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ONE measure of transport rates and residence times of organic carbon within the various organic carbon pools in the ocean is to follow perturbations from the 1952–58 and 1961–62 atmospheric thermonuclear bomb tests on the natural 14C activities of living, detrital and dissolved organic matter. We report here our measurement of the 14C activity of the total sedimentary organic carbon (SOC) in an abyssal red clay, as no such data were available for this organic carbon pool. Ultimately, these sedimentary organic 14C activities may be compared with the 14C activity of the source organic material in the water column and estimates made of the magnitude of organic carbon consumption at the seawater–sediment interface. What was found, unexpectedly, was evidence for the penetration of recent organic carbon from the 4 cm mixed layer at the sediment–seawater interface down to at least 12 cm in the sedimentary column. This suggests a different and more rapid mechanism for the sedimentation of some fraction of the SOC other than direct association with clay minerals.

THE release of 14C in nuclear explosions, particularly those detonated in 1961–1962, has caused a major disturbance of the steady-state natural distribution of this radioisotope. The resultant pulse of bomb 14C, detected initially in the atmosphere, has since been used extensively in evaluation of rates of exchange of carbon between major reservoirs of the dynamic cycle — notably atmospheric CO2and ocean water bicarbonate1–7. Within the biosphere, however, investigation of human 14C levels has been more limited despite the application of bomb 14C as early as 1959 by Broecker and co-workers8in the determination of residence times of carbon in several tissues. Perhaps the most specific result to emerge from initial research on this topic was the demonstration of almost total inertness of collagen in the adult human skeleton as evidenced by its failure to incorporate significant amounts of artificial 14C (ref. 9). In other related 14C measurements10–13, a non-correspondence between contemporary human and atmospheric 14C levels was often observed. Nydal and co-workers proposed a combination of three factors to account for such variations13and concluded from a long term analysis of 14C concentrations in human blood and hair samples that bomb 14C enters the human body 1.4 years after production in the atmosphere, and remains there with a mean lifetime of 10 years. This article describes the continuation at Glasgow of an intensive investigation of the presence of bomb 14C in humans, initiated by Harkness and Walton12. Comparison by these workers of blood plasma protein 14C data with predicted dietary 14C levels14has confirmed that a finite turnover of carbon in the human body does exist. Here, determination of 14C activities of atmospheric CO2, dietary and human tissue samples enables a more detailed evaluation of residence times.