LETTER: COMMENTS ON PROCESSES CONTRIBUTING TO THE ISOTOPE COMPOSITION OF 13C AND 18O IN CALCITE DEPOSITED TO SPELEOTHEMS O PROCESIH, KI VPLIVAJO NA IZOTOPSKO SESTAVO 13C IN 18O V KALCITNIH SIGAH Wolfgang DREYBRODT1 INTRODUCTION Isotope compositions of carbon and oxygen in calcite deposited to stalagmites are regarded as important proxies for paleo-climate. The number of papers reporting such proxies rises rapidly. Additionally cave monitoring is being performed to observe chemical properties of the water dripping to speleothems, the partial pressure of CO2 in the soil above the cave and in the cave atmosphere, and other parameters outside the cave to relate recent climate conditions to recently deposited calcite. There are, however, physical and chemical processes independent of climatic conditions, which also contribute to the isotope composition and which can add noise to the climate signal. In this letter I resume these from the current literature to open a more easy access to this problem than available from the current original publications. THE CHEMICAL EVOLUTION OF WATER ON ITS WAY FROM ENTERING THE CAVE TO THE DRIP SITE AND DURING PRECIPITATION OF CALCITE Water entering the cave mostly is in chemical equilibrium with respect to calcite. Its pH-value depends only on temperature and Ca-equilibrium-concentration as depicted in Fig. 1. For [Ca]eq = 2 mmol/L, pH is about 7.3. PCO2 in this solution exceeds PCO2 in the cave. Since CO2 in the solution is present as molecular carbon dioxide degassing occurs by molecular diffusion when the water gets into contact with the cave atmosphere. In most cases each drip site has a catchment area, e.g. a stalactite, from which the water flows in thin films of about 0.01 cm thickness to- wards the drip site contributing to drip formation. Fig. 2 illustrates the chemical evolution of the water on its way to the drip site. Degassing of CO2 from a thin water layer into the cave atmosphere is fast, with an exponential approach with time constant ^ = 462/(n2D ). 5 is film thick- deg ^ m^ ness and D is the coefficient of molecular diffusion (1 • 10- cm2 s-1). At 10° C T deg = 3.5 s for 5 = 0.01cm. To achieve 95% of equilibrium the time T = 3^ ^ is deg deg needed. (Buhmann & Dreybrodt 1985, Dreybrodt 1988) 1 Karst Processes Research Group, Fachbereich 1 , University of Bremen, 28359 Bremen, Germany 2 Karst Research Institute ZRC SAZU, Titov Trg 2, 6230 Postojna, Slovenia, e-maü: dreybrodt@t-online.de Fig. 1: Water entering the cave is in chemical equilibrium with respect to calcite. The corresponding pH-values are given as a function of the calcium- equilibrium- concentration for 10°C and 20°C. After degassing and equilibration of the carbonate system with respect to the PCO2 = 0.0004 atm in the cave atmosphere pH rises to values above 8. Calculated by use of PHREEQC version 2 (Parkhurst & Appelo 1999). After out gassing is completed (red line in Figure 2) pH, [HCO3-], and [CO32-] are no longer in equilibrium with respect to the lower concentration of CO2 in the solution. Establishing chemical and isotopic equilibrium (blue line) needs the time Te = 3 • Te = 321 s. (Dreybrodt & Scholz 2011; Zeebe et cel. 1999) sind causes an increase in pH and supersaturation with respect to calcite. The pH-values after completion of this process, higher by about one pH-unit are shown also in Fig. 1. Since the time Tlq of equilibration is one order of mag- nitude higher than the time Tdeg of degassing both Ca-concentration and pH stay almost constant during degassing. By measuring pH at the drip site with a micro- pH-electrode, which needs only one drop of solution, one is able to find out if degassing is already completed. For values of about 7.5 one can assume that one still deals with a solution saturated with respect to calcite, whereas pH-values above 8.3 indicate degassing and equilibration with respect to the PCO2 in the cave atmosphere. Fig. 2: Reaction pathways and the time needed to achieve equilibrium for each of them. In step 1 (degassing) pH and [Ca]-concentration stay constant. In step 2 (equilibration to new PCO2) pH increases, but Ca-concentration remains almost unaltered. During precipitation (step3) pH decreases slightly with decreasing calcium concentration. The dotted line schematically depicts the chemical pathway when the time of degassing is on the same order of magnitude as the time needed for equilibration. Fig. 3: Chemical equilibrium concentrations Ca-eq, pH, and saturation index SI after out gassing into a cave with PCO2cave and subsequent equilibration. The initial precipitation rates R = a • (C. - C ) at the onset of ^ tn eq' J precipitation and the pH-values after completion of precipitation are also shown. a = 1.5- 10-5 cm s-1. The values of Ca-eq are the calcite concentrations in equilibrium with PCO2cave. Note that the rates decline with increasing PCO2cave. Calculated by use of PHRICOiQC version 2 (Parkhurst & Appelo . After equilibrium and correspondingly supersaturation are attained calcite begins to precipitate (green line in fig. 2). Precipitation rates (blue line in Fig. 3) are given by R = a-(C - Ceq). a is a temperature dependent rate constant, C is the actual calcium-concentration, and C eq is the calcium concentration in equilibrium with the PCO2 in the solution. During precipitation Ceq is approached exponentially with time constant Tprec = 6/a. At 10°C T = 780 s and T = 3- t = 2340 s is much larger prec prec prec ^ than Teq (Buhmann & Dreybrodt 1985; Dreybrodt 1988; Baker et al. 1998). During precipitation the Ca-concen-tration decreases and pH becomes lower by less than 0.5 pH-units (Dreybrodt & Scholz 2011). For each CaCO- unit attached to the calcite surface one molecule of CO2 is released by the reaction HCO- + H+ ^ H2O + CO2 and degasses by molecular diffusion. This out gassing is driven by precipitation of calcite and must not be confused with degassing in step 1. Fig. 3 illustrates pH after degassing and equilibration as a function of the PCO2 in the cave atmosphere. Also are given the precipitation rates, the value of Ceq (Ca-eq), in equilibrium with the cave atmosphere and the saturation index SI. Note that this figure shows these values after equilibration at the onset of precipitation. The pH-value after completion of precipitation is also shown. Summarizing: Three steps determine the precipitation of calcite: 1) Physical out gassing in time scales of 10s for water films with thickness 5 <0.02 cm. 2) Subsequent equilibration of pH, [HCO-], and [CO2-] and establishment of supersaturation in time scales of 100s. 3) Precipitation of calcite in time scales of 1000s. Since these time scales are all different by one order of magnitude in a first approximation these 3 steps can be regarded as subsequent in time and Fig. 1 is an appropriate representation of reality. One comment must be given here. If the time of out gassing due to larger film thickness 5 is on the same order of magnitude as the equilibration time these two steps occur simultaneously as depicted schematically by the dotted curve in Fig. 1. If the time to attain equilibrium is still one order of magnitude smaller than the time needed for precipitation this has no consequences for the following arguments. If the water dripping to a stalagmite has not achieved full equilibration when it impinges to the stalagmite the saturation index SI has not reached its maximal value and consequently deposition rates also stay low. This applies for travel times T 3- Tprec both 13C and 18O can become more heavy by a few per mille due to Rayleigh-distillation (Dreybrodt 2008; Scholz et al. 2009; Dreybrodt and Scholz 2011). If the water needs a time much longer than Teq to reach the drip site, prior calcite precipitation (PCP) at the cave walls can occur as seen from many calcite crusts in the vicinity of drip sites. This can cause drastic enrichment in both 13C and 18O in the calcite precipitated at the apex of the stalagmite (Dreybrodt & Scholz 2011). Recently Sherwin and Baldini (2011) observed PCP on stalactites and on cave ceilings. Flow velocities along the walls of a cave or down a stalactite are on the order of 0.1 cm/s. Therefore water from sources, e.g. a joint, about half a meter away from the dripping site are ideal candidates for delivering drip water in chemical and isotopic equilibrium needed to record climate conditions at the surface above the cave. The catchment area and flow velocities can be determined by putting a tiny drop of uranine above the dripping site. Shining UV-light to it marks the pathway of the water from that point and the time until it reaches the drip site can be determined. We have done such experiments successfully. Fig. 4 shows an illustration. Fig. 4: flow of water film towards the tip of stalagmite. The water was dyed by fluorescent tracer. The situation at different times as denoted on the pictures was photographed with Uv illumination. (Photo: F. Gabrovšek) REFERENCES Baker, A., Genty, D., Dreybrodt, W., Barnes, W.L., Mock-ler, N.J. & J. 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