Andean Interglacial Climate and Hydrology Over the Last 650,000 Years

Abstract

Land temperatures and water supply control the distribution of life on Earth and link the global water and carbon cycles to the planet’s energy budget. Monsoons are at the nexus of these systems and function to transport water vapor and heat from the low latitudes towards the poles. However, the role of global climate on the position, intensity, and timing of monsoon rainfall remains poorly constrained in many regions, which together, are home to >70% of the world’s population.

In this dissertation, I use sediment records from Andean lakes to reconstruct the impact of global climate on the South American summer monsoon (SASM), regional water balance, and temperature. Specifically, I use isotope ratios of carbonate sediments to compare the abundance of heavy oxygen and carbon isotopes to their light counterparts, as these data preserve information about hydroclimate conditions.

Chapter 2 is a calibration study of two new isotope techniques, clumped and triple oxygen isotopes, that were measured in modern waters and carbonates from four lakes in the Junín region of central Peru. Combining the isotope data with numerical water balance models, I show that lake water temperature and evaporation are the primary drivers of carbonate clumped and triple oxygen isotope values, respectively. Accordingly, the interpretive framework established in Chapter 2 scaffolds the paleoclimate interpretations in Chapters 3 and 4, and can also be applied to lake systems beyond the central Andes.

In Chapters 3 and 4, I develop Andean temperature and water balance records during two interglacial periods to assess how hydrologic change manifests under distinct global climate conditions. Chapter 3 examines three Holocene (11.7 ka to present) lake records from the Junín region and Chapter 4 presents data from a single lake (i.e., Lake Junín) during MIS 15 (621 to 563 ka). Clumped isotopes reveal that temperatures during the Holocene and MIS 15 were indistinguishable from the present over both interglacials. These data suggest that temperature variability in the central Andes was minimal and not a primary driver of tropical hydroclimate change during these interglacials. Conversely, triple oxygen isotopes suggest water balance (i.e., surface water supply) was highly variable and follows a precession pacing (~23 ka cycles) during both interglacials. These observations are likely related to changes in summertime insolation, which is a well-documented control on SASM strength over the last ~350 ka. The strong correlation between triple oxygen isotope values and insolation suggests this relationship was the main factor driving water balance variability during both the Holocene and MIS 15. The triple oxygen isotope data reported in Chapters 3 and 4 represent some of the first evidence that water balance is tightly coupled to SASM. This finding illustrates that global climate forcings (e.g., orbital configurations) can have a strong impact on regional hydroclimate, and suggests changes in the planet’s energy balance would alter water availability throughout much of South America.

This dissertation represents the most extensive combined application of clumped and triple oxygen isotopes to lacustrine carbonates, to date, and contributes abundant data from modern and paleo-lake systems. Together with new numerical and conceptual models, these data illuminate a strong connection between Andean water balance, the SASM, and global climate which is explored through Chapters 2–4.