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

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 dC values of soil CO2 (d Cs) in grassland and savanna above these caves with dC values of cave drip water (d13CHCO3 ) and modern speleothem calcite grown on artificial substrates (dCcc). We compare d 13CHCO3 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 d 13CHCO3 values. The d13CHCO3 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 d13CHCO3 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 d13CHCO3 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 d13CHCO3 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 d13CHCO3 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 d13CHCO3 values ( 10.6 ± 0.5& and 9.7 ± 0.3& under savanna at NB and under grassland at IS, respectively). The relatively high d13CHCO3values of indirect drip sites during the winter (d13CHCO3 = 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 dC values from direct and indirect drip sites at IS. The dCcc 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 dCcc values of direct drip calcite are similar to dCcc values predicted from soil CO2 collected beneath trees above that drip site. The occurrence of highest dCcc values during the winter, when cave CO2 concentrations are low, highlights the significance of ventilation-driven changes in cave-air pCO2. Modern calcite d C values are also negatively correlated with drip rate, which suggests that dCcc variations are controlled by kinetic effects during degassing and calcite precipitation associated with the drip water http://dx.doi.org/10.1016/j.gca.2014.07.027 0016-7037/ 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Current address: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, United States. E-mail address: [email protected] (K.W. Meyer). 1 Current address: Environmental Services Business Group, CH2M Hill, Asheville, NC, United States. 282 K.W. Meyer et al. /Geochimica et Cosmochimica Acta 142 (2014) 281–298 exposure time to a low-pCO2 environment. In all, at the caves we investigated, variability in speleothem d Ccc values primarily reflect presence/absence of deep-rooted vegetation and kinetic isotope effects. We therefore infer that increased aridity may result in higher dC values of vegetation, lower drip rates and more drip water degassing, and thus higher dCcc values of speleothem calcite. 2014 Elsevier Ltd. All rights reserved.