Geoscience Reference
In-Depth Information
12.3.3 Denitrii cation
demand further down the water column, where
oxygen concentration tends to be higher. A second
mechanism is the increase of the C:N ratio induced
by ocean acidii cation, which enhances the down-
ward l ux of organic carbon, thereby increasing
the oxygen demand throughout the water column
(see Section 12.2.3.2).
Hofmann and Schellnhuber ( 2009 ) simulated the
impact of the i rst mechanism in a global model for
a business-as-usual scenario, and found a 75%
reduction in the export of CaCO
3
, a very substantial
expansion of hypoxia (i.e. O
2
concentrations below
60 μmol kg
-1
), and a moderate increase of anoxic/
suboxic regions. Their model did not include deni-
trii cation, but the relatively small expansion of
anoxic/suboxic waters would suggest that denitri-
i cation would not have increased substantially.
Oschlies
et al.
(2008) investigated the effect of
changes in the C:N ratio on marine oxygen in a glo-
bal model using a business-as-usual scenario, and
found a 50% increase in the ocean volume with sub-
oxic conditions by the end of this century, which
caused an increase in water-column denitrii cation
of about 60 Tg N yr
-1
. If sustained, such a loss of
i xed nitrogen would lead to a decrease in the
marine nitrogen inventory of the order of 10% in
1000 years, causing a corresponding decrease in
marine productivity.
In conclusion, while denitrii cation is probably
not directly affected by ocean acidii cation, it plays
an important role in the sequence of processes that
may ultimately cause substantial changes in the
earth system.
Denitrii cation is a dissimilatory process that occurs
only at extremely low to non-existent oxygen con-
centrations, but at appreciable levels of nitrate.
Under such circumstances, heterotrophic microor-
ganisms can use nitrate rather than dissolved oxy-
gen as a terminal electron acceptor, i.e. they
'breathe' nitrate instead of oxygen. The threshold
for the onset of this process occurs at oxygen con-
centrations of around 5 to 10 μmol kg
-1
, or at the
boundary between hypoxia and suboxia. In the
present-day ocean, this condition is only met in the
water column at a few locations, namely the east-
ern tropical North and South Pacii c, the Arabian
Sea, and a few more localized coastal regions. In
contrast, many sediments underlying productive
regions are completely void of oxygen (anoxic) at a
depth of a few centimetres, making them sites of
intense denitrii cation. Globally, denitrii cation is
two times higher in the sediments than in the water
column (180 ± 50 Tg N yr
-1
vs 65 ± 20 Tg N yr
-1
),
although there exists a considerable amount of
debate about the exact magnitude of these proc-
esses ( Gruber 2008 ).
Although so far unsupported by direct experi-
ments, Hutchins
et al.
( 2009 ) suggest that the
heterotrophic microorganisms responsible for deni-
trii cation are not directly affected by ocean acidii -
cation. This is consistent with the fact that these
organisms live and thrive in anoxic regions that
naturally have a much lower pH than the rest of
the ocean. However, ocean acidii cation-induced
changes in the l ux of organic matter that is entering
such anoxic regions and changes in the extent and
location of anoxic regions can lead to very substan-
tial changes in marine denitrii cation, both in the
water column and in the sediments. This may occur
as a result of several mechanisms: i rst, a shallower
remineralization of the organic matter sinking
downward in response to reduced ballasting may
lead to a higher oxygen demand in shallow waters
at the expense of a smaller oxygen demand in deep
waters (see Section 12.2.3.3). This may cause an
expansion of anoxia, since it leads to the enhanced
removal of oxygen in the upper thermocline where
oxygen is already low in many locations (e.g.
Keeling
et al.
2010), while reducing the oxygen
12.3.4
Nitrous oxide production
Nitrous oxide (N
2
O) is produced in the ocean
through at least two pathways (Fig.12.3; Gruber
2008). It is an intermediary product of denitrii ca-
tion, and under suboxic, but not completely anoxic,
conditions its further reduction to N
2
tends to pro-
ceed less efi ciently. N
2
O is also produced during
the oxidation of ammonium, and the fraction of the
ammonium transformed into N
2
O instead of nitrite
also tends to increase with lower oxygen concentra-
tions. Most of the N
2
O produced in the ocean is
emitted to the atmosphere, as only a small fraction
is consumed in the anoxic regions of the ocean.
