Climatic dependence of stable carbon and oxygen isotope signals recorded in speleothems: From soil water to speleothem calcite

Abstract

Understanding the relationship between stable isotope signals recorded in speleothems (δ 13C and δ 18O) and the isotopic composition of the carbonate species in the soil water is of great importance for their interpretation in terms of past climate variability. Here the evolution of the carbon isotope composition of soil water on its way down to the cave during dissolution of limestone is studied for both closed and open-closed conditions with respect to CO 2. The water entering the cave flows as a thin film towards the drip site. CO 2 degasses from this film within approx. 10 s by molecular diffusion. Subsequently, chemical and isotopic equilibrium is established on a time scale of several 10–100 s. The δ 13C value of the drip water is mainly determined by the isotopic composition of soil CO 2. The evolution of the δ 18O value of the carbonate species is determined by the long exchange time T ex, between oxygen in carbonate and water of several 10,000 s. Even if the oxygen of the CO 2 in soil water is in isotopic equilibrium with that of the water, dissolution of limestone delivers oxygen with a different isotopic composition changing the δ 18O value of the carbonate species. Consequently, the δ 18O value of the rainwater will only be reflected in the drip water if it has stayed in the rock for a sufficiently long time. After the water has entered the cave, the carbon and oxygen isotope composition of the drip water may be altered by CO 2-exchange with the cave air. Exchange times, τ ex CO 2 , of about 3000 s are derived. Thus, only drip water, which drips in less than 3000 s onto the stalagmite surface, is suitable to imprint climatic signals into speleothem calcite deposited from it. Precipitation of calcite proceeds with time constants, τ p, of several 100 s. Different rate constants and equilibrium concentrations for the heavy and light isotopes, respectively, result in isotope fractionation during calcite precipitation. Since T ex ≫ τ p, exchange with the oxygen in the water can be neglected, and the isotopic evolution of carbon and oxygen proceed analogously. For drip intervals T d < 0.1 τ p the isotopic compositions of both carbon and oxygen in the solution evolve linearly in time. The calcite precipitated at the apex of the stalagmite reflects the isotopic signal of the drip water. For long drip intervals, when calcite is deposited from a stagnant water film, long drip intervals may have a significant effect on the isotopic composition of the DIC. In this case, the isotopic composition of the calcite deposited at the apex must be determined by averaging over the drip interval. Such processes must be considered when speleothems are used as proxies of past climate variability.