Temporal Variations of Bomb-Produced Radiocarbon Inventory in the Pacific Ocean

Principal Investigator: Tsung-Hung Peng
Collaborating scientists:
Robert Key, Princeton University

Objective: To obtain the temporal variations of bomb-produced radiocarbon inventory in the Pacific Ocean for the purpose of providing constraints in calibrating the three-dimensional global ocean carbon cycle models before such models can be properly used for predicting the oceanic CO2 uptake.
Rationale: Because of the complexity and dynamic nature of the oceanic carbon reservoir, realistic models of the ocean carbon cycle on the basis of three dimensional ocean general circulation models (OGCM) are essential tools for gaining an understanding of the CO2 system in the ocean and its interaction with the atmosphere. Before these models can be used to determine the CO2 holding capacity of the ocean and the net flux of oceanic uptake of anthropogenic CO2, they have to be calibrated with the distribution of geochemical tracers, such as radiocarbon, in the ocean. The global distribution of bomb 14C in the ocean has been summarized on the basis of radiocarbon measurements made during GEOSECS, TTO, and SAVE ocean survey programs. To obtain the temporal variations of the distribution of bomb-produced 14C, we need to analyze the radiocarbon measurements made during the recent WOCE program. Both spatial and temporal distributions of the bomb radiocarbon are considered as the most powerful constraints in calibrating and adjusting the architecture of OGCMs.
Method: The natural and anthropogenic components of the radiocarbon measurements from seawater samples can be separated by an improved method, which is based on a very well-defined relationship between natural radiocarbon and dissolved silica observed mainly during the GEOSECS survey for waters beneath 1000 m depth. This relationship is further reconfirmed by the 14C measurements from large volume samples taken in the deep waters in the Pacific Ocean during the recent WOCE survey program. The anthropogenic 14C in the water column is then integrated to obtain the bomb 14C inventory. For this analysis, results from upper ocean 14C measurements made along 152o W, and north of 20o N, in the northeastern Pacific Ocean during the NOAA's CGC91 cruise are used. More analysis are expected when more WOCE 14C results become available in the future.
Accomplishment: Results of analyzing CGC91 radiocarbon data indicate that the bomb 14C inventory in this part of the ocean has increased by 22% since the GEOSECS measurements made in 1974. This increase is consistent with the box-model prediction of 25% for the northern hemisphere ocean. Change of the surface water bomb 14C values during this period is insignificant. This feature is also consistent with the model simulation.
Key reference:

Peng, T.-H., R. Key, and H. G. "Ostlund, Temporal variations of bomb radiocarbon in the Pacific Ocean. Marine Chemistry. (in press).

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