Processes affecting the stable isotope composition of calcite during precipitation on the surface of stalagmites: Laboratory experiments investigating the isotope exchange between DIC in the solution layer on top of a speleothem and the CO 2 of the cave atmosphere
Processes affecting the stable isotope composition of calcite during precipitation on the surface of stalagmites: Laboratory experiments investigating the isotope exchange between DIC in the solution layer on top of a speleothem and the CO 2 of the cave atmosphere
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DOI：
10.1016/j.gca.2015.11.012
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发表时间：
2016-02
影响因子：
5
通讯作者：
W. Dreybrodt;M. Hansen;D. Scholz
中科院分区：
文献类型：
--
作者：
W. Dreybrodt;M. Hansen;D. Scholz
我们提出了交换时间 τex 的理论推导，需要在洞穴中的大气 CO2 和溶解在覆盖洞穴表面的薄水膜中的 HCO3− 之间建立同位素平衡。结果为 τex=τredex·[HCO3-]KH·pCO2cave，其中 τredex 取决于水膜的深度 a 和温度。 [HCO3-]是碳酸氢盐的浓度，pCO2是CO2的分压，KH是亨利常数。为了测试该理论，我们制备了静止或流动的 NaHCO3 溶液薄膜，并将其在 20°C 下暴露于含有 pCO2 500、12,500 或 25,000 ppmV 和确定同位素组成的 CO2 气氛中。测量溶液中 DIC 的 δ13C 和 δ18O 值作为暴露时间的函数。对于深度在 0.06 至 0.2 cm 之间的静止薄膜，δ13C 值随着交换时间 τex 呈指数接近与大气 CO2 的同位素平衡。 δ18O 值首先向与大气 CO2 的同位素平衡演化，达到最小值，然后偏离与大气 CO2 的同位素平衡，由于氧与水的同位素交换而接近稳定状态。实验结果与理论预测吻合良好。为了进一步研究洞穴模拟条件下的同位素演化，将含有 5 mmol/L NaHCO3、深度为 0.013 cm 的水膜沿着倾斜的硼硅酸盐玻璃板流下，暴露在温度为 20 °C 的 pCO2 = 500 ppmV 的大气中。 δ13C 和 δ18O 值作为流动（暴露）时间 t 的函数进行测量。水膜 DIC 中的同位素组成随时间线性减小，δDIC(t)=δDIC(0)-(δDIC(0)-δDIC(∞))·t/τex，其中 δDIC(0) 是水膜中溶解的无机碳 (DIC) 的初始同位素组成，δDIC(∞) 是其最终值。从这些数据可以得出大约的交换时间 τex。获得了 7000 秒，与理论预测完全一致。交换时间可以通过 τex=τredex·[HCO3-]KH·pCO2cave 计算，其中 τredex 由理论给出，作为温度和水膜深度 a 的函数。通过这种方式，可以获得洞穴中石笋生长的各种条件的交换时间。
We present a theoretical derivation of the exchange time, τex, needed to establish isotopic equilibrium between atmospheric CO2 in a cave and HCO3− dissolved in a thin water film covering the surface of a speleothem. The result is τex=τredex·[HCO3-]KH·pCO2cave, where τredex depends on the depth, a, of the water film and on temperature. [HCO3-] is the concentration of bicarbonate, pCO2cave the partial pressure of CO2, and KH is Henry’s constant. To test the theory we prepared stagnant or flowing thin films of a NaHCO3 solution and exposed them at 20 °C to an CO2 containing atmosphere of pCO2 500, 12,500, or 25,000 ppmV and defined isotope composition. The δ13C and δ18O values of the DIC in the solution were measured as a function of the exposure time. For stagnant films with depths between 0.06 and 0.2 cm the δ13C values exhibit an exponential approach towards isotope equilibrium with the atmospheric CO2 with exchange time, τex. The δ18O values first evolve towards isotopic equilibrium with atmospheric CO2, reach a minimum value and then drift away from the isotopic equilibrium with atmospheric CO2 approaching a steady state caused by isotopic exchange of oxygen with water. The experimental findings are in satisfactory agreement with the theoretical predictions.To further investigate isotope evolution in cave analogue conditions, a water film containing 5 mmol/L of NaHCO3 with a depth of 0.013 cm flowing down an inclined borosilicate glass plate was exposed to an atmosphere with pCO2 = 500 ppmV at a temperature of 20 °C. The δ13C and δ18O values were measured as a function of flow (exposure) time, t. The isotope compositions in the DIC of the water film decrease linear in time by δDIC(t)=δDIC(0)-(δDIC(0)-δDIC(∞))·t/τex where δDIC(0) is the initial isotope composition of dissolved inorganic carbon (DIC) in the water film and δDIC(∞) its final value. From these data an exchange time τex of ca. 7000 s was obtained, in satisfactory agreement with the theoretical predictions. The exchange times can be calculated by τex=τredex·[HCO3-]KH·pCO2cave, where τredex is given by the theory as function of temperature and the depth, a, of the water film. This way it is possible to obtain exchange times for various conditions of stalagmite growth as they occur in caves.