portion of these programs was directed by W.
Broecker. O ¨ stlund made the D
14
C measurements
with d
13 C provided by Stuiver using the GEOSECS
procedures. Comparison of TTO results to GEOSECS gave the first clear evidence of the penetration
of the bomb-produced radiocarbon signal into the
subsurface North Atlantic waters. The French carried out a smaller scale (INDIGO)
14 C program in
the Indian Ocean during this time with O ¨ stlund and
P. Quay (University of Washington) collaborating.
These data also quantified upper ocean changes since
GEOSECS and relied on the same techniques.
The most recent oceanic survey was carried out
during the 1990s as part of the World Ocean Circulation Experiment (WOCE). This program was a
multinational effort. The US
14 C sampling effort was
heavily focused on the Pacific (1991–1993) and Indian oceans (1995–1996) since TTO and SAVE had
provided reasonable Atlantic coverage. R. Key
(Princeton University) directed the US radiocarbon
effort with collaboration from P. Schlosser (LDEO)
and Quay. In the deep Pacific where gradients were
known to be small, most radiocarbon sampling was
by the proven large-volume b technique. The Pacific
thermocline, however, was sampled using the AMS
technique. Shifting techniques allowed thermocline
waters to be sampled at approximately 2–3 times the
horizontal density used for large volume sampling.
O ¨ stlund and Stuiver again measured the large-volume samples while the AMS samples were measured
at the National Ocean Sciences AMS facility
(NOSAMS) at Woods Hole Oceanographic Institution. By 1994 the analytical precision at NOSAMS
had improved to the point that all US Indian Ocean
WOCE
14 C sampling used this technique. WOCE
sampling increased the total number of
14
C results
for the Pacific and Indian Oceans by approximately
an order of magnitude. Analysis of the Pacific Ocean
samples was completed in 1998. US WOCE
14
C
sampling in the Atlantic was restricted to two zonal
sections in the north-west basin using the AMS
technique. Analysis of the Atlantic and Indian Ocean
samples is expected to be finished during 2000–2001.
D
14 C Distribution and Implications for
Large-scale Circulation
The distribution of radiocarbon in the ocean is
controlled by the production rate in the atmosphere,
the spatial variability and magnitude of
14
CO 2 flux
across the air–sea interface, oceanic circulation and
mixing, and the carbon cycle in the ocean. Figure 4
shows average vertical radiocarbon profiles for the
Pacific, Atlantic, Southern, and Indian oceans with
the dotted line being southern basin and solid line
northern basin. All of the profiles have higher D
14
C
in shallow waters, reflecting proximity to the atmospheric source. The different collection times
combined with the penetration of the bomb-produced signal into the upper thermocline negate the
possibility of detailed comparison for the upper 600–
800 dB (deeper for the North Atlantic). Detailed
comparison is justified for deeper levels. The strongest signal in deep and bottom waters is that the
North Atlantic is significantly younger (higher D
14 C)
than the South Atlantic, while the opposite holds for
the Pacific. Second, the average age of deep water
increases (D
14 C decreases) from Atlantic to Indian to
Pacific. Third, the Southern Ocean D
14
C is very
uniform below approximately 1800 dB at a level
(B À 160 ppt). This is similar to the near bottom
water values for all three southern ocean basins. All
three differences are directly attributable to the largescale thermohaline circulation.
Figure 5 shows meridional sections for the Atlantic, Indian and Pacific oceans using subsets of the
data from Figure 4. As with Figure 4, the D
14
C values
in the upper water column have been increased by
invasion of the bomb signal. The pattern of these
contours, however, is generally representative of the
natural D
14
C signal. The D
14 C ¼ À 100% contour
can be taken as the approximate demarcation between the bomb-contaminated waters and those
having only natural radiocarbon.
Comparison of the major features in each section
shows that the meridional D
14
C distributions in the
Pacific and Indian Oceans are quite similar. The
greatest difference between these two is that the Indian Ocean deep water (1500–3500 m) is significantly
younger than Pacific deep waters. In both oceans:
• The near bottom water has higher D
14 C than the
overlying deep water.
• The deep and bottom waters have higher D
14 C at
the south than the north.
• The lowest D
14 C values are found as a tongue
extending southward from the north end of the
section at a depth of B2500 m.
• Deep and bottom water at the south end of each
section is relatively uniform with D
14
CB À 160 ppt.
• The D
14
C gradient with latitude from south to
north is approximately the same for both deep
waters and for bottom waters.
• The D
14 C contours in the thermocline shoal both
at the equator and high latitudes. (This feature is
suppressed in the North Indian Ocean owing to
the limited geographic extent and the influence of
flows through the Indonesian Seas region and
from the Arabian Sea.)
240 RADIOCARBON
Broecker. O ¨ stlund made the D
14
C measurements
with d
13 C provided by Stuiver using the GEOSECS
procedures. Comparison of TTO results to GEOSECS gave the first clear evidence of the penetration
of the bomb-produced radiocarbon signal into the
subsurface North Atlantic waters. The French carried out a smaller scale (INDIGO)
14 C program in
the Indian Ocean during this time with O ¨ stlund and
P. Quay (University of Washington) collaborating.
These data also quantified upper ocean changes since
GEOSECS and relied on the same techniques.
The most recent oceanic survey was carried out
during the 1990s as part of the World Ocean Circulation Experiment (WOCE). This program was a
multinational effort. The US
14 C sampling effort was
heavily focused on the Pacific (1991–1993) and Indian oceans (1995–1996) since TTO and SAVE had
provided reasonable Atlantic coverage. R. Key
(Princeton University) directed the US radiocarbon
effort with collaboration from P. Schlosser (LDEO)
and Quay. In the deep Pacific where gradients were
known to be small, most radiocarbon sampling was
by the proven large-volume b technique. The Pacific
thermocline, however, was sampled using the AMS
technique. Shifting techniques allowed thermocline
waters to be sampled at approximately 2–3 times the
horizontal density used for large volume sampling.
O ¨ stlund and Stuiver again measured the large-volume samples while the AMS samples were measured
at the National Ocean Sciences AMS facility
(NOSAMS) at Woods Hole Oceanographic Institution. By 1994 the analytical precision at NOSAMS
had improved to the point that all US Indian Ocean
WOCE
14 C sampling used this technique. WOCE
sampling increased the total number of
14
C results
for the Pacific and Indian Oceans by approximately
an order of magnitude. Analysis of the Pacific Ocean
samples was completed in 1998. US WOCE
14
C
sampling in the Atlantic was restricted to two zonal
sections in the north-west basin using the AMS
technique. Analysis of the Atlantic and Indian Ocean
samples is expected to be finished during 2000–2001.
D
14 C Distribution and Implications for
Large-scale Circulation
The distribution of radiocarbon in the ocean is
controlled by the production rate in the atmosphere,
the spatial variability and magnitude of
14
CO 2 flux
across the air–sea interface, oceanic circulation and
mixing, and the carbon cycle in the ocean. Figure 4
shows average vertical radiocarbon profiles for the
Pacific, Atlantic, Southern, and Indian oceans with
the dotted line being southern basin and solid line
northern basin. All of the profiles have higher D
14
C
in shallow waters, reflecting proximity to the atmospheric source. The different collection times
combined with the penetration of the bomb-produced signal into the upper thermocline negate the
possibility of detailed comparison for the upper 600–
800 dB (deeper for the North Atlantic). Detailed
comparison is justified for deeper levels. The strongest signal in deep and bottom waters is that the
North Atlantic is significantly younger (higher D
14 C)
than the South Atlantic, while the opposite holds for
the Pacific. Second, the average age of deep water
increases (D
14 C decreases) from Atlantic to Indian to
Pacific. Third, the Southern Ocean D
14
C is very
uniform below approximately 1800 dB at a level
(B À 160 ppt). This is similar to the near bottom
water values for all three southern ocean basins. All
three differences are directly attributable to the largescale thermohaline circulation.
Figure 5 shows meridional sections for the Atlantic, Indian and Pacific oceans using subsets of the
data from Figure 4. As with Figure 4, the D
14
C values
in the upper water column have been increased by
invasion of the bomb signal. The pattern of these
contours, however, is generally representative of the
natural D
14
C signal. The D
14 C ¼ À 100% contour
can be taken as the approximate demarcation between the bomb-contaminated waters and those
having only natural radiocarbon.
Comparison of the major features in each section
shows that the meridional D
14
C distributions in the
Pacific and Indian Oceans are quite similar. The
greatest difference between these two is that the Indian Ocean deep water (1500–3500 m) is significantly
younger than Pacific deep waters. In both oceans:
• The near bottom water has higher D
14 C than the
overlying deep water.
• The deep and bottom waters have higher D
14 C at
the south than the north.
• The lowest D
14 C values are found as a tongue
extending southward from the north end of the
section at a depth of B2500 m.
• Deep and bottom water at the south end of each
section is relatively uniform with D
14
CB À 160 ppt.
• The D
14
C gradient with latitude from south to
north is approximately the same for both deep
waters and for bottom waters.
• The D
14 C contours in the thermocline shoal both
at the equator and high latitudes. (This feature is
suppressed in the North Indian Ocean owing to
the limited geographic extent and the influence of
flows through the Indonesian Seas region and
from the Arabian Sea.)
240 RADIOCARBON