Insights Into Early Eocene Global Biological Productivity and Mammal Ecology Using Novel Methods to Analyze Triple Oxygen Isotopes in Phosphatic Tooth Enamel

Abstract

This dissertation focuses on analytical developments to triple oxygen isotope measurements (Δ′17O) and their potential for new applications in the paleosciences. In this dissertation, I explore the utility of Δ′17O as a paleoecological tool using the fossil teeth of early Eocene mammals. Furthermore, I use these Δ′17O measurements to probe global biological productivity in the early Eocene.

Chapter 2 introduces a new analytical method for making high-precision (< 10 ppm 1σ uncertainty) Δ′17O measurements in an array of diverse Earth materials, including organics, sulfates, phosphates, nitrates, carbonates, silicates, and waters. The method uses a 3-step process to quantitatively convert sample oxygen to molecular oxygen (O2) for mass spectrometric analysis. Sample oxygen is first converted to oxygen in CO via a high temperature conversion (HTC) furnace (commonly known by the popular Thermo brand “TC/EA”), then oxygen in CO is converted to oxygen in water via a Bosch-type methanation reaction, and finally oxygen in water is converted to O2 by fluorination. In this chapter, I report the first ever high-precision Δ′17O measurements for organic materials and nitrates along with new measurements for various other standard reference materials, including phosphates, sulfates, silicates, carbonates, and waters.

Chapter 3 applies the triple oxygen isotope system to a taxonomically diverse group of fossil mammal teeth from the early Eocene aged strata of the Bighorn Basin in northwest Wyoming. I use Δ′17O as a tracer for evaporative signals that yield information about the types of water resources each animal used during its life. This chapter offers new insights into the water-use strategies, dietary preferences, and habitats of the animals sampled. I found evidence for semi-aquatic lifestyles in Homogalax and Coryphodon, while Hyracotherium appeared to be substantially more water independent. Esthonyx and Cantius recorded more variable intermediate Δ′17O values, potentially suggesting more generalized water-use in these taxa. Additionally, the relatively small range of Δ′17O values observed in these teeth suggest that the Bighorn Basin may have been humid during this time.

Chapter 4 explores the utility of Δ′17O in mammalian bioapatite to reconstruct biological productivity at the global scale. Here, I used animal body water models to deconvolve the Δ′17O signal of respired atmospheric O2 from the fossil mammal teeth sampled in chapter 3. I then leveraged the relationship between global gross primary productivity (GPP) and Δ′17O of atmospheric O2 to reconstruct global GPP. The results from this analysis suggest that global GPP may have been substantially elevated relative to present day, providing additional context for the state of life during a warm greenhouse world unlike the modern.

The advances to triple oxygen isotope geochemistry outlined here offer new, highly versatile methodologies and applications that lay the foundation for routine Δ′17O measurements in a number of new Earth materials. This will fundamentally allow for new investigations into previously underexplored topics in the geosciences and expand our understanding of ancient Earth and its inhabitants.