Bomb 14C from nuclear tests in the atmosphere has proved to be a particularly useful tool in the study of the carbon cycle. We provide here a ca. 30-yr time series of 14C concentrations in the atmosphere between 28°N and 71°N and in the ocean surface between 45°S and 45°N. More recently (since 1990), a north-south profile also has been obtained for 14C in the surface waters of the Atlantic Ocean. The measurements were performed using the conventional technique of beta counting or large samples (4 to 5 liter CO2) in CO2 proportional counters. These data show that the 14C concentration in the atmosphere is leveling off with a time constant of 0.055 yr-1, and is now approaching that of the ocean surface at lower latitudes.

Additional tracer studies have been concerned especially with the penetration of bomb 14C into the deep ocean. The Norwegian and Greenland seas are of interest as a sink for atmospheric CO2 and also a source of water for the deep Atlantic Ocean. During the last five years, several 14C depth profiles have been measured from the Fram Strait (79°N) to south of Iceland (62°N), using the AMS technique available at the University of Arizona AMS Facility. We considered it important to repeat and compare a few of the profiles with those produced by the GEOSECS expedition in 1972 and the TTO expedition in 1981. The profiles show that water descending to the deep Atlantic Ocean is originating mainly from intermediate and surface depths in the Nordic Seas. However, the ventilation rate of the Norwegian Sea deepwater is too slow to be an important component in the transfer of water over the Greenland-Scotland Ridge.

The distribution of bomb-produced 14C in the ocean provides a powerful constraint for circulation models of upper ocean mixing. We report 14C measurements from an east-west section of the main thermocline at 24°N latitude in the subtropical North Atlantic Ocean in summer 1992, and one profile from the Gulf of Mexico in 1993. Observed gradients reflect the transient invasion of bomb 14C into the thermocline via mixing along isopycnals from the poleward outcrop, with progressively more sluggish mixing at greater depths. A slight deepening of the profile is observed over the 20-yr period since the GEOSECS survey at one location where the comparison is possible.

Fieldwork for the World Ocean Circulation Experiment (WOCE) radiocarbon program was recently completed. Ca. 9000 samples were collected for analysis using both conventional β-counting techniques and the newer AMS technique. The mean uncertainty for the β analyses is 3‰; for AMS analyses, ca. 4.5‰.

AMS radiocarbon results from the World Ocean Circulation Experiment in the Pacific Ocean show dramatic changes in the inventory and distribution of bomb-produced 14C since the time of the GEOSECS survey (8/73–6/74). Near-surface Δ14C values for the eastern portion of both the northern and southern subtropical gyres decreased by 25–50‰, with the change being greater in the north. Equatorial near-surface values have increased by ca. 25‰. Changes in the 250–750-m depth range are dramatically different between the northern and southern basins. The intermediate and mode waters of the southern basin have increased by as much as 75‰ since GEOSECS. Waters of similar density in the northern hemisphere are not exposed to the Southern Ocean circulation regime and are significantly less ventilated, showing maximum changes of ca. 50‰.

At the University of Miami Tritium Laboratory and the University of Washington Quaternary Isotope Laboratory, more than 1000 large-volume Pacific Ocean radiocarbon samples were measured for the WOCE program. Here we present a comprehensive data set, and a brief discussion of our findings.

Δ14C records are reported for post-bomb corals from three sites in the tropical Atlantic Ocean. In corals from 18°S in the Brazil Current, Δ14C values increased from ca. −58% in the early 1950s to +138% by 1974, then decreased to 110‰ by 1982. Shorter records from 8ºS off Brazil and from the Cape Verde Islands (17°N) showed initially higher Δ14C values before 1965 than those at 18ºS, but showed lower rates of increase of Δ14C during the early 1960s. There is general agreement between the coral results and Δ14C of dissolved inorganic carbon (DIC) measured in seawater previously for locations in the tropical Atlantic Ocean. Δ14C 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 DIG Δ14C 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 Δ14C values in Bermuda corals that showed higher post-bomb values than those predicted using a constant water mass renewal rate, hence indicating that ventilation in the western north Atlantic Ocean had decreased by a factor of 3 during the 1960s and 1970s (Druffel 1989).

We have refined marine reservoir age estimates for eastern Pacific coastal waters with radiocarbon measurements of mollusk shells collected prior to 1950. We have also investigated interspecific variability in 14C ages for historic and ancient shells from San Francisco Bay.

In order to verify sediment trap samples as indicators of upper ocean 14C concentrations, particulate inorganic radiocarbon (PICΔ14C) collected by time-series sediment traps in the Sea of Okhotsk and the Bering Sea was measured by accelerator mass spectrometry (AMS). All of the PICΔ14C measurements were < 0‰, in contrast to GEOSECS 14C data in the upper ocean from the northwestern North Pacific. This difference is attributed to the upwelling of deepwater that contains low Δ14C of dissolved inorganic carbon (DICΔ14C) and to the decrease over time of surface DICΔ14C owing to the decrease of atmospheric Δ14C values. In addition, PICΔ14C values showed significant seasonal variability: PICΔ14C collected in the fall was the greatest (-22‰ on average), whereas PICΔ14C collected in winter showed an average minimum of −48‰. It is likely that this difference was caused by changes in mixed layer thickness. Although some uncertainties remain, further study on PICΔ14C will enable us to estimate seasonal variability in DICΔ14C and air-sea CO2 exchange rate.

Accelerator mass spectrometry (AMS) measurement of 25 single-year tree rings from AD 1861–1885 at ca. ±3.5‰ precision shows no evidence of an anomalous 11-yr cycle of 14C near the Arctic Circle in the Mackenzie River area. However, the Δ14C measurements are lower on average by 2.7 ± 0.9 (ō)‰ relative to 14C measurements on tree rings from the Pacific Northwest (Stuiver and Braziunas 1993). We attribute this depression of Δ14C to thawing of the ice and snow cover followed by melting of frozen earth that releases trapped 14C-depleted CO2 to the atmosphere during the short growing season from May through August. Correlation of Δ14C with May–August estimated temperatures yields a correlation index of r = 0.60. The reduction in Δ14C is dominated by seven years of anomalous depletion. These years are 1861, 1867–1869, 1879–1880 and 1883. The years 1867–1869 are coincident with a very strong ENSO event.

Radiocarbon measurements on organic carbon from the varved cores of Elk Lake, Minnesota suggest that the varve count may underestimate calendar years by 18% for the most recent 3800 varve years.

Recently, in an intercomparison of the Hohenheim German oak chronology (Becker 1993) and the Göttingen chronology (Leuschner and Delorme 1988), an error was discovered in the former (Leuschner, in preparation). Due to an error in adding sections at 5241 bc, 41 yr are missing in the published Hohenheim chronology. After correction of the error, the two chronologies synchronize over their entire common length, back to 7200 bc.