CO
derived from the burning of fossil fuels and deforestation
is depleted in the isotope 
C due to isotope fractionation
during photosynthesis. This has caused the 
C/
C ratio
of atmospheric carbon to decline at the same time that CO
levels have gone up. The change in atmospheric 
C/
C
has penetrated into the ocean. In principle, the 
C signal
in the ocean should be easier to detect that the CO
signal
itself (Tans et al., 1993). The ratio 
C/
C is usually
expressed in terms of a quantity 
C which is
referenced to an arbitrary standard, i.e. 
C =
(
C/
C
) / (
C/
C
) -- 1.
Quay et al. (1992) compiled a set of 
C profiles at
eight locations in the Pacific Ocean which had been measured about
20 years apart. They found that 
C levels had indeed
declined over the 20 year period. They used the depth-integrated
change in 
C to estimate how much anthropogenic
CO
had gone into the ocean. They found 2.1 GtC/yr, a result
which agrees fairly well with ocean models.
Tans et al. (1993) and Broecker and Peng (1993) pointed out a major
problem with Quay et al.'s 
C observations. The observed
decrease in atmospheric 
C over the 20-year period was
about 0.4 per mil. The average 
C decrease in ocean
surface waters was also about 0.4 per mil. If changes in
atmospheric 
C are indeed driving the changes in the
ocean one would expect the 
C change in the surface
ocean to be substantially smaller than the change in the
atmosphere. This is because the isotope signal coming into the
ocean should be diluted as it is stirred into the ocean's upper few
hundred meters. If changes in atmospheric 
C really
are driving 
C changes in the ocean one would expect
the ocean's uptake of lighter atmospheric carbon to be analogous to
the ocean's uptake of bomb 
C.
There are a number of factors which might contribute to this
discrepancy. One possibility is that eight station pairs do not
constitute an appropriate average for monitoring 
C
changes in the ocean. There could also be temporal changes in
oceanic 
C brought about by changes in photosynthetic
isotope fractionation (due to higher CO
(g) concentrations in
the ocean) or temporal changes brought about by growth or shrinkage
in the ocean's large dissolved organic carbon reservoir. Either of
these factors could alter the temporal evolution of 
C
in the ocean without any effect on bomb 
C uptake.
Ideally one would want to set the oceanic 
C changes in
the context of a complete isotopic budget for the atmosphere. A
major problem here is knowing the isotope exchange between the
atmosphere and terrestrial biosphere. As atmospheric
C values fall over time terrestrial ecosystems should
respire older or heavier carbon back to the atmosphere. The
atmosphere's isotope budget is quite sensitive to the
C difference between old carbon being respired and new
carbon being fixed by photosynthesis. No one really knows how much
the isotope exchange is currently out of balance.