Abstract Understanding carbon and oxygen isotope systematics in cave environments is a prerequisite for the interpretation of stable isotopes in speleothem-based paleoclimate records. Here we present a series of experimental data collected under laboratory conditions with controlled temperature, relative humidity, drip water chemistry, flow rate and cave pCO2, simulating the growth of speleothems in natural cave settings. Drip water with high pCO2 and low calcium concentration (Ca2+ = 2 mmol/L) flowed along a three-step glass path, similar to a stalactite-stalagmite-pool route in natural caves, forming a thin water film that allowed CO2 degassing and CaCO3 precipitation as a result of the pCO2 gradient between the drip water and ambient cave atmosphere. The experiments were conducted at 15, 25 and 32 °C and flow rates of 700, 270 and 125 ml/d. The growth rate of calcite on the stalagmite-like settings increases linearly with increasing flow rate and/or temperature. The δ13CCc and δ18OCc of calcite formed on the stalagmite-like settings increases with decreasing flow rate (corresponding to increasing exposure time of water) at a given temperature, indicating non-equilibrium isotope effects between calcite, water and dissolved inorganic carbon (DIC). Nevertheless, these non-equilibrium isotope effects still display regular temperature dependence under a constant flow rate. This suggests that non-equilibrium isotope effects in natural stalagmites might be used to provide useful qualitative paleoclimate information (such as differentiating wet/dry and warm/cold climate conditions). The non-equilibrium carbon and oxygen isotope effects in the stalagmite-like settings were most likely caused by rapid CO2 degassing and CaCO3 precipitation that rapidly consume the available DIC pool in the thin water film. Furthermore, CO2 exchange between DIC and cave atmosphere quickly amplified the observed non-equilibrium carbon isotope effects in the precipitated calcite. In the pool-like settings, calcite was buffered by oxygen isotope exchange between DIC species and water, and slowly precipitated at or near to oxygen isotopic equilibrium with the temperature dependence of 1000 l n 18 α C c - H 2 O = 18.33 (103/T) – 33.31 regardless of flow rate. This fractionation relation agrees with that determined by Kim and O’Neil (1997) when a newly recommended value for the acid fractionation factor for calcite is used (i.e., Kim et al., 2015 ). Carbon isotope fractionation between calcite and bicarbonate was temperature-independent between 15 and 32 °C and the average magnitude was 1000ln13αCc–HCO3− = 1.7 ± 0.7‰. Observed variability of 1000 l n 18 α C c - H 2 O in modern calcite speleothems from natural cave settings lies in the range predicted by this study, between the predicted maximum non-equilibrium deviation at the stalagmite-like settings and the equilibrium 1000 l n 18 α C c - H 2 O achieved in the pool-like settings.

中文翻译：

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洞穴环境中的碳氧同位素系统学：来自人造洞穴“麦克马斯特洞穴”的教训

摘要 了解洞穴环境中的碳氧同位素系统学是解释基于洞穴环境的古气候记录中稳定同位素的先决条件。在这里，我们展示了一系列在实验室条件下收集的实验数据，这些数据具有受控的温度、相对湿度、滴水化学、流速和洞穴 pCO2，模拟天然洞穴环境中洞穴动物的生长。具有高 pCO2 和低钙浓度 (Ca2+ = 2 mmol/L) 的滴水沿三步玻璃路径流动，类似于天然洞穴中的钟乳石-石笋-池路线，形成一层薄薄的水膜，使 CO2 脱气和 CaCO3由于滴水和周围洞穴大气之间的 pCO2 梯度而导致的降水。实验在 15、25 和 32 °C 和 700、270 和 125 ml/d 的流速下进行。类石笋环境中方解石的生长速率随着流速和/或温度的增加而线性增加。在给定温度下，在类石笋环境中形成的方解石的 δ13CCc 和 δ18OCc 随着流速的降低（对应于水暴露时间的增加）而增加，表明方解石、水和溶解无机碳 (DIC) 之间存在非平衡同位素效应。尽管如此，这些非平衡同位素效应在恒定流速下仍显示出规律的温度依赖性。这表明天然石笋中的非平衡同位素效应可用于提供有用的定性古气候信息（例如区分湿/干和暖/冷气候条件）。类石笋环境中的非平衡碳和氧同位素效应很可能是由快速 CO2 脱气和 CaCO3 沉淀引起的，它们迅速消耗了薄水膜中可用的 DIC 池。此外，DIC 和洞穴大气之间的 CO2 交换迅速放大了在沉淀方解石中观察到的非平衡碳同位素效应。在池状环境中，方解石被 DIC 物种和水之间的氧同位素交换缓冲，并在氧同位素平衡或接近氧同位素平衡时缓慢沉淀，温度依赖性为 1000 ln 18 α C c - H 2 O = 18.33 (103/ T) – 33.31 与流速无关。该分馏关系与 Kim 和 O'Neil (1997) 在使用方解石酸分馏因子的新推荐值时确定的结果一致（即 Kim 等，2015）。方解石和碳酸氢盐之间的碳同位素分馏在 15 至 32 °C 之间与温度无关，平均震级为 1000ln13αCc–HCO3− = 1.7 ± 0.7‰。在来自天然洞穴环境的现代方解石洞穴中观察到的 1000 ln 18 α C c - H 2 O 的变异性处于本研究预测的范围内，介于石笋状环境中预测的最大非平衡偏差与平衡 1000 ln 18 α C c - H 2 O 在类似池的设置中实现。