Lead concentrations and isotopic ratios in the sediments of the Sea of Galilee

The isotopic composition and concentrations of Pb in the sediments of the Sea of Galilee (Lake Kinneret) were measured. The studied sediments have been deposited in the lake since the early 1900s (ca. 1920), hence Pb data record the transition from a period when the lake vicinity was sparsely populated to the present (approximately 100 000 people living in the area around the lake). In general, there is either a constant or a relatively slow increase in Pb concentrations from 40 cm depth (3.5-4.4 μg/g; ca. 1920) to 17 ± 2 cm below the sediment-water interface (3.7-7.2 μg/g;), which was deposited in the mid-1960s. From 17 ± 2 cm below the surface, there is a much faster increase up to 7 ± 2 cm below the surface (from 6.5 to 11.5 μg/g; 1982-1983), and from 7 ± 2 cm there is a gradual decrease in Pb concentrations toward the sediment-water interface. At station G, near the outlet of the Jordan River, Pb concentrations drop between 29 and 25 cm below the surface, probably reflecting changes in the particulate load of the Jordan River due to the drying out of the Hula Swamp in the early 1950s. ²⁰⁶Pb/²⁰⁷Pb values in all the stations record most of the shifts displayed by Pb concentrations in the sediment. The estimated value of total Pb deposited annually in the lake sediment in the early 1990s is very close to the value obtained from measurements of Pb fluxes to the lake from eolian and fluvial sources. On the basis of the linear relationship between ²⁰⁶Pb/²⁰⁷Pb (or ²⁰⁸ Pb/^207Pb) and 1/[Pb], we argue that two end-members contribute most of the Pb to the lake sediments. Sources of Pb to the lake include (i) the weathering of basalt from the eastern Galilee and the Golan Heights contributing 2.6 ± 0.5 μg/g Pb to the sediment and (ii) anthropogenic Pb that is affecting both surface and deep (from 30 to 40 cm) lake sediments. At station S, a third source, Pb released from soils developed on carbonates, should be considered as well.