Oxygen isotope variations in rainfall, drip-water and speleothem calcite from a well-ventilated cave in Texas, USA: Assessing a new speleothem temperature proxy

Measurements of the oxygen isotopic composition (delta O-18 value) of rainfall, drip-water and associated calcite (grown on artificial substrates and in a natural speleothem) at multiple drip sites in a well-ventilated cave in central Texas were conducted to investigate the potential use of speleothem delta O-18 for quantitative temperature reconstructions. From 2009 to 2011, rainfall delta O-18 varied by 11.5 parts per thousand (-10.5 parts per thousand to 1.0 parts per thousand, V-SMOW), whereas drip-waters had a much narrower range of 0.3 parts per thousand (-4.7 parts per thousand to -4.4 parts per thousand, V-SMOW). This contrast indicates that mixing processes along flow paths in the vadose zone above the cave produce a well-homogenized water reservoir that supplies drip-water to the cave. The delta O-18 values for calcite grown on substrates over the same time period show seasonal variations (summer: similar to-6 parts per thousand; winter: similar to-3 parts per thousand) that are strongly correlated with surface air temperatures (t(a)) at all three monitored sites (r(2) = 0.88-0.96; p < 0.001). These results indicate that the dominant control on calcite delta O-18 is temperature. An empirical relationship was established for one monitored site: t(a) = -9.1 (+0.9) x delta O-18 X 10(-3) - 20.6 (+/- 4.1); r(2) = 0.88, and applied to a delta O-18 time series of the top 6.7 mmof a stalagmite that grew at this drip site. This yields a temperature record that appears to reflect seasonal variations for the period 2005-2009. This speleothem-derived temperature record is offset to lower values (by 0-8 degrees C) compared to the instrumental temperature record. This offset may be a result of differences between substrate and speleothem calcite in terms growth mechanism, extent of non-equilibrium isotopic effects, or temporal shifts in drip-water delta O-18 values. Despite this offset, the speleothem-derived record reconstructs the amplitudes of seasonal variations and changes in inter-annual summer peak temperatures in the instrumental record. Results of this study have implications for reconstructing past temperatures, and for establishing a speleothem chronology with seasonal resolution, using speleothem calcite delta O-18. The results also suggest that relatively seldom-studied speleothem samples near the entrances of caves, where environmental conditions may be similar to surface conditions, could prove valuable as paleo-temperature archives. (C) 2013 Elsevier Ltd. All rights reserved.