Interpretation of speleothem calcite δ13C variations: Evidence from monitoring soil CO2, drip water, and modern speleothem calcite in central Texas

Abstract

We studied the sources and transport of carbon in two active karst systems in central Texas, Inner Space Cavern (IS) and Natural Bridge North and South Caverns (NB), to provide new insights into the interpretation of speleothem (cave calcite deposit) carbon isotope compositions. We have determined the δ13C values of soil CO2 (δ13Cs) in grassland and savanna above these caves with δ13C values of cave drip water (δ13CHCO3−) and modern speleothem calcite grown on artificial substrates (δ13Ccc). We compare δ13CHCO3− values from direct drip sites, where water was sampled immediately upon discharging from the cave ceiling, to values from indirect sites, where water was sampled after flowing along a prolonged path within the cave that allowed for longer CO2 degassing and have found that direct drip sites yield consistently lower δ13CHCO3− values. The δ13CHCO3− values of direct drip water below savanna (−10.6±0.5‰ and −12.6±0.2‰, in NB and IS, respectively) are indistinguishable from (IS) or similar to (NB) calculated δ13CHCO3− values in equilibrium with measured soil CO2 beneath trees (−13.5‰ to −11.3‰ for juniper trees above NB, and −13.6‰ to −12.6‰ for mixed oak and elm trees above IS, respectively). At IS, the δ13CHCO3− values of direct drip water are higher below grassland (−9.7±0.3‰) than below savanna (12.6±0.2‰). These results suggest that the δ13CHCO3− values of drip waters that initially enter the caves are controlled by deep-rooted plants, where present, and are minimally influenced by host-rock dissolution and/or prior calcite precipitation (PCP). The δ13CHCO3− values of indirect drip water vary seasonally with relatively low values during the summer (−10.8±0.8‰ and −9.2±0.4‰ under juniper savanna at NB and under grassland at IS, respectively) that are similar to the direct drip δ13CHCO3− values (−10.6±0.5‰ and −9.7±0.3‰ under savanna at NB and under grassland at IS, respectively). The relatively high δ13CHCO3− values of indirect drip sites during the winter (δ13CHCO3−=−8.6±0.8‰ at NB and 8.0±0.1‰ at IS) result from CO2 degassing of water along in-cave flow paths. We also present decade-long records of modern calcite δ13C values from direct and indirect drip sites at IS. The δ13Ccc values vary seasonally with lower values during the summer and higher values during the winter, and with smaller amplitude variations at the direct drip site. Such seasonal variations can be used as a geochronological tool in some speleothems that do not contain visible lamina. The summer δ13Ccc values of direct drip calcite are similar to δ13Ccc values predicted from soil CO2 collected beneath trees above that drip site. The occurrence of highest δ13Ccc values during the winter, when cave CO2 concentrations are low, highlights the significance of ventilation-driven changes in cave-air pCO2. Modern calcite δ13C values are also negatively correlated with drip rate, which suggests that δ13Ccc variations are controlled by kinetic effects during degassing and calcite precipitation associated with the drip water exposure time to a low-pCO2 environment. In all, at the caves we investigated, variability in speleothem δ13Ccc values primarily reflect presence/absence of deep-rooted vegetation and kinetic isotope effects. We therefore infer that increased aridity may result in higher δ13C values of vegetation, lower drip rates and more drip water degassing, and thus higher δ13Ccc values of speleothem calcite.