Oxygen (O)
| Atomic number: |
8 |
| Atomic weight: |
15.9994 |
| Average concentration in ocean: |
O2 - 175 µmol/kg |
| Residence time: |
yrs |
Distribution in ocean
The vertical profile of
dissolved O2 concentration is high at the surface and it reaches a
mid-depth minimum (data).
Dissolved O2 is present at the sea surface in concentrations
that are near equilibrium
with the atmosphere. It may be produced by photosynthesis in the euphotic
zone and it is consumed at depth as organisms respire organic carbon that sinks
from the surface. The O2 minimum occurs because more organic
carbon is respired near the surface than at great depths. Because surface
concentrations are near equilibrium with the atmosphere, we can estimate the
amount of O2 that has been consumed (or produced near the surface)
due to respiration as the difference between the saturation concentration and
the observed concentration. This quantity is termed the Apparent Oxygen
Utilization (AOU). AOU
increases as deep seawater circulates from its sources in the Atlantic to the
Pacific.
When the oxygen is strongly depleted, then bacteria may use other oxidized
compounds such as NO3-, SO42-, MnO2
and Fe2O3 as terminal electron acceptors in the oxidation
of organic carbon.
Speciation
Molecular oxygen is present in seawater as O2
Analysis
Oxygen is determined most precisely by a Winkler titration in which oxygen
reacts with I- to produce I2 and the amount of I2
is determined with thiosulfate. Oxygen can also be determined in situ with
an oxygen electrode.
References & Notes