Physics and chemistry of co2 outgassing from a solution precipitating calcite to a speleothem: Implication to13c,18o, and clumped13c18o isotope composition in dic and calcite

Abstract

UDC 552.545:556.114 Wolfgang Dreybrodt: Physics and chemistry of CO2 outgassing from a solution precipitating calcite to a speleothem: Implication to13C,18O, and clumped13C18O isotope composition in DIC and calcite Outgassing of CO2 from thin water layers of a solution of CaCO3 in an H2O-CO2 system plays a crucial role in the precipitation of calcite. Understanding the process of outgassing of CO2 during precipitation of calcite to the surface of stalagmites is important for the interpretation of isotope signals in the calcite deposited to the speleothem. There is, however, some confusion in the literature about the physics and chemistry of this process. Indistinct terms like forced, enhanced, rapid, intense, slow, increased, equilibrium and progressive outgassing are used widely in the literature to explain the impact on isotope composition of the calcite deposited. It is shown that in all the variety of conditions occurring in nature only two distinct processes of outgassing exist. 1. Diffusion controlled outgassing: In the first step, whenever a thin water layer of calcareous solution is present, either on the cave wall or on the surface of a stalagmite, molecular CO2 escapes within several seconds by physical diffusion and after about 40 seconds pH and DIC in the solution achieve chemical equilibrium with respect to the CO2 in the cave atmosphere. 2.) Controlled by precipitation: In the second step this supersatu-rated solution precipitates calcite, whereby for each unit CaCO3 deposited one molecule of CO2 is generated and escapes from the solution by molecular diffusion. This precipitation controlled outgassing is active during precipitation only. All variations of outgassing mentioned in the literature can be explained by one of these two types of outgassing. Furthermore it is shown that the first step of outgassing driven by diffusion has no influence on the isotope composition of the HCO3- reservoir in the solution and consequently on that of calcite precipitated from it. The isotope composition of HCO3 for13C as well as for18O solely is determined by the second step of precipitation controlled outgassing. An experiment is presented proving that the amount of CO2 escaping from the solution during precipitation of calcite at any time is equal to the amount of calcite precipitated. The results are used for a critical application to the Δ47 clumped isotope thermometer that explains why in most stalagmites the calcite is not a good candidate to obtain correct temperatures at the time of its deposition.