210Pb and210Po during the destruction of stratification in the Dead Sea

The progressive weakening and final disappearance (in 1979) of the long-term meromictic structure of the Dead Sea are clearly reflected in the depth profiles of²¹⁰Pb and²¹⁰Po. In 1977/78, prior to overturn, dissolved²¹⁰Pb (35–50 dpm kg^?1) predominated over particulate²¹⁰Pb (1–2 dpm kg^?1) in the oxic upper waters, whereas the reverse was true in the anoxic deep waters (16–20 dpm kg^?1 particulate vs. 2–5 dpm kg^?1 dissolved). The exact extent of the disequilibrium between²¹⁰Pb and²²⁶Ra is hard to evaluate in the upper oxic layers, because the progressive deepenings resulted in mixing with deep waters. By contrast, one can estimate the residence time of dissolved²¹⁰Pb in the unperturbed anoxic deepest layers, because these remained isolated, at about 3 years. Following the overturn of 1979, dissolved²¹⁰Pb exceeded particulate²¹⁰Pb at all depths. The²¹⁰Po profiles of the stratified lake resembled in shape those of its grandparent²¹⁰Pb, but with distinct characteristics of their own in the oxic upper waters where particulate²¹⁰Po (8–12 dpm kg^?1) was greatly in excess over particulate²¹⁰Pb, while dissolved²¹⁰Po (25–40 dpm kg^?1) was slightly deficient. Immediately following the overturn, dissolved and particulate²¹⁰Po were similar (about 15 dpm kg^?1), at all depths. The destruction of the lake's meromictic structure was accompanied by a reduction of its²¹⁰Pb inventory, while that of²¹⁰Po was almost unaffected. Thus, at overturn a transient state was created with the inventory of²¹⁰Po exceeding that of²¹⁰Pb.