A comparison of ocean tracer dating techniques on a meridional section in the eastern North Atlantic

Tritium and excess3He data from a 1988 meridional section in the eastern North Atlantic are presented and related to hydrographic data for the region. The computed tritium -3He ventilation ages are well correlated over a wide range of water masses with an independent tracer age estimate from a contemporaneous chlorolluorocarbon CFC-12 data set, with residuals only slightly greater than those expected from sampling error alone. The tritium -3He ages are slightly younger (∼ 1 year) than the CFC-12 ages in recently ventilated water, and the disparity between the two tracer ages increases sharply for ages approaching 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 simple 2-D gyre circulation model and are related to the timescales of the transient for each tracer and the pre-anthropogenic and primordial backgrounds of3He. Regions of anomalously low tritium -3He age relative to the overall section tracer age-age curve are observed near the tropical-subtropical transition and in the deep thermocline in the southern part of the subtropical gyre. These areas contain steep tracer gradients on isopycnal surfaces and elevated non-linear tracer age mixing effects, based on diagnostic calculations.