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The list of 46 research cruises in Table 3 is not comprehensive and data for a few
cruises completed by the USSR (and later Russia) are not available to the author.
However, based on an estimated 48 research cruises from 1981 through 2001, it is
suggested that minimum expenditures were about US $40 million for ship and associated
scientific operations related to field work, and $50 million for shore-based research, for a
total investment of about $90 million.
2. Fe-Mn CRUST CHARACTERISTICS
2.1. Textures and Physical Properties
Crust surfaces exposed directly to the seafloor are botryoidal, with botryoids
varying in size from microbotryoidal (millimetre size) to botryoidal (centimetre size).
Fresh growth surfaces are characterized by a fractal distribution of botryoids with
extreme surface area. Under conditions of high current flow, the botryoids are modified,
either by smoothing or by accentuation of the relief by erosion around the margins of the
botryoids, in places producing mushroom-shaped forms. With strong uni-directional
flow, the botryoids become polished and fluted. Crusts on the sides of rocks are
commonly more protected from current activity and grow at a slower rate thereby
acquiring very high cobalt contents; textures of those side crusts are very porous and
granular.
Crust profiles vary according to thickness and regional oceanographic
conditions. Thin crusts (80 mm) may have up to 8 distinct macroscopic layers, the lower several
layers of which are commonly phosphatised. In polished thin sections, textures consist of
alternating laminated, columnar, botryoidal, and mottled textures. Columns range in
height from millimetres to 50 mm 34, with detrital grains separating columns and actually
promoting columnar growth 35. Mottled layers are the most porous and detritus-rich.
The various textures probably reflect bottom-water conditions at the time of precipitation
of the oxides. Mottled, columnar, botryoidal, and laminated textures likely represent
progressively decreasing energy in the depositional environment 36.
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