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

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.

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

Study Site: Hollow Ridge Cave, FL

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

HRC Keeling Plot Develop this slide

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

Temperature Controls on δ18OCaCO3 Kinetic Precipitation: High supersaturation (Ω: 10-100) 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

Temperature Controls on δ18OCaCO3 Kinetic Precipitation: High supersaturation (Ω: 10-100) 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

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)

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)

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

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

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-1032403) Florida State University Francis Eppes Foundation