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 (⁸⁸Sr/⁸⁶Sr) during precipitation of inorganic (primary) calcite from the lake's water. It was found that the absolute value of the ⁸⁸Sr/⁸⁶Sr fractionation factor, Δ^88/86Sr, increases as a function of calcite accumulation rate (Δ^88/86Sr [‰] = −0.05 to 0.042·log(R) [μmol·m⁻²·d⁻¹], where R is the accumulation rate). Furthermore, the ⁸⁷Sr/⁸⁶Sr and ⁸⁸Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr and ⁸⁸Sr/⁸⁶Sr ratios were measured in primary calcite, aragonite shells of live Melanopsis, lake water and various water sources to the lake. While the lake's ⁸⁷Sr/⁸⁶Sr ratios are determined by the mixture of freshwater of the Jordan River and saline springs, the ⁸⁸Sr/⁸⁶Sr 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 ⁸⁸Sr, results in an increase of the δ^88/86Sr 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 ⁸⁸Sr/⁸⁶Sr water-CaCO₃ fractionation of Δ^88/86Sr = −0.21‰. Similar Δ^88/86Sr values were reported for the precipitation of coralline and inorganic aragonite from seawater and the precipitation of inorganic calcite from various continental waters. The Δ^88/86Sr 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 Δ⁸⁸Sr/⁸⁶Sr 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 ⁸⁸Sr/⁸⁶Sr ratio in the primary CaCO₃ recovered from the lake's sedimentary archives.