“白云岩问题”与“前寒武纪之谜”研究进展

白云岩的成因是沉积学界倍受关注的研究主题;众多的白云岩化模式可以用来解释各种成岩白云岩的成因;然而原生白云岩的成因一直是困扰沉积学界的难题,被称为“白云岩问题”。究其原因主要在于：在近地表环境的常温、常压实验条件下不能生成完美有序的白云石矿物。近年来,野外观测和实验模拟研究发现某些微生物的生命活动可导致白云石于地表常温、常压条件下发生沉淀。如硫酸盐还原细菌和产烷菌的调节作用可以克服白云石晶核形成的动力学障碍,在这些厌氧菌的参与下,白云石晶核形成和沉淀并不需要高镁离子和过饱和状态的溶液环境。这种微生物学和沉积学的结合代表了一个新的研究方向,同时也为解决“白云岩问题”带来了新希望。泥晶白云岩化作用（mimetic dolomization）可以保留原矿物（文石或方解石）的晶形、原岩的微细组构,对解释地史时期保存原生构造的白云岩具重要的启示。“前寒武纪之谜”是指前寒武纪叠层石中缺乏钙化蓝细菌化石的现象。参与碳酸盐岩叠层石建造的微生物群组成可能随着地质历史的演化发生变化,蓝细菌在显生宙的叠层石建造过程中起主导作用,细胞体积更小的真细菌很可能是参与前寒武纪叠层石建造的主要微生物。

Dolomite Problem and Precambrian Enigma

The origin of dolomites is a research topic attracting great attention in sedimentology.Many dolomitization models have been invoked to interpret the origination of virious diagenetic dolomites.However,the genesis of early-formed dolomites has long been an enigma in sedimentology,often referred to as the "Dolomite Problem".This problem arises from the fact that scientists have not yet been successful in the laboratory in precipitating perfectly ordered dolomite at the normal temperatures and pressure that occur at Earth's surface.The recent field and laboratory experiments show that some microbes play important roles in precipitation of dolomite under conditions of Earth's surface.For example,the direct mediation of sulfate-reducers and methanogens can overcome the kinetic barrier to dolomite nucleation,and that they may play an active role in the formation of this mineral in natural environments.In these anaerobes involved system,neither extremely Mg-rich fluids nor highly supersaturated conditions are required for the nucleation and precipitation of dolomite.This integrating microbiology into the carbonate sedimentology opens a new research direction and also throws a new light on the "Dolomite Problem".Mimetic dolomization,where the precursor fabrics are excellently preserved,has important implications on interpreting the genesis of dolostone with original fabrics preserved.The "Precambrian enigma" refers to the scarcity of calcified cyanobacteria in Precambrian stromatolites.There have been subsequent changes in the composition stromatolite biota through Earth's history.Smaller eubacteria may have greater involvement than cyanobacteria in stromatolite formation in the Precambrian,and cyanobacteria may enter stromatolite building biota until the latest Neoproterozoic.