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.