Simulation of Oxygen Isotopes in a Global Ocean Model
657
The focus of this paper is on oxygen isotopes,
because oxygen-I 8 variations are directly related
to paleotemperature studies. It is in fact for this
reason that most of the oceanographic work to date
has concentrated on oxygen-18 and only few deuterium measurements have been performed (Craig
and Gordon 1965).
Applications in Present-Day Oceanography
The distributions of both salinity S and oxygen isotope composition of seawater 0 18 0 w or simply Ow
are mainly controlled by the same two processes
precipitationP and evaporationE. Locally S and Ow
of surface waters are linearly related. However, the
slope of the 0w-S relationship varies between 0.1
for tropical surface waters and 1 for high-latitude
surface waters. This reflects the so-called temperature effect: in high latitudes, precipitation occurs at
lower temperatures and is more enriched in oxygen-18 than in low latitudes, which leads to a strong
depletion of the remaining cloud moisture. A slope
of 1 is observed for East Greenland surface waters, which are diluted by adding meltwater from
the permanent ice cap. Freshwater input from rivers can also modify the Ow -S relationship. Hence S
and Ow label the surface waters in different ways.
Mixing of water masses that originate from the sea
surface creates the characteristic vertical sections
known from GEOSECS (Birchfield 1987; Ostlund
et al. 1987).
The isotopic composition of sea water is also
affected by the formation of sea ice, but the
fractionation effect is so small that the freezing
process leads to an increase in salinity, with essentially no observable influence on ow' This gives a
near-zero slope in a 0w-S diagram (Craig and
Gordon 1965). As a result, the surface waters of
the Southern Ocean show a wide range of salinities,
although Ow is nearly constant. For Antarctic Surface Water (AASW), Jacobs et al. (1985) report a
° value of -0.31 %0 with a standard deviation of
w
0.08 %0.
In summary, in present-day oceanography Ow
allows to distinguish between different freshwater
sources. Thus it is possible to investigate the influence of local P - E, river-runoff and glacial meltwater, sea ice formation and decay on a water mass
of interest (Weiss et al. 1979; Jacobs et al. 1985;
Bauch 1995; Toggweiler and Samuels 1995).
To give an explicit example, we reproduced the
detailed freshwater budget for the Ross Sea continental shelf constructed by Jacobs et al. (1985).
From the difference between the 0 18 0 content
of shelf water and its 'source they estimated that
36 cm a-I of glacial meltwater is introduced to the
shelf. They found that surface and deep waters
move onto the shelf with an average 0 18 0 content
of -0.22 %0. On the shelf, its 0 18 0 content will be
lowered by marine precipitation, glacial meltwater
and the net freezing of sea ice, such that the average 0 18 0 content of shelf water becomes -0.42 %0:
(1) Marine precipitation at a rate of 15 cm a-I with
an 0 18 0 content of -16.5 %0 will lower the 0 18 0
content of shelf water by 0.005 %0 a-I. (2) Further
depletion of 0.025 %0 a-I will result from the addition of glacial meltwater at a rate of36 cm a-I with
an 0 18 0 content of -36 %0. (3) The decrease in
shelf water Ow from sea ice freezing will be only
0.002 %0 a-I, due to the near-zero slope of the freezing line. The net change in 0 18 0 by -0.20 %0 a-I and
the glacial addition of meltwater at a rate of 3 6 cm
a-I require a shelf water residence time of 6.2 a.
An important application of a detailed freshwater budget is the study of the role of sea ice in the
formation of Antarctic Bottom Water (AABW)
(Toggweiler and Samuels 1995; Stossel et al. 1996).
In the example of Jacobs et al. (1985), the on shelf
flow has an average salinity of 34.455 and the
offshelf flow has an average salinity of 34.589,
which leaves a salinity enrichment of 0.134 units.
This salinity enrichment corresponds to the brine
rejected during the formation of sea ice at a rate of
37 cm a-I, with a residual salinity of 4.1, from Antarctic Surface Water (AASW) with a salinity
of 34.1. Furthermore salt must be drained from
58 cm a· 1 of sea ice to balance IS cm a-I of marine
precipitation and 36 cm a-I of glacial meltwater.
Toggweiler and Samuels (1995) pointed out that on
the one hand brine rejection causes a net salinity
enrichment of only 0.134 units, which is much
smaller than the value proposed by Broecker (1986)
on the basis of open ocean GEOSECS Ow measurements, but on the other hand brine rejection is
essential in compensating the effects of marine precipitation and glacial meltwater on the shelf.
657
The focus of this paper is on oxygen isotopes,
because oxygen-I 8 variations are directly related
to paleotemperature studies. It is in fact for this
reason that most of the oceanographic work to date
has concentrated on oxygen-18 and only few deuterium measurements have been performed (Craig
and Gordon 1965).
Applications in Present-Day Oceanography
The distributions of both salinity S and oxygen isotope composition of seawater 0 18 0 w or simply Ow
are mainly controlled by the same two processes
precipitationP and evaporationE. Locally S and Ow
of surface waters are linearly related. However, the
slope of the 0w-S relationship varies between 0.1
for tropical surface waters and 1 for high-latitude
surface waters. This reflects the so-called temperature effect: in high latitudes, precipitation occurs at
lower temperatures and is more enriched in oxygen-18 than in low latitudes, which leads to a strong
depletion of the remaining cloud moisture. A slope
of 1 is observed for East Greenland surface waters, which are diluted by adding meltwater from
the permanent ice cap. Freshwater input from rivers can also modify the Ow -S relationship. Hence S
and Ow label the surface waters in different ways.
Mixing of water masses that originate from the sea
surface creates the characteristic vertical sections
known from GEOSECS (Birchfield 1987; Ostlund
et al. 1987).
The isotopic composition of sea water is also
affected by the formation of sea ice, but the
fractionation effect is so small that the freezing
process leads to an increase in salinity, with essentially no observable influence on ow' This gives a
near-zero slope in a 0w-S diagram (Craig and
Gordon 1965). As a result, the surface waters of
the Southern Ocean show a wide range of salinities,
although Ow is nearly constant. For Antarctic Surface Water (AASW), Jacobs et al. (1985) report a
° value of -0.31 %0 with a standard deviation of
w
0.08 %0.
In summary, in present-day oceanography Ow
allows to distinguish between different freshwater
sources. Thus it is possible to investigate the influence of local P - E, river-runoff and glacial meltwater, sea ice formation and decay on a water mass
of interest (Weiss et al. 1979; Jacobs et al. 1985;
Bauch 1995; Toggweiler and Samuels 1995).
To give an explicit example, we reproduced the
detailed freshwater budget for the Ross Sea continental shelf constructed by Jacobs et al. (1985).
From the difference between the 0 18 0 content
of shelf water and its 'source they estimated that
36 cm a-I of glacial meltwater is introduced to the
shelf. They found that surface and deep waters
move onto the shelf with an average 0 18 0 content
of -0.22 %0. On the shelf, its 0 18 0 content will be
lowered by marine precipitation, glacial meltwater
and the net freezing of sea ice, such that the average 0 18 0 content of shelf water becomes -0.42 %0:
(1) Marine precipitation at a rate of 15 cm a-I with
an 0 18 0 content of -16.5 %0 will lower the 0 18 0
content of shelf water by 0.005 %0 a-I. (2) Further
depletion of 0.025 %0 a-I will result from the addition of glacial meltwater at a rate of36 cm a-I with
an 0 18 0 content of -36 %0. (3) The decrease in
shelf water Ow from sea ice freezing will be only
0.002 %0 a-I, due to the near-zero slope of the freezing line. The net change in 0 18 0 by -0.20 %0 a-I and
the glacial addition of meltwater at a rate of 3 6 cm
a-I require a shelf water residence time of 6.2 a.
An important application of a detailed freshwater budget is the study of the role of sea ice in the
formation of Antarctic Bottom Water (AABW)
(Toggweiler and Samuels 1995; Stossel et al. 1996).
In the example of Jacobs et al. (1985), the on shelf
flow has an average salinity of 34.455 and the
offshelf flow has an average salinity of 34.589,
which leaves a salinity enrichment of 0.134 units.
This salinity enrichment corresponds to the brine
rejected during the formation of sea ice at a rate of
37 cm a-I, with a residual salinity of 4.1, from Antarctic Surface Water (AASW) with a salinity
of 34.1. Furthermore salt must be drained from
58 cm a· 1 of sea ice to balance IS cm a-I of marine
precipitation and 36 cm a-I of glacial meltwater.
Toggweiler and Samuels (1995) pointed out that on
the one hand brine rejection causes a net salinity
enrichment of only 0.134 units, which is much
smaller than the value proposed by Broecker (1986)
on the basis of open ocean GEOSECS Ow measurements, but on the other hand brine rejection is
essential in compensating the effects of marine precipitation and glacial meltwater on the shelf.