Raw Materials from Shelves: Phosphorites 285
Fig. 9.20), and favors burial of organic matter. We have briefly mentioned the Mesozoic structures identified as salt domes in the eastern South Atlantic, off Africa
(Sect. 2.3, Fig. 2.4) where organic-rich Cretaceous sediments were found by deep
drilling. Clearly, from the Gulf of Mexico experience, petroleum may be expected in
such a situation.
Regardless of the promise of certain continental slope regions, activity on the
shelves, even in regions with heavy seas or in polar areas, is going to dominate
petroleum recovery from the sea floor in the foreseeable future. Expenses of drilling
increase rapidly beyond the shelf break because platforms either have to have extremely long legs, or they have to be positioned dynamically, that is, by constant
calculated application of power to several thrusters. Also, safety problems rapidly
increase with depth. Beyond the continental slopes, in the deep sea proper, where
sedimentation rates are low, the chances of finding oil rapidly diminish anyhow. This
may be one reason that the Exclusive Economic Zone extends "only" to 200 nautical
miles (371 km) offshore.
10.3 Raw Materials from Shelves
10.3.1 Phosphorites. The naturalist on HMS Challengel~ John Murray, became a
wealthy man (unlike his modem counterparts) as a result of joining an oceanographic
expedition, as a result of his interest in a mining venture which resulted from the
discovery of phosphorite on Christmas Island in the western equatorial Pacific. What
is more, the British Crown's Treasury recovered the expenses of the expedition,
through the taxes derived from the phosphorite produced.
Phosphorites were briefly mentioned in Section 3.8.2. They are cryptocrystalline
apatites and vary in composition. A general formula is CalO (P04, C03 )6 F2-3, with
increases in carbonate running parallel to increases in fluoride (or hydroxide). Modem phosphorites typically occur in areas of high productivity - off Southern and
Baja California, off Peru, off South Africa, for example (Fig. 10.7). They occur as
black or brown nodules from pellets up to head size, and as irregularly shaped cakes.
They are interpreted as replacement products of fine-grained lithified carbonates, as
mineralization of pre-existing organic matter, as precipitates from microorganisms
filling cavities within carbonates, or as direct precipitates from interstitial waters.
Off California these deposits contain about 25 to 30 % P2 Os and 40 to 45 % CaO,
but these values vary in other regions. The common depth of deposition is on the
shelf and upper slope. Fossil phosphorites are abundant in Florida and Georgia.
Miocene phosphorites are mined on a large scale; in West Africa, Eocene deposits are
being exploited. Seamounts bearing Cretaceous carbonates commonly carry phosphorites.
The association of geologically young phosphorite deposits with present-day regions of upwelling (Fig. 4.19) suggests that the source of the phosphorus is organic
matter. Apparently, the algae growing in these regions, in the surface waters, extract
the phosphorus from the water. Also, crustaceans and fish concentrate it further in
their bodies and excrement. During decomposition of organic debris on the sea floor,
Fig. 9.20), and favors burial of organic matter. We have briefly mentioned the Mesozoic structures identified as salt domes in the eastern South Atlantic, off Africa
(Sect. 2.3, Fig. 2.4) where organic-rich Cretaceous sediments were found by deep
drilling. Clearly, from the Gulf of Mexico experience, petroleum may be expected in
such a situation.
Regardless of the promise of certain continental slope regions, activity on the
shelves, even in regions with heavy seas or in polar areas, is going to dominate
petroleum recovery from the sea floor in the foreseeable future. Expenses of drilling
increase rapidly beyond the shelf break because platforms either have to have extremely long legs, or they have to be positioned dynamically, that is, by constant
calculated application of power to several thrusters. Also, safety problems rapidly
increase with depth. Beyond the continental slopes, in the deep sea proper, where
sedimentation rates are low, the chances of finding oil rapidly diminish anyhow. This
may be one reason that the Exclusive Economic Zone extends "only" to 200 nautical
miles (371 km) offshore.
10.3 Raw Materials from Shelves
10.3.1 Phosphorites. The naturalist on HMS Challengel~ John Murray, became a
wealthy man (unlike his modem counterparts) as a result of joining an oceanographic
expedition, as a result of his interest in a mining venture which resulted from the
discovery of phosphorite on Christmas Island in the western equatorial Pacific. What
is more, the British Crown's Treasury recovered the expenses of the expedition,
through the taxes derived from the phosphorite produced.
Phosphorites were briefly mentioned in Section 3.8.2. They are cryptocrystalline
apatites and vary in composition. A general formula is CalO (P04, C03 )6 F2-3, with
increases in carbonate running parallel to increases in fluoride (or hydroxide). Modem phosphorites typically occur in areas of high productivity - off Southern and
Baja California, off Peru, off South Africa, for example (Fig. 10.7). They occur as
black or brown nodules from pellets up to head size, and as irregularly shaped cakes.
They are interpreted as replacement products of fine-grained lithified carbonates, as
mineralization of pre-existing organic matter, as precipitates from microorganisms
filling cavities within carbonates, or as direct precipitates from interstitial waters.
Off California these deposits contain about 25 to 30 % P2 Os and 40 to 45 % CaO,
but these values vary in other regions. The common depth of deposition is on the
shelf and upper slope. Fossil phosphorites are abundant in Florida and Georgia.
Miocene phosphorites are mined on a large scale; in West Africa, Eocene deposits are
being exploited. Seamounts bearing Cretaceous carbonates commonly carry phosphorites.
The association of geologically young phosphorite deposits with present-day regions of upwelling (Fig. 4.19) suggests that the source of the phosphorus is organic
matter. Apparently, the algae growing in these regions, in the surface waters, extract
the phosphorus from the water. Also, crustaceans and fish concentrate it further in
their bodies and excrement. During decomposition of organic debris on the sea floor,