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ates. Elements that form carbonate complexes in seawater behave independently from
those that form hydroxide complexes, which indicates their different modes of removal
from seawater onto crust surfaces 113. Very slow growth rates promote enrichment of
minor elements by allowing time for extensive scavenging by the major oxyhydroxides.
Accretion of oxyhydroxides will be slower where the OMZ intersects the seafloor than it
will be above and below that zone, because manganese is more soluble in low-oxygen
seawater (Fig. 2). Crusts exposed at the seafloor may not necessarily be actively accreting
oxyhydroxides 114 because of mechanical erosion or, less commonly in the contemporary
oceans, because of seawater oxygen contents that are insufficient to permit oxidation of
the major metals.
4. BIOLOGICAL COMMUNITIES AND CURRENTS ON
SEAMOUNTS
It is essential to understand the nature of biological communities that inhabit
seamounts so that that information can be incorporated into environmental impact
recommendations. It is also essential to understand the movement of water masses
around seamounts so that appropriate mining equipment and techniques can be
developed and dispersal routes of resuspended particles and wastes can be determined.
Very few studies have addressed seamount currents and biology, especially the latter.
Fe-Mn crusts occur on many different kinds of topographic features throughout the
global ocean, but in this section, we concentrate on seamounts of the type that occur in
the equatorial Pacific, where the most economically promising Fe-Mn crust deposits
occur.
Seamounts obstruct the flow of oceanic water masses, thereby creating a wide
array of seamount-generated currents of generally enhanced energy relative to flow
away from the seamounts. Seamounts interact simultaneously with large-scale currents,
mesoscale jets and eddies, and tidal flows 115, the combined effect of which produces
seamount-specific currents. Those seamount-generated currents can include anticyclonic
currents (Taylor column), internal waves, trapped waves, vertically propagating vortextrapped
waves, Taylor caps (regions of closed circulation or stagnant water above a
seamount), attached counter-rotating mesoscale eddies, and others (e.g., 116). The effects
of these currents are strongest at the outer rim of the summit region of seamounts, the
area where the thickest crusts are found. However, the seamount-generated currents can
be traced for at least several hundred meters above the summit of seamounts. Other
water column features produced by the interaction of seamounts and currents are density
inversions, isotherm displacement, enhanced turbulent mixing, and upwelling; the latter
process moves cold, nutrient-rich waters to shallower depths. Upwelling increases
primary productivity, which in turn increases the size and magnitude of the OMZ, and
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