The Seafloor - The Changing Earth: Exploring Geology and Evolution - page 239

Geo-Focus

Oceanic Circulation and Resources from the Sea

Earth's oceans are in constant motion.
Huge quantities of water circulate in
surface and deep currents as water is
transferred from one part of an ocean
basin to another. The Gulf Stream and
South Equatorial Current carry great
quantities of water toward the poles
and have an important modifying ef-
fect on climate. In addition to surface
and deep currents that carry water
horizontally, vertical circulation takes
place when upwelling slowly transfers
cold water from depth to the surface
and downwelling transfers warm sur-
face water to depth.
Upwelling is of more than academic
interest. It not only transfers water
from depth to the surface, but also car-
ries nutrients, especially nitrates and
phosphate, into the zone of sunlight
penetration. Here, these nutrients sus-
tain huge concentrations of fl oating

tive that even though constituting less
than 1% of the ocean surface, they sup-
port more than 50% (by weight) of all
fi shes.
Scientists recognize three types of
upwelling, but only coastal upwelling
need concern us here. Most coastal
upwelling takes place along the west
coasts of Africa, North America, and
South America, although one notable
exception is in the Indian Ocean.
Coastal upwelling involves move-
ment of water offshore, which is
replaced by water rising from depth
(

minute quantities in many sedimentary
rocks, most commercial phosphorus is
derived from phosphorite , a sedimentary
rock with such phosphate-rich miner-
als as fl uorapatite [Ca 5 (PO 4 ) 3 F]. Areas
of upwelling along the outer margins of
continental shelves are the depositional
sites of most of the so-called bedded
phosphorites, which are interlayered
with carbonate rocks, chert, shale, and
sandstone. Vast deposits in the Permian-
age Phosphoria Formation of Montana,
Wyoming, and Idaho formed in this
manner.
Upwelling accounts for most of
Earth's phosphate-rich sedimentary
rocks, but some forms by other pro-
cesses. In phosphatization , carbonate
grains such as animal skeletons and
ooids are replaced by phosphate, and
guano is made up of calcium phosphate
from bird or bat excrement. Another
type of phosphate deposit is essentially
a placer deposit where the skeletons
of vertebrate animals are found in
large numbers [vertebrate skeletons
are made up mostly of hydroxyapatite,
[Ca 5 (PO 4 ) 3 OH]. The 3- to 15-million-
year-old Bone Valley Formation in
Florida is a good example.
The United States is the world
leader in production and consumption
of phosphate rock, most of it coming
from deposits in Florida and North
Carolina, but some is also mined
in Idaho and Utah. More than 90%
of all phosphate rock mined in this
country is used to make chemical fer-
tilizers and animal feed supplements.
It also has several other uses in metal-
lurgy, preserved foods, ceramics, and
matches.

Figure 1). Along the coast of Peru,
for example, the winds, coupled with
the Coriolis effect,* transport surface
water seaward, and cold, nutrient-rich
water rises to replace it. This area is a
major fi shery, and changes in the sur-
face-water circulation every three to
seven years adjacent to South America

◗

Water moving offshore
due to Coriolis effect

Wind from
north

Upwelling

◗

Figure 1 Wind from the north along the west coast of a continent, coupled with the
Coriolis effect, causes surface water to move offshore, resulting in upwelling of cold,
nutrient-rich deep water.

organisms, which in turn support other
organisms. Other than the continental
shelves and areas adjacent to hydro-
thermal vents on the seafl oor, areas
of upwelling are the only parts of the
oceans where biological productivity is
very high. In fact, they are so produc-

are associated with the onset of El
Niño, a weather phenomenon with far-
reaching consequences.
Among the nutrients in upwelling
oceanic waters is considerable phos-
phorus, an essential element for animal
and plant nutrition. Although present in

*The Coriolis effect is the apparent defl ection
of a moving object from its anticipated course
resulting from Earth's rotation. Oceanic
currents are defl ected clockwise in the Northern
Hemisphere and counterclockwise in the
Southern Hemisphere.