Meteoric and mixing-zone diagenesis can dramatically alter the geochemical signatures of shallow marine carbonates. Most preserved pre-Cretaceous carbonates were deposited in shallow marine environments and thus may have been susceptible to meteoric and mixing-zone diagenesis. However, a quantitative understanding of how the geochemical composition of carbonates changes during diagenesis still requires further development. Meteoric and mixing-zone diagenesis can dramatically alter the geochemical signatures of shallow marine carbonates. Most preserved pre-Cretaceous carbonates were deposited in shallow marine environments and thus may have been susceptible to meteoric and mixing-zone diagenesis. However, a quantitative understanding of how the geochemical composition of carbonates changes during diagenesis still requires further development. Here, we present a new two-dimensional (2D) reactive transport model coupled with a 2D coastal hydrology model to simulate carbonate diagenesis and provide insights into its impact on the isotopic and elemental compositions of carbonates in the geological record. Using this model, we have simulated the stratigraphic trends and relationship between isotopic records (for example, δ13C and δ18O values) observed in modern (Recent-Miocene) sections where the impact of meteoric diagenesis has been clearly characterized. Our model can also reproduce anomalous Neoproterozoic carbonate geochemical profiles where the effects of meteoric diagenesis have been debated. Further, our model indicates that linear carbonate C-O isotope co-variations can either be generated in the mixing zone between freshwater and marine pore waters, or in the freshwater phreatic zone with a downward decrease in recrystallization (with no net carbonate dissolution or precipitation) rate. In addition, numerous processes were observed to decouple δ18Ocarb values from other isotopic and elemental signatures during carbonate diagenesis, indicating that a lack of linear correlation between δ18Ocarb values and other geochemical variables does not necessarily suggest limited meteoric alteration. Sensitivity analyses show that the steady-state timescale is controlled by compositional differences between fluid endmembers, the calcite-water element distribution coefficient, the recrystallization rate, porosity, and the groundwater discharge rate. Given that reactive transport models have proven to be powerful theoretical tools in many disciplines of Earth sciences, our hope is that this model will promote a more quantitative understanding of meteoric and mixing zone diagenesis of marine carbonates.

中文翻译：

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用二维反应输运模型模拟大气和混合带碳酸盐岩成岩作用

陨石和混合带成岩作用可以显着改变浅海碳酸盐岩的地球化学特征。大多数保存下来的白垩纪前碳酸盐沉积在浅海环境中，因此可能容易受到大气和混合带成岩作用的影响。然而，对碳酸盐岩地球化学成分在成岩作用过程中如何变化的定量理解仍需要进一步发展。陨石和混合带成岩作用可以显着改变浅海碳酸盐岩的地球化学特征。大多数保存下来的白垩纪前碳酸盐沉积在浅海环境中，因此可能容易受到大气和混合带成岩作用的影响。然而，对碳酸盐岩地球化学成分在成岩作用过程中如何变化的定量理解仍需要进一步发展。这里，我们提出了一种新的二维 (2D) 反应输运模型，结合二维沿海水文模型来模拟碳酸盐岩成岩作用，并深入了解其对地质记录中碳酸盐岩同位素和元素组成的影响。使用该模型，我们模拟了在现代（最近 - 中新世）剖面中观测到的同位素记录（例如，δ13C 和 δ18O 值）之间的地层趋势和关系，其中大气成岩作用的影响已得到明确表征。我们的模型还可以再现异常的新元古代碳酸盐地球化学剖面，其中大气成岩作用的影响一直存在争议。此外，我们的模型表明，线性碳酸盐 CO 同位素协变可以在淡水和海洋孔隙水之间的混合区产生，或在淡水潜水区，重结晶（无碳酸盐净溶解或沉淀）速率下降。此外，在碳酸盐岩成岩过程中，观察到许多过程将 δ18Ocarb 值与其他同位素和元素特征解耦，表明 δ18Ocarb 值与其他地球化学变量之间缺乏线性相关性并不一定表明有限的陨石蚀变。敏感性分析表明，稳态时间尺度受流体端元之间的成分差异、方解石-水元素分布系数、再结晶速率、孔隙度和地下水排放速率的控制。鉴于反应输运模型已被证明是地球科学许多学科中强大的理论工具，