Experimental evaluation of oxygen isotopic exchange between inclusion water and host calcite in speleothems
Experimental evaluation of oxygen isotopic exchange between inclusion water and host calcite in speleothems
复制标题
DOI：
10.5194/cp-16-17-2020
复制
发表时间：
2019-08
影响因子：
4.3
通讯作者：
R. Uemura;Yudai Kina;Chuan‐Chou Shen;K. Omine
中科院分区：
文献类型：
--
作者：
R. Uemura;Yudai Kina;Chuan‐Chou Shen;K. Omine
抽象的。洞穴沉积物流体包裹体中水的氢氧同位素组成是重要的水文气候指标，因为它们提供了过去雨水同位素组成的信息。此外，由于流体包裹体水和主机方解石之间的同位素差异提供了过去的同位素分馏因子的信息，他们也是有用的定量估计过去的温度变化。包裹体水的氧同位素比值（δ 18 Ofi）可能受到水与寄主碳酸盐岩之间的同位素交换的影响。因此，有必要估计这种沉积后效应所造成的偏差，以精确重建古温度。在这里，我们评估的同位素交换反应之间的包裹体水和主机方解石的实验室实验的基础上，涉及天然石笋。从同一深度段切取的多个石笋样品在实验室中在105 ℃下加热0 - 80 h。然后，对包裹体水的同位素组成进行了测定。在105 ℃加热实验中，δ 18 Ofi值从初始值增加了0.7 ‰，约100 ℃后保持稳定。20小时水的氢同位素比值没有表现出响应于加热实验的趋势，这表明流体包裹体水的氢同位素组成有效地反映了过去滴水的组成。然后，我们使用部分平衡模型评估了所观察到的同位素变化背后的过程。实验结果最好解释的假设，薄CaCO 3层周围的夹杂物与水反应。与水反应的CaCO 3的量以摩尔计相当于水夹杂物的2%。这些结果表明，同位素交换效应的大小对第四纪气候重建的古温度估计有轻微的影响。
Abstract. The oxygen and hydrogen isotopic compositions of water in fluid inclusions in speleothems are important hydroclimate proxies because they provide information on the isotopic compositions of rainwater in the past. Moreover, because isotopic differences between fluid inclusion water and the host calcite provide information on the past isotopic fractionation factor, they are also useful for quantitative estimation of past temperature changes. The oxygen isotope ratio of inclusion water (δ18Ofi), however, may be affected by isotopic exchange between the water and the host carbonate. Thus, it is necessary to estimate the bias caused by this postdepositional effect for precise reconstruction of paleotemperatures. Here, we evaluate the isotopic exchange reaction between inclusion water and host calcite based on a laboratory experiment involving a natural stalagmite. Multiple stalagmite samples cut from the same depth interval were heated at 105 ∘C in the laboratory from 0 to 80 h. Then, the isotopic compositions of the inclusion water were measured. In the 105 ∘C heating experiments, the δ18Ofi values increased from the initial value by 0.7 ‰ and then remained stable after ca. 20 h. The hydrogen isotope ratio of water showed no trend in response to the heating experiments, suggesting that the hydrogen isotopic composition of fluid inclusion water effectively reflects the composition of past drip water. We then evaluated the process behind the observed isotopic variations using a partial equilibration model. The experimental results are best explained by the assumption that a thin CaCO3 layer surrounding the inclusion reacted with the water. The amount of CaCO3 that reacted with the water is equivalent to 2 % of the water inclusions in molar terms. These results suggest that the magnitude of the isotopic exchange effect has a minor influence on paleotemperature estimates for Quaternary climate reconstructions.