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 Türkiye, 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.

中文翻译：

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水化学混合区引发碱性湖中白云石的形成

白云石在过去的地质记录中广泛存在于全球范围内，但在现代环境中却很少见。在环境条件下有利于其降水的机制仍然存在激烈争论。这项研究调查了来自碱性凡恩湖（土耳其共和国，正式土耳其）的沉积物，其中含有高浓度的早期成岩钙质白云石，其历史可以追溯到 252 ka bp。粉末 X 射线衍射和扫描电子显微镜证据表明，白云石的形成与文石和低镁方解石的先前溶解以及随后与高镁方解石共沉淀和/或部分转变为白云石有关。白云石包裹的硅藻壳很少出现，这表明凡湖在白云石形成时孔隙水 pH 值异常低。有利于二氧化硅保存以及碳酸盐相溶解和随后再沉淀的条件可能是由于范湖深水的定期再流通以及具有对比氧化还原电位和化学浓度梯度的孔隙流体的平流而产生的。这种沿海“混合区”白云石化模型的大陆模拟认为，并不是克服单一的特定水化学阈值，而是高度动态和波动的条件触发了凡湖白云石的形成。