Geoscience Reference
In-Depth Information
In the ocean, the concentration of O
2
dissolved in the surface layer
is usually high due to exchanges with the atmosphere (which occur in
both directions) [
12
] and phytoplankton photosynthesis [
1
]. This is
also the case at depth due to the thermohaline circulation
(see section 2.1.2 in Chapter 2 of [MON 14b]) that transports oxygen
from surface waters to ocean depths [
13
] (the vertical movements of
water that transport oxygen from the ocean surface toward
intermediate and deep waters are described in section 2.1.2 in Chapter
2 of [MON 14b]). Between the surface and deep waters, there is often
a minimum in oxygen concentration, at least at low and mid latitudes.
This minimum is located at depths where the respiration and
decomposition of organic matter consume O
2
faster than its
replenishment from the surface. In some cases, the concentration of O
2
at intermediate depths is very low, in which case oceanographers use
the expression oxygen minimum zone (OMZ; see next paragraph).
Ocean deoxygenation is an important, and still little known
phenomenon even by oceanographers [KEE 10]. This phenomenon
refers to the decrease in O
2
in the open-ocean OMZs, which must not
be confused with local losses of oxygen in shallow coastal waters
incorrectly known as dead zones (see section 4.5.2). Over the last half-
century since the 1960s, the concentration of O
2
between 300 and
700 m depths has greatly decreased in several regions of the ocean.
This phenomenon is likely due to the increase in surface water
temperature, which is associated with global warming. Indeed, higher
surface temperature causes both a decrease in the amount of
atmospheric gas that can dissolved in the seawater (the solubility
of gases, including O
2
, is inversely proportional to the temperature of
water) and an increase in the density gradient between surface
waters - whose density decreases as they warm up - and underlying
waters - which are cooler than those in the surface layer, and hence,
denser; increased density stratification reduces vertical mixing, and
thus, the O
2
replenishment of intermediate waters. What does the
future hold? It appears that, as global warming proceeds, the O
2
content of intermediate waters will continue to decrease throughout
the world's oceans. The deep ocean would experience a decrease in O
2
of 20-40%, without creating large oxygen-depleted zones. It follows
