The North Atlantic tritium and radiocarbon transients 1972–1983

Abstract

Deep ventilation and water mass transformation processes in the North Atlantic, on decadal time scales, are illustrated by the evolving distribution patterns of anthropogenic tritium and radiocarbon. Data from two quasisynoptic surveys of the interior of the North Atlantic, the Geochemical Ocean Sections Study (GEOSECS) and complementary oceanographic observation projects in 1972, and the Transient Tracers in the Oceans (TTO) in 1981 and 1983, are presented in comparative cross sections and maps representing conditions roughly one and two decades, respectively, after the first major tracer injections. The discussion emphasizes comparisons of the decay corrected tritium concentration fields, which show several distinct regimes of transient evolution, including examples of surprising constancy in some regional patterns. While largely supportive of previous qualitative ideas about transport patterns, these results also suggest that the patterns of deep water injection in high latitudes must have undergone a major change around or soon after the time of GEOSECS (1972), involving a major increase in supply of Upper North Atlantic Deep Water. The near‐surface waters show strong signatures of tropical (low tritium) influence in the southwestern and western part of the subtropical gyre, while the northeastern part is influenced by continued tritium input from Arctic surface waters. Strong evidence for distinct sub‐basin‐scale interior circulation domains is found in the fact that the major distribution regime transitions seen in the GEOSECS data are observed again in TTO, and thus maintained through the second decade of the transient. The radiocarbon data set, which reflects the addition to the natural background distribution, supports the significance also on longer time scales of the mid‐depth regime transition around 30°N latitude.