The transient distributions of nuclear weapon-generated tritium and its decay product ³ He in the Mediterranean Sea, 1952–2011, and their oceanographic potential

Abstract. We present a comprehensive account of tritium and ³He in the Mediterranean Sea since the appearance of the tritium generated by the atmospheric nuclear-weapon testing in the 1950s and early 1960s, based on essentially all available observations. Tritium in surface waters rose to 20–30 TU in 1964 (TU = 10¹⁸ × [³H]/H]), a factor of about 100 above the natural level, and thereafter declined 30-fold up to 2011. The decline was largely due to radioactive tritium decay, which produced significant amounts of its stable daughter ³He. We present the scheme by which we separate the tritiugenic part of ³He and the part due to release from the sea floor (terrigenic part). We show that the tritiugenic component can be quantified throughout the Mediterranean waters, typically to a ± 0.15 TU equivalent, mostly because the terrigenic part is low in ³He. This fact makes the Mediterranean unique in offering a potential for the use of tritiugenic ³He as a tracer. The transient distributions of the two tracers are illustrated by a number of sections spanning the entire sea and relevant features of their distributions are noted. By 2011, the ³He concentrations in the top few hundred metres had become low, in response to the decreasing tritium concentrations combined with a flushing out by the general westward drift of these waters. Tritium-³He ages in Levantine Intermediate Water (LIW) were obtained repeated in time at different locations, defining transit times from the LIW source region east of Rhodes. The ages show an upward trend with the time elapsed since the surface-water tritium maximum, which arises because the repeated observations represent increasingly slower moving parts of the full transit time spectrum of LIW. The transit time dispersion revealed by this new application of tritium-³He dating is considerable. We find mean transit times of 12 ± 2 yr up to the Strait of Sicily, 18 ± 3 yr up to the Tyrrhenian Sea, and 22 ± 4 yr up into the Western Mediterranean. Furthermore, we present full Eastern Mediterranean sections of terrigenic ³He and tritium-³He age in 1987, the latter one similarly showing an effect of the transit time dispersion. We conclude that the available tritium and ³He data, particularly if combined with other tracer data, are useful for constraining the subsurface circulation and mixing of the Mediterranean Sea.