Temporal Variations of Bomb Radiocarbon Inventory in the Ocean
Principal Investigator:
Tsung-Hung Peng
Collaborating scientist(s):
Objective:
Establish a data base for information with regard to evolution of
geochemical tracers in the ocean for the purpose of constraining the
architecture and circulation dynamics of the ocean general circulation
models, especially when such models are used for estimating the
uptake of fossil fuel CO2 by the ocean.
Rationale:
Narrowing the uncertainty of the estimates of fossil fuel CO2
uptake by the ocean is the focus of current improvement in quantification
of the
contemporary carbon budget. The use of ocean general circulation models
(OGCM) is considered to be the most effective method in accomplishing
this goal. However, before the OGCM can be used to compute the
anthropogenic CO2 uptake with confidence, it has to be calibrated with
the distribution of bomb produced radiocarbon in the ocean. The reason
is that radiocarbon distribution offers information including the entire
range of oceanic time scales. The characteristic ocean penetration time
for fossil fuel CO2 is 3 decades. The mean penetration time for bomb
radiocarbon is also about 30 years. In addition, we have available to
us a set of measurements of radiocarbon covering the entire ocean. Most
of all, the data include repeat surveys documenting the evolution of
changes with time. Such temporal variations provide crucial constraints
in modeling the ocean carbon cycle.
Method:
The distribution of natural (i.e. cosmic ray-produced) radiocarbon is
most sensitive to mixing processes operating on century time scale while
that of anthropogenic (i.e. bomb-testing produced) radiocarbon is most
sensitive to processes operating on decadal time scales. If we do not
separate the bomb-produced radiocarbon from the natural radiocarbon, it
will be impossible to determine whether the disagreements between model
and observation reflect the deficiencies in the model's simulation of
the steady state distribution of natural radiocarbon or in its
simulation of the distribution of the bomb radiocarbon transient.
Hence, the separation of bomb produced radiocarbon from the natural
radiocarbon in the overall radiocarbon measurements is essential if
radiocarbon distribution is to be used to constrain the ocean models.
The improved method has been developed for such a separation purpose.
This method involves the very strong correlation between natural
radiocarbon and dissolved silica. For global coverage, radiocarbon
measurements made on samples collected during the Geochemical Ocean
Sections Study (GEOSECS) in 1972-1978 are analyzed for bomb radiocarbon
inventory in the ocean. Subsequent measurements made in the Atlantic
Ocean during Transient Tracers in the Ocean (TTO) in early 1980s and
South Atlantic Ventilation Experiment (SAVE) in late 1980s are also
analyzed for temporal variations in bomb radiocarbon inventory. Recent
radiocarbon measurements on the NOAA/OACES 1993 cruise in the Atlantic have
also been analyzed for bomb radiocarbon inventory. In the Pacific
Ocean, radiocarbon measurements from samples collected during the WOCE
program are slowly becoming available for bomb radiocarbon inventory
analysis. Continuing efforts in analyzing WOCE data has been proposed.
Accomplishment:
Comparison of results between GEOSECS and TTO/SAVE indicates that the
bomb radiocarbon inventory has increased by 36% for the region north of
10N, by 69% for the equatorial region, and by 71% for the region south
of 10S. These results reflect the C14 uptake for the Atlantic Ocean
between 1973 (GEOSECS) and 1985 (TTO/SAVE). In addition to global bomb
C14 distribution, this information provides crucial constraints for the
carbon cycle in the ocean. Preliminary results from CGC-91, one of the WOCE
cruises, show that the observed increase in bomb
radiocarbon inventory
from 1974 to 1991 in the northern Pacific Ocean is consistent with the first
order prediction from a box-diffusion ocean model.
Key reference:
Peng, T-H, R. Key, W. S. Broecker, and J. P. Severinghaus, Temporal
variations of bomb radiocarbon inventory in the ocean, Maurice Ewing
Symposium on Applications of Trace Substance Measurements to Oceanographic
Problems, October 16-20, 1995, Biosphere 2, Oracle, Arizona, p. 28 (1995).
Broecker, W. S., S. Sutherland, W. Smethie, T-H Peng, and G. Ostlund, 1995.
Oceanic Radiocarbon: Separation of the natural and bomb components, G.
Biogeochem. Cycle, V. 9, p. 263-288.
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