Nature and causes of Quaternary climate variation of tropical South America

To further evaluate climate forcing over the last glacial cycle (∼125 ka), we developed a climate forcing model that combines summer insolation forcing and a proxy for North Atlantic SST forcing to reconstruct long-term precipitation variation in the SASM domain. The success of this model reinforces our confidence in assigning causation to observed reconstructions of precipitation. In addition, we propose a critical correction for speleothem stable oxygen isotopic ratios, which are among the most significant of paleoclimate proxies in tropical South America for reconstruction of variation of paleo-precipitation (or SASM intensity). However, it is already well known that any particular δ18O value observed in speleothem carbonate is affected by two processes that have nothing to do with changes in precipitation amount-the influence of temperature on carbonate-water isotopic fractionation in the cave and the influence of changing δ18O of seawater. Quantitatively accounting for both “artifacts” can significantly alter the interpretations of speleothem records. In tropical South America, both adjustments act in the same direction and have the tendency to increase the true amplitude of the paleo-hydrologic signal (but by different amounts in glacial and inter-glacial stages). These corrections have even graver implications for the interpretation of tropical Northern Hemisphere speleothem records (e.g. Chinese speleothems) where the combined adjustments tend to decrease or even eliminate the “true” signal amplitude.