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Topic

Numerical and stratigraphic expression of anomalous carbon cycling in the terminal Neoproterozoic, and Phanerozoic

School of Earth and Ocean Sciences

Date & location

• 9:00 A.M.
• Tuesday, December 9, 2025
• Clearihue Building, Room B017

Examining Committee

Supervisory Committee

• Dr. Blake Dyer, School of Earth and Ocean Sciences, ӣ��Ӱ�� (Co-Supervisor)
• Dr. Jon Husson, School of Earth and Ocean Sciences, UVic (Co-Supervisor)
• Dr. Anne-Sofie Ahm, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Eva Kwoll, Department of Geography, UVic (Outside Member)

External Examiner

• Dr. David Fike, Department of Earth, Environmental, and Planetary Sciences, Washington University

Chair of Oral Examination

• Prof. Martin Segger, Department of Art History and Visual Studies, UVic

Abstract

Carbon on Earth’s surface plays a central role in regulating climate. Most surface carbon in the ocean-atmosphere exists as CO₂, sourced from the mantle and released through volcanism. Over geologic timescales, this carbon is removed from the ocean-atmosphere system by burial as organic matter or as carbonate sediments (CaCO₃). The isotopic composition of marine carbonates (δ¹³C_carb) records the balance between these two sinks and, because organic carbon is predominantly produced through oxygenic photosynthesis, also records information about net oxygen release to the atmosphere. The global carbon cycle thus links the deep Earth, surface climate, and the redox state of the ocean-atmosphere system. Tracking variations in δ¹³C_carb through time provides insight into the co-evolution of life and environment, planetary habitability, and the development of Earth’s biosphere. This dissertation investigates intervals of Earth history when carbonate δ¹³C values were highly variable, potentially marking major changes to biogeochemical cycles. My thesis has two focii: (1) the Neoproterozoic Era (∼1000-539 Ma), particularly the Ediacaran Period (∼635-539 Ma), and (2) modern shallow-water carbonate platforms, where local processes shape the stratigraphic record of δ¹³C change over time. The results of my thesis research are presented in three data chapters. In chapter 2, I present data from Ediacaran sediments of the Old Fort Point Formation in the southern Canadian Cordillera that record a highly negative excursion in δ¹³C_carb values (below –12‰). I argue that this excursion is linked to the globally observed Shuram excursion, and that the geochemistry of these carbonates was acquired at or near the time of deposition, ruling out late-stage alteration. This chapter highlights the global nature of the Shuram and underscores the potential for local changes to seawater chemistry, rather than late diagenesis, as drivers for excursions such as the Shuram. In chapter 3, I examine how sediment transport and redistribution influence geochemical records such as values of δ¹³C_carb in shallow-water carbonate platforms, using numerical modelling. Model simulations show that sediment transport and offshore gradients in δ¹³C_DIC can lead to correlative isotope excursions. These results demonstrate that δ¹³C excursions, such as the Hirnantian carbon isotope excursion (HICE) of the terminal Ordovician (∼487-443 Ma), can arise without requiring changes to global biogeochemical cycles. In chapter 4, I expand upon the numerical model I developed in chapter 3 to investigate the potential for growth of early diagenetic carbonate cements and organic matter respiration as a driver for highly negative δ¹³C_carb values over Earth history. Here I show that simple interactions between high TOC margins and local sea-level cycles naturally generate large negative δ¹³C_carb excursions, with magnitudes reaching ∼12‰ when respired carbon is incorporated into authigenic cements. This framework provides an alternative mechanism for the generation of highly negative carbon isotope excursions, including the Shuram, throughout Earth history. Together, my findings demonstrate how local depositional and diagenetic processes, operating at basin scales, can generate isotopic signals that mimic global perturbations. Recognizing these systematic spatial patterns in basin δ¹³C_carb is critical for distinguishing local diagenetic overprints from true global perturbations, and for more reliably linking carbon isotope excursions to changes in the global carbon cycle.