Variations in tropical west Pacific climate and its role in glacial-interglacial cycles
Final Report Abstract
The aim of this project was to investigate tropical climate changes through the late Pleistocene using speleothems from Borneo. To date, our understanding of global climate changes during this time period relies to a large part on data from the high latitudes (mainly Antarctic ice cores) and marine sediments, due to a lack of long terrestrial records, especially from the low latitudes. This study has contributed a new multi-proxy dataset from a key region in the tropics, the West Pacific Warm Pool, covering three glacial-interglacial cycles from around 550-200 thousand years before present (ka). In addition to the traditional oxygen isotope (δ18O) measurements and trace element analyses, we applied the new ‘clumped isotope’ paleo-thermometer to the stalagmites. U-Th dating of the samples has turned out to be more challenging than anticipated, due to the specific characteristics of the stalagmites, including extremely low initial 234U/238U isotope ratios and a slight open system behavior. The age constraints are therefore not sufficient to assess the exact timing of climatic changes in Borneo. Nonetheless, several findings from this study that are less dependent on precise age control have important implications for our view on tropical climate. This study has yielded two unexpected results: First, the ‘clumped isotope’ temperatures were indistinguishable from present-day cave temperatures, unlike all other speleothem samples analyzed to date. The data further suggest that there was minimal temperature change in the cave throughout our record, despite significant temperature changes in the surrounding ocean. Second, despite an observed shift in maximum atmospheric pCO2 between interglacials before and after 450 ka, corresponding to proportional changes in global ice volume and Antarctic temperatures, in our record δ18O values (interpreted as amount of precipitation) are similar for all interglacials. This finding suggests a partial decoupling of tropical and high latitude climate under stable interglacial conditions. In contrast, we observe strong responses of the hydrological cycle at our site to millennial-scale cooling events in the North Atlantic region during glacial terminations. The apparent lack of a response in tropical West Pacific climate to changing interglacial pCO2 levels suggests that pCO2 might not be a strong forcing factor for tropical climate under interglacial conditions. Furthermore, the suggested different response of high and low latitude climate implies larger latitudinal climatic gradients during interglacials before 450 ka than today.
