Radium redux in the Dead Sea: profiles and transient Ra/Ba models

Abstract

Two radium-226 profiles were measured on 31 Dead Sea water samples collected in February, 1978 before the 1979 overturn. These two profiles, located in the central Northern Basin, are practically identical and show a distinct two-layer feature. The Upper Water Mass (UWM) has a uniform Ra activity at 114.5 dpm/kg and the Lower Water Mass (LWM) has a fairly constant value at 97.8 dpm/kg, being separated by a 25 m transition layer (150–175 m depth). Radon measurements in the LWM indicate that there is no radon in excess over radium except for the very bottom samples at 300 m depth. These Ra activity levels are at least two orders of magnitude greater than that of the Pacific deep water. That the UWM has significantly higher Ra activity than the LWM is a unique feature, suggesting major inputs into the UWM with very limited mixing between the two water masses. This feature was used to estimate duration of the meromixis since the last overturn based on Ra decay in a closed system [Stiller and Chung, Limnol. Oceanogr. 29 (1984) 574–586]. A first-order removal rate or particulate scavenging process was invoked later to interpret the observed barium and radium data [Chan and Chung, Earth Planet. Sci. Lett. 85 (1987) 41–53]. These two studies assumed that no mixing occurred between the two water masses and the LWM was thus in a closed system. In this study we apply a transient box model in a closed system with varying scavenging rates of Ra and Ba to evaluate the isolation time since the previous overturn based on the Ra and Ba data measured in 1978 and 1979 as reported in this paper and in Chan and Chung [Earth Planet. Sci. Lett. 85 (1987) 41–53]. We also evaluate the fluid residence time of the LWM with respect to mixing and exchange with the UWM, τ m, which characterizes the mixing rate between the two water masses in an open system with particulate scavenging, assuming that the 1978 data represent a steady-state condition and that the 1979 data represent the previous overturn condition. The transient Ra/Ba model in a closed system shows that the isolation time of the LWM is a linear function of the scavenging rate ratio of Ra to Ba with a maximum of 317 years as the ratio reaches zero and 207 years as the ratio becomes 1, and the scavenging residence time for Ba is about 20 times the isolation time at any scavenging rate ratio. The scavenging residence time for Ra increases exponentially as the scavenging rate ratio decreases. In the open-system steady-state model, τ m is also a linear function of the scavenging rate ratio with a maximum of 339 years as the ratio vanishes. The scavenging residence time for Ba is also about 20 times greater than τ m. Similarly, the scavenging residence time for Ra increases exponentially as the ratio decreases. Since the same set of data are used in these calculations, the isolation time in a closed system is quite comparable to but slightly less than τ m in an open system at any given scavenging rate ratio. Both the isolation time and τ m are quite sensitive to the changes of Ra and Ba values in the LWM and the overturned water. If the mean Ra value of the 1979 overturned water is increased by 2% and that of the 1978 LWM is decreased by 2% while the Ba values remain unchanged, this 4% deviation in Ra values between the overturned water and the LWM leads to an increase of 28–30% in the isolation time and τ m, respectively. In comparison with the earlier calculations [Stiller and Chung, Limnol. Oceanogr. 29 (1984) 574–586; Chan and Chung, Earth Planet. Sci. Lett. 85 (1987) 41–53], the isolation time calculated with no Ra scavenging is about 55 years longer, probably because the concentrations of Ba and Ra are used for the present calculations with no constraint on the inventories and the lake volume changes.