Abstract
Extensive mats of Fe oxyhydroxides and associated Fe-oxidizing microbial organisms form in diverse geochemical settings – freshwater seeps to deep-sea vents – where ever opposing Fe(II)-oxygen gradients prevail. The mineralogy, reactivity, and structural transformations of Fe oxyhydroxides precipitated from submarine hydrothermal fluids within microbial mats remains elusive in active and fossil systems. In response, a study of Fe microbial mat formation at the Loihi Seamount was conducted to describe the physical and chemical characteristics of Fe-phases using extended X-ray absorption fine structure spectroscopy, powder X-ray diffraction, synchrotron radiation X-ray total scattering, low-temperature magnetic measurements, and Mössbauer spectroscopy. Particle sizes of 3.5–4.6 nm were estimated from magnetism data, and coherent scattering domain (CSD) sizes as small as 1.6 nm are indicated by pair distribution function (PDF) analysis. Disorder in the nanostructured Fe-bearing phases results in limited intermediate-range structural order: less than that of standard two-line ferrihydrite (Fh), except for the Pohaku site. The short-range ordered natural Fh (FhSRO) phases were stable at 4°C in the presence of oxygen for at least 1 year and during 400°C treatment. The observed stability of the FhSRO is consistent with magnetic observations that point to non-interacting nanoparticles. PDF analyses of total scattering data provide further evidence for FhSRO particles with a poorly ordered silica coating. The presence of coated particles explains the small CSD for the mat minerals, as well as the stability of the minerals over time and against heating. The mineral properties observed here provide a starting point from which progressively older and more extensively altered Fe deposits may be examined, with the ultimate goal of improved interpretation of past biogeochemical conditions and diagenetic processes.
Highlights
Iron (Fe) microbial mats form in opposing iron and oxygen gradients in a wide variety of settings (Emerson and Revsbech, 1994; Chan et al, 2004; Rentz et al, 2007; Druschel et al, 2008; Edwards et al, 2011)
Point measurements from different particle aggregates within the mat samples indicate that Fe speciation was homogeneous at a spatial scale of 10 μm2
Comparison between the microprobe and bulk Fe extended X-ray absorption fine structure (EXAFS) spectra for these mat flocs shows that the Fe speciation measured at the microscale is representative of the bulk Fe speciation (Figure 1; Table 2)
Summary
Iron (Fe) microbial mats form in opposing iron and oxygen gradients in a wide variety of settings (Emerson and Revsbech, 1994; Chan et al, 2004; Rentz et al, 2007; Druschel et al, 2008; Edwards et al, 2011). Structural and compositional variability and complexity is intrinsic to Fe oxyhydroxides formed in settings such as microbial biofilms (Edwards et al, 2003; Chan et al, 2004; Toner et al, 2009) or in the presence of strongly sorbing inorganic and organic ligands (Cornell and Schwertmann, 1979; Waychunas et al, 1996; Rose et al, 1997; Vilge-Ritter et al, 1999; Doelsch et al, 2003; Mikutta, 2011). One viable strategy for describing Fe microbial mat mineralogy is to measure a suite of fundamental properties: (1) short-range structural order and valence state; (2) degree of structural defects relative to standard synthetic minerals; (3) degree of particle interaction and aggregation; www.frontiersin.org
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
