Abstract Oxygen isotopes are the most commonly applied speleothem proxy for reconstructing Quaternary changes in precipitation and/or temperature. These interpretations are either limited to qualitative wetting and drying trends or rely on theoretical, experimental and/or empirical equilibrium isotope fractionation factors for more quantitative constraints. These various fractionation factors have similar temperature sensitivities, but their absolute values differ, and cave calcite does not appear to generally precipitate in isotopic equilibrium with its drip water. Rapid CO2 degassing paired with calcite precipitation, both occurring under disequilibrium conditions, are a set of mechanisms commonly invoked to explain offsets between observed and equilibrium isotopic fractionation between cave calcites and drip waters. However, the relevance of these disequilibrium mechanisms to speleothem records remains unresolved. Here, we compare measured δ18O values of modern speleothem calcite from a tropical cave in Guam to calcite δ18O values predicted by a modified version of the ISOLUTION proxy system model. This extends the global comparison of cave drip water and modern calcite δ18O values to higher temperatures. We initialize the model using contemporaneous measurements of drip water (δ18O values, [Ca+], and pH), and cave air (CO2, and T) from four drip sites over 3.5 years of monitoring in the cave. Through this comparison, we show that for a slow drip-rate site, ventilation-driven CO2 degassing can explain seasonal variations in calcite oxygen isotope composition. At faster-dripping sites in this cave, the seasonal effect is limited. At these sites, the DIC reservoir is replenished by new drips faster than its isotopic composition can be modified by degassing CO2 and calcite precipitation, whether occurring each is occurring as an equilibrium or kinetic process. For the slow drip rate site, however, this is the first observation of cave air CO2 variations exerting a control on cave calcite oxygen isotope values. The confirmation of ventilation-driven processes controlling oxygen isotope values at a slow-drip site advances the process-based understanding of stalagmite formation that is required to move beyond the wetter-or-drier paradigm and make quantitative interpretations of speleothem oxygen isotope records.

中文翻译：

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限制由快速 CO2 脱气和方解石沉淀驱动的 speleothem 氧同位素不平衡：来自监测和建模的见解

摘要 氧同位素是重建第四纪降水和/或温度变化的最常用的洞穴测温指标。这些解释要么仅限于定性润湿和干燥趋势，要么依赖于理论、实验和/或经验平衡同位素分馏因子以获得更多定量约束。这些不同的分馏因子具有相似的温度敏感性，但它们的绝对值不同，洞穴方解石一般不会与其滴水在同位素平衡中沉淀。快速 CO2 脱气与方解石沉淀配对，两者都发生在不平衡条件下，是一组通常用于解释洞穴方解石和滴水之间观察到的和平衡同位素分馏之间的偏移的机制。然而，这些不平衡机制与洞穴记录的相关性仍未得到解决。在这里，我们比较了来自关岛热带洞穴的现代洞穴方解石的 δ18O 测量值与 ISOLUTION 代理系统模型的修改版本预测的方解石 δ18O 值。这将洞穴滴水和现代方解石 δ18O 值的全球比较扩展到更高的温度。在 3.5 年的洞穴监测中，我们使用来自四个滴水点的滴水（δ18O 值、[Ca+] 和 pH 值）和洞穴空气（CO2 和 T）的同期测量值来初始化模型。通过这种比较，我们表明，对于缓慢滴速站点，通风驱动的 CO2 脱气可以解释方解石氧同位素组成的季节性变化。在这个洞穴中滴水较快的地点，季节性影响是有限的。在这些站点，DIC 储层被新滴补充的速度比其同位素组成可以通过脱气 CO2 和方解石沉淀来改变的速度更快，无论它们是作为平衡过程还是动力学过程发生的。然而，对于缓慢滴速站点，这是第一次观察到洞穴空气 CO2 变化对洞穴方解石氧同位素值施加控制。在慢滴点位置控制氧同位素值的通风驱动过程的确认推进了对石笋形成的基于过程的理解，这是超越潮湿或干燥范式所需的，并对洞穴氧同位素记录进行定量解释。是否发生每个是作为一个平衡或动力学过程发生。然而，对于缓慢滴速站点，这是第一次观察到洞穴空气 CO2 变化对洞穴方解石氧同位素值施加控制。在慢滴点位置控制氧同位素值的通风驱动过程的确认推进了对石笋形成的基于过程的理解，这是超越潮湿或干燥范式所需的，并对洞穴氧同位素记录进行定量解释。是否发生每个是作为一个平衡或动力学过程发生。然而，对于缓慢滴速站点，这是第一次观察到洞穴空气 CO2 变化对洞穴方解石氧同位素值施加控制。在慢滴点位置控制氧同位素值的通风驱动过程的确认推进了对石笋形成的基于过程的理解，这是超越潮湿或干燥范式所需的，并对洞穴氧同位素记录进行定量解释。