Disentangling climatic, tectonic, and proxy-system controls on clumped isotope temperature records from lacustrine carbonates: A case-study from the Miocene Bidahochi Formation on the Colorado Plateau, Navajo Nation, AZ, U.S.A.

Abstract

Landscapes are shaped by climate and tectonic forces over geologic timescales. To understand these processes, there is a need to reconstruct paleo-environments and their topographic and climatic conditions. The Colorado Plateau is a place where the mechanisms of uplift and the development of the modern climate system are debated, and thus there is a need for proxy records of climate and topography in this region. A relatively novel and potentially powerful proxy for reconstructing topography and climate is the clumped isotope thermometer of lacustrine carbonate (T?47). This T?47 proxy is assumed to record lake water temperature and, in turn, surface temperature which is controlled by both topography and climate. This thesis reconstructed a T?47 record from a Miocene lacustrine basin on the Colorado Plateau, and interpreted the record by integrating multiple lines of geologic evidence, geochemical proxies, and proxy-system models. In Chapter 1 I compiled existing paleo-elevation estimates and geologic constraints for the Colorado Plateau region and reviewed recent advances in paleo-elevation methods. This work concluded that existing estimates permitted multiple hypotheses of Colorado Plateau uplift, and that there was potential to leverage proxy method advancements to refine and re-evaluate a record from the 16-6 Ma Bidahochi Fm. In Chapter 2 I collected field data and micritic lacustrine samples that I characterized sedimentologically and analyzed for their bulk isotope, clumped isotope, and dual-clumped isotope compositions. This work found that the T?47 record preserved a significant cooling trend of ~10 ± 6 °C. In Chapter 3 I used climate (CESM) and proxy-system (PRYSM) models to explore hypotheses to explain the observed proxy record and the sensitivity of T?47 to variable lake conditions. This work found that the proxy record could be explained by climate or topographic change, but likely requires changes within the lake carbonate proxy-system and thus could record a combination of different forcings. Altogether, this thesis presents an integral framework for interpreting paleo-environmental records and highlights the importance of thoroughly understanding proxy-systems. There is great potential for interdisciplinary collaborations on future research of the Bidahochi formation, and in basins similar to it, to reconstruct paleoclimate and tectonic change in the geologic past via collections of multiple proxy datasets and field observations paired with model experiments.