Abstract
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.
Highlights
Introduction and BackgroundOne 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
I suspect this bears on McConnaughey's [1986] observation that slow growing species from depth (11 m) contain an isotopic composition that is closer to the equilibrium value than fastergrowing specimens (M. annularis) from shallower depths, the isotopic composition of which is controlled to a greater extent by photosynthesis of the symbiotic algae within the coral polyp
(6Z'O, salinity, sea surface temperature (SST))are not significant, covariance with the water mass renewal rate record presented later in this paper suggests that records in banded corals may be potential integrators of ventilation rate in the upper ocean
Summary
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. Decade time scale variability of water mass renewal in the upper 600 m of the Sargasso Sea was reported by Jenkins [1982] based on a 27-year record of salinity and oxygen observed at the Panulirus station (station "S") southeast of Bermuda. Demonstration of decade time scale variability in water mass renewal rate in the Sargasso Sea is presented here using high-precision bomb radiocarbon records obtained from annually banded corals. An isopycnal transport model is used here to represent mixing of water and radiocarbon in the upper waters of the Sargasso Sea. Radiocarbon records in banded corals from Bermuda are used to represent the time history of C in a wellmixed subtropical gyre surface box. Micromass 602E mass spectrometer was used and the precision obtained for each was +0.07%0
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