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

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

Amorphous iron oxyhydroxides gathered from dormant and actively venting chimneys on the seafloor at Franklin Seamount contain volumetrically significant filamentous microbial features, although apparently non-bacterial agglomerates are also common. Spatial relationships between the two micro-textures suggest that the iron-microbe filaments are either forming subsequent to or together with the agglomerates.

The f(O2) of low temperature vent fluids emanating from hydrothermally active chimneys at Franklin Seamount may be much lower than the surrounding seawater. This is based on the in situ observation that yellow mixed-valence ferrosic hydroxide [Fe3(OH)8], red–orange ferric hydroxide [Fe(OH)3] and light green ferrous trioctahedral smectite appear to be precipitating on the surface of active chimneys coincident with the observation in the samples of fossils of Leptothrix and Gallionella, which prefer low f(O2) habitats. These observations are consistent with investigations into the thermodynamics of low temperature iron systems which show that, with increased cooling by mixing with seawater, the f(O2) of the fluid will drop to extremely low levels. In addition, the measured pH of a vent fluid sample (6.26) obtained from a chimney is similar to the pH of the triple point for Fe2+, Fe3(OH)8 and Fe(OH)3 which is calculated to occur at a log f(O2) of -44. Consideration of the thermodynamics and kinetics of iron in the vent fluid suggests that large amounts of amorphous iron oxyhydroxides are capable of precipitating by a combination of abiotic hydrothermal processes. Microbial mediation, however, possibly played a diagenetic role in transforming ferrosic hydroxides to ferric hydroxides resulting in older chimneys being dominated by filamentous textures.

Comparisons with experiments conducted on iron-rich soils and analyses of fluids gathered from actively venting oxyhydroxide chimneys at other seafloor sites provide supporting evidence of the importance of iron species and phases in the redox equilibria of low temperature iron systems. Comparisons with thermodynamic calculations of mineral equilibria in ancient iron formation suggest that the processes which are forming seafloor oxyhydroxide deposits today can be applied to understanding the origin of ancient BIF. Abundant magnetite and a relative paucity of microfossils in Archean iron formation are consistent with the preservation of the primary anoxic depositional environment. In contrast, hematite and abundant microfossils in Phanerozoic BIF are primarily diagenetic in origin reflecting their aerobic and microbial-rich surroundings that resulted in the oxidation of the initially precipitated ferrosic hydroxides.

Future work is needed to confirm the presence of ferrosic hydroxides or natural green rusts in actively venting low temperature sulfur-free springs by collecting samples of the associated iron hydroxide material in anaerobic chambers.


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