along the South Pacific island arch–trench system
near the dateline is strongly implied by the relatively high ⌬
14 C values. Across the equator most
of the water appears to turn westward and then
flows clockwise around the basin. Details of the
near-bottom ⌬
14 C distribution in the North Pacific
are difficult or impossible to determine from this
map due to the colour scale, the large south-tonorth ⌬
14
C gradient, and the limited data resolution. However, the implied flow pattern can be
traced to at least the Aleutian Island Arch in manually drawn maps combined with meridional vertical sections. Eastward flow over the Aleutian
Trench is consistent with detailed current studies
in this region (Warren and Owens, 1988). If the
bottom flow is continuous around the edge of the
basin from the far western edge to the northern
edge, the circulation pattern is different from that
described by Wijffels et al. (1998), who used data
from the WOCE P10 section at 149°E. They concluded that the deep and bottom water flow was
clockwise and against the western boundary until
it reached approximately 20°N. At that point they
describe the flow as turning directly eastward and
ascribe the deep chemical front found at 20°N to
the juxtaposition of this current and one representing southward flow along the westward boundary
up to 20°N, where it turns eastward. Based on the
near-bottom ⌬
14 C distribution, some fraction of
the bottom water crossing the equator splits from
the main flow and continues to flow toward the
northeast where it passes south of the Hawaiian
Island chain.
Figure 5.8.17 (see Plate 5.8.17, p. 428) indicates
that the oldest bottom waters are found in the
northeastern Pacific (⌬
14
C Ϸ9220‰), but not
immediately adjacent to the North American continent. The separation of the oldest waters from both
the Aleutians to the north and North America to
the east can be clearly seen in meridional sections
through the region (P16 and P17; Ϸ170 and
135°W, respectively). The displacement of the oldest water away from the boundary conflicts with
what was implied by the GEOSECS section in
the eastern Pacific, which did not extend north
of Ϸ35°N. It is also inconsistent with the results
from all numerical ocean models that include simulations of
14 C (e.g. Toggweiler et al., 1991).
These model simulations suggest that the oldest
waters should be found adjacent to the continental rise.
Old bottom water (⌬
14 C Ϸ9210‰) extends
southward to at least 20°S in the eastern basin.
South of this point, the meridional ⌬
14 C gradient
is quite strong, indicating significant mixing with
waters of southern origin. Northward transport of
Circumpolar Deep Water in the southeastern
South Pacific is topographically blocked initially
by the Chile Rise at about 40°S and subsequently
by the Sala Y T Gomez Ridge at about 25°S.
Key et al. (1996) used an advection–diffusion
model and the WOCE data available at the time to
calculate a net bottom water transport across the
equator of Ϸ20 10
6 m
3 s
91 and a resulting
upwelling rate of 9 m yr
91 . These values agree
with Warren (1973), but are somewhat higher
than more recent estimates (Whitworth et al.,
1999). They are inconsistent with inversion results
using most of the WOCE zonal sections and some
meridional sections (Ganachaud and Wunsch,
1998; A. Ganachaud, personal communication,
1999). The results from this inversion of hydrographic lines predict a total northward transport
of Circumpolar Deep Water of (8 <2)
10
6 m
3 s
91 . Further analysis will be required to
resolve this discrepancy.
Between the northward-flowing Circumpolar
Deep Water and the thermocline waters, Pacific
Deep Water is observed. North Pacific Deep Water
is the oldest and most isolated water mass in the
World Ocean. There is significant information in
the
14 C data on the return flow of northward
flowing bottom waters. Strong evidence for this
return flow is obtained by examining data from
WOCE cruise P6 (zonal section at Ϸ32°S). Figure
5.8.18 shows the P6 ⌬
14 C results for depths
greater than 1000 m. It contains three prominent
features: (1) The waters with the highest ⌬
14
C values included in the figure are observed at 1000 m
depth near the base of the thermocline. (2) Two
low-⌬
14 C areas around longitudes 155°W and
75°W and centred around Ϸ2600 m depth with
⌬
14 CϷ9210‰. (3) A relatively high-concentration region centred at Ϸ175°W and 5000 m depth
with ⌬
14 C values of Ϸ9170‰.
The relatively high ⌬
14 C values of 9150‰ at
the base of the thermocline are not surprising.
They appear as a ‘feature’ simply because they are
significantly higher than values normally found in
deep and bottom waters. The relatively high values
found between 180°W and 150°W are in the
core of the northward-flowing Circumpolar Deep
5.8 Transformation and Age of Water Masses
445
Schlosser et al.
near the dateline is strongly implied by the relatively high ⌬
14 C values. Across the equator most
of the water appears to turn westward and then
flows clockwise around the basin. Details of the
near-bottom ⌬
14 C distribution in the North Pacific
are difficult or impossible to determine from this
map due to the colour scale, the large south-tonorth ⌬
14
C gradient, and the limited data resolution. However, the implied flow pattern can be
traced to at least the Aleutian Island Arch in manually drawn maps combined with meridional vertical sections. Eastward flow over the Aleutian
Trench is consistent with detailed current studies
in this region (Warren and Owens, 1988). If the
bottom flow is continuous around the edge of the
basin from the far western edge to the northern
edge, the circulation pattern is different from that
described by Wijffels et al. (1998), who used data
from the WOCE P10 section at 149°E. They concluded that the deep and bottom water flow was
clockwise and against the western boundary until
it reached approximately 20°N. At that point they
describe the flow as turning directly eastward and
ascribe the deep chemical front found at 20°N to
the juxtaposition of this current and one representing southward flow along the westward boundary
up to 20°N, where it turns eastward. Based on the
near-bottom ⌬
14 C distribution, some fraction of
the bottom water crossing the equator splits from
the main flow and continues to flow toward the
northeast where it passes south of the Hawaiian
Island chain.
Figure 5.8.17 (see Plate 5.8.17, p. 428) indicates
that the oldest bottom waters are found in the
northeastern Pacific (⌬
14
C Ϸ9220‰), but not
immediately adjacent to the North American continent. The separation of the oldest waters from both
the Aleutians to the north and North America to
the east can be clearly seen in meridional sections
through the region (P16 and P17; Ϸ170 and
135°W, respectively). The displacement of the oldest water away from the boundary conflicts with
what was implied by the GEOSECS section in
the eastern Pacific, which did not extend north
of Ϸ35°N. It is also inconsistent with the results
from all numerical ocean models that include simulations of
14 C (e.g. Toggweiler et al., 1991).
These model simulations suggest that the oldest
waters should be found adjacent to the continental rise.
Old bottom water (⌬
14 C Ϸ9210‰) extends
southward to at least 20°S in the eastern basin.
South of this point, the meridional ⌬
14 C gradient
is quite strong, indicating significant mixing with
waters of southern origin. Northward transport of
Circumpolar Deep Water in the southeastern
South Pacific is topographically blocked initially
by the Chile Rise at about 40°S and subsequently
by the Sala Y T Gomez Ridge at about 25°S.
Key et al. (1996) used an advection–diffusion
model and the WOCE data available at the time to
calculate a net bottom water transport across the
equator of Ϸ20 10
6 m
3 s
91 and a resulting
upwelling rate of 9 m yr
91 . These values agree
with Warren (1973), but are somewhat higher
than more recent estimates (Whitworth et al.,
1999). They are inconsistent with inversion results
using most of the WOCE zonal sections and some
meridional sections (Ganachaud and Wunsch,
1998; A. Ganachaud, personal communication,
1999). The results from this inversion of hydrographic lines predict a total northward transport
of Circumpolar Deep Water of (8 <2)
10
6 m
3 s
91 . Further analysis will be required to
resolve this discrepancy.
Between the northward-flowing Circumpolar
Deep Water and the thermocline waters, Pacific
Deep Water is observed. North Pacific Deep Water
is the oldest and most isolated water mass in the
World Ocean. There is significant information in
the
14 C data on the return flow of northward
flowing bottom waters. Strong evidence for this
return flow is obtained by examining data from
WOCE cruise P6 (zonal section at Ϸ32°S). Figure
5.8.18 shows the P6 ⌬
14 C results for depths
greater than 1000 m. It contains three prominent
features: (1) The waters with the highest ⌬
14
C values included in the figure are observed at 1000 m
depth near the base of the thermocline. (2) Two
low-⌬
14 C areas around longitudes 155°W and
75°W and centred around Ϸ2600 m depth with
⌬
14 CϷ9210‰. (3) A relatively high-concentration region centred at Ϸ175°W and 5000 m depth
with ⌬
14 C values of Ϸ9170‰.
The relatively high ⌬
14 C values of 9150‰ at
the base of the thermocline are not surprising.
They appear as a ‘feature’ simply because they are
significantly higher than values normally found in
deep and bottom waters. The relatively high values
found between 180°W and 150°W are in the
core of the northward-flowing Circumpolar Deep
5.8 Transformation and Age of Water Masses
445
Schlosser et al.