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).

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.

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.

Radiocarbon (Δ¹⁴C) and stable isotope (δ¹⁸O and δ¹³C) records are presented for biannual samples from a 323-year banded coral series collected from the southern Great Barrier Reef, Australia. The high-precision Δ¹⁴C record contains variations on an interannual timescale, that are particularly large between A.D. 1680 and 1730. By comparison with tree ring Δ¹⁴C records [Stuiver and Quay, 1980; M. Stuiver, personal communication, 1992), it is clear that these shifts were not caused by changes in the Δ¹⁴C of atmospheric CO₂. Changes in vertical mixing and large scale advective changes involving source waters to the western Coral Sea region are likely processes that could account for these large Δ¹⁴C variations. Most low Δ¹⁴C values for the period A.D. 1635-1875 coincide with El Niño/Southern Oscillation (ENSO) events as reported by Quinn et al. [1987] for the eastern tropical Pacific. However, ENSO does not explain all of the variations, especially during 1875–1920 when Δ¹⁴C values remained high. Cross-spectral analysis of the early half of the Δ¹⁴C and δ¹⁸O records (A.D. 1635–1795) reveals that the 6-year period is coherent; this coherency is not present in the latter half (A.D. 1797–1957) of the isotope records. These data support the concept of century timescale changes in the nature of ENSO, as it is manifest in the southwestern Pacific. Our coral record shows no evidence of a Suess effect, the lowering of Δ¹⁴C from late 1800s through 1955 due mainly to CO₂ input from fossil fuel burning. This is coincident with the change we observe in the nature of ENSO and is further evidence that a long-term change in mixing of upper waters occurred in this region.

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.

Dissolved organic carbon (DOC) is the largest actively exchanging pool of organic carbon in the ocean, yet its sources and sinks are not well constrained. The average ¹⁴C ages of DOC in the deep N. Atlantic and N. Pacific Oceans are 4,000 [Bauer et al., 1992; Druffel et al.,1992] and 6,000 years [Williams and Druffel, 1987], respectively, and represent the beginning and end of the deep ocean conveyor [Broecker, 1991]. Here we report that the deep Southern Ocean DOC has a ¹⁴C age (5,600 y) much closer to that of the deep N. Pacific, but its concentration in seawater (41±2 µM) is nearly equal to that of the deep N. Atlantic. The radiocarbon and concentration data indicate that most, but not all, deep DOC is transported conservatively with the ocean's conveyor. A younger (post-bomb) source of DOC to the N. Atlantic is the most likely explanation for the large age difference we observe between deep DOC in the Atlantic and Southern Oceans. Other possibilities are a source of older DOC or a smaller microbial sink in the S. Ocean, or perhaps a possible slowdown of S. Ocean deep water formation during the past century [Broecker et al., 1999].

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.