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Phosphatisation of the older Fe-Mn crust layers caused changes in their
chemistry and mineralogy 86. Besides dilution of the primary crust contents by CFA,
certain elements were added and others removed. Depletion of elements in the
phosphatised crust generation compared to non-phosphatized crust layers occurred in
the approximate order silicon>iron= aluminum=thorium> titanium=
cobalt>manganese=lead=uranium; in contrast, nickel, copper, zinc, yttrium, REEs,
strontium, platinum, and commonly barium are enriched in phosphatised crusts 87.
2.6. Local and Regional Variations in Composition
The iron/manganese ratios are lowest in crusts from the central and west-central
Pacific and highest for crusts collected in the north Atlantic, Indian, south Atlantic, and
near continental margins and volcanic arcs in decreasing magnitude of the ratio. The
detrital-associated element (silicon, aluminium) contents increase in crusts with
proximity to continental margins (off western North America, far South Pacific) and
volcanic arcs in the west Pacific, which have contents equivalent to those found in most
crusts from the Atlantic and Indian Oceans (Table 6; Fig. 8A). Within the central Pacific
region, detrital-related elements are most abundant in the eastern part, along the
Hawaiian and Line Islands. In contrast, cobalt, nickel, and platinum contents are
generally highest in crusts from the central and northwest Pacific and lowest in crusts
from along the spreading centres in the southeast Pacific, the continental margins, and
along the volcanic arcs of the west Pacific (Table 6; Fig. 8B, C) 89. Cobalt contents are low
and nickel contents are the lowest for crusts from the Atlantic and Indian Oceans
compared to crusts from other regions (Fig. 8B, C). Copper contents generally follow the
trend for cobalt, nickel, and platinum, except for the Indian Ocean, where a high mean
value (1254 ppm) is found. The reason for those high values is the much greater mean
water depth for crusts collected from the Indian Ocean. Shatsky Rise crusts, midlatitudes
of the north Pacific, have a surprisingly high mean copper content, as well as
the highest copper value measured in a single bulk crust, 0.4% (4000 ppm). Mean barium
content is much higher in northeast Pacific crusts than anywhere else in the global
oceans. Those high barium contents are the result of intense upwelling and high
bioproductivity in that region 90, Hein et al., unpublished data). Trends for mean titanium
contents do not follow those of aluminium and silicon, but rather follow those of cobalt,
nickel, and platinum, which supports the idea that much of the titanium in crusts is a
hydrogenetic phase 91. Another interesting distribution is seen with phosphorus because
it is not most enriched in areas where upwelling and bioproductivity are greatest (east
Pacific, east equatorial Pacific), but rather is highest in crusts from the Marshall Islands
and the northwest Pacific (Fig. 8A; see also Hein et al. 92). It is not clear why phosphorus
does not reflect the high bioproductivity in the east Pacific as does barium, or why CFA
does not occur in crusts from that region. Phosphorus is relatively high in north Atlantic
crusts, where bioproductivity is also high. Cerium is generally lower in south Pacific
crusts than it is in north Pacific crusts and has moderate contents in Atlantic and Indian
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