1. Calcite farming in Florida caves: Calibrating modern calcite δ18O and δ13C to ventilation and in situ air temperature
Darrel M. Tremaine, Brian Kilgore, Philip N. Froelich
Florida State University

2. Scientific Problem & Approach
Cave speleothems are valuable archives of paleoclimate information, however in order to produce quantitative paleo-records it is ideal to establish in situ calibrations between cave microclimate and calcite composition.
Approach to Solution:
We “farm” modern calcite precipitates from dripwater of measured isotopic and elemental composition in a cave that is continuously monitored for microclimate and hydrologic conditions.

3. Study Site: Hollow Ridge Cave, FL
30°N 85°W – Subtropical
21m ASL
Rainfall ~ 1370mm yr-1
Temperature ~ 18.3°C
Subject to Sub-arctic Jet Stream and NGOM Convection
Oligocene (38 Myr) Limestone

4. Study Site: Hollow Ridge Cave, FL

5. Calcite Farming Methods
Standard glass microscope slides (1” x 3”) attached to stalagmites with flexible wire mesh under active drips
Slides tilted to prevent water pooling
Slides deployed for 3-month intervals, then rinsed to remove residual salts, dried, and stored for analyses
Drip waters collected every two weeks above farming slides and analyzed for cations, anions, TE, TCO2, pH, δ18O, δD, and δ13C

7. HRC Keeling Plot
Develop this slide

8. Temperature Controls on δ18OCaCO3
Equilibrium Precipitation:
Low supersaturation (Ω ~ 1) where Ω = [Ca2+][CO32-] / K’SP
Very slow precipitation rates, i.e. RF<< RB
Largest discrimination against 16O into calcite
Highest possible αCaCO3 – H2O
Equilibrium Fractionation

9. Temperature Controls on δ18OCaCO3
Kinetic Precipitation:
High supersaturation (Ω: ) where Ω = [Ca2+][CO32-] / K’SP
Very fast precipitation rates, i.e. RF ≥ RB
Smallest discrimination against 16O into calcite (calcite = bulk water 16O)
Lower αCaCO3 – H2O
Equilibrium Fractionation
Kinetic Fractionation

10. Temperature Controls on δ18OCaCO3
Kinetic Precipitation:
High supersaturation (Ω: ) where Ω = [Ca2+][CO32-] / K’SP
Very fast precipitation rates, i.e. RF ≥ RB
Smallest discrimination against 16O into calcite (calcite = bulk water 16O)
Lower αCaCO3 – H2O
Equilibrium Fractionation
Kinetic Fractionation

11. Ventilation Controls on δ13CCaCO3
Carbon Equilibrium Systematics:
Ca2+ + 2HCO3- 
CO2(g) + CaCO3 + H2O
Degassing controlled by cave air pCO2 which is a function of ventilation, rapid CO2 degassing enriches δ13CDIC
After drip splash, chemical & isotopic equilibration between drip and air happens in ~ 40 seconds enriching δ13CDIC (Dreybrodt & Scholz, 2011)

12. Ventilation Controls on δ13CCaCO3
Carbon Equilibrium Systematics:
Ca2+ + 2HCO3- 
CO2(g) + CaCO3 + H2O
Degassing controlled by cave air pCO2 which is a function of ventilation, rapid CO2 degassing enriches δ13CDIC
After drip splash, chemical & isotopic equilibration between drip and air happens in ~ 40 seconds enriching δ13CDIC (Dreybrodt & Scholz, 2011)

13. Ventilation Controls on δ13CCaCO3
The “Hendy” Test
Normally stalagmite δ13C vs. δ18O is used to determine whether fractionation occurs along laminations
We use it as a whole-cave ventilation proxy
Calcite within the ventilation pathways have a +1.9‰ shift in δ13C composition, while the δ18O is only a function of temperature

14. Conclusions
Empirical equilibrium temperature-dependent fractionation line for δ18OCaCO3:
1000 ln α =16.1(103 T-1) -24.6
Hollow Ridge Cave calcite chemistry is a function of ventilation driven gradients in temperature and cave air CO2
Two mechanisms for δ13CCaCO3 enrichment:
- Drip re-equilibration with heavy atmospheric CO2 during strong ventilation
- Rapid dripwater degassing of light CO2
By applying the “Hendy” Test to the entire cave, we can gain information about the ventilation regime(s) at each set of formations
- In HRC δ13CCaCO3 is a tracer for DIC and ventilation strength
- Rapid shifts in δ13CCaCO3 might indicate cave collapse events

15. Thank You for Your Attention
Research at Hollow Ridge Cave was conducted under operating agreements with the Southeast Cave Conservancy Inc., Chattanooga, TN 37407
National Science Foundation – Paleoclimate division (NSF# AGS )
Florida State University Francis Eppes Foundation