Continuous CO₂ escape from the hypersaline Dead Sea caused by aragonite precipitation

Chemical precipitation of CaCO₃ occurs in diverse marine and lacustrine environments. In the hypersaline Ca-chloride lakes that have been occupying the Dead Sea basin since the late Pleistocene, CaCO₃ precipitated, mostly as aragonite. The aragonite sediments precipitated mainly during periods of high lake level stands as a result of mixing of bicarbonate-rich freshwater runoff with Dead Sea brine, that is Ca-rich and have high Mg/Ca ratio. During periods of arid conditions with limited freshwater inflow, water level declined, salinity increased and gypsum and halite became the dominant evaporitic minerals to precipitate. The present study investigates the carbon cycle of the Dead Sea under the current limited water and bicarbonate supply to the brine, representing periods of extremely arid conditions. The decrease of inflows to the Dead Sea in recent years stems mainly from diversion of freshwater from the drainage basin and results in dramatic water level decline and massive halite precipitation. During 2013–2014, bi-monthly depth profiles of total alkalinity, dissolved inorganic carbon (DIC) and its isotopic composition (δ¹³C) were conducted in the Dead Sea, from surface down to the bottom of the lake (290 m). Mass balance calculations conducted for the period 1993–2013 show that while inventories of conservative ions such as Mg²⁺ remained constant, the net DIC inventory of the lake decreased by ∼10%. DIC supply to the lake during this period, however, amounted to ∼10% of lake's inventory indicating that during 20 years, the lake lost ∼20% of its 1993s inventory. Compilation of historical data with our data shows that during the past two decades the lake's low DIC (∼1 mmol kg⁻¹) and very high PCO₂ (1800 ppm V) remained relatively constant, suggesting that a quasi-steady-state situation prevails. In spite of the surprisingly stable DIC and CO₂ concentrations, during this 20 year period δ¹³C_DIC increased significantly, from 1.4‰ to 2.7‰. An isotopic mass balance calculation together with the high PCO₂ of the brine show that during that period the lake lost about 3.7 · 10¹⁰ moles of DIC, of which ∼60% by CO₂ degassing and ∼40% by aragonite precipitation. The deviation from the common 1:1 CO₂:CaCO₃ ratio in most aqueous systems is facilitated by the dominance of the borate alkalinity of the Dead Sea. Nucleation and crystal growth experiments suggest that throughout this time the Dead Sea remained supersaturated with respect to aragonite, a situation facilitated by combination of slow nucleation rates and absence of growth surfaces. The very high PCO₂ of the Dead Sea is maintained by the low CO₂ piston velocity of the brine, calculated to be only ∼4.5 m yr⁻¹, more than an order of magnitude slower than seawater value.