DECADE TIME SCALE VARIABILITY OF VENTILATION IN THE NORTH ATLANTIC: HIGH-PRECiSION MEASUREMENTS OF BOMB RADIOCARBON IN BANDED CORALS

. The first high-precision radio- carbon measurements for the upper ocean are presented for banded corals from two sites in the North Atlantic Ocean. The striking dis- similarities 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(cid:127)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 con- strain 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 (cid:127)4C-derived water mass renewal about 4 to a large extent by changes in ocean circulation rather than by atmospheric exchange of C02.


Introduction and Background
One of the few positive outcomes of the nuclear weapons testing era of the 1950s and early 1960s was the production of bomb radiocarbon and tritium, which offers geochemists the opportunity to study ocean circulation on relatively short time scales.
Numerous studies have been conducted to determine mixing rates in the upper ocean (for example, see Michel and Suess Nonetheless, study of variability on both spatial and temporal scales is needed to define the ocean-atmosphere coupling so critical for understanding climate. This approach is also important for quantifying anthropogenic perturbations on climate such as those associated with the greenhouse gas carbon dioxide. Until recently, little attention has been paid to variability of oceanic parameters other than those on very long (glacial-interglacial) or very short (diurnal or seasonal) time scales. Decade  and oxygen observed at the Panulirus station (station "S") southeast of Bermuda. The ocean is not impervious to changes in primary production, circulation, and water chemistry on decade time scales.
Efforts to understand and apply these long-term variations of the ocean-atmosphere system to geochemical models, especially in connection with global climate, must accompany studies of real-time    These factors, which implicate nonsteady state conditions with respect to mixing on an annual basis, must be considered when interpreting the Bermuda and Florida X4C records.
In order to use bomb radiocarbon to quantify water mass renewal rate in the Sargasso Sea, a model is constructed that uses transport along isopycnals as its major mixing mode. This model attempts to reproduce the actual mixing processes that occur in the upper ocean, unlike vertical diffusion models which have been used in the past to quantify the distribution of bomb radiocarbon.      This coincides with very high surface A•nC ( Figure 9) and the lowest W•(t) values ( Figure  10). It appears that less bomb radiocarbon penetrated the subsurface isopycnals during this period.

ZD•(t) = [D•(t)+Sz*D•(t)+S3*D3(t)+S•*D•(t) + Ss*Ds(t)+SG*D6(t)+S?*D?(t)]*[ZSi
In order to determine the significance of the W•(t) record, we submit the model to a suite of sensitivity analyses which consist of unreasonably large changes in several parameters.  This decoupling implies that •4C is primarily a water mass tracer, sensitive to changes in the renewal rate between surface and deeper isopycnal levels.

First, if we interject a random noise to the
Systematic variations of salinity on surfaces of constant density, caused by changes in latent heat of evaporation and significant correlations with oxygen, led Jenkins [1982] to calculate that water mass renewal or the rate of ventilation in the Sargasso Sea had changed by a factor of 2 over the past 3 decades.
Helium-3 distributions during two time periods were consistent with this record.
The changes in isopycnal salinity with time (>0.10ø/oo) were equal to or greater than the changes observed in isopycnal salinity throughout the entire gyre [Bainbridge, 1981]. This observation, coupled with changes in climatology, suggest that drifting or flopping of the gyre with time was not the major cause of the changes in salinity at the Panulirus station and that changes in the latent heat of evaporation are at least partially responsible for the change of isopycnal salinity. A