Interpretation of orbital scale variability in mid-latitude speleothem δ18O: Significance of growth rate controlled kinetic fractionation effects

Oxygen isotopes have been the most widely used climate indicator in stalagmites, applied to reconstruct past chahges in rainfall delta O-18 and cave temperature. However, the delta O-18 signal in speleothems may also be influenced by variable kinetic fractionation effects, here conceived broadly as fractionation effects not arising from temperature variation. The regional reproducibility of speleothem delta O-18 signals has been proposed as a way to distinguish the delta O-18 variations arising directly from changes rainfall delta O-18 and cave temperature, from variations due to kinetic effects which may nonetheless be influenced by climate. Here, we compare isotopic records from 5 coeval stalagmites from two proximal caves in NW Spain covering the interval 140 to 70 ka, which experienced the same primary variations in temperature and rainfall delta O-18, but exhibit a large range in growth rates and temporal trends in growth rate. Stalagmites growing at faster rates near 50 mu m/yr have oxygen isotopic ratios over 1 parts per thousand more negative than coeval stalagmites with very slow (5 mu m/yr) growth rates. Because growth rate variations also occur over time within any given stalagmite, the measured oxygen isotopic time series for a given stalagmite includes both climatic and kinetic components. Removal of the kinetic component of variation in each stalagmite, based on the dependence of the kinetic component on growth rate, is effective at distilling a common temporal evolution of among the oxygen isotopic records of the multiple stalagmites. However, this approach is limited by the quality of the age model. For time periods characterized by very slow growth and long durations between dates, the presence of crypto-hiatus may result in average growth rates which underestimate the instantaneous speleothem deposition rates and which therefore underestimate the magnitude of kinetic effects. The stacked growth rate-corrected speleothem delta O-18 is influenced by orbital scale variation in the cave temperature and the delta O-18 of the ocean moisture source, but also by temporally variable fractionation in the hydrological cycle. The most salient trend is increased hydrological fractionation during the GI-22 period, when warmer sea surface temperatures in the subtropical Atlantic moisture source region may have favored greater precipitation amounts. (C) 2015 Elsevier Ltd. All rights reserved.