Interpretation of speleothem calcite delta C-13 variations: Evidence from monitoring soil CO2, drip water, and modern speleothem calcite in central Texas

摘要

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 delta C-13 values of soil CO2 (delta C-13(s)) in grassland and savanna above these caves with delta C-13 values of cave drip water (delta C-13(HCO3)) and modern speleothem calcite grown on artificial substrates (delta C-13(cc)). We compare delta(CHCO3)-C-13 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 delta C-13(HCO3) values. The delta C-13(HCO3) values of direct drip water below savanna (-10.6 +/- 0.5 parts per thousand and -12.6 +/- 0.2 parts per thousand, in NB and IS, respectively) are indistinguishable from (IS) or similar to (NB) calculated delta C-13(HCO3) values in equilibrium with measured soil CO2 beneath trees (-13.5 parts per thousand to -11.3 parts per thousand for juniper trees above NB, and -13.6 parts per thousand to -12.6 parts per thousand for mixed oak and elm trees above IS, respectively). At IS, the delta C-13(HCO3) values of direct drip water are higher below grassland (-9.7 +/- 0.3 parts per thousand) than below savanna (12.6 +/- 0.2 parts per thousand). These results suggest that the delta C-13(HCO3) 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 delta C-13(HCO3) values of indirect drip water vary seasonally with relatively low values during the summer (-10.8 +/- 0.8 parts per thousand and -9.2 +/- 0.4 parts per thousand under juniper savanna at NB and under grassland at IS, respectively) that are similar to the direct drip delta C-13(HCO3) values (-10.6 +/- 0.5 parts per thousand and -9.7 +/- 0.3 parts per thousand under savanna at NB and under grassland at IS, respectively). The relatively high delta C-13(HCO3) values of indirect drip sites during the winter (delta C-13(HCO3) = -8.6 > 0.8 parts per thousand at NB and 8.0 +/- 0.1 parts per thousand at IS) result from CO2 degassing of water along in-cave flow paths. We also present decade-long records of modern calcite delta C-13 values from direct and indirect drip sites at IS. The delta C-13(cc) 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 delta C-13(cc) values of direct drip calcite are similar to delta C-13(cc) values predicted from soil CO2 collected beneath trees above that drip site.