(
3 He/
4
He: 3–410
95 ) is a factor of Ϸ3 higher
than that of the helium injected at Mid-Ocean
Ridges (Lupton, 1996). This raises the possibility
that helium from seamounts such as Loihi could
be distinguished from Mid-Ocean Ridge helium
plumes using high-precision helium isotopic measurements.
It is of interest to compare the pathways of specific water masses that can be inferred from the
helium pattern with other descriptions of the intermediate and deep circulation. The helium distribution requires a circulation pattern which differs
from the classical model for the deep Pacific circulation (Stommel and Arons, 1960b), but is consistent (Lupton, 1998) with the circulation patterns
predicted by Reid (1997) on the basis of a steric
height analysis of hydrographic data. In particular,
all of the major pathways predicted by the helium
plumes agree with Reid’s steric height analysis.
This comparison would not have been possible
before the WOCE Pacific sections were completed,
since neither the steric height maps nor the helium
distribution had sufficient resolution for this
comparison.
5.8.3.5 Deep and bottom water circulation
in the Pacific derived from
14 C data
After initial studies of the deep water renewal
times using
14 C, the first attempt to determine the
global distribution of radiocarbon in the ocean
was made during the GEOSECS expedition
(1971–78). Much of the utility of the GEOSECS
⌬
14 C data set has been drawn from its assessment
of the ocean’s uptake of bomb
14 C (Broecker et al.,
1985c, 1995), as well as from refinement of the
deep water renewal times. Defining the limits of
the bomb
14
C ‘dye patch’ offers a direct way of
visualizing the ventilation of the thermocline over
a decadal time scale. Quantifying the total inventory of bomb
14 C in the ocean has been used to set
limits on the rate at which
14 CO 2 invades the
ocean across the air–sea boundary. During the
1980s, TTO and SAVE increased the Atlantic
Ocean radiocarbon data set significantly and provided the first opportunity to study how the
bomb-produced inventory and distribution had
changed during the years since GEOSECS. No
large-scale radiocarbon programmes were carried
out in the Pacific during the 1980s and only the
relatively limited INDIGO expeditions provided
14 C data for the Indian Ocean.
Although the WOCE ⌬
14
C data is still sparse
compared with the standard hydrographic survey,
it represents an unprecedented global data set
for studies of the deep circulation and the penetration of the bomb signal into the deep ocean.
It is generally accepted that Pacific bottom
waters are ventilated by the northward flow of
Circumpolar (Common) Deep Water. The primary
pathway is as a deep western boundary current
passing along the Tonga–Kermadec trench (e.g.
Whitworth et al., 1999). As it moves northward,
part of the bottom water is lost due to slow
upwelling. Significant questions exist about the
depth to which this upwelling occurs and about
the primary return pathway of the modified
bottom water to the south.
14 C is an ideal tracer to
study the long-term mean of this flow pattern.
Additionally, the
14 C distribution can be used to
estimate mean net transports and residence times.
Based on the GEOSECS radiocarbon data,
Stuiver et al. (1983) estimated a net transport of
circumpolar water to the North Pacific of 25
10
6 m
3 s
91
. This transport implied a deep water
mean residence time of 510 years and an
upwelling rate of 5 m yr
91
. The calculations were
based on the difference in ⌬
14 C between circumpolar source waters (⌬
14 CϷ9160‰) and the lowest
values found at the presumed mid-depth return
flow in the North Pacific (⌬
14 CϷ9256‰). Given
this relatively simple approach, the WOCE Pacific
measurements confirm the GEOSECS results
(⌬
14
C rangeϷ9160 to 9260‰). These transport
and upwelling estimates were in reasonable agreement with similar estimates based on geostrophic
calculations (e.g. Warren, 1973).
However, due to the lack of spatial resolution,
the GEOSECS data set is of limited value for studies of the overall flow pattern. The WOCE data set
represents a major step forward with respect to
this problem. The spatial resolution of the WOCE
data is high enough that a reasonably detailed
picture of the mean large-scale bottom water flow
can now be inferred. Figure 5.8.17 (see Plate
5.8.17, p. 428) shows an objectively contoured
distribution of the near-bottom ⌬
14 C values for
the entire Pacific. Both GEOSECS and WOCE data
were used in constructing the map. The deepest
data points from stations where the bottom depth
was at least 3500 m and where the deepest data
point was within 250 m of the bottom were used.
The northward flow of Circumpolar Deep Water
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
444
3 He/
4
He: 3–410
95 ) is a factor of Ϸ3 higher
than that of the helium injected at Mid-Ocean
Ridges (Lupton, 1996). This raises the possibility
that helium from seamounts such as Loihi could
be distinguished from Mid-Ocean Ridge helium
plumes using high-precision helium isotopic measurements.
It is of interest to compare the pathways of specific water masses that can be inferred from the
helium pattern with other descriptions of the intermediate and deep circulation. The helium distribution requires a circulation pattern which differs
from the classical model for the deep Pacific circulation (Stommel and Arons, 1960b), but is consistent (Lupton, 1998) with the circulation patterns
predicted by Reid (1997) on the basis of a steric
height analysis of hydrographic data. In particular,
all of the major pathways predicted by the helium
plumes agree with Reid’s steric height analysis.
This comparison would not have been possible
before the WOCE Pacific sections were completed,
since neither the steric height maps nor the helium
distribution had sufficient resolution for this
comparison.
5.8.3.5 Deep and bottom water circulation
in the Pacific derived from
14 C data
After initial studies of the deep water renewal
times using
14 C, the first attempt to determine the
global distribution of radiocarbon in the ocean
was made during the GEOSECS expedition
(1971–78). Much of the utility of the GEOSECS
⌬
14 C data set has been drawn from its assessment
of the ocean’s uptake of bomb
14 C (Broecker et al.,
1985c, 1995), as well as from refinement of the
deep water renewal times. Defining the limits of
the bomb
14
C ‘dye patch’ offers a direct way of
visualizing the ventilation of the thermocline over
a decadal time scale. Quantifying the total inventory of bomb
14 C in the ocean has been used to set
limits on the rate at which
14 CO 2 invades the
ocean across the air–sea boundary. During the
1980s, TTO and SAVE increased the Atlantic
Ocean radiocarbon data set significantly and provided the first opportunity to study how the
bomb-produced inventory and distribution had
changed during the years since GEOSECS. No
large-scale radiocarbon programmes were carried
out in the Pacific during the 1980s and only the
relatively limited INDIGO expeditions provided
14 C data for the Indian Ocean.
Although the WOCE ⌬
14
C data is still sparse
compared with the standard hydrographic survey,
it represents an unprecedented global data set
for studies of the deep circulation and the penetration of the bomb signal into the deep ocean.
It is generally accepted that Pacific bottom
waters are ventilated by the northward flow of
Circumpolar (Common) Deep Water. The primary
pathway is as a deep western boundary current
passing along the Tonga–Kermadec trench (e.g.
Whitworth et al., 1999). As it moves northward,
part of the bottom water is lost due to slow
upwelling. Significant questions exist about the
depth to which this upwelling occurs and about
the primary return pathway of the modified
bottom water to the south.
14 C is an ideal tracer to
study the long-term mean of this flow pattern.
Additionally, the
14 C distribution can be used to
estimate mean net transports and residence times.
Based on the GEOSECS radiocarbon data,
Stuiver et al. (1983) estimated a net transport of
circumpolar water to the North Pacific of 25
10
6 m
3 s
91
. This transport implied a deep water
mean residence time of 510 years and an
upwelling rate of 5 m yr
91
. The calculations were
based on the difference in ⌬
14 C between circumpolar source waters (⌬
14 CϷ9160‰) and the lowest
values found at the presumed mid-depth return
flow in the North Pacific (⌬
14 CϷ9256‰). Given
this relatively simple approach, the WOCE Pacific
measurements confirm the GEOSECS results
(⌬
14
C rangeϷ9160 to 9260‰). These transport
and upwelling estimates were in reasonable agreement with similar estimates based on geostrophic
calculations (e.g. Warren, 1973).
However, due to the lack of spatial resolution,
the GEOSECS data set is of limited value for studies of the overall flow pattern. The WOCE data set
represents a major step forward with respect to
this problem. The spatial resolution of the WOCE
data is high enough that a reasonably detailed
picture of the mean large-scale bottom water flow
can now be inferred. Figure 5.8.17 (see Plate
5.8.17, p. 428) shows an objectively contoured
distribution of the near-bottom ⌬
14 C values for
the entire Pacific. Both GEOSECS and WOCE data
were used in constructing the map. The deepest
data points from stations where the bottom depth
was at least 3500 m and where the deepest data
point was within 250 m of the bottom were used.
The northward flow of Circumpolar Deep Water
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
444