微生物(有机)白云石成因模式研究进展与思考

Advance and Review on Microbial/Organogenic Dolomite Model

白云石的成因机制一直是地球科学领域备受争论的议题.传统研究多认为白云石为次生成因.新近提出的"微生物(有机)白云石模式"认为该矿物在微生物或有机物作用下可以从溶液中直接沉淀,从而为白云石的成因研究提供了新思路.总结了该模式近20年来取得的重要进展,并指出尚待弥补的环节.微生物的催化机理主要表现为其代谢作用可提高胞外微环境中白云石的饱和度,同时其携带负电荷的细胞外壁可作为白云石晶体的成核位点.微生物成因白云石呈球状、哑铃状和花椰菜形等外貌.高盐度有利于微生物介导低温白云石沉淀,而硫酸根扮演的角色则仍需进一步验证.最新的研究表明微生物胞外聚合物有助于Mg^(2+)摆脱水合作用的束缚,是微生物催化白云石形成的关键.非微生物源的羧基化合物也可通过与胞外聚合物类似的途径在白云石饱和溶液中促进低温白云石形成.微生物(有机)成因白云石为原白云石,而非有序白云石.目前对微生物(有机)成因白云石在成岩改造中的演化过程仍然缺乏认识,是"微生物(有机)白云石模式"的缺失环节,亟待完善.

The origin of dolomite has remained an enigmatic issue of earth sciences. Dolomite mineral is generally considered as a replacement phase of pre-exsisting carbonate. However, a microbial/organogenic dolomite model has been recently proposed, suggesting that dolomite can also be directly precipitated from solutions with the aid of microorganisms or organic matters, thus providing new insight into the old enigma. This paper reviews the latest progress on the microbial/organogenic dolomite model and discusses how to improve such model. Microbes facilitate lowtemperature dolomite formation primarily by enhancing the saturation state with respect to dolomite. Furthermore, the negatively-charged microbial cell walls can be functioned as template for dolomite nucleation. Microbial dolomite normally displays spherical, dumbbell and cauliflower-like morphologies. Microbially-induced dolomite formation can be facilitated by elevated salinity. The role of sulfate on microbial dolomite formation still awaits resolution. The recent studies have also demonstrated that microbial exopolymeric substances（EPS） play an important role in the dewatering of Mg-H2O complexes. In the similar manner with microbial EPS, natural organic matters can also catalyze dolomite precipitation.However, growing evidence shows that microbes could only precipitate proto-dolomite rather than ordered-dolomite. More studies are required to improve our understanding of the alteration of microbial/organogenic dolomite during burial diagenesis.