210Pb and 210Po during the destruction of stratification in the Dead Sea

Abstract

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 of 210Pb and 210Po. In 1977/78, prior to overturn, dissolved 210Pb (35–50 dpm kg −1) predominated over particulate 210Pb (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 between 210Pb and 226Ra 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 dissolved 210Pb in the unperturbed anoxic deepest layers, because these remained isolated, at about 3 years. Following the overturn of 1979, dissolved 210Pb exceeded particulate 210Pb at all depths. The 210Po profiles of the stratified lake resembled in shape those of its grandparent 210Pb, but with distinct characteristics of their own in the oxic upper waters where particulate 210Po (8–12 dpm kg −1) was greatly in excess over particulate 210Pb, while dissolved 210Po (25–40 dpm kg −1) was slightly deficient. Immediately following the overturn, dissolved and particulate 210Po were similar (about 15 dpm kg −1), at all depths. The destruction of the lake's meromictic structure was accompanied by a reduction of its 210Pb inventory, while that of 210Po was almost unaffected. Thus, at overturn a transient state was created with the inventory of 210Po exceeding that of 210Pb.