• More precise determination of the half-life by
three different laboratories.
• Recognition by Hans Suess, while at the USGS and
Scripps Institution of Oceanography, that radiocarbon in modern samples (since the beginning of
the industrial revolution) was being diluted by
anthropogenic CO 2 addition to the atmosphere
and biosphere.
• Recognition that atmospheric and oceanic D
14
C
levels were increasing as a result of atmospheric
testing of nuclear weapons.
During the 1970s and 1980s incremental changes
in technique and equipment further increased the
precision and lowered the counting background.
With respect to the ocean, this was a period of sample
collection, analysis, and interpretation. The next
significant change occurred during the 1990s with
application of the accelerator mass spectrometry
(AMS) technique to oceanic samples. This technique
counts
14
C atoms rather than detecting the energy
released when a
14
C atom decays. The AMS technique allowed reduction of the sample size required for
oceanic D
14
C determination from approximately 250
liters of water to 250 milliliters! By 1995 the AMS
technique was yielding results that were as good as
the best prior techniques using large samples and
decay counting. This size reduction and concurrent
automation procedures had a profound effect on sea
water D
14
C determination. Many of the AMS techniques were developed and most of the oceanographic AMS D
14
C measurements have been made at
the National Ocean Sciences AMS facility in Woods
Hole, Massachusetts, by Ann McNichol, Robert
Schneider, and Karl von Reden under the initial direction of Glenn Jones and more recently John Hayes.
The natural concentration of
14
C in sea water is
extremely low (B1 Â 410
9 atoms kg
À1
). Prior to
AMS, the only available technique to measure this
low concentration was radioactive counting using
either gas proportional or liquid scintillation detectors. Large sample were needed to obtain high
precision and to keep counting times reasonable.
Between about 1960 and 1995 most subsurface
open-ocean radiocarbon water samples were collected using a Gerard–Ewing sampler commonly
known as a Gerard barrel. The final design of the
Gerard barrel consisted of a stainless steel cylinder
with a volume of approximately 270 liters. An external scoop and an internal divider running the
length of the cylinder resulted in efficient flushing
while the barrel was lowered through the water on
wire rope. When the barrel was returned to the ship
deck, the water was transferred to a gas-tight container and acidified to convert carbonate species to
CO 2 . The CO 2 was swept from the water with a
stream of inert gas and absorbed in a solution of
sodium hydroxide. The solution was returned to
shore where the CO 2 was extracted, purified, and
counted. When carefully executed, the procedure
produced results which were accurate to 2–4 ppt
based on counting errors alone. Because of the expense, time, and difficulty, samples for replicate
analyses were almost never collected.
With the AMS technique only 0.25 liter of sea
water is required. Generally a 0.5 liter water sample
is collected at sea and poisoned with HgCl 2 to halt
all biological activity. The water is returned to the
laboratory and acidified, and the CO 2 is extracted
and purified. An aliquot of the CO 2 is analyzed to
determine d
13
C and the remainder is converted to
carbide and counted by AMS. Counting error for the
AMS technique can be o2 ppt, however, replicate
analysis shows the total sample error to be approximately 4.5 ppt.
Sampling History
Soon after the radiocarbon dating method was developed, it was applied to oceanic and atmospheric
samples. During the 1950s and 1960s most of the
oceanographic samples were limited to the shallow
waters owing to the difficulty of deep water sampling
combined with the limited analytical precision. The
majority of the early samples were collected in the
Atlantic Ocean and the South-west Pacific Ocean.
Early sample coverage was insufficient to give a good
description of the global surface ocean radiocarbon
content prior to the onset of atmospheric testing of
thermonuclear weapons; however, repeated sampling
at the same location was sufficient to record the
surface water increase due to bomb-produced fallout.
A very good history of radiocarbon activity, including
the increase due to bomb tests and subsequent decrease, exists primarily as a result of the work of R.
Nydal and co-workers (Trondheim) and K. Munnich
and co-workers (Heidelberg).
The primary application of early radiocarbon results was to estimate the flux of CO 2 between the
atmosphere and ocean and the average residence
time in the ocean. Sufficient subsurface ocean measurements were made, primarily by W. Broecker
(Lamont–Doherty Earth Observatory LDEO) and H.
Craig (Scripps Institution of Oceanography SIO), to
recognize that radiocarbon had the potential to be an
important tracer of deep ocean circulation and
mixing rates.
During the 1970s the Geochemical Ocean Sections
(GEOSECS) program provided the first full water
238 RADIOCARBON
three different laboratories.
• Recognition by Hans Suess, while at the USGS and
Scripps Institution of Oceanography, that radiocarbon in modern samples (since the beginning of
the industrial revolution) was being diluted by
anthropogenic CO 2 addition to the atmosphere
and biosphere.
• Recognition that atmospheric and oceanic D
14
C
levels were increasing as a result of atmospheric
testing of nuclear weapons.
During the 1970s and 1980s incremental changes
in technique and equipment further increased the
precision and lowered the counting background.
With respect to the ocean, this was a period of sample
collection, analysis, and interpretation. The next
significant change occurred during the 1990s with
application of the accelerator mass spectrometry
(AMS) technique to oceanic samples. This technique
counts
14
C atoms rather than detecting the energy
released when a
14
C atom decays. The AMS technique allowed reduction of the sample size required for
oceanic D
14
C determination from approximately 250
liters of water to 250 milliliters! By 1995 the AMS
technique was yielding results that were as good as
the best prior techniques using large samples and
decay counting. This size reduction and concurrent
automation procedures had a profound effect on sea
water D
14
C determination. Many of the AMS techniques were developed and most of the oceanographic AMS D
14
C measurements have been made at
the National Ocean Sciences AMS facility in Woods
Hole, Massachusetts, by Ann McNichol, Robert
Schneider, and Karl von Reden under the initial direction of Glenn Jones and more recently John Hayes.
The natural concentration of
14
C in sea water is
extremely low (B1 Â 410
9 atoms kg
À1
). Prior to
AMS, the only available technique to measure this
low concentration was radioactive counting using
either gas proportional or liquid scintillation detectors. Large sample were needed to obtain high
precision and to keep counting times reasonable.
Between about 1960 and 1995 most subsurface
open-ocean radiocarbon water samples were collected using a Gerard–Ewing sampler commonly
known as a Gerard barrel. The final design of the
Gerard barrel consisted of a stainless steel cylinder
with a volume of approximately 270 liters. An external scoop and an internal divider running the
length of the cylinder resulted in efficient flushing
while the barrel was lowered through the water on
wire rope. When the barrel was returned to the ship
deck, the water was transferred to a gas-tight container and acidified to convert carbonate species to
CO 2 . The CO 2 was swept from the water with a
stream of inert gas and absorbed in a solution of
sodium hydroxide. The solution was returned to
shore where the CO 2 was extracted, purified, and
counted. When carefully executed, the procedure
produced results which were accurate to 2–4 ppt
based on counting errors alone. Because of the expense, time, and difficulty, samples for replicate
analyses were almost never collected.
With the AMS technique only 0.25 liter of sea
water is required. Generally a 0.5 liter water sample
is collected at sea and poisoned with HgCl 2 to halt
all biological activity. The water is returned to the
laboratory and acidified, and the CO 2 is extracted
and purified. An aliquot of the CO 2 is analyzed to
determine d
13
C and the remainder is converted to
carbide and counted by AMS. Counting error for the
AMS technique can be o2 ppt, however, replicate
analysis shows the total sample error to be approximately 4.5 ppt.
Sampling History
Soon after the radiocarbon dating method was developed, it was applied to oceanic and atmospheric
samples. During the 1950s and 1960s most of the
oceanographic samples were limited to the shallow
waters owing to the difficulty of deep water sampling
combined with the limited analytical precision. The
majority of the early samples were collected in the
Atlantic Ocean and the South-west Pacific Ocean.
Early sample coverage was insufficient to give a good
description of the global surface ocean radiocarbon
content prior to the onset of atmospheric testing of
thermonuclear weapons; however, repeated sampling
at the same location was sufficient to record the
surface water increase due to bomb-produced fallout.
A very good history of radiocarbon activity, including
the increase due to bomb tests and subsequent decrease, exists primarily as a result of the work of R.
Nydal and co-workers (Trondheim) and K. Munnich
and co-workers (Heidelberg).
The primary application of early radiocarbon results was to estimate the flux of CO 2 between the
atmosphere and ocean and the average residence
time in the ocean. Sufficient subsurface ocean measurements were made, primarily by W. Broecker
(Lamont–Doherty Earth Observatory LDEO) and H.
Craig (Scripps Institution of Oceanography SIO), to
recognize that radiocarbon had the potential to be an
important tracer of deep ocean circulation and
mixing rates.
During the 1970s the Geochemical Ocean Sections
(GEOSECS) program provided the first full water
238 RADIOCARBON