Geology topics

Estella A. Atekwana

Publications and source records attributed to Estella A. Atekwana.

4 recordsLinked to original sources

USGS2022

The 180-km-long Meers-Willow Fault System in the Southern Oklahoma Aulacogen: A potential U.S. mid-continent seismic hazard

We integrate new high-resolution aeromagnetic data with seismic reflection data, well logs, satellite remote sensing, and field observations to provide a regional view of buried and exposed structures in the Southern Oklahoma Aulacogen and to assess their potential for future seismicity. Trends ranging from NW−SE to ∼E−W, peaking at 330° ± 4.5° and 280° ± 3°, dominate the magnetic lineaments of the Southern Oklahoma Aulacogen, reflecting basement contacts, dikes, and faults, including a previously unmapped ∼100-km-long basement fault, which is herein referred to as the Willow fault. The fault disrupts, truncates, and vertically offsets basement-related seismic reflectors and overlying Paleozoic strata up through the Permian reflectors. Surface deformation along the trend includes fault-parallel monoclinal folds, pervasive fractures, and fracture-hosted mud dikes in Permian evaporite units. These structures indicate a Permian or post-Permian reactivation of the fault. Along-strike, the Willow fault connects to the NW-trending, seismically active Meers Fault to comprise the ∼180-km-long Meers-Willow fault system, which potentially represents a major seismic hazard along the Southern Oklahoma Aulacogen. Fault slip potential analyses of the mapped potential fault traces show that seismic hazards are elevated where faults have steeper dips. Given some uncertainty in the regional stress state, we also show that hazards along the NW−SE to E−W trending faults vary considerably within the uncertainty range. We propose that the Meers-Willow fault system originated as a Cambrian aulacogen-scale, basement-rooted fault that was later reactivated as a left-lateral strike-slip fault (with ∼40 km displacement) during the late Paleozoic Ancestral Rocky Mountain orogeny, highlighting that lateral offset accommodated a major component of deformation during the orogen.

Oklahoma↗

USGS2022

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.

Minnesota↗

USGS2021

Methanogens and their syntrophic partners dominate zones of enhanced magnetic susceptibility at a petroleum contaminated site

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.

Frontiers in Earth Science↗

USGS2017

Evidence of coupled carbon and iron cycling at a hydrocarbon-contaminated site from time lapse magnetic susceptibility

Conventional characterization and monitoring of hydrocarbon (HC) pollution is often expensive and time-consuming. Magnetic susceptibility (MS) has been proposed as an inexpensive, long-term monitoring proxy of the degradation of HC. We acquired repeated down hole MS logging data in boreholes at a HC-contaminated field research site in Bemidji, MN, USA. The MS data were analyzed in conjunction with redox conditions and iron availability within the source zone to better assess whether MS can serve as a proxy for monitoring HC contamination in unconsolidated sediments. The MS response at the site diminished during the sampling period, which was found to coincide with depletion of solid phase iron in the source zone. Previous geochemical observations and modeling at the site suggest that the most likely cause of the decrease in MS is the transformation of magnetite to siderite, coupled with the exhaustion of ferrihydrite. Although the temporal MS response at this site gives valuable field-scale evidence for changing conditions of iron cycling and stability of iron minerals it does not provide a simple proxy for long-term monitoring of biodegradation of hydrocarbons in the smear zone.

Minnesota↗