海相沉积岩中钙质结核成因综述

Origin of calcareous concretions in marine sedimentary rocks

目的由自生碳酸盐矿物集合体构成的钙质结核是深时海相地层中较为常见的沉积构造,因其特殊的生长条件,已成为研究沉积环境、古气候和沉积物孔隙流体演化的重要载体。为深入了解不同钙质结核的成因,突出其地质意义,方法本文介绍了不同成因类型钙质结核的宏观形态、微观组构和同位素等特征,并对其成岩时限、生长模式和涉及的生物地球化学过程进行了综述。结果钙质结核可形成于准同生沉积阶段、早期成岩阶段和晚期成岩阶段,包括同心圈层状生长和弥漫性生长两种生长模式。钙质结核的形成机制主要包括3种:(1)在厌氧环境中,沉积物中的有机质为硫酸盐还原菌提供电子供体,发生氧化,生成碳酸氢根,导致孔隙水中碱度升高和钙质结核的形成;(2)在硫酸盐还原菌和甲烷氧化古菌共同作用下,冷泉流体中的甲烷发生厌氧氧化作用,孔隙水中过饱和的碳酸盐发生沉淀,形成钙质结核;(3)深埋藏环境下,孔隙水中的流体超压形成的晚期成岩构造,以发育“牛肉”和“锥中锥”为特征。结论目前对参与钙质结核形成的微生物过程已有了较为深入的认识,但仍需更多高分辨率原位分析,以揭示不同的生物地球化学过程(如有机质矿化作用、硫酸盐还原与甲烷厌氧氧化耦合作用等)如何控制结核生长,并对其成因机制作进一步探究。

Objective As a common sedimentary structure in deep-time marine strata,calcareous concretions,which is mainly composed of authigenic carbonate mineral aggregates,have become an important archive for studying sedimentary environment,paleoclimate,and sedimentary pore fluid evolution due to their special growth conditions.In order to deeply understand the genesis of different calcareous concretions and highlight their geological significance,Methods this paper reviews the macroscopic morphology,microfabrics,and isotopic characteristics of different calcareous concentrations,and elaborates the timing and mode of concretionary growth,and the related biogeochemical processes during their formation.Results Calcareous concentrations can be formed in the quasi-syndepositional stage,early or late diagenetic stage,with concentric or pervasive growth mode.Its formation mechanism mainly have three ways:(1)In anaerobic environment,organic matters in the sediment are oxidized by providing electron donors for sulfate-reducing bacteria,which undergo oxidation and generate bicarbonate ions,leading to an increase in alkalinity in pore water and the formation of calcareous concentrations;(2)Methane in cold-seep fluids upward to the seafloor is oxidized via consortia of methanotrophic archaea and sulfate-reducing bacteria,increasing carbonate alkalinity in sediment porewaters,and promoting in-situ carbonate precipitation;and(3)In deep burial settings,over-pressured porewaters in the sediment trigger the formation of late digenetic carbonates in fractures,which are characterized by the development of"beef"and"cone in cone"structures.Conclusion In recent years,microbial processes involved in the formation of calcareous concentrations have been extensively studied,but more high resolution in-situ analysis is needed to clarify how different biogeochemical processes(e.g.,organic mineralization,sulfate reduction and methane anaerobic oxidation coupling)control on calcareous concretion growth,and to further indicate its genetic mechanism.