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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