UC Irvine

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 ø 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 A•4C signal at Bermuda during the 1960's. 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 •4C-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 •C record is controlled to a large extent by changes in ocean circulation rather than by atmospheric exchange of C02.