Speleothems (stalagmites) record temperature and precipitation signals as they grow. They do this through the oxygen and trace metals in their crystal structure. Oxygen has heavy and light isotopes, and the ratio between them (δ18O) changes with climate. δ18O in speleothems is related to regional climate effects like ice age cycles, moisture source variability, condensation temperature, and precipitation amount. Conversely, trace metals are related to local effects like bedrock composition and groundwater residence time. With careful analysis, we can determine which variables contribute to δ18O and trace metal concentrations of the speleothem and use that information to examine climate patterns over time.
I used a micromill to collect samples from speleothem PH23-2 for this climate analysis. PH23-2 is from a cave in the Peruvian Andes, and is estimated to have grown between 423 and 372 ka. based on U/Th dating. I used isotope ratio mass spectrometry to analyze the samples for δ18O, and I used inductively-coupled mass spectrometry to test for trace element concentrations. PH23-2 grew during an interglacial known as Marine Isotope Stage (MIS) 11. When combined with data from stalagmite PH19-4, the climate record extends back to glacial MIS 12 and captures Termination V, the transition from glacial to interglacial climate. δ18O shows an abrupt rise at Termination V and an abrupt drop in the middle of MIS 11. This is unexpected because interglacials are warmer, which tends to strengthen precipitation over the Andes; however, an increase in δ18O indicates drying. I propose that this is due to a decreased land-sea temperature gradient which weakened precipitation over South America. Additionally, the drop in δ18O is of similar magnitude to changes typical of MIS boundaries, and yet it falls in the middle of MIS 11. I propose that this is due to the extended duration of MIS 11 leading to ice sheet collapse in the N. Atlantic. The influx of cold water strengthened the land-sea gradient and increased precipitation.
Upon analysis of many global climate records, I found a global trend of increased temperatures from approximately 424 to 395 ka. While Termination V is agreed to take place at 424 ka., the MIS 11/10 boundary is thought to be at 374 ka. despite a lack of extreme climate shift at that time. My record indicates that the MIS 11/10 boundary could be shifted to 395 ka. to better align with global trends and reflect the best timing constraints available.
Acknowledgements: National Science Foundation Rodbell and Gillikin EAR-2102996, Union College's isotope ratio mass spectrometer and peripherals were also funded by the U.S. National Science Foundation (NSF-MRI #1229258).