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Mineralogy of iron microbial mats from loihi seamount.

Toner BM, Berquó TS, Michel FM, Sorensen JV, Templeton AS, Edwards KJ - Front Microbiol (2012)

Bottom Line: 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 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Soil, Water, and Climate, University of Minnesota-Twin Cities St. Paul, MN, USA.

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 (Fh(SRO)) 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 Fh(SRO) is consistent with magnetic observations that point to non-interacting nanoparticles. PDF analyses of total scattering data provide further evidence for Fh(SRO) 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.

No MeSH data available.


Related in: MedlinePlus

X-ray microprobe fluorescence mapping of an intact Fe/Mn iron microbial mat from Ula Nui. Center maps for Regions 1, 2, and 3 (R1, R2, R3) show total Fe and Mn distributions, while outer maps are tricolor maps that include the redox distribution for Fe(II)/Fe(III), and total Ti (left), as well as Mn(II)/Mn(IV), showing total Ca (right).
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Figure 2: X-ray microprobe fluorescence mapping of an intact Fe/Mn iron microbial mat from Ula Nui. Center maps for Regions 1, 2, and 3 (R1, R2, R3) show total Fe and Mn distributions, while outer maps are tricolor maps that include the redox distribution for Fe(II)/Fe(III), and total Ti (left), as well as Mn(II)/Mn(IV), showing total Ca (right).

Mentions: Iron and Mn oxidation state maps, coupled with total Fe, Mn, Ca, and Ti distributions, were collected for an intact surface crust sample from the Ula Nui site (Figure 2). This sample was frozen shipboard under N2 and analyzed at the synchrotron in the absence of O2. Noticeable, discontinuous black banding can be observed visually in the laminated mats and these bands are represented in the X-ray fluorescence maps as regions of higher Mn fluorescence intensity [e.g., top of Region I (R1) and II (R2) in Figure 2]. XANES fitting of multiple energy maps collected across the Mn K-edge also indicates that these Mn-rich regions are Mn(IV)-oxides. The valence state is confirmed by analysis of Mn K-edge XANES spectra for this sample (data not shown). The majority of the mat profile is then dominated by Fe(III) particles, although a zone dominated by reduced Fe(II) was mapped at the top of R2. Calcium is also enriched by more than an order of magnitude at the top of R2, and these Ca-rich zones envelop the particles in which Fe(II) is present. No micro-EXAFS spectra were collected on this sample and therefore the full speciation of the Fe in discrete regions is unknown, but possible candidate solid-phase Fe species include carbonates, sulfides, or basalt. The Fe(II)-rich particles are likely not basalt given the lack of direct correlation to enrichments in Mn(II) or Ti expected in basaltic glass; there is also no Si as expected for phenocrysts of olivine or feldspar. Instead, carbonates may be forming due to the strong accumulation of Fe(II) and Ca.


Mineralogy of iron microbial mats from loihi seamount.

Toner BM, Berquó TS, Michel FM, Sorensen JV, Templeton AS, Edwards KJ - Front Microbiol (2012)

X-ray microprobe fluorescence mapping of an intact Fe/Mn iron microbial mat from Ula Nui. Center maps for Regions 1, 2, and 3 (R1, R2, R3) show total Fe and Mn distributions, while outer maps are tricolor maps that include the redox distribution for Fe(II)/Fe(III), and total Ti (left), as well as Mn(II)/Mn(IV), showing total Ca (right).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3316996&req=5

Figure 2: X-ray microprobe fluorescence mapping of an intact Fe/Mn iron microbial mat from Ula Nui. Center maps for Regions 1, 2, and 3 (R1, R2, R3) show total Fe and Mn distributions, while outer maps are tricolor maps that include the redox distribution for Fe(II)/Fe(III), and total Ti (left), as well as Mn(II)/Mn(IV), showing total Ca (right).
Mentions: Iron and Mn oxidation state maps, coupled with total Fe, Mn, Ca, and Ti distributions, were collected for an intact surface crust sample from the Ula Nui site (Figure 2). This sample was frozen shipboard under N2 and analyzed at the synchrotron in the absence of O2. Noticeable, discontinuous black banding can be observed visually in the laminated mats and these bands are represented in the X-ray fluorescence maps as regions of higher Mn fluorescence intensity [e.g., top of Region I (R1) and II (R2) in Figure 2]. XANES fitting of multiple energy maps collected across the Mn K-edge also indicates that these Mn-rich regions are Mn(IV)-oxides. The valence state is confirmed by analysis of Mn K-edge XANES spectra for this sample (data not shown). The majority of the mat profile is then dominated by Fe(III) particles, although a zone dominated by reduced Fe(II) was mapped at the top of R2. Calcium is also enriched by more than an order of magnitude at the top of R2, and these Ca-rich zones envelop the particles in which Fe(II) is present. No micro-EXAFS spectra were collected on this sample and therefore the full speciation of the Fe in discrete regions is unknown, but possible candidate solid-phase Fe species include carbonates, sulfides, or basalt. The Fe(II)-rich particles are likely not basalt given the lack of direct correlation to enrichments in Mn(II) or Ti expected in basaltic glass; there is also no Si as expected for phenocrysts of olivine or feldspar. Instead, carbonates may be forming due to the strong accumulation of Fe(II) and Ca.

Bottom Line: 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 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Soil, Water, and Climate, University of Minnesota-Twin Cities St. Paul, MN, USA.

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 (Fh(SRO)) 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 Fh(SRO) is consistent with magnetic observations that point to non-interacting nanoparticles. PDF analyses of total scattering data provide further evidence for Fh(SRO) 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.

No MeSH data available.


Related in: MedlinePlus