Abstract:

The Neoproterozoic represents a critical interval in the evolution of the Earth surface system,during which oceans transitioned from low-oxygen,microbially dominated conditions to a stratified and progressively oxygenated state,accompanied by profound reorganization of seawater chemistry and carbonate depositional systems. Marine dolomite cements,owing to their synsedimentary to early diagenetic modes of formation,have been widely used to reconstruct Neoproterozoic seawater chemistry;however,their formation mechanisms and principal controlling factors remain debated. This study synthesizes petrographic observations together with multiple geochemical proxies,including C,O,Sr,Mo,U,and Mg isotopes,from globally representative Neoproterozoic carbonate successions,to systematically evaluate the petrographic characteristics and paleoenvironmental significance of marine dolomite cements. The stratigraphic comparisons indicate that,despite regional variations in cement morphologies and isotopic compositions,the timing of dolomite precipitation and associated genetic patterns are broadly comparable,suggesting a dominant control by a common global seawater chemical background. Multi-isotopic records further demonstrate that Neoproterozoic seawater chemistry did not remain in a long-term steady state,but evolved dynamically in response to tectonic reorganization,glacial-interglacial oscillations,and variations in continental weathering fluxes. The widespread occurrence of dolomite therefore does not reflect a single,persistently existing “dolomite sea,”but instead records the episodic emergence of favorable chemical conditions during different tectonic-climatic stages. Dolomite precipitation windows are inferred to have been opened under a relatively elevated Mg/Ca background,when low sulfate concentrations,high alkalinity,and appropriate redox conditions acted in concert. Neoproterozoic tectonic-climatic processes modulated seawater end-member inputs and material cycling,allowing such precipitation windows to recur at the global scale and thereby accounting for the anomalous abundance of marine dolomite during this interval. This study systematically clarified the key controlling mechanisms of Neoproterozoic marine dolomite formation from a multi-scale and multi-isotope perspective,providing important constraints for reconstructing the chemical evolution of ancient oceans and understanding the co-evolution of the Earth’s surface system in deep time.

Key words: Neoproterozoic, seawater chemical evolution, marine dolomite cements, dolomite precipitation, isotope geochemistry

摘要：

新元古代是地球表层系统由低氧、微生物主导海洋向分层氧化海洋转型的关键时期,海水化学结构与碳酸盐沉积体系发生了显著重组。海相白云石胶结物因其同生—准同生的特征,被广泛用于重建古海水化学条件,但其形成机制及控制因素仍存在争议。以全球典型新元古代碳酸盐岩地层为研究对象,综合岩石学及碳、氧、锶、钼、铀和镁同位素等多重地球化学证据,系统梳理了海相白云石胶结物的岩石学特征及其环境意义。地层对比表明,不同地区白云石胶结物在形态组合和同位素组成上虽存在区域差异,但其沉积时序与成因模式具有良好的可比性,指示白云石发育主要受全球性海水化学背景控制。多同位素记录显示,新元古代海水化学并非长期稳定,而是在构造重组、冰期—间冰期振荡及大陆风化通量变化的共同作用下持续演化。白云石的广泛发育反映的并非单一、长期存在的“白云石海”,而是特定海水化学条件在不同构造-气候下的阶段性出现。研究认为,在相对偏高的 Mg/Ca 值背景下,低硫酸盐浓度、高碱度及适宜的氧化还原条件协同作用,可开启白云石沉淀“窗口”。新元古代构造-气候过程通过调控海水化学端元输入与物质循环方式,使该沉淀窗口在全球多次出现,从而解释了该时期白云石的异常富集。本研究从多尺度、多同位素角度系统厘清了新元古代海相白云石形成的关键控制机制,为重建古海洋化学演化及理解深时地球表层系统协同演变提供了重要约束。

关键词: 新元古代, 海水化学演化, 海相白云石胶结物, 白云石沉淀, 同位素地球化学

CLC Number: 

• P588.245