Estimation of epikarst air [formula omitted] using measurements of water δ13CTDIC, cave air [formula omitted] and [formula omitted

Abstract

When present, an epikarst represents the starting point (the first karst compartment) of water flow through a karst system. The air characteristics in a karst, and especially in an epikarst, determine the initial water characteristics, e.g., water aggressiveness, which depends on the partial pressure of CO2 (PCO2) in equilibrium with water. This paper proposes a method to estimate PCO2 in epikarst air using spring water measures as HCO3-, temperature, pH and δ13C of Total Dissolved Inorganic Carbon (TDIC) and cave air measures as PCO2 and δ13C of CO2. This method accounts for the TDIC variations of δ13C that are caused by CO2 degassing and calcite precipitation from water. The calculations are based on the influence of pH variations and carbon loss on δ13C of TDIC. Measurements are taken at two sites: Lascaux cave and the Cussac cave sites located in Perigord, southwest of France. Four water springs are presented in this case study: two springs from an epikarst compartment, one spring from an unsaturated zone and one spring from a saturated zone. The PCO2 in epikarst air is estimated to be from 4.4% (44,000ppm) in winter to 10% in summer. These values are higher than the values of air PCO2 measured in the soil (0.27–1.60%) or in the caves (0.30–3.1%, up to 7.50% in some parts of Lascaux). We show that in epikarst air, PCO2 and δ13CCO2 are not constant values but vary annually with high PCO2 and depleted values (−22.31‰ VPDB) in the winter and higher PCO2 and more depleted values in the summer (−24.20‰ VPDB).