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

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


We postulate that, at the low temperatures of the vent fluids, chemical reactions involving iron phases are controlled by reaction kinetics which slow the attainment of true equilibria in these systems. In order to account for this thermodynamically, Lindsay[24] and Ponnamperuma et al. [25] used the standard free energies of formation for unstable hydroxides rather than their crystalline counterparts when modeling the behavior of iron in soils at a temperature and pH similar to those of the vent fluids at Franklin Seamount. This is because, according to the Oswald step rule, an unstable phase does not necessarily transform directly to a stable phase, but may pass through successive intermediate phases due to there being lower activation energy barriers via that route. These intermediate unstable phases are preserved in the form of amorphous iron oxyhydroxides in both soils[26] and in the submarine environment at Franklin Seamount. Iron has a greater solubility relative to amorphous iron oxyhydroxides than to their crystalline counterparts but the higher reactivity of the former results in their preferential precipitation from hydrothermal solutions. [17]

For the purposes of thermodynamic modeling, Fe(OH)3 or ferric hydroxide is used to describe unstable amorphous ferric oxyhydroxides or two-XRD-line ferrihydrite. This material is well characterized in the literature and displays a variety of brown to red colors depending on its crystallinity. [27] The true composition of ferric hydroxide is uncertain and variable depending upon its crystallinity with proposed examples such as Fe5HO8·4H2O (ref. [28]), Fe5(O4H3)3 (ref. [29]), 5Fe2O3·9H2O (ref. [27]), Fe4(O,OH,H2O)12 (ref. [30]) and VIFe2.9IV (Si,Fe,Al)1.3-(O,OH,H2O)12 (ref. [8]).

Fe3(OH)8 is used to describe a mixed-valence iron compound named ferrosic hydroxide. [25,26,31] It is variable in color ranging from yellow to olive green to green-blue in color[25,31,32] and is a major constituent in reduced iron-rich carbonate-poor soils. [26,33] Ferrosic hydroxide is believed to be the unstable precursor to magnetite. [25] Ferrosic hydroxide belongs to a variable and wide range of mixed valence compounds named green rusts for which general compositions have been proposed which vary from Fe2(OH)5 (ref. [32]) to (Fe11(1 - x)Fe111x-(OH)2)x+ (x/nAn- m/nH2O)x-, (ref. [34]) where A can be the anions OH-, Cl-, CO3 2- or SO4 2-.

At Franklin Seamount, the dominance of amorphous iron oxyhydroxide was demonstrated by XRD and electron diffraction combined with bulk chemical and spot EDS analyses. [8] Knowledge of the composition of the vent fluid at Franklin Seamount (Table 1, ref. [5]) provides an opportunity to place constraints on the physico-chemical environment of the iron precipitation assuming the fluid and the minerals are in equilibrium.

Table 2 is a compilation of some of the available thermodynamic data applicable to low temperature Fe–Si–O–H systems and Table 3 lists the equations used to model the system at 25°C and 220 bar (the pressure at 2200 m depth) at Franklin Seamount. There is considerable variability in the tabulated free energies of oxyhydroxide and oxide phases (Table 2). Experimental solubility measurements suggest that differences in particle size and crystallinity can account for as much as several kcal mole- 1 in their free energy of formation. [35] Langmuir[36] estimated that free energy decreased with increase in grain size consistent with the trend of decreasing free energy from -169.25 to -170.40 to -177.85 kcal mole-1 and decreasing solubility of iron in water among materials of increasing crystallinity for Fe(OH)3 (amorphous iron hydroxide), Fe(OH)3 (in soil) and hematite, respectively. [24,26] Fig. 6 shows the great difference in the calculated position of the phase boundary between Fe(OH)3 and Fe3(OH)8 from 10-44 to 10-68 bar f(O2) at Franklin Seamount, a range of 24 orders of magnitude depending upon the type of ferric iron hydroxide used and/or source of the data!

When choosing the appropriate thermodynamic parameters for the amorphous iron oxyhydroxides at Franklin Seamount, it should be noted that these phases are actively precipitating from hydrothermal fluids and thus are very fresh when initially deposited. Accordingly, the free energy values chosen to model the formation of ferrosic hydroxide (Table 2) were those obtained from the laboratory experiments of Arden,[31] as these conditions are most similar to the vent conditions on the seafloor. The solubility product for this fresh ferrosic hydroxide was obtained by potentiometric titration. The phase was precipitated at room temperature from an initial ferrous iron solution under rising pH, between 6.4–6.8, with the addition of a ferric sulphate solution. Hansen et al. [37] repeated the experiment, obtained a similar titration curve and the material formed was identified as sulphate-interlayed green rust. Frederickson et al. [38] concluded that the optimal pH range for the development of green rust is between 6.5 to 7.5. These conditions are similar to those in the low temperature vent fluid at Franklin Seamount so this value is incorporated in the thermodynamic database for soil phases given in Table 2 by Sadiq and Lindsay. [39] Free energy values for ferric hydroxide Fe(OH)3 in the Lindsay database, such as those of Schindler et al. [40] and Sadiq and Lindsay[39] were not considered because these were obtained on aged material whose conditions of formation are far removed from the vent conditions at Franklin Seamount.

Fig. 6 demonstrates that the fresher the Fe(OH)3 in equilibrium with fresh Fe3(OH)8, the higher the f(O2) of the boundary between the two compounds. Since the free energy of amorphous materials has a negative correlation with grain size and because freshly precipitated primitive crystallites agglomerate together and grow in size with aging, it would be expected that the material's free energy would decrease as it becomes older. The free energy of formation provided by Langmuir [36] for Fe(OH)3 in Table 2 was obtained from solubility experiments conducted on a freshly precipitated phase and it has the least negative free energy among the Fe(OH)3 compounds shown. For this reason, this value best matches the precipitation of amorphous ferric oxyhydroxides at Franklin Seamount. Dzombak and Morel[41] reported that freshly precipitated hydrous ferric oxyhydroxides are made up of spherical particles 10–100 Å in size, similar to samples from the actively precipitating vents at Franklin Seamount described by Boyd and Scott. [8]

In summary, the free energy database of Sadiq and Lindsay,[39] excepting the value for fresh Fe(OH)3 from Langmuir[36] substituting for the value of the 4 day old material, is used here to model the mineral equilibria of amorphous iron oxyhydroxides at an actively venting chimney at Franklin Seamount as shown in Table 3.


Silicon makes up on average 12.2 wt.% of the bulk samples and 7.0 wt.% of ferrihydrite from Franklin Seamount (the content of the iron end-member sample, M2192-loc. 1 106848, Boyd and Scott 1999, ref. [8]) so the element must be incorporated into the thermodynamic model of the vent sites. Based on XRD patterns of all the collected samples,[8] excepting two, discrete silica mineralization of hydrothermal origin appears to be minor. However, as a caution Cremer[42] found for the oxyhydroxide deposits at Loihi Seamount that the XRD response to amorphous silica is commonly masked by the iron oxyhydroxide. In the two non-conforming samples from Franklin Seamount, amorphous silica mantles iron minerals having filamentous textures (Fig. 3b) in a manner similar to that described by Juniper and Fouquet[9] for samples from Explorer Ridge and the East Pacific Rise. The dynamic depositional environment and low temperature of the vent fluids at Franklin Seamount promote the precipitation of unstable amorphous silica rather than quartz in a similar fashion to the behavior of iron.

In general, the relative scarcity of amorphous silica phases at Franklin Seamount can be explained by the silicon content of the vent fluid (Table 1), which is calculated to be under-saturated with respect to amorphous silica by a factor of 5 at a temperature of 25°C. The presence of nontronite in the samples at Franklin Seamount, however, suggests that the silicon in the fluid does exert some control on the mineralogy of the deposits. Thermodynamic data for the phase ferrosilite (FeSiO3) in the Winters and Buckley[43] compilation (originally from Helgeson et al. [35] in Table 2 is used to represent nontronite. This is a reasonable substitute provided the iron in nontronite is initially ferrous. Harder[44] synthesized a ferrous trioctahedral smectite precursor to nontronite under Eh-pH conditions similar to those for which ferrosilite was expected, but was not successful in precipitating lembergite at higher Eh or lower pH.

Oxygen and hydrogen

The f(O2) of the vent fluid is not known but is believed to be much lower than that of seawater (0.06 atm at 2000 m depth). [45] End-member hydrothermal vent fluids at >300°C are extremely acidic before mixing with seawater. [46,47] As the fluid cools, whether by mixing with seawater or conduction, the pH steadily increases and element concentrations decrease with the precipitation of phases. [48] Hannington and Scott[49] used mineral equilibria to model the decrease in log f(O2) from -32 to -43 as the fluid cooled from 350 to 180°C at 21°N East Pacific Rise and Axial Seamount. A strong redox buffering capacity between the H2S in the vent fluid and SO4 in seawater was invoked to explain the down-temperature decrease in f(O2) with the increased mixing of the fluid with the oxygenated bottom seawater. The authors suggested that below 180°C, as the H2S content of the fluid dropped with the precipitation of sulfides, the redox buffer broke down.

There are, however, other possible redox buffers which could become important. Reaction between Fe2+ in the fluid and the precipitated iron phases would cause the oxygen fugacity of the evolving vent fluid to continue to fall with increased mixing. The equations in Table 3 demonstrate that for a fluid precipitating an iron phase, such as shown by the univariant boundaries in Fig. 6, a rise in pH due to mixing with seawater is accompanied by a concurrent drop in f(O2). The fluid would be held or buffered to that univariant line providing the Fe2+ is maintained in or close to mineral equilibria with the precipitating phases. These mineral equilibria would be maintained as long as the active vent was constantly replenishing ferrous iron to the system.

There have been no direct measurements of oxygen or Eh of vent fluids on the seafloor. However, it is likely that vent fluids at Franklin Seamount are considerably more reduced than seawater despite possessing no H2S and being more than 90% diluted by seawater. Bowers et al. [48] thermodynamically modeled the cooling of end-member hydrothermal fluids by mixing with seawater and found that log f(O2) steadily falls to below -70 at 25°C. The f(O2) then climbs sharply back up in response to the ambient seawater flooding the system. The calculations indicate that all the sulfides are precipitated between 300 and 150°C and yet the f(O2) continues to drop at temperatures below 150°C. Bowers et al. [50] analysed the composition of high temperature vent fluids obtained from 11–13°N and 21°N and found that a 50% decrease in iron in solution accompanied by a pH rise can be accounted for by a 15°C drop in temperature and the buffering of the solution by greenschist-facies mineral assemblages. The authors found that vent fluid compositions at 21°N were uniform with respect to time, based upon expeditions in 1979, 1981 and 1985, and concluded that the compositions of vent fluids and especially the iron are controlled by equilibria with solid phases.

In conclusion, the low temperature redox conditions at Franklin Seamount are consistent with results of studies of higher temperature seafloor hydrothermal systems. These results support the contention that iron participates in the redox balance between the vent fluid and its precipitated products, and are consistent with iron phases commonly being the dominant mineralization in both high and low temperature modern hydrothermal deposits.

Carbon and sulfur

No analyses of carbon in the vent fluid are available from Franklin Seamount, however, the pH drifted from 6.1 to 6.26 when exposed to the atmosphere indicating probable exsolution of CO2 from the fluid. [5] Based on the estimate that P(CO2) of surface seawater in equilibrium with the atmosphere is 3.3 × 10-4 bar, then doubling the P(CO2) will lower the pH by 0.28 units. [51] A drift of 0.16 pH units therefore is consistent with an estimated P(CO2) of 5.2 × 10-4 bar for the vent fluid providing its overall composition is not considerably different from seawater, as is the case (Table 1). A P(CO2) of at least 10-2 bar is required before iron carbonates such as siderite will start to precipitate in the hydrothermal system at Franklin Seamount. This is supported by the SEM and TEM examination of the samples that showed an absence of any hydrothermal carbonate minerals.

No H2S was detected in the vent fluid so sulfur was not incorporated into the thermodynamic calculations. The low level of sulfur in the system is consistent with the absence of hydrothermal sulfur bearing minerals in the samples.

Kinetic parameters

Kinetic constraints on the hydrothermal precipitation of iron have been invoked by many researchers in proposing microbial mediation as an alternative explanation for the formation of voluminous iron oxyhydroxides on the seafloor. [10,21] The rate of oxidation of ferrous iron in low temperature hydrothermal solutions is very slow. For example, at pH = 5 and P(O2) = 0.06 bar (2000 m depth in seawater), the half-life for the oxidation of Fe2+ to Fe3+ is 30 years. [10] The ferrous iron oxidation rate constant (k) in seawater, determined by Millero et al.,[52] as a function of temperature (T/K) and ionic strength (I) is:

log k = 21.56 - 1545/T - 3. 291(I) 1/2 + 1.52(I)     (1)

Eqn. (1) is incorporated into an equation used to calculate the oxidation rate of iron as a function of P(O2), pH and the concentration of Fe2+ in the fluid:[52]

-d[Fe2+]/dt = k[Fe2+][O2][OH-]2     (2)

The activity coefficient of H2O in seawater at 25°C and less than 1 kbar pressure is 0.98, the difference from unity being a function of ionic strength, so for the purposes of these calculations fugacity and activity are assumed to equal partial pressure and concentration, respectively.

The oxidation rate is extremely sensitive to changes in pH. From eqn. (2), the half-life for the oxidation of ferrous iron is calculated to be 44 min for the pH, temperature and iron concentration of the measured vent fluid at Franklin Seamount (Table 1) if that fluid had an oxygen content equal to that of surrounding seawater with P(O2) of 0.06 bar. This assumes the iron in the fluid is all Fe2+. The half-life decreases to 2.3 min for the pH (7.6) of the bottom water in Woodlark Basin (Table 1). Even at these accelerated rates of oxidation, however, almost all of the iron would be dispersed in the water column by the time the oxidation reactions were completed. Voluminous ferric oxyhydroxides dispersed in plumes have been found above areas of active hydrothermal deposition[53] supporting the notion that most of the vented iron is lost to the seawater column.

Submersible observations during the sampling of the vent fluid at Franklin Seamount suggest that the fluid was exiting at a rate approximately 0.11 s-1 (ref. [54]). This is calculated from an estimated rate of emission of 5 cm s-1 from an orifice approximately 5 cm in diameter located on an oxyhydroxide mound at the base of a meter high spire. Based on the calculated oxidation rate of iron for bottom seawater at a pH of 6.26 and assuming that only the iron oxidized in the first 20 s (the approximate length of time a fluid is within a 1 m high chimney) is precipitated, then only 0.4 wt.% of the iron in the vent fluid would be accumulating within and on the chimneys and mounds. The rest would still be dissolved in the fluid and thus be dispersed by the ocean currents. At these discharge and oxidization rates, about 26 g of iron would be deposited around that single orifice in one year.

This amount is compared to the calculated oxyhydroxide accumulation rates at Loihi Seamount based upon radio-chemical dating reported in Cremer. [42] The average iron accumulation rate at the hydrothermally active Peles Vent area is 9.9 g cm-2 kyr-1 and at the less vigorously venting 1000 Fingers Field is 0.6 g cm-2 kyr. A marker, recovered in the Peles Vent area after six months deployment, was found to have accumulated iron at a rate of 23 g cm-2 kvr-1. [42]

Assuming the precipitates from that single vent at Franklin Seamount cover an area of one square meter, based on submersible observations of the chimneys, the rate of precipitation is too low by about an order of magnitude when compared to the iron accumulation at Peles Vent, but five times higher than at 1000 Fingers.

Other actively venting orifices were seen in the same oxyhydroxide mound and slowly emitting water was observed from numerous smaller crevices[54] indicating that the amount of emitted fluid associated with the building of the spire and mound was probably considerably greater than that used in the calculation. Nevertheless, the presence of the large oxyhydroxide deposits at Franklin Seamount suggests that iron is being precipitated in the vent fluid at a much faster rate than that indicated by the kinetic equations. This could occur through: (1) passive or active microbial mediation overcoming the kinetic barriers to ferrous iron oxidation as suggested by previous investigators (e.g., refs. [10,16] and [55]); or (2) by initially precipitated iron acting as a catalyst for the accelerated auto-catalytic hydrothermal deposition of voluminous iron oxyhydroxides. The initial precipitates occurring as mixed-valence ferrosic compounds in the latter process facilitate the decrease in the amount of ferrous iron required to be oxidized to complete the reaction.

Yariv and Cross[56] state that the oxidation of an element is catalyzed by the presence, in suspension of colloidal materials, of that same element, and that the surface of a precipitated element will behave as a seed for additional deposition. This auto-catalytic precipitation will occur at a lower supersatura-tion ratio than that for the element in isolation. As previously mentioned, scanning transmission electron microscope images of amorphous iron oxyhydroxide samples from Franklin Seamount show agglomerated crystallites of < 100 Å diameter at high magnification which suggests that the drawn vent fluid sample contained numerous fine colloidal particles of iron. These particles would be highly reactive to hydrothermal chemical processes.

In summary, kinetic inhibitions are not a major barrier to the initial abiotic chemical oxidation and precipitation of iron at Franklin Seamount. Although the calculated rate of iron oxidation is somewhat too low to explain the large size of the deposits, this rate could be accelerated by auto-catalytic reactions and/or the deposition of mixed valence compounds, as well as by microbial mediation. The predominance of formless textures with no associated bacterial forms in some samples gathered from actively venting chimneys suggests that most of the initial precipitation could be abiotic, however an explanation is required for the dominance of biotic textures in other samples. Nevertheless, calculations show that the kinetic barriers are minimal enough to allow for the formation of seed crystallites that would act as nuclei for the more efficient auto-catalytic reactions to follow and investigation into the kinetic and thermodynamic conditions at the vent sites at Franklin Seamount permit the possibility that microbial processes are not necessary for the oxidation of ferrous iron in the fluid. This legitimizes the examination of hydrothermal explanations for the precipitation of the iron.

Chemical conditions of formation

The sample descriptions and analyses plus an understanding of the microbiology, thermodynamics and kinetics of low temperature vent fluid environments suggest a hydrothermal mechanism is viable for the initial deposition of the Franklin Seamount amorphous iron oxyhydroxides. A geochemical model for the Fe–Si–O–H system, based on the previously discussed chosen values for fresh hydroxides and using the reactions in Table 3, is graphically presented in Fig. 7. As the temperature and f(O2) drop and pH rises in the vent fluid with dilution by seawater, Fe2+ dissolved in the fluid oxidizes and then precipitates as both ferric and ferrosic hydroxide compounds. Fig. 7 shows that the measured pH of 6.26 of the vent fluid (Table 1) is similar to that of the calculated triple point for Fe2+, Fe(OH)3 and Fe3(OH)8 at a log f(O2) = -44 at 25°C. This is consistent with the contention that the precipitation of both ferrosic and ferric iron phases in equilibrium with aqueous ferrous iron appears to be holding the fluid at that triple point. The redox habitats of the iron bacteria Gallionella and Leptothrix (from Fig. 5) plot in the same region. It is possible that some primary microbially mediated oxidation of Fe2+ is aiding the maintenance of the mineral equilibrium between the fluid and the phases since even samples from the most active vents possess some biotic micro-textures (Fig. 3c,d).

Genin et al. [34] constructed similar models using thermodynamic data for various types of green rusts, identified as GR1 and GR2, such as Cl-bearing GR1 (Fe4(OH)8Cl), CO3-bearing GR1 (Fe6(OH)12CO3) and SO4-bearing GR2 (Fe6(OH)12SO4) in equilibrium with lepidocrocite or goethite and Fe2+. They obtained comparable results to this paper when examined with respect to the iron content in the vent fluid at Franklin Seamount (3.9 × 10-5 m). The ferric and ferrosic oxyhydroxides, Fe2+ triple point varies from pH of 7.8 and log f(O2) of -59 for Fe4(OH)8Cl, to a pH of 8.2 and log f(O2) of -64 for Fe6(OH)12CO3, and to a pH of 8.5 and log f(O2) of -70 for Fe6(OH)12SO4. The lower f(O2) and higher pH values in comparison to the results obtained in this paper are mostly due to the choice by Genin et al. of well ordered ferric oxyhydroxide minerals for their model which are not found at Franklin Seamount. [34] This is consistent with the aforementioned observed trend in Fig. 6 of lower f(O2) and higher pH with greater ferric iron oxyhydroxide crystallinity. It is probable, however, that the true composition of ferrosic hydroxide at Franklin Seamount is closer to that of the Cl-bearing than the SO4- or CO3-bearing varieties of green rusts because the former is in much higher concentration in seawater than the latter.

Although Si(OH)4 in the fluid was calculated to be under-saturated with respect to amorphous silica at the sampled vent site, Fig. 7 shows that silica can precipitate as FeSiO3 at a pH higher than 7.7 and lower f(O2) than for Fe3(OH)8. This would occur with increased seawater dilution of the fluid providing the redox balance between Fe2+ and the solid phases is maintained. Such an explanation is consistent with the relative rarity of hydrothermal Fe–Si smectites as opposed to amorphous iron oxyhydroxides in active vents since most of the time the seawater would flood the system and break down the equilibria before the fluid became very reduced.

The initial abiotic deposition of ferric and ferrosic hydroxides would accelerate the precipitation of the same resulting in a greater accumulation of iron than the 26 g yr-1 previously calculated. Tunnicliffe and Fontaine[57] found much of the iron oxyhydroxides at the southern Juan de Fuca vent field were deposited non-specifically as orbs and formless masses suggesting that the oxidation of metals is auto-catalytic.

An understanding of the precipitation of the iron in low temperature systems, however, must account for the evidence of microbial activity at Franklin Seamount and other seafloor oxyhydroxide deposits. It is suggested here that the initially deposited ferrosic hydroxide and ferrous trioctahedral smectite would immobilize the ferrous iron into colloidal particles and thus provide a favorable ferrous substrate for the subsequent growth of iron oxidizing bacteria such as Leptothrix. Most of the filamentous textures observed in the oxyhydroxides probably formed in this fashion. This interpretation is consistent with the dominance of formless iron agglomerates in samples from some of the most active vents and the observation of filaments emanating from the agglomerates (Fig. 3f,g).

The filamentous textures in most samples taken from inactive vents appear very uncontaminated, that is, lacking large amounts of additional deposition of iron on their walls (Figs. 3a,c,d,e,f,g, 4c,d,e). This suggests that the paragenesis of the filaments is relatively late. Additional iron encrustations on sheaths may fill spaces between filaments and eventually coalesce to form irregular iron oxyhydroxide spheres but this texture is uncommon at Franklin Seamount. Difficulties in the identification of filamentous textures arise due to their degradation (Fig. 3h), rather than due to additional iron deposition obscuring the shapes.

This explanation invokes a mostly diagenetic role for the bacteria in the formation of the preserved ferric oxyhydroxides. The ferrosic hydroxides deposited within the relatively aerobic environment of the seawater would also be undergoing abiotic oxidation and transformation to ferric phases. The presence of ferrosic hydroxide on the seafloor is consistent with the observation of the yellowish actively forming deposits at Franklin Seamount. The change to a patchy orange-red coloration in some in situ chimneys on the seafloor is indicative that this transformation, whether microbial or abiotic driven, is occurring rapidly after deposition. Indications of the presence of ferrosic hydroxides therefore would only be found in samples from the most hydrothermally active areas. Iron oxyhydroxide samples obtained from older seafloor deposits would be dominated by filaments. The few hours it took for the gathered samples to darken and turn red on the ship's deck are consistent with oxidation rates of ferrous iron at an atmospheric oxygen concentration and neutral pH. [58]

Comparisons can be made to investigations into mixed valence hydroxides in soil environments. Ponnamperuma et al.25 stated that reduced soils which are high in iron and low in both sulfur and organic matter have a grey to green color similar to that of ferrosic hydroxide. Trolard et al. [32] reported that a waterlogged soil containing green rust once exposed to air was observed to change color rapidly over a number of stages from bluish-grey to greenish-grey to olive to grey over one day. Lindsay and Sadiq[59] and Genin[34] concluded, based on their own experiments and others studies, that mixed-valence iron hydroxide acts as a control on many aqueous systems even though the compound is thermodynamically unstable and very reactive in ambient conditions.

In summary, Fig. 7 presents the thermodynamic and microbial environment at Franklin Seamount. Iron oxyhydroxides at Franklin Seamount are formed as a result of a complex interaction of microbial, abiotic, syngenetic and diagenetic processes occurring at the seawater/solid interface. This study demonstrates the importance of avoiding the rigid application of abiotic vs. microbial explanations. The complexity of the possible interrelationships between biotic and hydrothermal processes at an iron oxyhydroxide chimney is illustrated in Fig. 8. It should be noted, however, at Franklin Seamount the chemical hydrothermal processes appear to be robust enough that the absence of microbiological activity would slow but not prevent the formation of the deposit. This has implications for understanding the genesis of ancient iron oxide deposits that are interpreted to have formed in an analogous setting.



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