a potential magnetic proxy for identifying hydrocarbon migration pathways. In this paper we test this idea by magnetically measuring core samples from the Tay Fan in the Western Central Graben in the Central North Sea. In a companion paper, 3D petroleum systems modelling has been carried out to forward model migration pathways within the Tay Fan. Rock magnetic experiments identified a range of magnetite, maghemite, iron sulphides, siderite, goethite and titanohematite, some of which are part of the background signal, and some due to the presence of hydrocarbons. Typical concentrations of the magnetic minerals were ∼10–200 ppm. Importantly, we have identified an increasing presence of authigenic iron sulphides (likely pyrite and greigite) along the identified lateral hydrocarbon migration pathway (east to west). This is likely caused by biodegradation resulting in the precipitation of iron sulphides, however, though less likely, it could alternatively be caused by mature oil generation, which subsequently travelled with the migrating oil to the traps in the west. These observations suggest mineral magnetic techniques could be a rapid alternative method for identifying the severity of biodegradation or oil maturity in core sample, which can then be used to calibrate petroleum systems models.more » « less
-
null (Ed.)The textures of outcrop and near-surface exposures of the massive magnetite orebodies (>90 vol % magnetite) at the Plio-Pleistocene El Laco iron oxide-apatite (IOA) deposit in northern Chile are similar to basaltic lava flows and have compositions that overlap high- and low-temperature hydrothermal magnetite. Existing models— liquid immiscibility and complete metasomatic replacement of andesitic lava flows—attempt to explain the genesis of the orebodies by entirely igneous or entirely hydrothermal processes. Importantly, those models were developed by studying only near-surface and outcrop samples. Here, we present the results of a comprehensive study of samples from outcrop and drill core that require a new model for the evolution of the El Laco ore deposit. Backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to investigate the textural and compositional variability of magnetite and apatite from surface and drill core samples in order to obtain a holistic understanding of textures and compositions laterally and vertically through the orebodies. Magnetite was analyzed from 39 surface samples from five orebodies (Cristales Grandes, Rodados Negros, San Vicente Alto, Laco Norte, and Laco Sur) and 47 drill core samples from three orebodies (Laco Norte, Laco Sur, and Extensión Laco Sur). The geochemistry of apatite from eight surface samples from three orebodies (Cristales Grandes, Rodados Negros, and Laco Sur) was investigated. Minor and trace element compositions of magnetite in these samples are similar to magnetite from igneous rocks and magmatic-hydrothermal systems. Magnetite grains from deeper zones of the orebodies contain >1 wt % titanium, as well as ilmenite oxyexsolution lamellae and interstitial ilmenite. The ilmenite oxyexsolution lamellae, interstitial ilmenite, and igneous-like trace element concentrations in titanomagnetite from the deeper parts of the orebodies are consistent with original crystallization of titanomagnetite from silicate melt or high-temperature magmatic-hydrothermal fluid. The systematic decrease of trace element concentrations in magnetite from intermediate to shallow depths is consistent with progressive growth of magnetite from a cooling magmatic-hydrothermal fluid. Apatite grains from surface outcrops are F rich (typically >3 wt %) and have compositions that overlap igneous and magmatic-hydrothermal apatite. Magnetite and fluorapatite grains contain mineral inclusions (e.g., monazite and thorite) that evince syn- or postmineralization metasomatic alteration. Magnetite grains commonly meet at triple junctions, which preserve evidence for reequilibration of the ore minerals with hydrothermal fluid during or after mineralization. The data presented here are consistent with genesis of the El Laco orebodies via shallow emplacement and eruption of magnetite-bearing magmatic-hydrothermal fluid suspensions that were mobilized by decompression- induced collapse of the volcanic edifice. The ore-forming magnetite-fluid suspension would have rheological properties similar to basaltic lava flows, which explains the textures and presence of cavities and gas escape tubes in surface outcrops.more » « less
-
SUMMARY Anisotropy of remanent magnetization and magnetic susceptibility are highly sensitive and important indicators of geological processes which are largely controlled by mineralogical parameters of the ferrimagnetic fraction in rocks. To provide new physical insight into the complex interaction between magnetization structure, shape, and crystallographic relations, we here analyse ‘slice-and-view’ focused-ion-beam (FIB) nano-tomography data with micromagnetic modelling and single crystal hysteresis measurements. The data sets consist of 68 magnetite inclusions in orthopyroxene (Mg60) and 234 magnetite inclusions in plagioclase (An63) were obtained on mineral separates from the Rustenburg Layered Suite of the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction was used to determine the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. Hysteresis loops were calculated using the finite-element micromagnetics code MERRILL for each particle in 20 equidistributed field directions and compared with corresponding hysteresis loops measured using a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. In plagioclase the ratio of remanent magnetization to saturation magnetization (Mrs/Ms) for both model and measurement agree within 1.0 per cent, whereas the coercivity (Hc) of the average modelled curve is 20 mT lower than the measured value of 60 mT indicating the presence of additional sources of high coercivity in the bulk sample. The VSM hysteresis measurements of the orthopyroxene were dominated by multidomain (MD) magnetite, whereas the FIB location was chosen to avoid MD particles and thus contains only particles with diameters <500 nm that are considered to be the most important carriers of palaeomagnetic remanence. To correct for this sampling bias, measured MD hysteresis loops from synthetic and natural magnetites were combined with the average hysteresis loop from the MERRILL models of the FIB region. The result shows that while the modelled small-particle fraction only explains 6 per cent of the best fit to the measured VSM hysteresis loop, it contributes 28 per cent of the remanent magnetization. The modelled direction of maximal Mrs/Ms in plagioclase is subparallel to [001]plag, whereas Hc does not show a strong orientation dependence. The easy axis of magnetic remanence is in the direction of the magnetite population normal to (150)plag and the maximum calculated susceptibility (χ*) is parallel to [010]plag. For orthopyroxene, the maximum Mrs/Ms, maximum χ* and the easy axis of remanence is strongly correlated to the elongation axes of magnetite in the [001]opx direction. The maximum Hc is oriented along [100]opx and parallel to the minimum χ*, which reflects larger vortex nucleation fields when the applied field direction approaches the short axis. The maximum Hc is therefore orthogonal to the maximum Mrs/Ms, controlled by axis-aligned metastable single-domain states at zero field. The results emphasize that the nature of anisotropy in natural magnetite does not just depend on the particle orientations, but on the presence of different stable and metastable domain states, and the mechanism of magnetic switching between them. Magnetic modelling of natural magnetic particles is therefore a vital method to extract and process anisotropic hysteresis parameters directly from the primary remanence carriers.more » « less
-
null (Ed.)Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere. International Ocean Discovery Program (IODP) Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel. Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 18 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access this zone. Active conduits, resulting from fractures in the forearc, are prompted by along- and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes. Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth. Their positions adjacent to or atop fault scarps on the forearc are likely related to the regional extension and vertical tectonic deformation in the forearc. Serpentinite mudflows at these volcanoes include serpentinized forearc mantle clasts, crustal and subducted Pacific plate materials, a matrix of serpentinite muds, and deep-sourced formation fluid. Mud volcanism on the Mariana forearc occurs within 100 km of the trench, representing a range of depths and temperatures to the downgoing plate and the subduction channel. These processes have likely been active for tens of millions of years at this site and for billions of years on Earth. At least 10 active serpentinite mud volcanoes have been located in the Mariana forearc. Two of these mud volcanoes are Conical and South Chamorro Seamounts, which are the furthest from the Mariana Trench at 86 and 78 km, respectively. Both seamounts were cored during Ocean Drilling Program (ODP) Legs 125 and 195, respectively. Data from these two seamounts represent deeper, warmer examples of the continuum of slab-derived materials as the Pacific plate subducts, providing a snapshot of how slab subduction affects fluid release, the composition of ascending fluids, mantle hydration, and the metamorphic paragenesis of subducted oceanic lithosphere. Data from the study of these two mud volcanoes constrain the pressure, temperature, and composition of fluids and materials within the subduction channel at depths of about 18 to 19 km. Understanding such processes is necessary for elucidating factors that control seismicity in convergent margins, tectonic and magma genesis processes in the forearc and volcanic arc, fluid and material fluxes, and the nature and variability of environmental conditions that impact subseafloor microbial communities. Expedition 366 centered on data collection from cores recovered from three serpentinite mud volcanoes that define a continuum of subduction-channel processes defined by the two previously cored serpentinite mud volcanoes and the trench. Three serpentinite mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoro) were chosen at distances 55 to 72 km from the Mariana Trench. Cores were recovered from active sites of eruption on their summit regions and on the flanks where ancient flows are overlain by more recent ones. Recovered materials show the effects of dynamic processes that are active at these sites, bringing a range of materials to the seafloor, including materials from the lithosphere of the Pacific plate and from subducted seamounts (including corals). Most of the recovered material consists of serpentinite mud containing lithic clasts, which are derived from the underlying forearc crust and mantle and the subducting Pacific plate. Cores from each of the three seamounts drilled during Expedition 366, as well as those from Legs 125 and 195, include material from the underlying Pacific plate. A thin cover of pelagic sediment was recovered at many Expedition 366 sites, and at Site U1498 we cored through serpentinite flows to the underlying pelagic sediment and volcanic ash deposits. Recovered serpentinites are largely uniform in major element composition, with serpentinized ultramafic rocks and serpentinite muds spanning a limited range in SiO2 , MgO, and Fe2 O3 compositions. However, variation in trace element composition reflects pore fluid composition, which differs as a function of the temperature and pressure of the underlying subduction channel. Dissolved gases H2 , CH4 , and C2 H6 are highest at the site furthest from the trench, which also has the most active fluid discharge of the Expedition 366 serpentinite mud volcanoes. These dissolved gases and their active discharge from depth likely support active microbial communities, which were the focus of in-depth subsampling and preservation for shore-based analytical and culturing procedures. The effects of fluid discharge were also registered in the porosity and GRA density data indicated by higher than expected values at some of the summit sites. These higher values are consistent with overpressured fluids that minimize compaction of serpentinite mud deposits. In contrast, flank sites have significantly greater decreases in porosity with depth, suggesting that processes in addition to compaction are required to achieve the observed data. Thermal measurements reveal higher heat flow values on the flanks (~31 mW/m2) than on the summits (~17 mW/m2) of the seamounts. The new 2G Enterprises superconducting rock magnetometer (liquid helium free) revealed relatively high values of both magnetization and bulk magnetic susceptibility of discrete samples related to ultramafic rocks, particularly in dunite. Magnetite, a product of serpentinization, and authigenic carbonates were observed in the mudflow matrix materials. In addition to coring operations, Expedition 366 focused on the deployment and remediation of borehole casings for future observatories and set the framework for in situ experimentation. Borehole work commenced at South Chamorro Seamount, where the original-style CORK was partially removed. Work then continued at each of the three summit sites following coring operations. Cased boreholes with at least three joints of screened casing were deployed, and a plug of cement was placed at the bottom of each hole. Water samples were collected from two of the three boreholes, revealing significant inputs of formation fluids. This suggests that each of the boreholes tapped a hydrologic zone, making these boreholes suitable for experimentation with the future deployment of a CORK-lite. An active education and outreach program connected with many classrooms on shore and with the general public through social media.more » « less
