Speleothem calcite farmed in situ: Modern calibration of δ 18O and δ 13C paleoclimate proxies in a continuously-monitored natural cave system

Abstract

Understanding the relationships between speleothem stable isotopes (δ 13C δ 18O) and in situ cave forcing mechanisms is important to interpreting ancient stalagmite paleoclimate records. Cave studies have demonstrated that the δ 18O of inorganically precipitated (low temperature) speleothem calcite is systematically heavier than the δ 18O of laboratory-grown calcite for a given temperature. To understand this apparent offset, rainwater, cave drip water, groundwater, and modern naturally precipitated calcite (farmed in situ) were grown at multiple locations inside Hollow Ridge Cave in Marianna, Florida. High resolution micrometeorological, air chemistry time series and ventilation regimes were also monitored continuously at two locations inside the cave, supplemented with periodic bi-monthly air gas grab sample transects throughout the cave. Cave air chemistry and isotope monitoring reveal density-driven airflow pathways through Hollow Ridge Cave at velocities of up to 1.2 m s −1 in winter and 0.4 m s −1 in summer. Hollow Ridge Cave displays a strong ventilation gradient in the front of the cave near the entrances, resulting in cave air that is a mixture of soil gas and atmospheric CO 2. A clear relationship is found between calcite δ 13C and cave air ventilation rates estimated by proxies pCO 2 and 222Rn. Calcite δ 13C decreased linearly with distance from the front entrance to the interior of the cave during all seasons, with a maximum entrance-to-interior gradient of Δδ 13C CaCO3 = −7‰. A whole-cave “Hendy test” at multiple contemporaneous farming sites reveals that ventilation induces a +1.9 ± 0.96‰ δ 13C offset between calcite precipitated in a ventilation flow path and calcite precipitated on the edge or out of flow paths. This interpretation of the “Hendy test” has implications for interpreting δ 13C records in ancient speleothems. Calcite δ 13C CaCO3 may be a proxy not only for atmospheric CO 2 or overlying vegetation shifts but also for changes in cave ventilation due to dissolution fissures and ceiling collapse creating and plugging ventilation windows. Farmed calcite δ 18O was found to exhibit a +0.82 ± 0.24‰ offset from values predicted by both theoretical calculations and laboratory-grown inorganic calcite. Unlike δ 13C CaCO3, oxygen isotopes showed no ventilation effects, i.e. Δδ 18O CaCO3 appears to be a function of growth temperature only although we cannot rule out a small effect of (unmeasured) gradients in relative humidity (evaporation) accompanying ventilation. Our results support the findings of other cave investigators that water–calcite fractionation factors observed in speleothem calcite are higher that those measured in laboratory experiments. Cave and laboratory calcite precipitates may differ mainly in the complex effects of kinetic isotope fractionation. Combining our data with other recent speleothem studies, we find a new empirical relationship for cave-specific water–calcite oxygen isotope fractionation across a range of temperatures and cave environments: 1000 ln α = 16.1 10 3 T - 1 - 24.6 with a fractionation temperature dependence of Δδ 18O/Δ T = −0.177‰/°C, lower than the currently accepted −0.206‰/°C.