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.

A14C records are reported for post-bomb corals from three sites in the tropical Atlantic Ocean. In corals from 18°S in the Brazil Current, A14C values increased from ca. -58% in the early 1950s to +138%o by 1974, then decreased to 110% by 1982. Shorter records from 8°S off Brazil and from the Cape Verde Islands (17°N) showed initially higher A14C val- ues before 1%5 than those at 18°S, but showed lower rates of increase of A14C during the early 1960s. There is general agree- ment between the coral results and &4C of dissolved inorganic carbon (DIC) measured in seawater previously for locations in the tropical Atlantic Ocean. Al4C values at our tropical ocean sites increased at a slower rate than those observed previously in the temperate North Atlantic (Florida and Bermuda), owing to the latter's proximity to the bomb 14C input source in the northern hemisphere. Model results show that from 1960-1980 the Cape Verde coral and selected DIC A14C values from the North Equatorial Current agree with that calculated for the North Atlantic based on an isopycnal mixing model with a constant water mass renewal rate between surface and subsurface waters. This is in contrast to A14C values in Bermuda corals that showed higher post-bomb values than those predicted using a constant water mass renewal rate, hence indicating that venti- lation in the western north Atlantic Ocean had decreased by a factor of 3 during the 1960s and 1970s Druffel 1989).

We present time series of tritium (³H) concentrations in seawater from stations in the eastern subpolar (50°N, 145°W) and subtropical (28°N, 122°W) North Pacific. In the eastern subpolar North Pacific the tritium gradient between surface water and North Pacific Intermediate Water is smallest during years that coincide with El Niño events. In the eastern subtropical North Pacific between 200 and 400 m the tritium signature suggests that the water is of subpolar origin during non-El Niño years. During El Niño years, the water at this location is devoid of tritium. We hypothesize that the El Niño phenomenon alters both the ventilation of thermocline and intermediate waters in the eastern subpolar North Pacific, as well as the transport of this water to the eastern subtropical gyre. Combined with satellite altimeter data, these results offer both a mechanism and a time frame by which subpolar water ventilates the vast reservoir of the subtropical North Pacific.

Bomb-produced 14C is a valuable tool for studying rates of short-term processes involving carbon cycling. This study shows that bomb 14C is an excellent tracer of a biochemical process that takes place in the human body, namely the accretion of stones in the gallbladder. The methods developed for obtaining time histories of 14C/12C and 13C/12C in concentric layers from a large gallstone (30mm diameter) are reported. Formation times are assigned by 14C/12C obtained from individual layers witbhy mtahtocshei ngf otuhned for known-aged tree rings. Results show that the gallstone grew over a period of 10 years and seems to have lain dormant within the gallbladder for a period of 11 years. The average growth rate was 1.5mm/year.

We present high-precision radiocarbon (Δ¹⁴C) results and stable isotope (δ¹⁸O and δ¹³C) records for a coral from Heron Island (23°S, 152°E) and new stable isotope (δ¹⁸O and δ¹³C) records for annual coral bands from Abraham Reef (22°S, 153°E) in the southern Great Barrier Reef studied earlier [Druffel and Griffin, 1993]. These tracers provide unique information on the regional water mass history, and together these data allow us to constrain the variability of circulation in the upper Pacific over the past four centuries. First, we observe decreases in δ¹⁸O of coral from Abraham Reef and Heron Island, indicating an increase in sea surface temperature and/or a decrease in surface salinity since 1850. Second, the small Suess effect value (Δ¹⁴C decrease from 1880 to 1955, due mostly to fossil fuel CO₂) observed previously at Abraham Reef[Druffel and Griffin, 1993] is confirmed in the measurements reported here from the Heron Island coral. This value is low compared to those observed in other areas of the ocean [Druffel, 1997; Druffel and Linick, 1978; Nozaki et al., 1978] between 1880 and 1955. Third, we report alterations in the correlation between El Niño events and the occurrence of low Δ¹⁴C, which is indicative of long-term change(s) in circulation in the SW Pacific. The Δ¹⁴C shifts reported here are not large, but even small temporal changes in prebomb Δ¹⁴C suggest that important changes in the large-scale state of the ocean have occurred, such as a temporal change in circulation.

Radiocarbon was measured in atmospheric CO2 from La Jolla, California and in living organic materials from six sites in the northern hemisphere. Atmospheric CO2 &C values from La Jolla agreed with those previously published records from China Lake, California (Berger et a1.1987) and Vermunt, Austria (Levin et a1.1985). &4C values of fruit and grain samples that grew during 1980 agreed with the atmospheric CO2 &4C measurements. Most of the A14C results of fruit and corn samples stored since the 1940s agreed with tree-ring &4C values for the same time period. In general, agreement was found between the atmospheric CO2 or tree-ring &4C records available for the Northern Hemisphere and the &4C signatures of rapidly exchanging organic matter pools examined in this study. Exceptions were the i 4C values of carbonate from egg shells and that of organic carbon from egg insides, which demonstrate that bicarbonate and organic carbon within the egg follow different biochemical pathways.