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This paper presents mineralogical, chemical and morphological information on the oxyhydroxides from …


Biology Articles » Geobiology » Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea » Description of samples

Description of samples
- Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea

The actively venting oxyhydroxide chimneys at Franklin Seamount, as seen in situ from a submersible, commonly have a yellowish appearance with reddish orange patches that become more common towards their inactive bases (Fig. 2a, see also Fig. 5a,b in ref. [5]). These observations have been corrected for the color changes caused by the longer wavelengths in the visible light spectrum being filtered out by seawater. Chimney samples removed from the water were observed to change to dark brown and reddish hues on the deck of the ship within a few hours of exposure to the atmosphere (Fig. 2b, see also Fig. 5d, 6a,c in ref. [5]).

The samples are very friable with a roughly laminated to stromatolitic to porous, clotted texture (Fig. 2b). Amorphous iron oxyhydroxides are commonly coated by the black Mn oxyhydroxides, birnessite and todorokite. Dark green non-tronite is found in the interiors of some samples although distinctive bright green patches were also observed from the submersible on the surface of an actively forming chimney near the vent orifice. [5] These bright green patches are suggestive of ferrous trioctahedral smectite, a reduced precursor to non-tronite. [20]

In thin section, the amorphous iron oxyhydroxide exhibits a complex mottled structure of hair-like strands or scattered circular patches together with filaments of probable biological origin that appear to emanate from the strands and patches (Fig. 2c,d). The filaments have a sinuous, apparently branching, morphology (Fig. 2e), or have the appearance of bunched streamers (Fig. 2f). The textures in Fig. 2 suggest that the iron-rich material forms a coating.

SEM photographs clearly show the distinctive, apparently branching, filamentous micro-texture (Fig. 3a). The filaments are 5–15 μm long and 0.8 to 2 μm wide but thicken to up to 5 μm where coated with silica (Fig. 3b). Less common are some filament types that appear to be bunched into parallel strands (Fig. 3c) or short twisted and braided stalks (Fig. 3d). Fig. 3c,d and eof samples from an actively venting chimney also show that much of the iron oxyhydroxide appears to consist of formless agglomerates made up of irregularly shaped blebs which surround the filaments. In some samples, filaments appear to have grown out of the agglomerates (Fig. 3f,g).

It is difficult to determine if samples containing voluminous apparently non-bacterial agglomerates are just masked filamentous textures. This is probably not the case because the observed filaments in such samples do not appear to be especially thickened by additional iron oxyhydroxide deposition (Fig. 3a,b,c,d,e,f,g). Filaments in some samples appear to be degraded making their distinction from the formless micro-textures uncertain (Fig. 3h). However, Fig. 3d,e,f,g show non-degraded filaments adjacent to the formless blebs. These images are different from that exhibited in Fig. 3h. No evidence was found for formless micro-textures replacing degraded filaments.

Examination of the samples at higher magnifications by TEM reveals the sinuous pseudo-branching habit of the filaments identified by SEM (Fig. 4c) as well as several other features. Filaments have a hollow structure with rough walls consisting of bands of amorphous iron oxyhydroxide parallel to the long axis (Fig. 4d). The twisted stalks observed by SEM (Fig. 3d) are made up of <0.5 μm interwoven strands (Fig. 4e). Amorphous iron oxyhydroxide coats the walls of some filaments (Fig. 4f) but most of the individual filaments contain relatively little additional iron deposition on their walls so it is thought that such iron is accidental.

In summary, the amorphous iron oxyhydroxide in the samples displays a variety of morphologies that can be divided into three groups: (1) morphologies which appear to form or mold the walls of filamentous and/or stalked structures (Fig. 3a,b,c,d, and 4c,d,e); (2) morphologies which form discrete irregularly shaped formless agglomerates (Fig. 3c,d,e,f); and (3) morphologies which appear to have accumulated on the walls of the filaments and stalks (Fig. 4f). The first two groups are more abundant but their proportions are highly variable from sample to sample. The second group predominates in freshly precipitated material sampled from the actively venting part of a chimney, whereas nearly all of the material appears to be filamentous in samples from other parts of active chimneys and from inactive chimneys.


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