Hydrochemical mixing-zones trigger dolomite formation in an alkaline lake

Dolomite is globally present in past geological records, but rare in modern environments. The mechanisms favouring its precipitation under ambient conditions remain highly debated. This study investigates sediments, containing high concentrations of early diagenetic calcian dolomite, from alkaline Lake Van (Republic of Turkiye, formally Turkey) dating back to 252 ka bp. Powder X-ray diffraction and scanning electron microscopy evidence suggests that dolomite formation is associated with prior dissolution of aragonite and low-Mg calcite and a subsequent co-precipitation with, and/or partial transformation of, high-Mg calcite into dolomite. The infrequent presence of diatom frustules encapsulated by dolomite suggests, for Lake Van, unusually low pore-water pH at the time of dolomite formation. Conditions facilitating the preservation of silica, as well as dissolution and subsequent reprecipitation of carbonate phases, could result from periodic reventilations of Lake Van's deep water and an advection of pore fluids with contrasting redox potential and chemical concentration gradients. This continental analogue of the coastal 'mixing-zone' dolomitization model argues that not overcoming a single specific hydrochemical threshold, but highly dynamic and fluctuating conditions trigger dolomite formation in Lake Van.