For more than two decades, coral skeletons have been used to reconstruct records of isotopic and chemical ratios in surface seawater (for reviews, see Druffel, 1997a; Gagan et al., 2000). Here I demonstrate how high precision 14C records (±3) reveal information regarding regional mixing of surface and subsurface waters. Climatic variability on many time scales (e.g. El Niño, Pacific Decadal Oscillation, and the North Atlantic Oscillation) are detected in the corals by small changes in Δ14C. Radiocarbon records in corals have also been used to reconstruct the input of fossil fuel CO2 into the subtropical and tropical regions of the Pacific and Atlantic Oceans.

The first high-precision radiocarbon measurements for the upper ocean are presented for banded corals from two sites in the North Atlantic Ocean. The striking dissimilarities between the post-1950 records at Bermuda in the Sargasso Sea and the Florida Straits in the Gulf Stream illustrate the different mixing processes in the upper ocean at each site. Convective overturn associated with 18° degree water formation during late winter in the northern Sargasso Sea facilitates storage of considerable quantities of bomb radiocarbon at depth, which accounts for the damping of the Δ¹⁴C signal at Bermuda during the 1960s. A multibox isopycnal mixing model is used to estimate the ventilation rate of the upper 700 m of the water column in the Sargasso Sea from 1950 to 1983. An inverse model is used; that is, the water mass renewal rate was calculated for the post-bomb period in order to satisfy the bomb radiocarbon time history in the corals. Sea water radiocarbon measurements made during the GEOSECS (1972–1973) and Transient Tracers in the Ocean (1980–1981) surveys are used to constrain the subsurface radiocarbon values calculated by the model. Results show that the rate of water mass renewal in the Sargasso Sea was high during 1963–1964, decreased during the late 1960s, and remained low during most of the 1970s. The ¹⁴C-derived record of water mass renewal precedes by about 4 years that derived from isopycnal salinity in the Sargasso Sea [Jenkins, 1982], illustrating that the coral ¹⁴C record is controlled to a large extent by changes in ocean circulation rather than by atmospheric exchange of CO₂.

Sixty radiocarbon measurements were performed on aragonite from annually banded corals collected from three sites in the Galápagos Islands. Preanthropogenic Δ¹⁴C values of coral that grew around A.D. 1930 averaged −70‰. This is substantially lower than average values previously reported (−51‰) for corals from Florida and Belize in the western North Atlantic Ocean. A decrease of 6‰ was noticed in coral that grew from 1930 to 1954. This decrease could be interpreted as a Suess effect in surface ocean water. The 100‰ increase in Δ¹⁴C for coral that grew from 1954 to 1973 is the result of bomb-produced ¹⁴C that was introduced to the surface ocean waters. The ¹⁴C levels in corals that grew during El Niño years were considerably higher than those for normal years. These higher values are attributed to the absence of up welling at the equator during El Niño events.