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 CO₂ of the cave atmosphere

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• 作者：Wolfgang Dreybrodt^a ; ; Maximilian Hansen^b ; Denis Scholz^b
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 February 2016
• 年：2016
• 卷：174
• 期：Complete
• 页码：247-262
• 全文大小：1263 K

文摘

We present a theoretical derivation of the exchange time, τ_ex, 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 ${\tau }_{\text{ex}}={\tau }_{\text{red}}^{\text{ex}}·[{\text{HCO}}_3^{-}]/\left(K_{\text{H}}·p_{{\text{CO}}_2}^{\text{cave}}\right)$, where ${\tau }_{\text{red}}^{\text{ex}}$ depends on the depth, a  , of the water film and on temperature. $[{\text{HCO}}_3^{-}]$ is the concentration of bicarbonate, $p_{{\text{CO}}_2}^{\text{cave}}$ the partial pressure of CO₂, and K_H 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 p_CO2$p_{{\text{CO}}_2}$ 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, τ_ex. 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 p_CO2$p_{{\text{CO}}_2}$ = 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 δ_DIC(t)=δ_DIC(0)-(δ_DIC(0)-δ_DIC(∞))·t/τ_ex${\delta }_{\text{DIC}}(t)={\delta }_{\text{DIC}}(0)-({\delta }_{\text{DIC}}(0)-{\delta }_{\text{DIC}}(\infty ))·t/{\tau }_{\text{ex}}$ where 1a20a9b31f9d11e7d" title="Click to view the MathML source">δ_DIC(0)${\delta }_{\text{DIC}}(0)$ is the initial isotope composition of dissolved inorganic carbon (DIC) in the water film and 41a3554c9dc4ef6b23cbe13b8f53291d" title="Click to view the MathML source">δ_DIC(∞)${\delta }_{\text{DIC}}(\infty )$ its final value. From these data an exchange time τ_ex of ca. 7000 s was obtained, in satisfactory agreement with the theoretical predictions. The exchange times can be calculated by ${\tau }_{\text{ex}}={\tau }_{\text{red}}^{\text{ex}}·[{\text{HCO}}_3^{-}]/\left(K_{\text{H}}·p_{{\text{CO}}_2}^{\text{cave}}\right)$, where ${\tau }_{\text{red}}^{\text{ex}}$ 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.