Speleothem calcite farmed in situ: Modern calibration of δ18O and δ13C paleoclimate proxies in a continuously-monitored natural cave system
Speleothem calcite farmed in situ: Modern calibration of δ18O and δ13C paleoclimate proxies in a continuously-monitored natural cave system
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DOI：
10.1016/j.gca.2011.06.005
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发表时间：
2011-09
影响因子：
5
通讯作者：
D. Tremaine;P. Froelich;Yang Wang
中科院分区：
文献类型：
--
作者：
D. Tremaine;P. Froelich;Yang Wang
了解洞穴稳定同位素（δ13C δ18O）与原位洞穴强迫机制之间的关系对于解释古代石笋古气候记录非常重要。洞穴研究表明，在给定温度下，无机沉淀（低温）洞穴方解石的 δ18O 比实验室生长的方解石的 δ18O 更重。为了了解这种明显的偏移，在佛罗里达州玛丽安娜空心岭洞穴内的多个地点种植了雨水、洞穴滴水、地下水和现代自然沉淀的方解石（就地养殖）。还在洞穴内的两个位置连续监测高分辨率微气象、空气化学时间序列和通风状况，并定期每两个月在整个洞穴内采集空气样本。洞穴空气化学和同位素监测揭示了通过空心岭洞穴的密度驱动气流路径，冬季速度高达 1.2ms−1，夏季速度高达 0.4ms−1。空心岭洞穴在入口附近的洞穴前部表现出强烈的通风梯度，导致洞穴空气是土壤气体和大气二氧化碳的混合物。方解石 δ13C 和通过代理 pCO2 和 222Rn 估计的洞穴空气通风率之间存在明显的关系。在所有季节中，方解石δ13C随着从前入口到洞穴内部的距离线性减小，入口到内部的最大梯度为Δδ13CCaCO3=-7‰。在多个同时期养殖场进行的全洞穴“Hendy 测试”表明，通风会导致通风流道中沉淀的方解石与流道边缘或流道外沉淀的方解石之间产生+1.9±0.96‰ δ13C 偏移。 “Hendy 测试”的这种解释对于解释古代洞穴中的 δ13C 记录具有重要意义。方解石 δ13CCaCO3 不仅可以代表大气 CO2 或上覆植被变化，还可以代表由于溶解裂缝和天花板塌陷产生并堵塞通风窗而导致的洞穴通风变化。研究发现，人工方解石 δ18O 与理论计算和实验室种植的无机方解石预测值存在 +0.82±0.24‰ 的偏移。与 δ13CCaCO3 不同，氧同位素没有表现出通风效应，即 Δδ18OCaCO3 似乎仅是生长温度的函数，尽管我们不能排除伴随通风的相对湿度（蒸发）梯度的小影响。我们的结果支持其他洞穴调查人员的发现，即在洞穴方解石中观察到的水-方解石分馏因子高于实验室实验中测量的水-方解石分馏因子。洞穴和实验室方解石沉淀物可能主要在动态同位素分馏的复杂效应方面有所不同。将我们的数据与其他最近的洞穴研究相结合，我们发现了在一系列温度和洞穴环境下洞穴特定的水-方解石氧同位素分馏的新经验关系：分馏温度依赖性为Δδ18O/ΔT=-0.177‰/°C，低于目前接受的-0.206‰/°C。
Understanding the relationships between speleothem stable isotopes (δ13C δ18O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ18O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ18O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave. Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2ms−1in winter and 0.4ms−1in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO2. A clear relationship is found between calcite δ13C and cave air ventilation rates estimated by proxies pCO2and222Rn. Calcite δ13C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ13CCaCO3=−7‰. A whole-cave “Hendy test” at multiple contemporaneous farming sites reveals that ventilation induces a +1.9±0.96‰ δ13C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the “Hendy test” has implications for interpreting δ13C records in ancient speleothems. Calcite δ13CCaCO3may be a proxy not only for atmospheric CO2or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating and plugging ventilation windows. Farmed calcite δ18O was found to exhibit a +0.82±0.24‰ offset from values predicted by both theoretical calculations and laboratory-grown inorganic calcite. Unlike δ13CCaCO3, oxygen isotopes showed no ventilation effects, i.e. Δδ18OCaCO3appears to be a function of growth temperature only although we cannot rule out a small effect of (unmeasured) gradients in relative humidity (evaporation) accompanying ventilation. Our results support the findings of other cave investigators that water–calcite fractionation factors observed in speleothem calcite are higher that those measured in laboratory experiments. Cave and laboratory calcite precipitates may differ mainly in the complex effects of kinetic isotope fractionation. Combining our data with other recent speleothem studies, we find a new empirical relationship for cave-specific water–calcite oxygen isotope fractionation across a range of temperatures and cave environments: with a fractionation temperature dependence of Δδ18O/ΔT=−0.177‰/°C, lower than the currently accepted −0.206‰/°C.