白云岩有机成因模式:机制、进展与意义

The organogenic dolomite model:Mechanism,progress and significance

白云岩是一种常见的碳酸盐岩,它广泛分布在古代碳酸盐岩台地中,却很少见于全新世沉积物中,这构成了一个未解之谜。近20多年的研究成果表明,早期成岩过程中微生物参与的硫酸盐还原反应、甲烷生成和厌氧氧化反应以及有氧呼吸作用能够促进白云石的沉淀:细菌细胞和胞外聚合物(EPS)带有负电荷,能够聚集溶液中Mg2+和Ca2+;同时上述氧化还原反应产生HCO3-,提高了孔隙水中反应物的浓度;这样在细菌细胞周围形成一个对白云石超饱和的微环境,有利于白云石的沉淀。这是一种新的白云石成因模式,微生物活动和有机质是影响白云石形成最重要的因素,特殊的球状形态和碳同位素特征是鉴别有机成因白云石的重要标志。白云岩有机成因模式为认识地质历史时期大套白云岩的成因、探索"白云岩之谜"提供了新的思路。

Dolomite is common in stratigraphic record.It is widely distributed in the carbonate platform in ancient times,however,rarely found in the Holocene sediments,which constitutes a mystery.The research achievements in last two decades indicate that sulfate reduction,methanogenesis, anaerobic methane oxidation and aerobic respiration in the early diagenetic process induce dolomite precipitation.Bacterial cells and extracellular polymeric substances（EPS）with negative charges could concentrate Mg and Ca ions in solution.At the same time,these three oxidation-reduction reactions generate bicarbonate and raise the concentration of reactants of the pore water.All of the above favor the formation of a micro-environment that is supersaturated respect to dolomite around cells,resulting in dolomite precipitation.This is a new dolomitization model.Microbial activity and organic matter are the most important factors for the precipitation of dolomite.The special spherical shape and carbon isotope characteristics of dolomite can be used to identify an organogenic origin.This organogenic dolomite model has significant implications for the origin of the massive dolomite in geological history and ＂Dolomite Problem＂ research.