Paper No. 6
Presentation Time: 2:40 PM
VEGETATION AND CLIMATE SIGNALS IN THE CARBON ISOTOPE COMPOSITIONS OF SPELEOTHEM CALCITE
We measured carbon (C) isotope compositions of CO2 gas, dissolved inorganic C (δ13CDIC) and calcite (δ13Ccc) 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 δ13CDIC. 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 δ13CDIC values are similar to the minimally degassed drip water DIC values (NBCH). Calcites precipitated on artificial substrates in IS reflect δ13CDIC 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 δ13Ccc baselines thus reflect vegetation distribution. The increase of δ13Ccc values with decreasing drip rate support δ13Ccc as a paleoaridity proxy. Peak δ13Ccc values during the winter, when cave pCO2 is lowest, suggest that in seasonally well-ventilated caves δ13Ccc fluctuations may resolve annual layers. Drip rate and pCO2 likely interact in their control on δ13Ccc values.