VEGETATION AND CLIMATE SIGNALS IN THE CARBON ISOTOPE COMPOSITIONS OF SPELEOTHEM CALCITE

We measured carbon (C) isotope compositions of CO₂ gas, dissolved inorganic C (δ¹³C_DIC) and calcite (δ¹³C_cc) in two central Texas caves (Natural Bridge Caverns, NB, and Inner Space Cavern, IS) and overlying soils in order to investigate how vegetation and climate influence speleothem δ¹³C values. Previous research in these caves showed that trees are the dominant C source for cave-air CO₂. Here we show that trees, where present, are also the C source for drip water DIC and speleothems. We also show that drip rate and cave-air pCO₂ modulate the magnitude of seasonal δ¹³C excursions from vegetation-controlled values. The δ¹³C values of minimally-degassed drip water in tree-covered regions of the caves (-11.0 ± 0.5 and -13.1 ± 0.4 ‰ for NBCH and ISLM) are similar to those expected for DIC in equilibrium with tree-dominated soil CO₂ (-11.3 ± 0.9 and -13.4 ± 2.3 ‰ for NB and IS). This similarity implies that drip water DIC is sourced from trees and that neither prior calcite precipitation nor host rock dissolution measurably influence δ¹³C_DIC. A 250-year BP conventional radiocarbon age for NB cave air CO₂ is likely explained by decomposition of ancient tree roots in the epikarst. Measured δ¹³C values of drip water from the distal end of flowstones (NBCT and ISST) vary seasonally, exhibiting higher winter values (-8.4 ± 0.8 and -7.6 ± 0.3 ‰ for NBCT and ISST) and lower, or ‘baseline’, summer values (-11.3 ± 0.8 and -9.3 ± 0.7 ‰ for NBCT and ISST). At NB, summer baseline δ¹³C_DIC values are similar to the minimally degassed drip water DIC values (NBCH). Calcites precipitated on artificial substrates in IS reflect δ¹³C_DIC values. Calcites at ISLM have lower baseline δ¹³C values and smaller amplitude winter excursions (baseline = -11.1; amplitude = 1.5‰) than calcites precipitated under the distal flowstone drip at ISST (-8.2‰; 4.5‰). The absence of trees above ISST explains the difference between C₄ grass-influenced (ISST) and tree-controlled (ISLM) δ¹³C values. The IS δ¹³C_cc baselines thus reflect vegetation distribution. The increase of δ¹³C_cc values with decreasing drip rate support δ¹³C_cc as a paleoaridity proxy. Peak δ¹³C_cc values during the winter, when cave pCO₂ is lowest, suggest that in seasonally well-ventilated caves δ¹³C_cc fluctuations may resolve annual layers. Drip rate and pCO₂ likely interact in their control on δ¹³C_cc values.