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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 » Introduction

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

Amorphous iron oxyhydroxides combined with variable amounts of manganese oxyhydroxides and silica are common in the oceans. [1-3] but have not been as extensively studied as sulfide deposits. They are typically found in areas of volcanic activity such as at mid-ocean and back-arc spreading centers and on intraplate seamounts. Some are spatially associated with sulfide deposits, usually around the margins and, from their high base metal content, are clearly oxidation products of the sulfides. Others, the subject of this paper, were precipitated as oxides directly from hydrothermal fluids. These can form large deposits up to hundreds of meters across. They are found both in proximity to sulfide deposits and completely isolated from them. A distinctive characteristic is that, regardless of their location, these primary oxide deposits have a much lower content of base (except iron) and precious metals than those that have formed by oxidation of sulfides [3].

Primary oxyhydroxide deposits cover extensive > 100 m2 areas of Franklin Seamount at 2143–2366 m water depth in Western Woodlark Basin, Papua New Guinea (Fig. 1). [4,5] The basaltic andesite volcano straddles an active spreading center propagating westward into the Papua New Guinea continental margin. [6,7] During the Soviet Union–Papua New Guinea–Australia–Canada (SUPACLARK) expedition in 1990, numerous actively venting oxyhydroxide chimneys and mounds were observed and sampled using the submersible Mir. Analysis of samples gathered from the Franklin Seamount deposits indicate they consist dominantly of amorphous iron oxyhydroxide (two-XRD-line ferrihydrite), locally contain major amounts of birnessite and nontronite, variable amounts of hydrothermal opal-A, variable amounts of incorporated volcanic and biologically-derived detritus, and minor vernadite and todorokite. [8] No authigenic mixed-valence or ferrous iron is preserved in the samples.

The observation of the active creation of these chimneys together with their distinctive chemical composition characterized by a paucity of hydrothermally immobile elements such as Al, Ti and Zr (ref. [3]) suggests a primary abiotic hydrothermal control on the formation of these types of deposits. Conversely, close examination of similar materials by Juniper and Fouquet,[9] Alt[10] and Fortin[11] found they possess complex filamentous micro-textures of apparent bacterial origin. The central role of filamentous bacteria in the precipitation of iron in acid mine drainage environments,[12] subterranean environments[13] and acidic surface hot springs[14] is well known. Fortin and Ferris[15,16] discussed the capability of bacteria to provide nucleation sites favorable for iron and manganese deposition. However, the importance of such sites, whether or not due to active metabolic processes, are apt to diminish under the near-neutral pH conditions of warm seafloor springs because kinetic barriers to the chemical precipitation of iron are greatly reduced at higher pH. [17] In general, both the microbial and abiotic chemical processes that occur at seafloor vent sites need to be considered in relation to the genesis of large amorphous iron oxyhydroxide deposits because the free energy of crystal nucleation (and thus precipitation) is constrained thermo-dynamically by the bulk free energy of the solution and the interfacial free energy of available surfaces. [16,17] Their study is important because such marine deposits are similar in their chemistry and geological setting to many ancient iron formations found on land. [8,18]

Iron, because of its multi-valent nature, is a sensitive indicator of its redox environment thus its behavior at the seafloor sediment/water interface can serve as a tool to understanding the physicochemical conditions for the precipitation of the oxyhydroxides at Franklin Seamount. This paper presents mineralogical, chemical and morphological information on the oxyhydroxides from Franklin Seamount that is interpreted in light of the thermodynamics and kinetics of low temperature hydrothermal iron systems, systematics of vent fluid chemistry and previous research on bacterial processes and habitats. These interpretations are used to explain the precipitation of iron locally around the vent site and to make comparisons with analogous environments associated with the precipitation of iron such as in other seafloor locations, soils and ancient iron formations.


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