PARTICULATE ORUANIC MATTER IN SEA WATER
87
mainly on expedition data of the " Discovery '), " Dana )', " Meteor ",
and " Atlantis ", and found that the oxygen minimum layer is quite
complicated in its structure, being compounded of several loci of
marked oxygen depletion which are imperfectly diffused and show
evidence of regional variation of the same kind that has been noted in
connection with particulate organic matter and organic phosphorus.
Oxygen minima are most strongly developed off the West African
coast at the roots of the North and South Equatorial Currents. They
are centred at a depth of about 400 m on ut surfaces 26.9-27.0. There
is also a secondary minimum at a depth of only 100m in the Guinea
Current according to " Dana " data. Another locus of low oxygen
concentration is found in the Sub-Antarctic at about ut 27.5. This
tends to spread northward but remains separate from the tropical
oxygen minimum above, so that stations between 25" and 4 0 ' 5 Lat.
sometimes show double minima a t about 400 and 1 800 m.
In the tropical North Atlantic the oxygen minima spread westward across the ocean and are centred at a ut surface of 27-0-27.1,
indicating that some local consumption occurs along the way at a
slightly deeper level. The great mass of northern water that drifts
down to the central North Atlantic has a weak and diffuse oxygen
minimum layer, and individual stations have minimum oxygen values
at various ut surfaces from 27.3-27.7. I n the vicinity of the Mediterranean outflow the minimum is also weak and is centred around the
27.5 surface.
These waters of various origins meet and mix in the western North
Atlantic, where current systems are relatively strong. Because two of
the sources have oxygen minima that are vertically diffuse, no secondary minima are formed. Instead there is coalescence into a single well
defined minimum at ut surface 27.2-27.3. To the southward it grades
upward toward the tropical minimum, while to the east and north it
grades toward denser water.
The fact that the oxygen minimum layers occur on different density
surfaces in different parts of the ocean is indicative of a more or less
local origin. More recently Menzel and Ryther (1968a) have suggested
that this layer is derived from the upwelling area of the tropical eastern
Atlantic and spreads from there by isentropic mixing. However, this
is clearly not the case except in a very limited area, and even in their
short north-south profile (ca 8-36"s) in the South Atlantic, which was
the main subject of their analysis, the northward intrusion of the deep
Antarctic oxygen minimum layer can be seen at the southern end.
In deep ocean waters vertical turbulence is believed to be slightly
stronger than in the mid-depth region of maximum stability. This
87
mainly on expedition data of the " Discovery '), " Dana )', " Meteor ",
and " Atlantis ", and found that the oxygen minimum layer is quite
complicated in its structure, being compounded of several loci of
marked oxygen depletion which are imperfectly diffused and show
evidence of regional variation of the same kind that has been noted in
connection with particulate organic matter and organic phosphorus.
Oxygen minima are most strongly developed off the West African
coast at the roots of the North and South Equatorial Currents. They
are centred at a depth of about 400 m on ut surfaces 26.9-27.0. There
is also a secondary minimum at a depth of only 100m in the Guinea
Current according to " Dana " data. Another locus of low oxygen
concentration is found in the Sub-Antarctic at about ut 27.5. This
tends to spread northward but remains separate from the tropical
oxygen minimum above, so that stations between 25" and 4 0 ' 5 Lat.
sometimes show double minima a t about 400 and 1 800 m.
In the tropical North Atlantic the oxygen minima spread westward across the ocean and are centred at a ut surface of 27-0-27.1,
indicating that some local consumption occurs along the way at a
slightly deeper level. The great mass of northern water that drifts
down to the central North Atlantic has a weak and diffuse oxygen
minimum layer, and individual stations have minimum oxygen values
at various ut surfaces from 27.3-27.7. I n the vicinity of the Mediterranean outflow the minimum is also weak and is centred around the
27.5 surface.
These waters of various origins meet and mix in the western North
Atlantic, where current systems are relatively strong. Because two of
the sources have oxygen minima that are vertically diffuse, no secondary minima are formed. Instead there is coalescence into a single well
defined minimum at ut surface 27.2-27.3. To the southward it grades
upward toward the tropical minimum, while to the east and north it
grades toward denser water.
The fact that the oxygen minimum layers occur on different density
surfaces in different parts of the ocean is indicative of a more or less
local origin. More recently Menzel and Ryther (1968a) have suggested
that this layer is derived from the upwelling area of the tropical eastern
Atlantic and spreads from there by isentropic mixing. However, this
is clearly not the case except in a very limited area, and even in their
short north-south profile (ca 8-36"s) in the South Atlantic, which was
the main subject of their analysis, the northward intrusion of the deep
Antarctic oxygen minimum layer can be seen at the southern end.
In deep ocean waters vertical turbulence is believed to be slightly
stronger than in the mid-depth region of maximum stability. This