88Sr/86Sr fractionation and calcite accumulation rate in the Sea of Galilee

This study used the Sea of Galilee (Lake Kinneret, northern Israel) as a "natural laboratory" to investigate the fractionation of the stable Sr isotope ratio ((88) Sr/(86) Sr) during precipitation of inorganic (primary) calcite from the lake's water. It was found that the absolute value of the Sr-88/(86) Sr fractionation factor,Delta Sr-88/86, increases as a function of calcite accumulation rate (Delta Sr-88/86 [%] = -0.05 to 0.042.log(R) [mu mol.m(-2).d(-1)], where R is the accumulation rate). Furthermore, the Sr-87/Sr-86 and Sr-88/(86) Sr ratios in the freshwater and brines that enter the lake were used to calculate the contributions of these sources to the lake Sr budget. The Sr-88/(86) Sr and Sr-88/(86) Sr ratios were measured in primary calcite, aragonite shells of live Melanopsis, lake water and various water sources to the lake. While the lake's Sr-88/(86) Sr ratios are determined by the mixture of freshwater of the Jordan River and saline springs, the Sr-88/(86) Sr ratios of the lake reflect a more complex mass balance that includes the effect of isotopic fractionation associated with the precipitation of primary calcite. Data analysis suggests that long-term accumulation of inorganic calcite depleted in the heavy isotope Sr-88, results in an increase of the delta Sr-88/86 value of the lake water by 0.05%. In contrast to the primary inorganic calcite, biogenic aragonite of the Melanopsis shells show a rather constant (88) Sr/(86) Sr water-CaCO3 fractionation of Delta Sr-88/86 = -0.21%. Similar Delta Sr-88/86 values were reported for the precipitation of coralline and inorganic aragonite from seawater and the precipitation of inorganic calcite from various continental waters. The Delta Sr-88/86 value of inorganic calcite is modulated by the rate of carbonate precipitation, as noted above and shown by precipitation experiments. Massive precipitation of primary calcite with a wide spread of accumulation rates occurs during the spring phytoplankton bloom in Lake Kinneret. The bloom dictates the degree of calcite saturation in the lake and hence the stable Sr isotopic composition of the precipitating calcite. The correlation between Delta(88) Sr/Sr-86 and the calcite accumulation rate can be therefore used as a tool to reconstruct environmental and ecological variations in the historical lake by analyzing the (88) Sr/(86) Sr ratio in the primary CaCO3 recovered from the lake's sedimentary archives. (C) 2017 Elsevier Ltd. All rights reserved.