Reconstructing ocean circulation using coral ¹⁴C time series

Tom Guilderson, Ph.D.
West Coast: Center for Accelerator Mass Spectrometry
Lawrence Livermore National Laboratory
East Coast: Harvard University

Wednesday, March 31, 1999
3:00 p.m.—Pacific Forum

The tropical Pacific plays an important role in the localization of deep convective activity and in the development of El Niņo-Southern Oscillation (ENSO) events. On both the seasonal and inter-annual timescale, the redistribution of tropical Pacific surface waters as a response to the mean wind field has a large impact globally via atmospheric teleconnections initiated through the transfer of sensible and latent heat. Unfortunately, direct oceanographic observations are generally limited to the last 25 years and contain spatial and temporal biases. Atmospheric nuclear testing in the 1950s and early 1960s resulted in an excess of ¹⁴C which has augmented the natural ¹⁴C gradient between surface and subsurface waters. We can use ¹⁴C as a passive tracer to directly study circulation, and through the use of biological archives (e.g. corals), reconstruct the temporal and spatial variations in Delta ¹⁴C.

We have generated high-resolution coral Delta ¹⁴C records from Nauru, the Galapagos, Guadalcanal, and Rarotonga. These new results exhibit a previously undocumented spatial and temporal dynamic Delta ¹⁴C range, which reflects the underlying circulation of the tropical Pacific. High-resolution Delta ¹⁴C time series such as these provide a powerful constraint on the rate of surface ocean mixing and hold great promise to augment one-time surveys such as Geochemical Ocean Section Study (GEOSECS) and World Ocean Circulation Experiment (WOCE). Time series such as these not only provide fundamental information about the shallow circulation of the Pacific, but can also be directly used as a benchmark for the next generation of high-resolution ocean models used in prognosticating climate.

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