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Topic

δ^13C of mid-Pacific carbonate sediments around the time of Ocean Anoxic Event 2

School of Earth and Ocean Sciences

Date & location

• Wednesday, February 5, 2025
• 9:00 A.M.
• Virtual Defence

Examining Committee

Supervisory Committee

• Dr. Laurence Coogan, School of Earth and Ocean Sciences, University of Victoria (Co-Supervisor)
• Dr. Blake Dyer, School of Earth and Ocean Sciences, UVic (Co-Supervisor)
• Dr. Anne-Sofie Ahm, School of Earth and Ocean Sciences, UVic (Member)

External Examiner

• Dr. Jeremy Owens, Department of Earth, Ocean, and Atmospheric Science, Florida State University

Chair of Oral Examination

• Dr. Kirstin Lane, School of Exercise Science, Physical and Health Education, UVic

Abstract

The δ¹³C of marine carbonates is commonly used as a proxy for the global burial fraction of carbonate vs. organic carbon. At the same time, carbonate δ¹³C is understood to vary locally from the global ocean average due to coeval spatial variation in seawater geochemistry, especially in shallow marine settings, and/or post-depositional alteration of sediments. In Cretaceous-age shallow marine sections, Ocean Anoxic Event 2 (OAE2) is identified by a positive δ¹³C excursion near the Cenomanian-Turonian boundary (∼93.9 Ma). However, there are currently no OAE2 δ¹³C records from the Pacific basin, which at the time was ∼65% of ocean area and included most of the world’s deep ocean. Here I present new (n = 255) and previously published (n = 13) bulk carbonate δ¹³C and δ¹⁸O data from 10 mid-Pacific sites, measured in sediments recovered by the Deep Sea Drilling Program, which I use to test the hypothesis that there was a global change in the δ¹³C_DIC of seawater at the time of OAE2. I use existing biostratigraphic data to target sediments of Cenomanian and Turonian age. Between sites, I compare δ¹³C_carbonate and δ¹⁸O_carbonate values associated with the same planktonic foraminifera species. My Pacific δ¹³C_carbonate values range from 0.88‰ to 4.20‰ VPDB (excluding 3 samples within a 7-cm interval with -2.7‰ to -2.5‰ VPDB). They are generally lower than shallow marine bulk carbonate δ¹³C values where the OAE2 positive δ¹³C excursion has been identified, e.g., the well-known Eastbourne, UK section where Cenomanian and Turonian δ¹³C values range from 2.6 to 5.4‰ VPDB. To explain δ¹³C_carbonate values within the Pacific, I invoke both factors local to the sites, such as proximity to a shallow semi-restricted carbonate platform in the case of the site with the shallowest paleodepth, and changes in global ocean δ¹³C_DIC over time. An OAE2 positive δ¹³C_carbonate excursion is not observed at most Pacific sites in this thesis, which has two possible explanations. One possible explanation is that a global ocean OAE2 δ¹³C excursion did occur, but that some process largely prevented its deposition, preservation, or recovery in the mid-Pacific. Another possible explanation is that the OAE2 δ¹³C excursion occurred only in the shallow epicontinental basins and continental slope environments where it is observed (including the Atlantic, Western Tethys, Neotethys, and Western Interior Seaway of North America) because organic carbon burial within these relatively restricted water masses changed their δ¹³C_DICs but did not affect δ¹³C_DIC in the mid-Pacific.