Dynamics of the carbon dioxide system in the Dead Sea

Because the Dead Sea is a Ca-chloride hypersaline lake and is virtually lif eless, it is an excellent system to investigate the physical and chemical p aths of the carbon cycle in terrestrial aquatic environments that are gener ally overwhelmed by biologic carbon fluxes. The Dead Sea is known to form m assive aragonite deposits in the form of varves at present and during the L isan period (late Pleistocene). The commonly accepted season for the main a ragonite precipitation was summer, and the trigger for precipitation was at tributed to evaporation and the warming of surface water ("whitening" event s). To determine the main carbon fluxes in the Dead Sea, we followed change s in the carbonate system of the mixed layer, from February 1993 to Decembe r 1994, after stratification formed due to heavy flooding during the winter of 1992. The stratification isolated the mixed layer, a relatively small r eservoir from the main brine body, causing an amplification of the chemical signals. The data show that partial pressure of CO2 in the lake was very high (appro ximate to 2000 mu atm). Total alkalinity and total carbon of the mixed laye r decreased with time, whereas delta C-13 increased. The high P-CO2 origina tes from precipitation of aragonite and implies that in many aquatic system s it may originate from an inorganic process and not only from degradation of organic matter. Thermodynamic calculation estimated the degree of aragonite saturation to b e approximate to 15 when 10% of Dead Sea brine with a high Ca content mixed with 90% runoff freshwater with high-bicarbonate content. Therefore, mixin g during winter flooding triggers massive aragonite deposition in the Dead Sea. The general conclusion is that inorganic carbonate precipitation by mi xing of two solutions, one supplying Ca2+ and the other HCO3- should be con sidered in the evaluation of the carbonate system in a wide range of aquati c environments. A mass-balance model for total alkalinity, total carbon, and carbon isotope s reveals two main carbon sinks and one carbon source. The sinks are a chem ical deposition of aragonite (1.4 mol.m(-2).y(-1)) andCO(2) escape to the a tmosphere (4 mol.m(-2).y(-1)), and the source is bicarbonate input by flood s (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 three fold the aragonite precipitation rate, indicating a net CO2 loss. We sugges t that such a scenario is possible if the Dead Sea was meromictic (stratifi ed) for a very long period of time. Copyright (C) 2001 Elsevier Science Ltd .