during the interglacial periods before 400 ka is not lower
than the concentration during the more recent interglacial. In
addition, N 2 O shows a millennial variability similar to that
of methane, with an amplitude very close to that of
glacial-interglacial transitions (Schilt et al. 2013).
There is little information available to explain the
glacial-interglacial and millennial variations in N 2 O. The
natural cycle of N 2 O over these time scales is governed by
two sources of approximately equal importance currently, an
ocean source (estimated at 4 ± 2 TgN/year) and a terrestrial
source (the natural part estimated at 7 TgN/year), and by an
atmospheric sink mainly linked to the photolysis of the N 2 O
molecule in the stratosphere (12 Tg/yr). Modeling studies
simulating atmospheric chemistry over the last few thousand
years suggest that the N 2 O sink during the LGM was similar
to now (Crutzen and Brühl 1993), resulting in a lifetime
of *120 years for N 2 O in the atmosphere. Therefore, it
appears that variations in the sources are responsible for the
changes in its atmospheric concentration during the
Quaternary.
Isotopic data, in particular measurements of the
15 N of
N 2 O, are used to separate the ocean source from the terrestrial source, as the former generally shows enriched isotopic signatures compared to the latter (probably due to a
relatively stronger denitrification process in N 2 O production
in an ocean environment). These data suggest that the ratio
of ocean to land sources did not change over the last
33,000 years (Sowers et al. 2003). These two sources appear
to vary in phase with each other, with an increase of nearly
40% during warm periods compared to cold periods.
The mechanisms for explaining these variations are still
largely hypothetical. The production of N 2 O in the ocean is
generally linked to the presence of suboxic zones, low in
oxygen, which are found directly below some of the major
productive regions (east of the tropical Pacific, the Arabian
Sea). In these zones deficient in dissolved oxygen, nitrates
are used by the microorganisms as a source of oxygen during
the denitrification reactions leading to the remineralization of
the organic matter; nitrous oxide (N 2 O) is a by-product of
these reactions. A decrease in the source of N 2 O during the
glacial period could be the consequence of a shrinking of
these zones, which would itself be a result of changes in
ocean dynamics or local marine productivity leading to an
increase in dissolved oxygen at the sub-surface. However,
this hypothesis is only partially supported by reconstructions
of paleoproductivity.
Another theory concerning the ocean source is related to
the expansion of flooded surfaces on the continental shelves.
Recent estimates suggest that a significant portion (between
0.6 and 2.7 TgN/year) of marine N 2 O production comes
from the continental shelves. The significant drop in sea
level (−120 m) during cold periods would have greatly
reduced the flooded areas at the continental edges and thus
the associated source of N 2 O.
Several authors (Sowers et al. 2003; Flückiger et al. 2004;
Schilt et al. 2013) also highlighted variations of around 40
ppbv in the N 2 O concentration in the atmosphere in phase
with Dansgaard-Oeschger events. These variations in N 2 O
are substantially different in amplitude from variations in
CH 4 . While methane recordings show a fairly strong correlation with insolation in the low and mid latitudes of the
Northern Hemisphere, this is not the case for N 2 O.
The concentration of N 2 O begins to increase before
methane (during the warming phase in the Southern Hemisphere), and then the concentrations of the two gases reach
their maximum at the peak of the hot phase of the D-O
event. This information is compatible with the idea that both
marine and terrestrial sources play an important part in the
evolution of N 2 O: the marine source is stronger in the
Southern Hemisphere, in phase with the warming period in
the south, and the terrestrial source is stronger in the
northern hemisphere, in phase with the warming period in
the north.
So far, very little modelling work has focused on changes
in the concentration of N 2 O in the atmosphere over the last
hundreds of thousands of years. A simulation for the
Younger Dryas episode suggests a combination of changes
in the marine and terrestrial sources of N 2 O to explain the
variations measured during this event. Simulations run by
Schmittner and Galbraith (2008) show that changes in ocean
circulation play a major role N 2 O variations. A reduction of
the AMOC leads to decreased productivity and better ventilation resulting in increased subsurface oxygen concentrations, which explains the decrease in N 2 O production.
Oxygen Cycle
The Oxygen Cycle and Its Ocean Component
Atmospheric free oxygen does not directly impact climate
since it does not absorb infrared radiations. Despite this fact,
the cycle of oxygen has gained lots of attention primarily
because of its tight relationship with life on Earth. Indeed,
the main source of free oxygen comes as a waste product of
photosynthesis by plants on land and phytoplankton in the
ocean. The main oxygen sink is due to respiration and/or
remineralization of organic matter by almost all living
organisms, which consume di-oxygen and release carbon
dioxide. Other minor sources include the photolosyis of N 2 O
and H 2 O in the atmosphere, whereas oxygen sinks are
numerous and include a number of oxidation and chemical
weathering pathways (see Walker 1980, for a review of the
global oxygen cycle).
286
N. Bouttes et al.
than the concentration during the more recent interglacial. In
addition, N 2 O shows a millennial variability similar to that
of methane, with an amplitude very close to that of
glacial-interglacial transitions (Schilt et al. 2013).
There is little information available to explain the
glacial-interglacial and millennial variations in N 2 O. The
natural cycle of N 2 O over these time scales is governed by
two sources of approximately equal importance currently, an
ocean source (estimated at 4 ± 2 TgN/year) and a terrestrial
source (the natural part estimated at 7 TgN/year), and by an
atmospheric sink mainly linked to the photolysis of the N 2 O
molecule in the stratosphere (12 Tg/yr). Modeling studies
simulating atmospheric chemistry over the last few thousand
years suggest that the N 2 O sink during the LGM was similar
to now (Crutzen and Brühl 1993), resulting in a lifetime
of *120 years for N 2 O in the atmosphere. Therefore, it
appears that variations in the sources are responsible for the
changes in its atmospheric concentration during the
Quaternary.
Isotopic data, in particular measurements of the
15 N of
N 2 O, are used to separate the ocean source from the terrestrial source, as the former generally shows enriched isotopic signatures compared to the latter (probably due to a
relatively stronger denitrification process in N 2 O production
in an ocean environment). These data suggest that the ratio
of ocean to land sources did not change over the last
33,000 years (Sowers et al. 2003). These two sources appear
to vary in phase with each other, with an increase of nearly
40% during warm periods compared to cold periods.
The mechanisms for explaining these variations are still
largely hypothetical. The production of N 2 O in the ocean is
generally linked to the presence of suboxic zones, low in
oxygen, which are found directly below some of the major
productive regions (east of the tropical Pacific, the Arabian
Sea). In these zones deficient in dissolved oxygen, nitrates
are used by the microorganisms as a source of oxygen during
the denitrification reactions leading to the remineralization of
the organic matter; nitrous oxide (N 2 O) is a by-product of
these reactions. A decrease in the source of N 2 O during the
glacial period could be the consequence of a shrinking of
these zones, which would itself be a result of changes in
ocean dynamics or local marine productivity leading to an
increase in dissolved oxygen at the sub-surface. However,
this hypothesis is only partially supported by reconstructions
of paleoproductivity.
Another theory concerning the ocean source is related to
the expansion of flooded surfaces on the continental shelves.
Recent estimates suggest that a significant portion (between
0.6 and 2.7 TgN/year) of marine N 2 O production comes
from the continental shelves. The significant drop in sea
level (−120 m) during cold periods would have greatly
reduced the flooded areas at the continental edges and thus
the associated source of N 2 O.
Several authors (Sowers et al. 2003; Flückiger et al. 2004;
Schilt et al. 2013) also highlighted variations of around 40
ppbv in the N 2 O concentration in the atmosphere in phase
with Dansgaard-Oeschger events. These variations in N 2 O
are substantially different in amplitude from variations in
CH 4 . While methane recordings show a fairly strong correlation with insolation in the low and mid latitudes of the
Northern Hemisphere, this is not the case for N 2 O.
The concentration of N 2 O begins to increase before
methane (during the warming phase in the Southern Hemisphere), and then the concentrations of the two gases reach
their maximum at the peak of the hot phase of the D-O
event. This information is compatible with the idea that both
marine and terrestrial sources play an important part in the
evolution of N 2 O: the marine source is stronger in the
Southern Hemisphere, in phase with the warming period in
the south, and the terrestrial source is stronger in the
northern hemisphere, in phase with the warming period in
the north.
So far, very little modelling work has focused on changes
in the concentration of N 2 O in the atmosphere over the last
hundreds of thousands of years. A simulation for the
Younger Dryas episode suggests a combination of changes
in the marine and terrestrial sources of N 2 O to explain the
variations measured during this event. Simulations run by
Schmittner and Galbraith (2008) show that changes in ocean
circulation play a major role N 2 O variations. A reduction of
the AMOC leads to decreased productivity and better ventilation resulting in increased subsurface oxygen concentrations, which explains the decrease in N 2 O production.
Oxygen Cycle
The Oxygen Cycle and Its Ocean Component
Atmospheric free oxygen does not directly impact climate
since it does not absorb infrared radiations. Despite this fact,
the cycle of oxygen has gained lots of attention primarily
because of its tight relationship with life on Earth. Indeed,
the main source of free oxygen comes as a waste product of
photosynthesis by plants on land and phytoplankton in the
ocean. The main oxygen sink is due to respiration and/or
remineralization of organic matter by almost all living
organisms, which consume di-oxygen and release carbon
dioxide. Other minor sources include the photolosyis of N 2 O
and H 2 O in the atmosphere, whereas oxygen sinks are
numerous and include a number of oxidation and chemical
weathering pathways (see Walker 1980, for a review of the
global oxygen cycle).
286
N. Bouttes et al.