Understanding the Changing Global Distribution of Radiocarbon: What are we learning from the WOCE and CLIVAR Repeat Hydrography Results

Radiocarbon (DI¹⁴C) is an important tracer for studies of ocean processes. It has been used to study mixing, ventilation rates, production rates, and residence times in the deep ocean, deep ocean biogeochemistry and oxygen utilization rates, air-sea gas exchange, thermocline ventilation rates, as a proxy for anthropogenic CO₂ in the ocean, to estimate deep water mass ages for anthropogenic CO₂ uptake and carbon studies, and to evaluate ocean general circulation model (OGCM) performance. DI¹⁴C has been used to study the aging of the water masses and calculate pre-bomb surface water values. DI¹⁴C has also been used with CFCs and anthropogenic CO₂ to investigate the influence of eddies on mixing/ventilation in moderately high resolution ocean. More recently, it has been used to demonstrate the evolving applications of radiocarbon as the shock of the initial bomb-produced radiocarbon spike has passed through the surface ocean and upper thermocline. The radiocarbon distribution is now illuminating mechanisms, pathways and rates of ¹⁴C transfer into the lower thermocline and deeper levels.

¹⁴C results from over 28,000 samples taken as part of the WOCE and CLIVAR programs have been reported. Significant changes, well above the analytical uncertainty, are seen in all three major ocean basins. Simply mapping the distribution measured during the WOCE program provided new insights. In the Southern Ocean near the Antarctic coast, DI¹⁴C values in the western-most Pacific transect are much lower than those observed in the eastern transects. If this is a real feature (rather than a gridding artifact), it has important implications for understanding the role of the Southern Ocean in global climate change. Another extremely interesting feature is a break observed in the southward pointing tongue (NPDW) along the P16 line just west of the islands near 20-25S. It seems quite likely that this unexpected distribution is related to or controlled by proximity to the extreme topography in that area of the Pacific just east of the section. More examples of the changes that repeated sampling has revealed will be presented as well as a discussion of the implications of some of the most interesting observations.