Abstract
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.
Highlights
The tritium and δ3He sections presented above visualise many features of the subsurface circulation and mixing of the Med
An important feature is the recent depletion in tritiugenic 3He in the top 1500 m of the Levantine Sea and in the Levantine Intermediate Water (LIW) throughout the Med (Fig. 9)
The depletion is caused by westward transfer of the residing waters, while in the waters replacing them the generation of new 3He has become small due to low tritium concentrations
Summary
The explanation is that the Med’s sub-floor lithosphere is characterised by high sediment load above crust that is largely of continental type (Morelli, 1985); in both these compartments, α-decay of the natural decay series is a source of pure 4He. Using the isotopic ratio of terrigenic He, measured He isotope and neon (Ne) concentrations allow one to correct for the terrigenic 3He contribution (Roether et al, 1998a, 2001). The potential of using tritium and 3He data jointly is based on the fact that these two tracers form a radioactive mother-daughter pair, which allows one to deduce ages, providing constraints on subsurface water transport and mixing. He dealt primarily with the North Atlantic subtropical gyre waters, which are replenished by surface water within at most a few decades, so that terrigenic 3He hardly interferes
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