Stacey Edmonsond

Topic

Spatiotemporal variability in the stratigraphic record reveals past global and local carbon cycling

School of Earth and Ocean Sciences

Date & location

• Monday, October 26, 2026
• 9:30 A.M.
• Clearihue Building, Room B007

Examining Committee

Supervisory Committee

• Dr. Blake Dyer, School of Earth and Ocean Sciences, University of Victoria (Supervisor)
• Dr. Anne-Sofie Ahm, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Roberta Hamme, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Terri Lacourse, Department of Biology, UVic (Outside Member)

External Examiner

• Dr. Chris Holmden, Department of Geological Sciences, University of Saskatchewan

Chair of Oral Examination

• Dr. Jaime Grimm, Department of Biology, UVic

Abstract

The sedimentary rock record is the primary archive of Earth’s long-term biogeochemical evolution. Translating vertical changes in the physical and chemical properties of stacked sedimentary rocks into information about Earth system change over time is the fundamental challenge of stratigraphy. Past perturbations to the global carbon cycle, which regulates Earth’s climate on geologic timescales, commonly are inferred from stratigraphic shifts in the carbon isotopic composition (δ¹³C) of ancient marine carbonate rocks. However, carbonate δ¹³C values are sensitive not only to global carbon cycling, but also to local depositional and diagenetic processes that vary among regions and environments, complicating interpretations of stratigraphic δ¹³C variations. In this thesis, I develop computational and field methodologies to untangle global and local geochemical signals in spatially heterogeneous stratigraphic records. I use these approaches to interpret evidence for global and local carbon cycling in carbonate sequences from two intervals of Earth history: the Paleoproterozoic era (ca. 2500 − 1600 million years ago), and the early to middle Pennsylvanian period (ca. 323 − 307 million years ago).

In Chapter 2, I introduce a novel Bayesian statistical approach for reconstructing global changes in geochemical proxy values over time using incomplete, time-uncertain, and locally variable stratigraphic observations. Using only age constraints and simple geologic principles, the model (StratMC) simultaneously correlates multiple stratigraphic sections, constructs an age model for each section, and estimates global and local signals for one or more sedimentary proxies. Synthetic experiments establish that StratMC can successfully recover proxy signals from complex stratigraphic data sets analogous to those encountered in the rock record.

Chapter 3 focuses on stratigraphic evidence for a perturbation to the global carbon cycle across the initial oxygenation of Earth’s atmosphere. Specifically, I apply StratMC to a global compilation of Paleoproterozoic geochemical and geochronological data to constrain the timing and magnitude of the Lomagundi-Jatuli carbon isotope excursion (LJE). The LJE is a large and long-lived positive excursion in the δ¹³C of marine carbonate rocks that is broadly co-eval with both the rise of atmospheric O₂ and global ‘Snowball’ glaciations. However, its structure, cause, synchroneity, and global versus local nature are widely debated due to the incomplete nature of the sedimentary record. I show that the LJE is a global excursion with an earlier onset, longer duration, and lower magnitude than previously thought, strengthening its temporal correlation with both oscillations in atmospheric O2 levels and swings in global climate.

Chapter 4 investigates lateral and stratigraphic variability in the geochemistry of early to middle Pennsylvanian marine carbonates. Using detailed sedimentological and geochemical cross-sections through an exposure of the Ely Limestone in eastern Nevada, I quantify variations in δ¹³C and facies over lengthscales of centimeters to hundreds of meters. I show that early diagenesis causes δ¹³C values to vary along beds and among sedimentary components, and that variations in isotope values are modulated by facies. The same local controls create stratigraphic shifts in δ¹³C that are unrelated to evolving global seawater chemistry, but that encode information about paleoenvironment. For example, I demonstrate that changes in sediment composition and pore fluid processes across episodes of sea level rise create negative δ¹³C excursions in deep-water carbonate muds. This case study underscores the importance of testing that stratigraphic signals are laterally reproducible before making interpretations about broader Earth system change, and illustrates the potential utility of local geochemical variability in paleoenvironmental reconstructions.