Reconstructing the Penultimate Glaciation: Marine Isotope Stage 6 Record from Speleothems in the Peruvian Andes

Marine Isotope Stage 6, the penultimate glaciation, provides key insights into past climate variability, and the interaction between global cooling and regional climate systems. Speleothems are regional proxies for recording long-term climate patterns and provides context for not only understanding, but predicting modern and future trends. Carbon and oxygen isotope and trace metal data (Mg/Ca, Sr/Ca, and Ba/Ca ratios) were used to determine the environmental conditions of the Peruvian Andes through speleothems (PH19-5a and PH19-11) collected from Huagapo and Pacupahuain cave systems. Within this, the influence of fluctuations in the Intertropical Convergence Zone and the South American Summer Monsoon on the regional precipitation patterns of the area. δ13C and δ18O stable isotope variations were collect using Micromilling techniques the Thermo Gas Bench II Thermo Delta Advantage Ion Ratio Mass Spectrometer. ICP-MS laser ablation system measured the trace metal ratios and U/Th dating was collected to create a chronology of speleothem development throughout MIS 6. The δ18O trends in the speleothems PH19-5a and PH19-11 show contrasting trends, with PH19-5a showing an increase in δ18O from older to younger, while PH19-11 records a decrease in δ18O over the latter half of the glaciation. The δ13C values following the same increasing trend over time from both PH19-5a and PH19-11. These contradictory trends represent variations in local climatic or environmental conditions that tie back to the regional drivers impacting the isotopic composition of the cave system and speleothem growth. The Amount Effect is one of the primary driving factors influencing the isotopic values and trace metal ratios in the speleothems. δ18O values in rainwater are lighter due to greater moisture availability, in regions with increased precipitation. During drier periods, heavier δ18O values are seen as a result of reduced rainfall. This relationship is influenced by the dynamics of the SASM and the ITCZ. Trace metal ratios show details into the vegetation type and bedrock geology of the region. δ13C values identify different carbon sources, including terrestrial plants and microbial processes, which can be influenced by changing local vegetation and environmental conditions. Combining this data, these proxies complete a comprehensive record of the environmental and climatic factors influencing speleothem formation in the Peruvian Andes cave systems during the penultimate glaciation