Geochemical indicators of redox conditions as a proxy for mid-Pleistocene climate change from a lacustrine sediment core, Valles Caldera, New Mexico
— Catrina M. Johnson, Peter J. Fawcett, and Abdul-Mehdi S. Ali

This study examines several characteristics of sediments as proxies for lake level and climate, from an 81-m core of mid-Pleistocene lacustrine sediments recovered from the Valle Grande, located within the Valles caldera, New Mexico. Due to the limited sources for the lake, this core is a valuable indicator of regional climatic variations during the mid-Pleistocene. The caldera’s limited watershed is dominated by rocks of rhyolitic composition with well-constrained geochemistry. Because of limited sediment sources, this record is significant, as any geochemical variations through the core will be due to aquatic chemistry and climate rather than a change in source terrain. The core spans ~180 kyr with a basal age of is 552 ± 3 ka and contains a record of two glacial-interglacial cycles spanning marine isotope stages 14 through 10. Samples taken at 20 cm intervals for geochemical analysis show significant systematic variation for 8 major and 13 trace elements coinciding with glacial-interglacial cycles. We see a significant decrease in Fe and Mn concentration at glacial terminations, followed by low values maintained throughout the interglacial interval, suggesting anoxic bottom waters. Al, Ba, Co, Mg, Ni, Pb, Ti, and Zn concentrations also exhibit similar trends with higher values during glacial intervals and lower values during interglacial intervals inversely correlating with total organic carbon (TOC). Higher TOC levels during interglacial intervals indicate increased biological productivity, the decomposition of which depletes oxygen concentrations in the lake.