Geology Reference
In-Depth Information
and for the standard state
F
pe
¼
2
:
303RT
E
ð
4
:
4
Þ
where F is Faraday's constant, R the universal gas constant and T the
absolute temperature in Kelvin. Whereas a high value of pe indicates
oxidising conditions, a low value signifies reducing conditions. Oxygen
plays a role via the reaction
O
2
þ
4H
þ
þ
4e
Ð
2H
2
O
At 20 1C, K
¼
10
83.1
and so water of pH
¼
8.1 in equilibrium with
atmospheric O
2
(pO
2
¼
0.21 atm) has pe
¼
12.5. This conforms to
surface conditions, but the pe decreases as the O
2
content diminishes
with depth. The oxygen minimum is particularly well developed beneath
the highly productive surface waters of the eastern tropical Pacific
Ocean, where there is a large flux of organic material to depth and
subsequently considerable oxidation. The O
2
becomes sufficiently de-
pleted (i.e., hypoxia) that the resulting low redox conditions causes
NO
3
to be reduced to NO
2
. Aeolian transport of nitrate to Chesa-
peake Bay can lead to low O
2
conditions. Similarly, intermittent hypoxia
develops in parts of the Gulf of Mexico due to the riverine transport of
nutrients derived from agricultural uses in the Mississippi catchment.
When circulation is restricted vertically due to thermal or saline stratifi-
cation and horizontally by topographic boundaries, the water becomes
stagnant and the oxygen may be completely utilised producing anoxic
conditions. Such regions represent atypical marine environments where
reducing conditions prevail. Well-known examples include the Black Sea,
which is permanently anoxic below 200 m, and the Cariaco Trench, a
depression in the Venezuelan continental shelf. Some fjords, such as Saanich
Inlet in western Canada and Dramsfjord of Norway, may be intermittently
anoxic. Periodic flushing of these inlets by dense, oxygenated waters
displaces deep anoxic water to the surface causing massive fish mortality.
O
2
can be used as a tracer to help identify the origin of water masses.
The warm, saline intrusion into the Atlantic Ocean from the Mediter-
ranean Sea is relatively O
2
deficient. Alternatively, the waters down-
welling from Polar Regions have elevated O
2
concentrations.
4.2.2.3 Carbon Dioxide and Alkalinity. Marine chemists sometimes
adopt activity conventions quite different to those traditionally used in
chemistry. It is useful to preface a discussion about the carbon dioxide-
calcium carbonate system in the oceans with a brief outline of pH scales.
