points are several tens of per cent higher than
background ␦
3
He values in the deep ocean.
Finally,
4 He is injected into the ocean through
the addition of glacial meltwater (Schlosser,
1986; Schlosser et al., 1990; Weppernig et al.,
1996).
4 As tests for numerical models of ocean circulation and mixing. Tracer distributions are controlled by their input to the ocean and by
advective and mixing processes. Thus, tracer
distributions in ocean circulation models are
sensitive to these parameters in the model.
The magnitude of the WOCE programme contributed to technological advances in tracer
oceanography. These advances reached from the
establishment of new laboratories (e.g. the Accelerator Mass Spectrometry Facility at the Woods
Hole Oceanographic Institution) to refinements of
existing technologies. Such refinements include,
for example, the preparation at sea of helium isotope samples for mass spectrometric measurement
(Jenkins et al., 1991) or the advancement of CFC
measurement techniques (e.g. Bulsiewicz et al.,
1998).
The establishment of the US National Oceanographic Sciences Accelerator Mass Spectrometry facility (NOSAMS) at the Woods Hole
Oceanographic Institution (e.g. Jones et al., 1994;
McNichol et al., 1994; Schneider et al., 1994; von
Reden et al., 1994) probably had the most significant impact on the WOCE tracer programme.
During GEOSECS,
14 C measurements required the
extraction of the total inorganic carbon from
approximately 250 litres of seawater. The collection of such large amounts of water was only possible through the use of Gerard–Ewing samplers,
which require extra casts at each
14 C station.
Given the large number of
14 C samples planned
for the WOCE survey, this method would have
required too much ship time and the
14 C programme would have had to be reduced significantly, with serious implications for the value of
the resulting data set. Therefore, during the US
Pacific WOCE field work (Key, 1996), both largevolume and small-volume AMS samples were collected along many of the WOCE sections. Both the
proven -counting Large-Volume (LV) technique,
which had a precision of <2–4‰, and the AMS
technique, with a precision of Ϸ<4‰ (Elder et al.,
1998), were utilized.
5.8.3 Exemplary results
5.8.3.1 Thermocline ventilation in the
Pacific Ocean using tritium/
3 He data
The subsurface transport of subducted surface
waters from the subtropics to the tropics has been
implicated in the long-term modulation of El Niño
events in the Pacific (e.g. Gu and Philander, 1997;
Guilderson and Schrag, 1997). Fine et al. (1987)
identified a tongue of tritium penetrating the tropics, tracing the cell in the North Pacific, and
thereby placing an upper bound on the time scales
associated with the transit of these waters. The
WOCE Pacific expedition provided us with an
opportunity to delineate this circulation more completely, and refine our knowledge of its character.
A 1991 tritium section along 135°W (P17C,
Fig. 5.8.1a, see Plate 5.8.1a, p. 428) traces the
equatorward movement from the subtropical
North Pacific of the tritium tongue, which extends
more than 3000 km meridionally. As it extends
southward, it follows isopycnals (26.0: :
26.5 kg m
93 ), rising from a depth of approximately 250 m at 30°N to about 75 m at 10°N,
then deepening about 100 m at the equator. The
tritium tongue is underlain by a
3 He maximum
(Fig. 5.8.1b, see Plate 5.8.1b, p. 428) at a depth
of about 450 m in the subtropics. This deeper,
tritiogenic
3 He maximum is characteristic of the
subtropical thermocline, and is a widespread feature in other regions of the world ocean such as
the North Atlantic (e.g. Jenkins, 1998) or the
Nordic seas (Schlosser et al., 1995). As the tongues
extend into the tropics, the tritium and
3 He maxima become largely depth-coincident, supporting a
net meridional advective origin for their existence.
The shallow distribution of excess
3 He, especially in the North Pacific, is dominated by in-situ
decay of tritium (Jenkins, 1996). This conclusion
is supported by fact that the shallow
3
He maximum is almost everywhere underlain by a deeper
3
He minimum (Fig. 5.8.1b, see Plate 5.8.1b, p. 428).
In addition, the tritiogenic
3
He flux is large compared with the mantle-derived flux (Jenkins,
1996), and there is little hydrographic evidence of
large-scale vertical exchange of water across these
horizons (e.g. see Wijffels et al., 1996a).
Figure 5.8.2 represents the same meridional section, with the exception that
3
He is plotted versus
potential density anomaly instead of depth. A
strong front in both Tritium and
3
He exists at the
5.8 Transformation and Age of Water Masses
433
Schlosser et al.
background ␦
3
He values in the deep ocean.
Finally,
4 He is injected into the ocean through
the addition of glacial meltwater (Schlosser,
1986; Schlosser et al., 1990; Weppernig et al.,
1996).
4 As tests for numerical models of ocean circulation and mixing. Tracer distributions are controlled by their input to the ocean and by
advective and mixing processes. Thus, tracer
distributions in ocean circulation models are
sensitive to these parameters in the model.
The magnitude of the WOCE programme contributed to technological advances in tracer
oceanography. These advances reached from the
establishment of new laboratories (e.g. the Accelerator Mass Spectrometry Facility at the Woods
Hole Oceanographic Institution) to refinements of
existing technologies. Such refinements include,
for example, the preparation at sea of helium isotope samples for mass spectrometric measurement
(Jenkins et al., 1991) or the advancement of CFC
measurement techniques (e.g. Bulsiewicz et al.,
1998).
The establishment of the US National Oceanographic Sciences Accelerator Mass Spectrometry facility (NOSAMS) at the Woods Hole
Oceanographic Institution (e.g. Jones et al., 1994;
McNichol et al., 1994; Schneider et al., 1994; von
Reden et al., 1994) probably had the most significant impact on the WOCE tracer programme.
During GEOSECS,
14 C measurements required the
extraction of the total inorganic carbon from
approximately 250 litres of seawater. The collection of such large amounts of water was only possible through the use of Gerard–Ewing samplers,
which require extra casts at each
14 C station.
Given the large number of
14 C samples planned
for the WOCE survey, this method would have
required too much ship time and the
14 C programme would have had to be reduced significantly, with serious implications for the value of
the resulting data set. Therefore, during the US
Pacific WOCE field work (Key, 1996), both largevolume and small-volume AMS samples were collected along many of the WOCE sections. Both the
proven -counting Large-Volume (LV) technique,
which had a precision of <2–4‰, and the AMS
technique, with a precision of Ϸ<4‰ (Elder et al.,
1998), were utilized.
5.8.3 Exemplary results
5.8.3.1 Thermocline ventilation in the
Pacific Ocean using tritium/
3 He data
The subsurface transport of subducted surface
waters from the subtropics to the tropics has been
implicated in the long-term modulation of El Niño
events in the Pacific (e.g. Gu and Philander, 1997;
Guilderson and Schrag, 1997). Fine et al. (1987)
identified a tongue of tritium penetrating the tropics, tracing the cell in the North Pacific, and
thereby placing an upper bound on the time scales
associated with the transit of these waters. The
WOCE Pacific expedition provided us with an
opportunity to delineate this circulation more completely, and refine our knowledge of its character.
A 1991 tritium section along 135°W (P17C,
Fig. 5.8.1a, see Plate 5.8.1a, p. 428) traces the
equatorward movement from the subtropical
North Pacific of the tritium tongue, which extends
more than 3000 km meridionally. As it extends
southward, it follows isopycnals (26.0: :
26.5 kg m
93 ), rising from a depth of approximately 250 m at 30°N to about 75 m at 10°N,
then deepening about 100 m at the equator. The
tritium tongue is underlain by a
3 He maximum
(Fig. 5.8.1b, see Plate 5.8.1b, p. 428) at a depth
of about 450 m in the subtropics. This deeper,
tritiogenic
3 He maximum is characteristic of the
subtropical thermocline, and is a widespread feature in other regions of the world ocean such as
the North Atlantic (e.g. Jenkins, 1998) or the
Nordic seas (Schlosser et al., 1995). As the tongues
extend into the tropics, the tritium and
3 He maxima become largely depth-coincident, supporting a
net meridional advective origin for their existence.
The shallow distribution of excess
3 He, especially in the North Pacific, is dominated by in-situ
decay of tritium (Jenkins, 1996). This conclusion
is supported by fact that the shallow
3
He maximum is almost everywhere underlain by a deeper
3
He minimum (Fig. 5.8.1b, see Plate 5.8.1b, p. 428).
In addition, the tritiogenic
3
He flux is large compared with the mantle-derived flux (Jenkins,
1996), and there is little hydrographic evidence of
large-scale vertical exchange of water across these
horizons (e.g. see Wijffels et al., 1996a).
Figure 5.8.2 represents the same meridional section, with the exception that
3
He is plotted versus
potential density anomaly instead of depth. A
strong front in both Tritium and
3
He exists at the
5.8 Transformation and Age of Water Masses
433
Schlosser et al.