The evolution of ¹³C and ¹⁸O isotope composition of DIC in a calcite depositing film of water with isotope exchange between the DIC and a CO₂ containing atmosphere, and simultaneous evaporation of the water. Implication to climate proxies from stalagmites: A theoretical model

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• 作者：Wolfgang Dreybrodt^a ; ^{dreybrodt@t-online.de" class="auth_mail" title="E-mail the corresponding author} ; Douchko Romanov^b
• 关键词：Stalagmite ; Isotope composition ; Isotope exchange
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 December 2016
• 年：2016
• 卷：195
• 期：Complete
• 页码：323-338
• 全文大小：739 K

文摘

The most widely applied climate proxies in speleothems are the isotope compositions of carbon and oxygen expressed by δ¹³C and δ¹⁸O values. However, mechanisms, which are not related to climate changes, overlay the climate signal. One is the temporal increase of both, δ¹³C and δ¹⁸O values by kinetic processes during precipitation of calcite. Isotope exchange between DIC in the water and the CO₂ in the surrounding cave atmosphere can also change isotope composition.

Here we present a theoretical model of the temporal isotope evolution of DIC in a thin water layer during precipitation of calcite and simultaneous isotope exchange with the cave atmosphere, and simultaneous evaporation of water. The exchange of oxygen isotopes in the DIC with those in the water is also considered.

For drip times for T_drip < 0.2τ, where τ is the precipitation time, we find for the change of the δ¹³C and δ¹⁸O values, respectively, after the time T_drip

The first term on the right hand side is the contribution from precipitation of calcite, the second stems from isotope exchange with the CO₂ of the cave atmosphere, and the third results from isotope exchange between oxygen in the DIC and the oxygen in the water.

λ, ε are kinetic parameters, τ   is the time scale of precipitation, 65dd887ef2d32">$({\delta }_{\text{eq}}^{\text{atm}}-{\delta }_0)$ and $({\delta }_{\text{eq}}^{\text{water}}-{\delta }_0)$ are the differences between the corresponding initial δ-value δ₀ and the value ${\delta }_{\text{eq}}^{\text{atm,water}}$ if DIC were in isotope equilibrium with the atmosphere or in the case of oxygen with the water, respectively. d15dbdc0e2b4c67b87904">${\tau }_{\text{in}}^{\text{atm}}$ and τ^water are the time scales of approach to isotope equilibrium by the exchange reactions. C_eq is the concentration of DIC in chemical equilibrium with the CO₂ in the cave atmosphere and C₀ is the initial concentration, when the water drips to the stalagmite. T_ev is the time needed for complete evaporation of the water layer. ε_W is the fractionation between water vapor and fluid water.

For times T_drip > 0.2τ we find a further increase of Δ_DIC(T_drip) until a maximum is passed and a final value is reached. If 65dd887ef2d32">$({\delta }_{\text{eq}}^{\text{atm}}-{\delta }_0)$ and $({\delta }_{\text{eq}}^{\text{water}}-{\delta }_0)$ are both zero, exchange has no influence on the isotope composition for drip times T_drip < 0.2τ_B. For carbon it is likely that 65dd887ef2d32">$({\delta }_{\text{eq}}^{\text{atm}}-{\delta }_0)$ is about zero in summer time when most of the CO₂ comes from the vadose zone (ground air), and about 12‰ in winter time in well ventilated caves, when most of the CO₂ stems from Earth’s atmosphere outside the cave.

Therefore δ¹³C may be affected by isotope exchange. For the ¹⁸O equilibrium between the DIC and the oxygen in the water as well as the oxygen in the CO₂ of the atmosphere is likely in summer time. In winter time CO₂ from the atmosphere outside the cave with δ of about 6‰ VPDB may imprint small signals.