Seasonal microclimate control of calcite fabrics, stable isotopes and trace elements in modern speleothem from St Michaels Cave, Gibraltar - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 332

Fig. 9. Observed and modeled variations in drip water Ca and coexisting calcite, drip water and cave air d 13 Casa
function of fluctuating cave air pCO 2 responding to seasonal reversals in cave ventilation. (a) Calcite solubility curve
and curves representing the compositions of solutions with calcite saturation indices (log of the ratio of ionic activity
product to calcite solubility product) of 0.5, 1.0 and 1.5 calculated for pure water at 18 8C using PHREEQC (Appelo &
Postma 2007). The loci of solution compositions evolving via coupled degassing - calcite precipitation from an initial
solution in equilibrium with calcite and air containing 70 000 ppmv CO 2 (pCO2 10 21 ) containing 4.5 mmole/l Ca, of
are shown for 'summer' degassing where final equilibrium would be attained under low cave air levels of 500 ppmv
CO 2 ; the four curves represent cases where degassing is 10, 10 2 , 10 3 (heavy line) and 10 4 greater than the rate of
calcite precipitation. The cumulative amount of calcite precipitated is shown for the 10 3 curve which mirrors the
evolution of dripwater at the flowstone site (triangles). The compositions of dripwater (open circles) for the Gib04a site
along with a hypothetical initial solution in equilibrium with calcite and air containing 10 000 ppmv CO 2 are shown for