Natural radiocarbon in the Atlantic Ocean

Abstract

By the use of suitable chemical and radiometric techniques the natural radiocarbon concentration in the dissolved bicarbonate of 135 samples representing the major water masses of the Atlantic Ocean has been determined with a precision ranging from 0.5 to 1.3 per cent. Whereas the results from a given water mass exhibit a standard deviation only slightly exceeding that predicted from the experimental error alone, measurable differences exist between the major water masses, the total range in C14/C12 ratio being about 10 per cent. Corrections for the bomb-produced C14 effect and the industrial CO2 effect have been applied where necessary. The surface water C14/C12 ratios show a progressive increase from south to north, ranging from 120 per mil lower than the preindustrial atmospheric value in the Antarctic to 50 per mil lower in the North Atlantic. Deep water masses originating in the high latitudes of the southern hemisphere have consistently lower C14/C12 ratios than those originating in the high latitudes of the northern hemisphere. A layer of water of high C14/C12 ratio found at depths between 1200 and 2400 meters in the western North Atlantic may well represent a wedge of young water penetrating the older North Atlantic deep water. Bottom water in the eastern basin of the North Atlantic has a 20 per mil lower C14/C12 ratio than the corresponding water in the western basin. According to a steady-state circulation model, most of the water below 600 meters in the North Atlantic remains at depth for an average of 650 years. Corresponding residence times for water masses of Antarctic origin are less than 350 years. A circulation model explaining the prominent features of the C14 distributions in the atmosphere-ocean system is based on a south to north transport of water along the surface of the Atlantic Ocean, with a return flow at depth. The Atlantic and Pacific communicate through the Antarctic. On the basis of this model, despite the lower ΔC14 values, the mean residence times of water in the deep reservoirs of the Pacific may not exceed those for the deep Atlantic by more than 30 per cent. Although results of C14 analyses on tree rings suggest that the oceans are reasonably close to steady state, the possibility of nonsteady-state circulation must be considered. It is shown that the present C14 distribution in the oceans could be achieved through the storage of C14 in the atmosphere and surface oceans during a relatively short period of greatly restricted bottom water formation. If nonequilibrium effects are important the residence times computed from the steady-state model could be considerably in error.