Dynamics of the carbon dioxide system in the Dead Sea

Abstract

–Because the Dead Sea is a Ca-chloride hypersaline lake and is virtually lifeless, it is an excellent system to investigate the physical and chemical paths of the carbon cycle in terrestrial aquatic environments that are generally overwhelmed by biologic carbon fluxes. The Dead Sea is known to form massive aragonite deposits in the form of varves at present and during the Lisan period (late Pleistocene). The commonly accepted season for the main aragonite precipitation was summer, and the trigger for precipitation was attributed to evaporation and the warming of surface water (“whitening” events). To determine the main carbon fluxes in the Dead Sea, we followed changes in the carbonate system of the mixed layer, from February 1993 to December 1994, after stratification formed due to heavy flooding during the winter of 1992. The stratification isolated the mixed layer, a relatively small reservoir from the main brine body, causing an amplification of the chemical signals. The data show that partial pressure of CO 2 in the lake was very high (≈2000 μatm). Total alkalinity and total carbon of the mixed layer decreased with time, whereas δ 13C increased. The high P CO 2 originates from precipitation of aragonite and implies that in many aquatic systems it may originate from an inorganic process and not only from degradation of organic matter. Thermodynamic calculation estimated the degree of aragonite saturation to be ≈15 when 10% of Dead Sea brine with a high Ca content mixed with 90% runoff freshwater with high-bicarbonate content. Therefore, mixing during winter flooding triggers massive aragonite deposition in the Dead Sea. The general conclusion is that inorganic carbonate precipitation by mixing of two solutions, one supplying Ca 2+ and the other HCO 3 − should be considered in the evaluation of the carbonate system in a wide range of aquatic environments. A mass-balance model for total alkalinity, total carbon, and carbon isotopes reveals two main carbon sinks and one carbon source. The sinks are a chemical deposition of aragonite (1.4 mol · m −2 · y −1), and CO 2 escape to the atmosphere (4 mol · m −2 · y −1), and the source is bicarbonate input by floods (2.1 mol · m −2 · y −1). The precipitation rate of the present Dead Sea is approximately sixfold lower than the average precipitation rate during the Lisan period, implying a wetter climate during that period. The present CO 2 escape rate from the mixed layer is twice the bicarbonate input and threefold the aragonite precipitation rate, indicating a net CO 2 loss. We suggest that such a scenario is possible if the Dead Sea was meromictic (stratified) for a very long period of time.