A COMPARISON OF OCEAN TRACER DATING TECHNIQUES ON A MERIDIONAL SECTION IN THE EASTERN NORTH-ATLANTIC

Tritium and excess He-3 data from a 1988 meridional section in the eas tern North Atlantic are presented and related to hydrographic data for the region. The computed tritium-He-3 ventilation ages are well corre lated over a wide range of water masses with an independent tracer age estimate from a contemporaneous chlorofluorocarbon CFC-12 data set, w ith residuals only slightly greater than those expected from sampling error alone. The tritium-He-3 ages are slightly younger (similar to 1 year) than the CFC-12 ages in recently ventilated water, and the dispa rity between the two tracer ages increases sharply for ages approachin g the elapsed period since the bomb-tritium input in the early 1960s. The overall features of the tracer age-age curve are captured by a sim ple 2-D gyre circulation model and are related to the timescales of th e transient for each tracer and the pre-anthropogenic and primordial b ackgrounds of He-3. Regions of anomalously low tritium-He-3 age relati ve to the overall section tracer age-age curve are observed near the t ropical-subtropical transition and in the deep thermocline in the sout hern part of the subtropical gyre. These areas contain steep tracer gr adients on isopycnal surfaces and elevated non-linear tracer age mixin g effects, based on diagnostic calculations. (C) 1997 EIsevier Science Ltd.