Tritium and 90Sr in North Atlantic surface water

Abstract

Areal distributions and complete time histories since 1952 are presented of the tritium and 90Sr concentrations in North Atlantic surface water. The distributions are based on a compilation of measured North Atlantic surface water tritium concentrations which is part of this paper and includes hitherto unpublished measurements, and on available 90Sr compilations. To reconstruct the insufficiently represented early concentrations, a two-box North Atlantic mixing model with tropospheric input is employed, for which the input is specified (on relative scales), and which is fitted to the available surface water observations. This procedure gives a natural tritium concentration of 0.2 TU (±30%), and furthermore suggests that part of the old oceanic tritium and 90Sr measurements are high. The fit requires characteristic model mixing times of 2.5 years (exchange with an intermediate-depth reservoir about three times the size of the surface box) and 30 years (loss into the deep ocean), and a tritium/ 90Sr input ratio of 310 Ci/Ci. The areal distributions and time histories can serve as boundary value data for evaluations of subsurface tritium and 90Sr measurements.